Note: Descriptions are shown in the official language in which they were submitted.
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PLASMA GUN AND METHODS FOR THE USE THEREOF
Field Of The Invention
This invention relates to plasma guns and more particularly to an improved
plasma gun
suitable for use as a space thruster or to produce radiation at selectable
wavelengths, including
extreme ultraviolet (EUV), vacuum ultraviolet (VUV) and/or soft x-ray
radiation at high pulse
repetition frequency bands. The invention also involves methods for utilizing
such plasma guns.
Background Of The Invention
t0 The improved plasma gun disclosed in U.S. Patent 5,866,871 (the Patent)
finds
application in a variety of environments for performing functions which either
could not be
performed previously, could not be performed well previously, or could only be
performed with
relatively large and expensive equipment. These functions include thrusters
for satellite or other
space station keeping and maneuvering applications, and the controlled
generation of radiation
t 5 at selected frequencies, generally within the extreme ultraviolet (EUV)
band. The plasma guns
disclosed for such applications were particularly advantageous in that they
provided high
reliability and pulse repetition frequency (PRF), and in particular a plasma
gun having a PRF in
excess of approximately 100 Hz and preferably a PRF in excess of 5,000 Hz for
space
applications and PRFs of at least 500 Hz and preferably 1,000 Hz for
lithography or other
20 applications requiring radiation generation.
In order to achieve these objectives, the plasma gun of the Patent had two
general
embodiments, one for space applications or other tlu-uster applications, and a
second embodiment
for radiation generator applications. In both cases, the plasma gun involved a
center electrode and
an outer electrode substantially coaxial with the center electrode, with a
coaxial column being
25 formed between the electrodes. A selected gas was introduced into the
column through an inlet
mechanism, and a plasma initiator was provided at the base end of the column.
Finally, a solid
state high repetition rate pulsed driver was provided which was operable on
pulse initiation at the
base of the column to deliver a high voltage pulse across the electrodes, the
plasma expanding
from the base end of the column and off the end thereof. For the thruster
embodiment, the voltage
30 of each of the pulses decreased over the duration of the pulse, and the
pulse voltage and electrode
length were selected such that the voltage across the electrodes reached a
substantially zero value
as the plasma exited the column. For this embodiment, the inlet mechanism
preferably introduced
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the gas radially from the center electrode at the base end of the column,
thereby enhancing plasma
velocity uniformity across the column, plasma exiting the column for this
embodiment at exhaust
velocities which are currently in the range of approximately 10,000 to 100,000
meters per second,
the exhaust velocity varying somewhat with application.
For the radiation source embodiment of the invention, the pulse voltage and
electrode
lengths are such that the current for each voltage pulse is at substantially
its maximum as the
plasma exits the column. The outer electrode for this embodiment of the
invention is preferably
the cathode electrode and may be solid or may be in the form of a plurality of
substantially evenly
spaced rods arranged in a circle. The inlet mechanism for this embodiment of
the invention
provides a substantially uniform gas fill in the column, resulting in the
plasma being initially
driven off the center electrode. the plasma being magnetically pinched as it
exits the column, to
produce a very high temperature at the end of the center electrode. A selected
gas/element fed
to the pinch as part of the gas, through the center electrode or otherwise, is
ionized by the high
temperature at the pinch to provide radiation at a desired wavelength. The
wavelength is achieved
~ 5 by careful selection of various plasma gun parameters, including the
selected gas/element fed to
the pinch. current from the pulse driver, plasma temperature in the area of
the pinch, and gas
pressure in the column. The Patent for example indicates combinations of
parameters for
generating radiation at a wavelength of approximately 13 nm using for example
lithium vapor as
the gas fed to the pinch.
2o In order for the invention to function effectively in either of the above
applications, it is
critical that the pre-ionization of the gas by the initiator provide an
absolutely uniform pre-
ionization of the gas. For the Patent, this was achieved by forming holes
evenly spaced around
the column, with the gas either being introduced through the holes or directed
at the holes.
Electrodes were provided which were preferably mounted in the holes or
otherwise at the base
25 of the column, and preferably out of the column or closely adjacent
thereto, which electrodes were
fired to initiate plasma. The trigger electrodes were preferably evenly spaced
around the base end
of the column and were fired substantially simultaneously to provide uniform
initiation of plasma
at the base end, a DC signal being used to fire the electrodes. While this
mechanism provides far
more uniform plasma initiation than is possible with any prior arrangements,
and is suitable for
3o most applications, there are applications, particularly when the plasma gun
is being used as a
radiation source, where even more uniform plasma initiation is desirable. This
more uniform
plasma initiation may be provided by using an RF signal to fire the
electrodes. However,
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currently available RF power sources such as magnetrons, klystrons or RF
amplifiers are
relatively expensive to operate, costing approximately $1 per peak power watt,
and are also
relatively large, requiring a cabinet sized enclosure to produce for example
20 kilovolts at 8
megawatts. It would therefore be desirable if the RF signal used to fire the
electrodes could be
generated in a way which produced the power at lower cost, and which also
permitted the RF
power to be generated utilizing a compact solid state circuit which, in
addition to reduced costs
and substantially smaller size, also presents a significantly lower heat
removal burden to the
system. While a simulated RF generator of the type just described would be
particularly useful
in the plasma gun application of this invention, such a simulated RF power
source. which does
t o IlOt currently exist in the art, would also be useful in other
applications.
It is also desirable that the electrodes used for plasma initiation provide a
high voltage
field over as large an area as possible at the base of the column between the
electrodes, and it is
also desirable that it be possible to energize the electrodes to produce the
requisite high voltage
field at the base of the column without needing to bring wires into the vacuum
environment of
~ 5 the column, the maintaining of the vacuum around such wires increasing the
cost of the plasma
gun.
Another problem with plasma guns is to get the requisite gas/material to the
pinch which
material is to be ionized to produce the desired radiation. An improved
technique for holding
such material and releasing it into the column to the pinch is therefore
desirable.
2o Further, while plasma guns of the type indicated above, can serve as a
radiation source
and provide useful radiation at a desired wavelength, the high velocity of the
plasma being driven
down the column and off the center electrode can cause a problem which
significantly limits the
usefulness of such sources. In particular, temperatures at the pinch in the
range of 100 eV (i.e.,
about 11,000°C) to 1000 eV, depending on the desired frequency of
radiation, require magnetic
25 compression fields which are sufficient to drive the plasma to velocities
of several centimeters
per microsecond. Plasmas moving at these velocities down the center conductor
and off the end
forming the pinch tend to continue moving out into space away from the end of
the center
conductor, the plasma sheath eventually losing electrical connection to the
pinch. This
prematurely ends the pinch after as little as 100 nanoseconds and also results
in a large voltage
3o transient in the thousands of volts range, resulting in a restrike which
can severely damage the
electrodes.
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Since a discharge can last for several microseconds, if premature loss of
electrical
connection between the plasma sheath and the electrode could be eliminated,
the pinch lifetime
could be extended dramatically and the potentially damaging restrike
eliminated. This could
result in significantly increased output efficiency for the plasma source and
a greatly expanded
electrode lifetime for the source, thus reducing source down time and
maintenance, both of which
can be expensive in for example a lithographic application. Significantly
better performance at
lower costs can thus be obtained.
Finally, it is desirable to achieve as uniform a breakdown as can be achieved,
and
techniques for enhancing such uniformity of breakdown, particularly by use of
an enhanced drive
signal are desirable.
A need therefore exists for an improved plasma gun and method for the use
thereof which
provides more uniform plasma initiation at lower cost than is possible in
prior art systems, which
facilitates introduction of the material to be ionized at the pinch into the
column, which prevents
premature ending of the pinch and/or restrike and which provides more uniform
breakdown when
high voltage is applied across the main electrodes.
Summary Of The Invention
In accordance with the above. this application provides a high PRF plasma gun
having a
center electrode, an outer electrode substantially coaxial with the center
electrode to form a
2o coaxial column between the electrodes having a closed base end and an open
exit end. an inlet
mechanism for introducing a selected gas into the column, a plasma initiator
at the base end of
the column, a solid state simulated IZF source selectively connected to drive
the plasma initiator,
and a solid state, high repetition rate pulsed driver operable on plasma
initiation at the base of the
column for delivering a high voltage pulse across the electrodes, the plasma
expanding from the
base end of the column and off the exit end thereof. The RF source may for
example operate at
a frequency in the range of 10 MHZ to 1,000 MHZ and may be used either alone
or in
conjunction with a DC source.
The simulated RF source may include an N stage non-linear magnetic pulse
compressor,
where N is an integer >_ 1; a solid state switch selectively operable for
connecting an energy
storage device to an input of a first stage of the compressor; an output stage
having a resonant
circuit at the RF frequency F to be simulated, the resonant circuit including
a capacitor CR and
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a saturable reactor LR, a last stage of the compressor having a capacitance
CN, at least one of CR
and LR being selected so that there is a reverse voltage on CN before CR is
fully charged; LR
successively saturating to cause oscillating of CR at frequency F; and a
coupling circuit for
coupling energy from C,t to drive the plasma initiator. For preferred
embodiments, the solid state
switch is an SCR, an IGBT or a MOSFET. CR may be selected such that C,z > CN
or LR may be
selected such that it saturates before transfer of charge from CN to CR is
completed. The output
stage is preferably a resonant saturable shunt to ground and the coupling
circuit preferably has
an impedance such that only a fraction of the energy stored in C,t is coupled
to the plasma initiator
during each oscillating cycle of C,z. For preferred embodiments, LR and CR are
selected such that
there are only three to four oscillating cycles of the output stage for each
plasma initiation. The
solid state simulated RF source described above may also be utilized
independent of the high PRF
plasma gun application.
The plasma gun of this invention, either in addition to or instead of having
the solid state
simulated RF source, may also have a plurality of electrodes affixed to an
insulator and spaced
substantially uniformly about the column, the electrodes producing a high
voltage field at a
surface of the insulator which surface is at the base end of the column. For
at least one
embodiment of the invention, the insulator surrounds the center electrode at a
base end thereof,
and the electrodes are mounted to the insulator near the base end of the
column. For another
embodiment, the insulator forms a base of the column and the electrodes are
mounted in the
2o insulator on a side thereof outside the column and spaced a short distance
from the column by the
insulator, energizing of the electrodes producing a high voltage field on the
side of the insulator
in the column.
Another feature of the invention which may be utilized either in conjunction
with the prior
features or independent thereof, is to form at least one of the center
electrode and the outer
electrode of a sintered powder refractory metal, both electrodes being formed
of such a sintered
powder refractory metal for a preferred embodiment. When a plasma gun is
operating as a
radiation source at a selected wavelength, the at least one electrode may be
saturated with a fluid
(i.e., liquid or gas) material suitable for generating radiation at such
wavelengths. For certain
embodiments of the invention, this fluid is liquid lithium. A preferred
embodiment of the
3o invention includes a mechanism which provides fluid material to the at
least one electrode on a
substantially continuous basis.
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Another feature of the invention, which again may be utilized either alone or
in
combination with one or more of the prior features, is for the pulse driver to
provide a high
voltage spike followed by a lower voltage, longer duration sustainer signal,
most of the driver
energy being provided by the sustainer signal. The pulse driver may include a
first non-linear
magnetic pulse driver for generating the high voltage spike and a second non-
linear magnetic
pulse driver for generating the sustainer signal. The second driver may have
at least two stages,
a saturable reactor of a last of the stages being normally biased to prevent
the spike from the first
driver entering the second driver, the spike partially desaturating the
reactor to inhibit initial flow
from the second driver until the reactor again saturates to pass the
sustaining signal.
Still another feature of the invention. which again may be utilized either
alone or in
combination with one or more of the prior features, is the provision of a high
PRF radiation
source at a selected wavelength. which source includes a center electrode, an
outer electrode
substantially coaxial with the center electrode, a coaxial column being formed
between the
electrodes, which column has a closed base end and an open exit end; an inlet
mechanism for
introducing a selected gas into the column; a plasma initiator at the base end
of the column; a
solid state high repetition rate pulse driver operable on plasma initiation at
the base of the column
for delivering a voltage pulse across the electrodes, the plasma expanding
from the base end of
the column and off the exit end thereof; the pulse voltage and electrode
lengths being such that
the current for each pulse is at substantially its maximum as the plasma exits
the column; the inlet
2o mechanism providing a substantially uniform gas fill in the column,
resulting in the plasma being
initially driven off the center electrode, the plasma being magnetically
pinched as it exits the
column. raising the temperature at the end of the center electrode sufficient
to cause an ionizable
element appearing at the end of the center electrode to produce radiation at
at least the selected
wavelength; and a component for redirecting plasma driven of the center
electrode back toward
the center electrode without substantially affecting passage of the radiation.
For preferred
embodiments, the component which redirects is a shield of a high temperature,
non-conductive
material positioned a selected distance from the exit end of the center
electrode and shaped to
reflect plasma impinging thereon back toward the center electrode, the shield
having an opening
positioned to permit the radiation to pass therethrough. For preferred
embodiments, the selected
3o distance that the shield is spaced from the center electrode is no more
than approximately 2R,
where R is the radius of the center electrode, and is not less than
approximately R. The shape of
the shield may for example be generally spherical, generally conical, or
generally parabolic. The
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opening for permitting passage of radiation is preferably substantially
circular and located at
substantially the center of the shield. More specifically, the opening is
sized and positioned such
that radiation exiting the center electrode at an angle of X15° from
the axis of the center electrode
passes through the opening. The material for the shield is preferably at least
one of a high
temperature ceramic, glass, quartz and/or sapphire. the material for a
preferred illustrative
embodiment being A1,0, (aluminum oxide).
The foregoing and other objects, features and advantages of the invention will
be apparent
from the following more particular description of preferred embodiments of the
invention as
illustrated in the accompanying drawings and otherwise discussed herein.
to
In The Drawings
Fig. 1 is a semi-schematic, semi cutaway side view of a first illustrative
thruster
embodiment of the invention;
Fig. 2 is a semi-schematic, semi-cutaway side view of alternative thruster
embodiment of
i 5 the invention;
Fig. 3 is semi-schematic, semi cutaway side view of a radiation source
embodiment of the
invention;
Fig. 4 is an enlarged cutaway view (not to scale) of the center electrode of
Fig. 3 for one
embodiment of the invention,
2o Fig. 5 is a semi-schematic, side cutaway view of an embodiment of the
invention, which,
depending on relative dimensions and other factors may be used either as a
thruster or radiation
source. having an RF initiator in accordance with the teachings of this
invention;
Fig. 6 is a schematic representation of a further implementation for obtaining
RF initiation
in a plasma gun of this invention;
25 Fig. 7A is a schematic diagram of a solid state simulated RF source
suitable for use as an
RF source to drive a plasma initiator;
Figs. 7B and 7C are diagrams illustrating the voltage across certain
capacitors in the
circuit of Fig. 7A;
Figs. 8A and 8B are cutaway partial side views of a portion of a plasma gun
illustrating
3o two different initiator electrode configurations suitable for use in
applying initiator voltage to the
plasma gun;
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Fig. 9A is a schematic diagram of a pulse driver circuit suitable for use in
driving the
plasma guns of this invention in accordance with an alternative embodiment:
and
Fig. 9B is a diagram of an illustrative output signal from the circuit of Fig.
9A.
Figs. 1 OA-l OC are enlarged side sectional views illustrating the end of the
center electrode
and a shield for a spherical. conical and parabolic embodiment of the
invention. respectively.
Detailed Description
Referring first to Fig. I, the thruster. 10 has a center electrode 12. which
for this
embodiment is the positive or anode electrode, and a concentric cathode.
ground or return
to electrode 14, a channel 16 having a generally cylindrical shape being
formed between the two
electrodes. Channel 16 is defined at its base end by an insulator 18 in which
center electrode 12
is mounted. Outer electrode I4 is mounted to a conductive housing member 20
which is
connected through a conductive housing member 22 to ground. Center electrode
12 is mounted
at its base end in an insulator 24 which is in turn mounted in an insulator
26. A cylindrical outer
i5 housing 28 surrounds outer electrode I4 and flares in area 30 beyond the
front or exit end of the
electrodes. The electrodes 12 and 14 may for example be formed of thoriated
tungsten, titanium
or stainless steel.
A positive voltage may be applied to center electrode t 2 from a do voltage
source 32
through a dc-do invertor 34, a nonlinear magnetic compressor 3G and a terminal
38 which
2o connects to center electrode 12. Dc-do inventor 34 has a storage capacitor
42, which may be a
single large capacitor or a bank of capacitors, a control transistor 44, a
pair of diodes 46 and 48
and an energy recovery inductor 50. Transistor 44 is preferably an insulated-
gate bipolar
transistor. Inventor 34 is utilized in a manner known in the art to transfer
power from do source
32 to nonlinear magnetic compressor 36. As will be discussed later, inventor
34 also functions
z5 to recover waste energy reflected from a mismatched load. and in particular
from electrodes 12
and 14, to improve pulse generation efficiency.
Nonlinear magnetic compressor 36 is shown as having two stages, a first stage
which
includes a storage capacitor 52, a silicon controlled rectifier 54 and an
inductor or saturable
inductor 56. The second stage of the compressor includes a storage capacitor
58 and a saturable
30 inductor 60. Additional compression stages may be provided if desired to
obtain shorter, faster'
rising pulses and higher voltages. The manner in which nonlinear magnetic
compression is
accomplished in a circuit of this type is discussed in U.S. Patent 5.142,166_
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Basically, circuit 36 uses the saturable cores
as inductors in a resonance circuit. The core of each stage saturates before a
significant fraction
of the energy stored in the capacitors of the previous stage is transferred.
The nonlinear saturation
phenomenon increases the resonance frequency of the circuit by the square root
of the decrease
of the permeability as the core saturates. Energy is coupled faster and faster
from one stage to
the next. It should be noted that compression circuit 36 is efficient at
transferring power in both
directions since it not only acts to upshift the frequency in the forward
direction, but also
downshifts the frequency as a voltage pulse is reflected and cascades back up
the chain. Energy
which reflects from the mismatched load/eIectrodes can cascade back up the
chain to appear as
a reverse voltage being stored in capacitor 42 and to be added to the next
pulse. In particular.
when the reflected charge is recommuted into initial energy storage capacitor
42. current begins
to flow in the energy recovery inductor 50. The combination of capacitor 42
and coil 50 forms
a resonant circuit. After a half point [where t~/(L~C,Z)''], the polarity of
the voltage on capacitor
42 has been reversed. and this energy will reduce the energy required to
recharge this capacitor
t 5 from voltage source 32.
The drive circuits shown in Fig. 1 can also be matched to very low impedance
loads and
can produce complicated pulse shapes if required. The circuits are also
adapted to operate at very
high PRFs and can be tailored to provide voltages in excess of one Kv.
Propellant gas is shown in Fig. I as being delivered from a line 64, through a
valve 66
2o under control of a signal on line 68. to a manifold 70 which feeds a number
of inlet ports 72 in
housing 28. There may, for example be four to eight ports 72 spaced
substantially evenly around
the periphery of housing 28 near the base end thereof. Ports 72 feed into
holes 74 formed in
electrode 14 which holes are angled to direct the propellant radiaIly and
inwardly toward the base
of channel 16 near center electrode 12. Propellant gas may also be fed from
the rear of channel
25 16:
Thruster 10 is designed to operate in space or in some other low pressure,
near vacuum
environment, and in particular at a pressure such that breakdown occurs on the
low pressure side
of the Paschen curve. While the pressure curve for which this is true will
vary somewhat with
the gas being utilized and other parameters of the thruster, this pressure is
typically in the 0.01
3o to I O Torr range and is approximately 1 Torr for preferred embodiments.
For pressures in this
range, increasing pressure in a region reduces the breakdown potential in that
region. therefor
enhancing the likelihood that breakdown will occur in such region. Therefor,
theoretically,
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merely introducing the propellant gas at the base of column 16, and therefor
increasing the
pressure at this point, can result in breakdown/plasma initiation, occurring
at this point as desired.
However, as a practical matter, it is difficult both to control the gas
pressure sufficiently to cause
predictable breakdown and to have the pressure sufficiently uniform around the
periphery of
column 16 for breakdown to occur uniformly in the column rather than in a
selected section of
the column.
At least two things can be done to assure that plasma initiation occurs
uniformly at the
base of column 16 and that such breakdown occurs at the desired time. To
understand how these
breakdown enhancements are achieved, it should be understood that the plasma
guns of this
i o invention typically operate at pressures between .O1 Torr and 10 Torr, and
in particular, operate
at pressures such that breakdown occurs on the low pressure side of the
Paschen curve. For
preferred embodiments, the pressure in column 16 is at approximately 1 Torr.
In such a low
pressure discharge, there are two key criteria which determine gas breakdown
or initiation:
1. Electric field in the gas must exceed the breakdown field for the gas which
depends on the gas used and the gas pressure. The breakdown field assumes a
source of electrons
at the cathode 14 that is known as the Paschen criteria. In the low pressure
region in which the
gun is operating, and for the dimensions of this device, the breakdown
electric field decreases
with increasing pressure (this occurring on the low pressure side of the
Paschen curve). Therefor,
breakdown occurs in column 16 at the point where the gas pressure is highest.
2. Second, there must be a source of electrons. Even if the average electric
field
exceeds the breakdown field, nothing will happen until the negative surface
begins to emit
electrons. In order to extract electrons from a surface, one of two conditions
must occur. For the
first condition, a potential difference must be produced near the surface
which exceeds the
cathode fall or cathode potential. The cathode fall/cathode potential is a
function of gas pressure
and of the composition and geometry of the surface. The higher the local gas
pressure, the lower
the required voltage. A re-entrant geometry such as a hole provides a greatly
enhanced level of
surface area to volume and will also reduce the cathode fall. This effect,
whereby a hole acts
preferentially as an electron source with respect to adjacent surface, is
denoted the hollow cathode
effect. The second condition is that a source of electrons can be created by a
surface flashous
3o trigger source. These conditions may be met individually or both may be
employed. However,
the voltage across the electrodes should be less than the sum of the gas
breakdown potential and
cathode fall potential to prevent spurious initiation.
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Thus, in Fig. I, a plurality of holes 74 are formed in cathode 14 through
which gas is
directed to the base of column 16, which holes terminate close to the base of
the column. For
preferred embodiments, a plurality of such holes would be evenly spaced around
the periphery
of column 16. The gas entering through these holes, coupled with the hollowed
cathode effect
resulting from the presence of these holes, results in significantly increased
pressure in the area
of these holes near the base of column 16, and thus in plasma initiation at
this place in the
column. While this method of plasma initiation is adequate for plasma
initiation in some
applications, for most applications of the plasma gun of this invention,
particularly high PRF
applications, it is preferable that trigger electrodes also be provided in the
manner described for
i o subsequent embodiments so that both conditions are met to assure both the
uniformity and
timeliness of plasma initiation.
When thruster 10 is to be utilized, valve 66 is initially opened to permit gas
from a gas
source to flow through manifold 70 into holes 74 leading to channel 16. Since
valve 66 operates
relatively slowly compared to other components of the system, valve 66 is left
open long enough
i 5 so that a quantity of gas flows into channel 16 sufficient to develop the
desired thrust through
multiple plasma initiations. For example, the cycle time of a solenoid valve
which might be
utilized as the valve 66 is a millisecond or more. Since plasma bursts can
occur in two to three
microseconds, and since gas can typically flow down the length of the 5 to I 0
cm electrodes used
for thrusters of preferred embodiments in approximately 1/4000th of a second.
if there was only
20 one pulse for each valve cycle, only about 1/10 of the propellant gas would
be utilized. Therefor,
to achieve high propellant efficiency, multiple bursts or pulses, for example
at least ten, occur
during a single opening of the valve. During each individual burst of pulses,
the peak power
would be in the order of several hundred kilowatts so as to create the
required forces. The peak
PRF is determined by two criteria. The impulse time must be long enough so
that the plasma
25 resulting from the previous pulse has either cleared the thruster exit or
recombined. In addition,
the impulse time must be shorter than the time required for cold propellant to
travel the length of
the electrodes. The latter criteria is determined to some extent by the gas
utilized. For argon,
with a typical length for the column 16 of 5 cm, the time duration for
propellant to spread over
the thruster electrode surface is only 0.1 msec, while for a heavier gas such
as xenon, the time
30 increases to approximately 0.2 msec. Therefor, a high thruster pulse
repetition rate (i.e.
approximately 5,000 pps or greater) will enable the plasma gun to achieve a
high propellant
efficiency approaching 90%. The burst lengths of the pulses during a single
valuing of the fluid
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can be varied from a few pulses to several million, with some fuel being
wasted and a lower
propellant efficiency therefor being achieved for short burst lengths.
Therefor, if possible, the
burst cycle should be long enough to allow at least full use of the propellant
provided during a
minimum-time cycling of the valve 66.
Before the propellant reaches the end of column 16, gate transistor 44 is
enabled or
opened, resulting in capacitor 58 becoming fully charged to provide a high
voltage across the
electrodes (400 to 800 volts for preferred embodiments) which, either alone or
in conjunction
with the firing of a trigger electrode in a manner to be described later
results in plasma initiation
at the base of column 16. This results in a sheath of plasma connecting the
inner and outer
conductors, current flowing readily between the electrodes through the plasma
sheath, and
creating a magnetic field. The resulting magnetic pressure pushes axially on
the plasma sheath
providing a ,lxB Lorentz force which accelerates the plasma mass as it moves
along the
electrodes. This results in a very high plasma velocity, and the electrode
length and initial charge
are selected such that the rms current across the electrodes which initially
increases with time and
~ 5 then decreases to zero, and the voltage which decreased as capacitor 58
discharges, both return
to zero just as the plasma is ejected from the tip of the electrodes. When the
plasma reaches the
end of the coaxial structure, essentially all of the gas has been entrained or
drawn into the plasma
and is driven off the end of the electrodes. This results in maximum gas mass
and thus maximum
momentum/thrust for each pulse. If the length of the structure has been chosen
so that the
2o capacitor is fully discharged when the plasma exits the electrode, then the
current and voltage are
zero and the ionized slug of gas leaves thruster 10 at a high velocity.
Exhaust velocity in for
example the range of 10,000 to 100,000 meters/second can be achieved with
thrusters operating
in this manner, with the exhaust velocity utilized being optimum for a given
thruster application.
Flared end 30 of the thruster, by facilitating controlled expansion of the
exiting gases allows for
25 some of the residual thermal energy to be converted to thrust via
isentropic thermodynamic
expansion, but this effect has been found to be fairly negligible and tapered
portion 30 is not
generally employed. In fact, except for protection of electrode 12, which is
not generally required
in space, the weight of thruster 10 may be reduced by completely eliminating
housing 28. A
pulse burst may be terminated by disabling gate transistor 44 or by otherwise
separating source
30 32 from circuit 36.
Fig. 2 illustrates an alternative embodiment thruster 10' which differs in
some respects
from that shown in Fig. 1. First, nonlinear magnetic compressor 36 has been
replaced by a single
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storage capacitor 80, which in practical applications would typically be a
bank of capacitors to
achieve a capacitance of approximately 100 microfarads. Second, cathode 14
tapers slightly
towards its exit end. Third, spark plug-like trigger electrodes 82 are shown
as being positioned
in each of the holes 74 with a corresponding drive circuit 86 for the trigger
electrodes; an internal
gas manifold 72' formed by a housing member 77 is provided to feed propellant
gas to holes 74,
a gas inlet hole (not shown) being provided in member 77, and gas outlet holes
84 are shown
formed in insulator 24 and in center electrode 12. As for the embodiment of
Fig. 1, there would
typically be a plurality of holes 74, for example four to eight, evenly spaced
around the periphery
of cathode 14, with a trigger electrode 82 in each hole 74, and a gas outlet
or outlets 84 preferably
t o opposite each hole 74 and directing gas thereat. For reasons to be
discussed later, most of the gas
inlet to chamber 16 flows from a suitable source, which may be the same source
as for manifold
72' and holes 74, through outlets 84 and into the chamber near center
electrode 12, gas flowing
through holes 74 being primarily to facilitate ignition by the trigger
electrodes.
While the capacitor 80 may be utilized in some applications in lieu of
nonlinear magnetic
compressor circuit 36 in order to store voltage to provide high voltage drive
pulses, such an
arrangement would typically be used in applications where either lower PRFs
and or lower
voltages are required, since compressor 36 is adapted to provide both shorter
and higher voltage
pulses. Circuit 36 also provides the pulses at a time determined by the
voltage across capacitor
58 and a saturation of nonlinear coil 60, which is a more predictable time
than can be achieved
2o with capacitor 80, which basically charges until breakdown occurs at the
base of column 16
permitting the capacitor to discharge.
Trigger electrodes 82 are fired by a separate drive circuit 86 which receives
voltage from
source 32, but is otherwise independent of invertor 34 and either compressor
36 or capacitor 80.
Drive circuit 86 has two non-linear compression stages and may be fired in
response to an input
signal to SCR 87 to initiate firing of the trigger electrodes. The signal to
SCR 87 may for
example be in response to detecting the voltage or charge across capacitor 80
and initiating firing
when this voltage reaches a predetermined value or in response to a timer
initiated when charging
of capacitor 80 begins, firing occurring when a sufficient time has passed for
the capacitor to
reach the desired value. With a compressor 36, firing could be timed to occur
when inductor 60
3o saturates. Controlled initiation at the base of the column 84 is enhanced
by the re-entrant
geometry of hole 74, and also by the fact that channel 16 is narrower at the
base end thereof,
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further increasing pressure in this area and thus, for reasons previously
discussed, assuring
initiation of breakdown in this area.
Each trigger electrode 82 is a spark-plug like structure having a screw
section which fits
in an opening 89 in housing 77 and is screwed therein to secure the electrode
in place. The
forward end of electrode 82 has a diameter which is narrower than that of the
opening so that
propellant gas may flow through hole 74 around the trigger electrode. For
example, the hole may
be 0.44 inches in diameter while the trigger electrode at its lowest point is
0.40 inches. The
trigger element 91 of the trigger electrode extends close to the end of hole
74 adjacent column 16,
but preferably does not extend into column 16 so as to protect the electrode
against the plasma
forces developed in column 16. The end of the electrode may, for example, be
spaced from the
end of hole 74 by a distance roughly equal to the diameter of the hole
(7/16").
While trigger electrode 82 and plasma electrodes 12 and 14 are both fired from
common
voltage source 32, the drive circuits for the two electrodes are independent
and, while operating
substantially concurrently, produce different voltages and powers. For
example, while the plasma
electrodes typically operate at 400 to 800 volts, the trigger electrode may
have a 5 Kv voltage
thereacross. However, this voltage is present for a much shorter time
duration, for example. 100
ns, so that the energy is much lower, for example 1/20 Joule.
Another potential problem with thrusters of the type shown in Figs. 1 and 2 is
that the
Lorenz forces across column 16 are not uniformed, being greatest near center
electrode 12 and
2o decreasing more or less uniformly outward therefrom to the cathode outer
electrode 14. As a
result, gas plasma exits along an angled front, with gas exiting first from
the center electrode and
later for gas extending out toward the outer electrode. The outer electrode 14
could therefor be
shorter to facilitate gas exiting the thruster uniformly across the thruster,
although this is not done
for preferred embodiments. The taper of this outer electrode is for the same
reason as the taper
in region 30 of housing 28 and is optional for the same reasons discussed in
connection with this
tapered region.
The problem of uneven velocity in column 16 is also dealt with in Fig. 2 by
having most
of the gas enter column 16 from and/or near the center electrode through holes
89, thereby
resulting in a greater mass of gas at the center electrode than at the outer
electrode. If this is done
3o carefully so that the greater mass near the center electrode offsets the
greater accelerating forces
thereat, a more nearly uniform velocity can be achieve radially across column
16 so that
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gas/plasma exits uniformly (i.e. with a front perpendicular to the electrodes)
off the end of the
thruster. This correction is one reason why a shorter outer electrode is not
generally required.
Except for the differences discussed above, the thruster of Fig. 2 operates in
the same way
as the thruster of Fig. 1. Further, while a single thruster is shown in the
figures, in a space or
other application, a plurality of such thrusters, for example twelve
thrusters, could be utilized,
each operating at less than 1 Joule/pulse and weighing less than 1 kg. All the
thrusters would be
powered by a central power supply, would use a central control system and
would receive
propellant from a common source. The latter is a particular advantage for the
thruster of this
invention in that maneuvering life of a space vehicle utilizing the thruster
is not dictated by the
t o fuel supply for the most frequently used thruster(s) as is the case for
some solid fuel thrusters. but
only by the total propellant aboard the vehicle.
Fig. 3 shows another embodiment of a plasma gun in accordance with the
teachings of
this invention. which gun is adapted for use as a radiation source rather than
as a thruster. This
embodiment of the invention uses a driver like that shown in Fig. 1 with a dc-
do invertor 34 and
a nonlinear magnetic compressor 36, and also has a manifold 72' applying gas
through holes 74
of the cathode and around trigger electrodes 82. However, for this embodiment,
propellant gas
is not inputted from center electrode 12. The cathode electrode also does not
taper for this
embodiment of the invention and is of substantially the same length as the
center electrode 12.
The length of the electrodes 12 and 14 are also shorter for this embodiment of
the invention than
for the thnister embodiments so that gas/plasma reaches the end the
electrodes/column when the
discharge current is at a maximum. Typically, the capacitor will be
approaching the one-half
voltage point at this time. Further, for the radiation source application,
outer electrode 14 may
be solid or perforated. It has been found that best results are typically
achieved with an outer
electrode that consists of a collection of evenly spaced rods which form a
circle. With the
configuration described above, the magnetic field as the plasma is driven off
of the end of the
center electrode creates a force that will drive the plasma into a pinch and
dramatically increase
its temperature. The higher the current, and therefor the magnetic field, the
higher will be the
final plasma temperature. There is also no effort to profile the gas density
so as to achieve more
uniform velocity across column 16 and a static, uniform, gas fill is typically
used. Therefor, the
3o gas need not be introduced at the base end of column 16, although this is
still preferred. The gas
not being profiled results in the velocity being much higher at center
conductor 12 than at the
outer conductor 14. The capacitance at the driver, gas density and electrode
length are adjusted
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to assure that the plasma surface is driven off the end of the center
electrode as the current nears
its maximum value.
Once the plasma is driven off the end of the center conductor, the plasma
surface is
pushed inward. The plasma forms an umbrella or water fountain shape. The
magnet field of the
current flowing through the plasma column immediately adjacent the tip of the
center conductor
provides an inward pressure which pinches the plasma column inward until the
gas pressure
reaches equilibrium with the inward directed magnetic pressure.
Temperatures more than 100 times hotter than surface of the sun can be
achieved at the
pinch using this technique. Radiation of a desired wavelength is obtained from
the plasma gun
90 by introducing an element, generally in gas state, having a spectrum line
at that wavelength
at the pinch. While this may be achieved by the plasma gas functioning as the
element. or by the
element being introduced at the pinch in some other way. for a preferred
embodiment. the element
is introduced through a center channel 92 formed in electrode 12. Center
electrode 12 is
preferably cooled at its base end by having cooling water, gas or other
substance flow over the
I 5 portion of the housing in contact therewith. This provides a large
temperature gradient with the
tip of the cathode which, when a plasma pinch occurs, can be at a temperature
of approximately
1,200°C. In particular, at high temperatures, radiation intensity is
inversely proportional to the
fourth power of wavelength (i.e., intensity ~ 1 /~,a; _ (f/c)~; where 7~ = the
wavelength of the desired
radiation, f = the frequency of the desired radiation. and c = speed of
light). Thus, for a given
2o gas/element being fed through channel 92 to the pinch or otherwise
delivered to the pinch,
maximum intensity is obtained for the shortest wavelength signal radiated from
the element,
during decay from the 2P~ 1 S state which signal is obtained for atoms of the
element in their
single electron state (i.e., atoms which have been raised to such a high
energy state that all but one
atom have been removed from the molecule). For atoms in the single electron
state, the
25 wavelength ~, is given by (~, = 121.5 nm/N', where N is the atomic number
of the element in
chamber 92 which is being vaporized). Using this equation, the wavelengths
having the highest
energy for the first six elements of the periodic table are indicated in Table
1 below:
Table 1
Element Atomic Number
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H 1 121.5 nm
He 2 30.375 nm
Li 3 13.5 nm
Be 4 7.6 nm
B 5 4.86 nm
C 6 3.375 nm
To the extent gas applied through channel 92 is not fully converted to its
single electron
state, and even at the temperatures existing at the pinch most of the gas will
not generally be
ionized to this state. radiation will also be outputted at the other spectrum
wavelengths for the
element; however. as is apparent from the above equation, these radiations
will be at much lower
intensity, the intensity being a small fraction of the intensity for the
single electron state. Thus,
for example, xenon with an atomic number of 54 has a single electron
wavelength of .04 nm
which is of little value, but also has, as will be discussed shortly, energy
at a wavelength of 13
nm which is useful. However, the energy at 13 nm is 1/10-'° of the
energy at the single electron
to wavelength for a temperature at the pinch optimized for the single electron
state and still
generally orders of magnitude lower even at lesser pinch temperatures. This is
because it is never
possible to force more than a small fraction of the energy (__<'/4) to be
emitted solely at 13 nm
because of the shape of the black body emission curve relied on to determine
the amplitude of
relative lines and the temperature vary significantly.
t 5 Therefore, to use radiation at a wavelength other than the optimum single
electron
wavelength for an element, it is necessary to filter out the shorter
wavelengths also being radiated
for the element, which wavelengths are at much higher intensity. Fig. 3 shows
one way of doing
this wherein the radiation 94 being emitted from plasma gun 90 is applied to a
mirror 96 of a type
known in the art which is constructed to absorb all wavelengths of radiation
except the desired
2o wavelength, which wavelength is reflected toward the desirable target.
Other filters, which are
at least high pass filters for the desired wavelength and above might also be
used.
Thus, if possible, it is desired to use an element for the gas or other
element supplied to
channel 92 which produces radiation at the desired wavelength in its highest
energy single
electron state. However, where either an element which emits radiation at a
desired wavelength
25 in its single electron state does not exist, and from Table 1 it is seen
that above about 7.6 nm very
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few wavelengths are in fact available for elements in their maximum energy
state, then an element
must be found which emits radiation at the desired wavelength and a suitable
filter, such as the
filter mirror 96, utilized to obtain radiation at the desired wavelength.
Since this radiation will
be at far lower intensity than for radiation at the single-electron state
wavelength, a larger and
generally more costly device 90 would generally be required to obtain
sufficient energy at the
lower intensity wavelength. The radiation intensity at a given wavelength is
given in terms of
watts/meter'/hertz and varies both as a function of the frequency or
wavelength of the radiation,
the temperature and the emissivity. Emissivity is a function which has a
maximum value of one
and it is important to choose a gas which has a maximum emissivity at the
desired output
frequency/wavelength. The optimum pinch temperature (T~",T) for a given
wavelength ~, can be
determined from Wiens displacement law, T~">T 0.2898 cm x K°/?~ where
K° is the temperature
of the plasma in Kelvin. Xenon may be flowed at a relatively slow rate through
channel 90,
since only a very small quantity of the gas is ionized to produce radiation
during each pinch, to
obtain 13 nm radiation. However, as discussed earlier, if xenon is used, the
output radiation at
~ 5 13 nm will be at relatively low intensity, and a filter such as filter 96
will be required to obtain
useful radiation at this wavelength. For this reason, lithium, which from
Table 1 can be seen to
have a maximum intensity wavelength substantially at the desired wavelength
(i.e., at 13.5 nm),
is the preferred element for radiation at this wavelength.
Fig. 4 illustrates a center electrode 12 for an embodiment utilizing lithium
vapor to
2o produce the desired radiation. Referring to this Figure, a solid lithium
core 98 is held in a tube
100 of a material such as stainless steel, the tip of tube 100 being at a
point along the center
electrode near the tip which. during a plasma pinch, is at a temperature of
approximately 900°C,
resulting in the production of lithium vapor at a pressure of about 1 Torr off
the end of lithium
core 98. This lithium vapor flows out of hole 102 in the end of electrode 12
at a rate which
25 displaces the argon or other plasma gas at the tip. this required flow rate
being in the range of
approximately 1-10 grams per year for an illustrative embodiment. Tube 100 may
be slowly
advanced in a suitable way to keep the forward tip of lithium core 98 at the
appropriate locations.
When core 98 is used up, it may be replaced. A small amount of helium gas is
preferably fed up
around tube 100 and out opening 102 to assure that only lithium and helium are
present at the
3o pinched zone, since argon, even in small quantities, would introduce higher
energy, shorter
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wavelength lines which, if not filtered, could interfere with the 13 nm
radiation at the desired
target.
Another way to get the lithium or other suitable material to the pinch is to
form at least
one of center electrode 12 and outer electrode 14 of a sintered powder
refractory metal saturated
with liquid lithium or with some other suitable material in a fluid (i.e.,
liquid or gas) state. A
metal such tungsten or molybdenum can be fabricated into the desired electrode
shape by pressing
the powdered refractory material, such as tungsten, with an appropriate
bonding agent and then
sintering the resulting mass at high temperatures. The resulting porous
refractory metal matrix
can be impregnated with the liquid lithium or other desired material,
providing improved lifetime
and an alternative means of introducing the lithium/material into the
discharge. Liquid lithium
could be constantly supplied to the metal matrix of the electrodes during
operation if desired so
as to provide a substantially infinite lifetime for the process without need
to replace the radiation
generating material. One constraint in selecting the powdered refractory metal
is to assure that
the metal is not soluble in the radiation generating material being wicked
therein.
~ 5 If xenon is used to obtain the 13 nm radiation, it must be confined to the
immediate
vicinity of the pinch because it is so absorptive at that wavelength. Where
the radiation used is
at a wavelength other than the single electron wavelength for the element/gas
in column 92, as
is the case for xenon, the temperature at the pinch may be controlled so as to
ionize less of the
element to its single electron state, thereby providing more radiation at the
longer wavelengths
20 and less radiation, although still much higher intensity radiation, at the
shorter wavelength.
It is also desirable that the cone angle for the emitted radiation be as small
as possible.
Small cone angle is achieved when the stimulated emission of radiation from
the radiating gas
at the pinch is much larger than the spontaneous emission, spontaneous
emission being more
dispersive. In particular, if it is assumed that the Boltzmann constant k
times the temperature at
25 the pinch is much larger than the frequency of the radiation f times the
Planck constant h, then
the ratio of spontaneous emission B to stimulated emission A is given as (B/A
= kT/hf). For
example, when this ratio equals 20 (i.e., the plasma temperature is 20 times
the photon energy of
interest), then the half cone angle is approximately 25°. The higher
the plasma temperature, the
narrower the cone angle; however, the shorter the wavelength of the radiation,
the harder it is to
3o achieve narrow cone angles. However, cone angle is one more factor to be
taken into account in
selecting current and other parameters to achieve a desired temperature at the
pinch.
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Fig. 5 illustrates another embodiment of the invention which, depending on
factors such
as electrode length and whether or not a radiation emitting element/gas is
introduced through the
center electrode 12, may be used as a thruster, radiation source, or other
function for which
plasma guns are utilized. The plasma gun is shown as being driven by a main
solid state driver
I 10 which, for preferred embodiments, includes voltage source 32, DC/DC
converter 34, and
NMC 36. However, while this embodiment utilizes spark plugs 82 set in holes 74
for plasma
initiation, it differs from prior embodiments in that the spark plug or other
electrode is driven
from a pulsed RF source 112 through a DC blocking capacitor 114 and a resonant
coaxial line 116
which functions as a matching transformer. For preferred embodiments, the RF
signal is at a
1o frequency of 10 MHZ to 1,000 MHZ and is energized approximately 1 to 10
microseconds prior
to energization of main driver 110. Figure 5 also shows an optional DC bias
source 118 which
is connected through an AC filter coil 120 to center electrode 12. Source I 18
may be voltage
source 32, generally applied through a shaping and control circuit such as
circuit 86, or may be
a separate source depending on application.
While in Fig. 5 only two trigger electrodes or spark plugs 82, 91 are shown
which are
positioned on opposite sides of cavity 16, a plasma gun would preferably have
at least four, and
could have six or eight (or possibly more) electrodes evenly spaced around the
periphery of
channel 16. With four electrodes, the RF signal applied to the electrodes
shown would be at a
first phase, and the RF signal applied to the electrodes at 90° to
those shown would be at a second
2o phase 90° out of phase with the first phase. For a plasma gun having
six trigger electrodes, a
three phase RF signal would be used, with each phase being applied to a pair
of electrodes on
opposite sides of chamber 16. With eight electrodes a two phase signal would
preferably be
utilized, with one phase being applied to every other electrode and the second
phase to the ones
in between, a four phase signal could also be used. The reason for using an RF
rather than a DC
25 signal for plasma initiation is that it has been found that RF applied to
the initiator electrodes
results in a more uniform, and nearly perfectly uniform, volumetric ionization
or initialization in
chamber 16. The DC bias from source 118, which is preferably applied
simultaneously with the
RF signal from source 112 in response to control signals on a line or lines
22, further contributes
to the uniform ionization, particularly near the center electrode, and reduces
the power
3o requirements on RF source 112. The DC bias may be applied to the center
electrode as shown,
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or may be applied to electrode 84 in series or parallel with the RF signal so
that, for example, the
RF signal modulates the DC bias.
Fig. 6 illustrates the connection of the RF source to two electrodes/spark
plugs 82. 82'
which are for example positioned 90° from each other. There would be
two additional
electrodes/spark plugs in the plasma gun, with a second electrode 82 being
positioned at 180° to
the electrode 82 shown and being connected in the manner shown for the
electrode 82 and a
second electrode 82' being positioned 180° from the electrode 82' shown
and being connected in
the same manner as this electrode. Source 112 is connected through quarter
waveguide coaxial
lines 124, 124' to a point near a shorted end of a coaxial line 126, 126', but
spaced from the
shorted end by a distance Ll, L2, respectively. Coaxial line 126 is a quarter
wavelength long and
has electrode 82 at the unshorted end thereof, while coaxial line 126' is a
half wavelength long
and has electrode 82' at the unshorted end thereof. With line 126 a quarter
wavelength long and
line 126' a half wavelength long, the desired phase difference for the RF
signal at electrodes 82
and 82' is achieved. The coaxial line also provide a large voltage step-up
and, if the coupling
t5 positions/distances L1, L2 are chosen correctly, will look to the source as
a matched load until
breakdown is achieved. Using good quality coaxial lines, voltage step-up
ratios on the order of
10-20:1 can easily be achieved. Once breakdown is achieved, the line appears
as a short circuit
at position L 1. At the input coupling to the source 7,,/4 away from L 1, the
apparent impedance
looks like an open circuit. Further. if the position L2 is chosen correctly.
this line will appear as
2o a matched load once breakdown is initiated. While it is desirable to keep
lines 126, 126' as short
as possible, desired phase and impedance matching could generally be achieved
for the line with
respective lengths of (2M-1)a./4 and M7J2. Therefore, the RF source always
sees a matched load.
first creating a voltage step-up at one pair of spark plugs, and then
providing a voltage step-down,
but current step-up, at the second pair of spark plugs 82' once the plasma is
initiated. The
25 following Table 2 gives parameters for the RF source of Fig. 6 for an
illustrative embodiment.
Table 2
Andrews Andrews
- F5J4-50B LD F4-50A
Velocity Velocity
= O.81C = 0.88C
50 MHZ 150 MHZ 440 MHZ 50 MHZ 150 MHZ 440 MHZ
Attn DB/100 M I 2.5 4.5 I 7.0 I 1.5 3.0 I 5.0
I I
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/=),/4 1.25M .405M .13M 1.32M 0.44M 0.150M
2.87 8.061 I .727 5.75810-
s'~~I = j/rr a ~ 10- 5.18 ~ 10- 10- -3/M 3/M
'~ Crir'rr ~ 3/M 10-3/M 31M 3/M 3.45
10
_ Pin
PreLircrrlrrrion 72 ~ 120 ~ 225 ~ 1 10 165 ~ 289 ~
Pin Pin Pin ~ Pin Pin Pin
V(~~/V;.(a/4) 8.5 10.95 15 I0.5 12.8 17.0
Sin'(V;~V,")(7J4)6.756 5.24 3.82 5.46 4.48 3.37
2.35810-~.857U0- 5.617~10-
/i (a/4) 9.12 2M 3M 8.00 2.19 3M
10-2M 10-2M 10-2M
= a/2 2.43 0.310 0.276 2.64 0.88 0.30
M M M M M M
Precirczrlate
_ J (~l2
Pin (J - ~-.i~n
36 60 112.5 55 82.5 144.5
Sin-'(V;"/V",)(7J2)9.59 7.4 5.41 7.7 6.32 4.77
/_(~2) M 12.9510--3.33 0.829 1 1.29 3.089 7.95
10-' 10-~ 10 ~ 10-= 10-'
The RF frequency and voltage, either from the RF source alone or from both the
RF
source and DC bias source 118, are determined from dimensions and operating
pressure to give
maximum uniformity. In general, the RF frequency must be chosen to be above a
critical
frequency, the critical frequency being the frequency below which electrons in
the gas have time
to be swept across the entire electrode gap in each one half cycle, and are
therefore lost. Above
the critical frequency, electrons oscillate back and forth between electrodes,
facilitating the
ionization of the gas. The critical frequency for a given plasma gun geometry
is determined by
Velocity
9wa-i~)E
= V (!i(-L - _> >
electron
first computing the mobility
1 o where v~ = collision frequency; cu = 2~f where f is the frequency of the
radiation; q = electronic
charge; E = electric field; m = electronic mass. Therefore, the time to
transit the gas is given by
d
Dt =
V(!~LL'
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I __ _I V '.m
.far,"~m - 2~Dt 2~c d
where d = distance between electrode
As for the thruster embodiments, it is required that the entire radiation
source 90 be
maintained in a near vacuum envirorunent (generally a gas pressure <_ I O
Torr), and this is further
required since radiation in the EUV band is easily absorbed and cannot be used
to do useful work
in other than a near vacuum environment. Since propellant efficiency is not as
critical for this
embodiment. there may be a single radiation burst for each valuing, or the
valuing duration and
number of pulses/bursts may be selected to provide the radiation for a desired
duration.
While a standard high voltage RF source 112 such as a magnetron, klystron or
RF
1 o amplifier may be utilized as the RF source for the prior embodiments, as
indicated previously,
such standard RF sources are expensive both to purchase and to use, are bulky
and produce
significant heat which adds to the heat management burden of the system where
utilized. It would
therefore be preferable if such source could be replaced with a smaller source
which is
significantly less expensive both to purchase and operate and would generate
significantly less
~ 5 heat. Fig. 7A illustrates a solid state simulated RF generator which
satisfies these requirements.
In particular, the circuit 130 has been found to produce RF power at a cost
which is
approximately 1 % of that for standard RF power sources and to take up the
space of a small
circuit board, for example "6" or by "8", rather than a large cabinet.
Referring to Fig. 7A, circuit 130 includes a capacitor 132 which is charged in
standard
20 fashion from a voltage source, for example the voltage source 32 previously
discussed. A solid
state switch 134, which may for example be an SCR, IGBT or MOSFET, when closed
or
conducting permits capacitor 132 to discharge into the input of a multi-stage
nonlinear magnetic
pulse compression circuit 136, which circuit is of the type previously
discussed. Circuit 136 may
include multiple stages and/or transformers, one example of such configuration
being shown, and
25 terminates in a specialized output section 138. Output section 138 forms a
resonant saturable
shunt to ground, the resonant circuit of this section including a capacitor CR
and a saturable
inductor LR. Capacitor C,t is charged resonantly from capacitor CN of the nth
stage of nonlinear
magnetic pulse compressor 130. CN is chosen to be smaller in capacitance than
CR so that C,z
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reverses during the charge of CN. Alternatively, LR can be chosen to saturate
before the transfer
of charge from CN to C,t is completed. With either one or both of these
conditions satisfied, a
reverse voltage is created for CN before L,z saturates and C,t reaches its
peak charge. Under these
conditions, successive saturations of LR cause C,t to oscillate as shown in
Fig. 7C. While for the
plasma initiation application of this invention, only three or four cycles of
the source are required
as shown in Fig. 7C, the parameters of the circuit could be selected to
provide a desired number
of cycles, depending on application. The resonant frequency F of output
section 132 is
determined by the values of C,t and LR, either one of which may be made
adjustable to permit
tuning of the circuit. An output coupling circuit 140 is provided consisting
of a resistive element
R~, and/or a capacitive element C~,, each of which may be formed of a number
of elements
suitably interconnected. Output coupling circuit 140 couples some of the
energy out of capacitor
CR to output terminal 142. the impedance of the coupling circuit being chosen
so as to remove
only a fraction of the energy stored in CR for each cycle (e.g., 20% per
cycle). Further, while the
circuit shown in Fig. 7A is particularly adapted for use in the plasma guns of
this invention, a
~ 5 solid state simulated RF generator having the performance characteristics
of the circuit shown in
Fig. 7A does not currently exist, and such circuit may therefore also find use
in other applications.
This circuit is also therefore part of the invention.
Two potential problems in delivering the RF initiator signal into the plasma
gun are to
have the high voltage field occur over a relatively large uniform area at the
base of column 16.
2o so that breakdown occurs in this area and to get the RF field to this point
with minimum
disruption to the vacuum required in chamber 16. While the latter is not a
problem in space
applications, it is a potential problem in the more common applications of the
plasma gun as a
radiation source. Fig. 8A illustrates one way of accomplishing both
objectives, while Fig. 8B
illustrates a way of accomplishing the second objective only.
25 Referring first to Fig. 8A. a ceramic dielectric 150 is provided between
electrodes 12 and
14 at the base of column 16. A plurality of electrodes 152 are mounted in the
surface of dielectric
separator 150 which is outside of channel 16 and are spaced a short distance
from surface 154 of
the dielectric inside column 16 by the ceramic dielectric. The thickness of
dielectric between
electrodes 152 and surface 154 might typically be less than 1/8th inch, and is
selected to be as thin
3o as possible while assuring that the ceramic dielectric will not crack or
break. When an RF and
or DC signal is applied to electrodes 152, it results in a high voltage field
appearing at surface 154
to initiate the desired plasma breakdown.
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The device of Fig. 8B differs from that of Fig. 8A in that the ceramic
dielectric 150' is
formed as a collar over the bottom portion of center electrode 12 and
extending a short distance
into column I 6. Electrodes I 52 are mounted to external surface I 54' of the
dielectric and the high
voltage field is formed on surface I 54' when an RF and/or DC signal is
applied to the electrodes.
For applications where the entire plasma gun is not in a vacuum environment,
the configuration
of Fig. 8A is preferable in that it does not require an electric lead to be
brought into vacuum
column 16, a lead 156 being brought into the column for the embodiment of Fig.
8B.
It has also been found that more uniform breakdown can be achieved in the
plasma gun
by applying an initial high voltage spike to main electrodes 12 and 14 after
initiation. Fig. 9A
1o illustrates circuit for achieving the desired waveform which waveform is
shown on in Fig. 9B.
In particular, this waveform has an initial spike 160 followed by a sustaining
signal 162. The
initial spike may be as much as 10 times the voltage of the sustaining signal
162, but being of
much shorter duration, delivers as little as 1/10 of the energy supplied to
the electrodes 12 and
I 4.
Referring to Fig. 9A, the circuit consists of a first non-linear magnetic
compression circuit
164, only the last stage of which is shown in Fig. 9A, and a second non-linear
magnetic
compression circuit 166, only the last stage of which is also shown in the
figure. Circuit 166
generates the spike signal 160 which is a high voltage short duration pulse
while circuit 164
generates the sustaining signal 162 which is a lower voltage signal of much
longer duration which
20 occurs at the end of spike I 60. Reactor 168 for the last stage of circuit
I 64 is normally biased by
a bias signal applied through bias winding 170 so as to be saturated in a
direction to permit signal
to flow from the sustainer circuit 164 but to block signal flow in the reverse
direction from spiker
circuit 166. Thus, this signal is not applied to circuit 164, and in
particular not to capacitor 172
of the last stage thereof, thereby protecting circuit 164 from the voltage
spike and assuring that
25 all of this signal is applied to electrodes 12 and 14. The spike 160 starts
to reverse the bias of
saturable reactor 168, partially overcoming the bias applied thereto, and
simultaneously causes
avalanche breakdown to occur at the electrodes. This permits the optimum
voltage and drive
impedance level for the main discharge chain to be chosen without concern for
exceeding the
breakdown voltage. The reverse bias of saturable reactor 168 provides a delay
for the sustainer
3o signal from circuit 164 until the reactor resaturates, thereby providing a
smooth transition between
the two signals.
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One problem with a plasma source of the type shown for example in Fig. 3 is
that, in
order to achieve the desired pinch temperatures, which are in the range of 100
eV to 1000 eV
depending on the desired frequency of radiation, magnetic compression fields
on the order of
Tesla are required which are sufficient to drive the plasma to velocities of
several centimeters per
microsecond. These high velocities result in the plasma being driven down the
center conductor
12 and off the end of the center conductor, the plasma sheath continuing to
move out into space
away from the end of the center conductor. This results in the plasma sheath
eventually losing
electrical connection to the pinch, thus ending the pinch and causing a large
voltage transient.
This voltage transient can result in a high voltage restrike which can
severely damage the
t o electrodes. The loss of electrical contact with the plasma sheath also
results in a substantial
decrease in output efficiency from the source, the pinch lasting for only
approximately 100 ns,
rather than for the substantially longer duration of the electrical discharge,
which can be several
microseconds (for example 2-4 microseconds).
In accordance with the teachings of this invention, this problem of plasma
separation is
t 5 overcome by providing a blast shield or focussing device 194 adjacent the
exit end of center
electrode 12 to redirect the plasma sheath back toward the center electrode.
Figs. l0A-l OC show
three possible embodiments for such a shield or focusing device (hereinafter
collectively referred
to as shield) 194A, 194B, 194C which differ from each other primarily in the
shape of the
focusing cavity 196A, 196B. 196C respectively. In particular, cavity 196A has
a generally
2o spherical shape. the cavity being mounted by suitable mounting components
(not shown) to outer
electrode 14 or to suitable housing components of the source such that the
walls of cavity 196A
are spaced from the tip of center electrode 12 by a distance sufficient so
that there is no contact
between the shield and center electrode, but close enough so that redirection
of the plasma back
to the center electrode occurs before plasma separation. These objectives are
achieved with a
25 spacing which is generally in the range of R to 2R, where R is the radius
of center electrode 12.
However, these distances may vary to some extent depending on other parameters
of the source
10. Cavity 196B has a conical shape and cavity 196C has a parabolic shape. The
parameters
previously indicated for spacing of the cavity from the end of center
electrode 12 apply for all
three cavity shapes.
3o While it is desired to prevent separation of the plasma sheath and to
contain the sheath
with shield 194, it is important that shield 194 not interfere with the
exiting of the desired
radiation from source 10. Each shield 194 thus has a center opening 198A,
198B, 198C formed
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at the top of a corresponding cavity and having a center coaxial with the
center line of the center
electrode. Opening 198 is preferably circular and has a sufficient diameter
such that radiation
emitted from the pinch at the tip of the center electrode at an angle of
X15°. which is roughly the
angle of the emitted radiation, will pass through the opening unobstructed.
The upper portion of
each opening 198 is tapered outward to facilitate exiting of the radiation
while substantially
limiting any escape of the plasma sheath.
The material of shield 194 must be a high temperature. non-conductive material
capable
of withstanding temperatures in the range of approximately 1000°C and
higher. A variety of high
temperature ceramics have the desired characteristics, with Al_O, (aluminum
oxide) being utilized
For an illustrative embodiment. Various glasses, quartz and sapphire also have
the desired
characteristics to serve as the material for shield 194.
While in the discussion above, the plasma redirecting shield 194 has been
illustrated for
use with a particular configuration of radiation source, this shield is
suitable for use with any
radiation source where plasma separation is a potential problem and the
invention is therefore in
~ 5 no way limited by the specific radiation source configuration of Fig. 3.
Similarly, while three
cavity configurations have been shown in Figs l0A-l OC for redirecting
radiation to the cathode.
other cavity shapes adapted for performing this function could also be
utilized. The specific
materials described are also by way of illustration only.
Further, while parameters have been discussed above for producing radiations
at 13 nm,
2o radiation at other wavelengths within the EUV band, or in some cases
outside this band, may be
obtained by controlling various parameters of the radiation source 90, and
particularly by careful
selection of the element/gas utilized, the maximum current from the high
voltage source. the
plasma temperature in the area of the pinch, the gas pressure in the column,
and in some cases the
radiation filter utilized.
25 While a large number of gases can be used as the plasma gas for the plasma
guns
described above, inert gases such as argon and xenon are frequently preferred.
Other gases which
may be used include nitrogen, hydrazine, helium, hydrogen, and neon. As
indicated above, when
the plasma gun is used as a radiation source, as for the Fig. 3 embodiment. a
variety of
elements/gases might also be utilized to achieve selected EUV or other
wavelengths, the plasma
30 and radiation gas in some cases being the same gas. For example, H2 gas
might be selected for
efficiently obtaining radiation in the VUV band at 121.5 nm. Further, while
various embodiments
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have been discussed above, it is apparent that these embodiments are by way of
example only and
are not limitations on the invention. For example, while the drivers
illustrated are advantageous
for the applications, other high PRF drivers having suitable voltage and rise
times, and not
requiring high voltage switching, might also be utilized. Similarly, while a
variety of plasma
initiation mechanisms have been described, with the simulated solid state RF
driver electrode
trigger being preferred, other methods for initiating plasma breakdown might
also be utilized in
suitable applications. The configurations of the electrodes and the
applications given for the
plasma gun are also by way of illustration. Thus, while the invention has been
particularly shown
and described above with respect to preferred embodiments, the foregoing and
other changes in
forming detail may be made therein by one skilled in the art while still
remaining within the spirit
and scope of the invention and the invention is only to be limited by the
following claims.
What is claimed is:
