Note: Descriptions are shown in the official language in which they were submitted.
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Description
Larqe Volume Gaseous Electric Dischar~e System
Technical Field
This invention relates generally to a method
and an apparatus for controlling gas flow within a laser
channel to provide a uniform large volume plasma, and
more particularly, to a method and apparatus for
providing a mixed and diffused gas flow via recirculation
of only a portion of the gas at a velocity substantially
higher than that of the remaining gas flow.
Backqround Art
One of the problems encountered when a laser
system is designed for use in an industrial environment
is the compactness, size and reliability of the system.
Lasing action is obtained by subjecting a gas filled
vessel or channel to an electric discharge to form a
plasma. The electrons provided by the discharge collide
with active gas molecules thereby exciting them to higher
energy levels, from which they descend to lower energy
levels and emit excess energy in the form of photons or
light quanta. The density of particles in the higher
energy level must exceed that in the lower energy level
to achie~e optical gain. The addition of helium to a
mixture of carbon dioxide and nitrogen has been found to
yield increased output.
An electrical discharge having a large cross
sectional area which will uniformly fill large volume
cavities regardless of size or shape is mandatory if
powerful and reliable lasers suitable for industrial
applications are to be developed. An electrical
discharge is normally very restricted in diameter because
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the temperature in different parts of the discharge is
not uniform. This results in lower density and higher
current at the inside of the plasma column, thus
constricting the column. Individually ballasting a
plurality of electrodes offers a partial solution by
spreading the emission, but the streams tend to
recombine. Judicious use and design of aerodynamic
forces to control the ion and electron distribution in a
large volume discharge have achieved some success and
have resulted in a degree o~ compactness and reduction in
size for the same power output, as measured against the
very long discharge systems known in the prior art.
Lasers have been used to reduce production
costs in those applications requiring cutting, welding,
drilling, heat treating, and other processes. However,
according to industry publications, laser sales are only
about three percent of the machine tool industry.
Industrial application of lasers has been limited in the
past because of the limitations of poor reliability, poor
beam quality, large size, and heavy weight. ~hese
problems, coupled with the high cost of lasers per watt
of output, have caused the machine tool industry and
other industries, which can use lasers, to proceed very
slowly in their implementation. As an example of the
size and weight problem, current high power (1 kw and
up), continuous wave lasers measure about 22 feet long by
7 feet wide by 5.5 feet high and weigh several tons.
Improved reliability would enable industry to
utilize lasers on production lines. Improved beam
quality would make industry applications such as
hardening or softening of metals, or adhering materials
economical~ Miniaturization of structure would open the
field of robotics to new applications. Lower costs would
mean other equipment could be replaced by lasers.
U.S. Patent 3,581,146 issued May 25, 1971, to
A. E~ Hill relates to a method of ballasting a gaseous
discharge-tube system wherein a plurality of tubes are
excited from a single power source.
U.S. Patent 3,735,2~4 issued May 22, 1973, to
A. E. Hill teaches the use of aerodynamic forces to
control the spatial distribution of charge in a laser
system to obtain a uniform plasma.
U.S. Patent 3,795,838 issued March 5, 1974, to
A. E. Hill also shows the use of aerodynamic forces to
obtain uniform plasma in a laser system. It is important
to note that both this and the previously mentioned
patent obtain the desired uniform plasma by recirculation
of the entire lasing medium. Accordingly, a portion of
the gains obtained in reduced laser size and weight are
offset by the necessity oE the large volume compressors.
Refsrence is made to the following publications
for those relationships and definitions used herein:
"An investigation of Ejector Design by Analysis
and Experiment" by Keenan, Neumann, and Lustwerk; Journal
of Applied Mechanics; September 1950; page 299;
"Gaseous Conductors - Theory and Engineering
Applications"; James D. Cobine Ph.D.; Dover Publications,
Inc., 1958 Edition; and
"Basic Data of Plasma Physics" by Sanborn C.
Brown, M.I.T. Press; 1959 Edition.
The present invention is directed to overcoming
one or more of the problems as set forth above.
Disclosure of the Invention
In accordance with one aspect of the present
invention, a laser system having a high power output is
comprised of a housing, a gas lasing medium in the
housing, a laser channel in the housing, and means for
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providing continuous recirculating flow of the gas lasing
medium through the channel. A plurality of electrodes
are positioned at the upstream end of the housing
adjacent one end of the channel, and a plurality of
electrodes are positioned at the downstream end of the
housing adjacent the other end of the channel. A source
of elactrical power connects the electrodes for
establishing and maintaining discharges in the channel,
and means draws forth a beam of light energy from the
channel. A diffusing means provides a uniform plasma
flow in the channel and a pressure rise to balance the
pressure drop from recirculating flow through the
channel. The diffusing means is comprised of an ejector
disposed downstream from each respective upstream
electrode for mi~ing and diffusing gas streams into the
laser channel.
In accordance with another aspect of the
present invention, a laser system having a high power
output comprises a housing having a gas lasing medium
therein, a laser channel in the housing. A means
provides continuous recirculating flow of the gas lasing
medium through the channel. An electrode is positioned
at one end of the channel. An electrode is positioned at
the other end of the channel and a source of electrical
power is connected to the electrodes for establishing and
maintaining discharges in the channel. A means provides
a plasma flow in the laser channel, and means surrounding
the laser channel generates magnetic fields to cross
couple thermal and attachment instabilities.
In accordance with another aspect of the
present invention, a laser system for providing a uniform
large volume gaseous discharge comprises a housing and a
laser channel in fluidic communication with the housing
and forming a primary recirculating gas flow path through
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the housing and the laser ~hannel. A gaseous lasing
medium is contained within the housing and laser channel.
A means pressurizes only a portion of the gaseous lasing
medium and delivers the pressurized portion of the
gaseous lasing medium via a secondary recirculating gas
flow path into the lasing channel at a preselected
velocity substantially greater than the velocity of the
primary recirculating gas.
Thus the large pump previously needed for state
of the art lasers is sharply reduced in size, weight, and
capacity, leading to higher system efficiency, lower
manufa~turing costs, lower input energy requirements, and
lower cost per watt of output. The laser has high
reliability and exceptionàl beam quality at high power.
Brief Description of the Drawin~s
Fig. l is a schematic view of a gas laser
embodying the present invention;
Fig. 2 is a sectional view of an ejector
diffusing a plurality of lasing gas streams;
Fig. 3 is a sectional view of the ejector of
Fig. 2 showing the blending of lasing gas streams;
Fig. 4 is a sectional view of the ejector of
Fig. 2 showing the radial component of gas stream and
showing the uniform velocity distribution of the lasing
gas; and
Figs. 5 through 8 are longitudinal sectional
views of the laser showing four different means for
applying magnetic fields to the lasing gas stream.
Best Mode for Carryinq Out the Invention
As more fully disclosed hereinafter, this
embodiment provides for mixing and pumping lasing gas
streams in a laser having a primary, recirculating, gas
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stream. Fig. 1 shows a gas laser having the structure
shown in more detail in Figs. 2, 3 and 4. The laser
assembly has a c~lindrical housing 10 sealed to end walls
12 and 14 to form an enclosed housing. A laser channel
16 is disposed concentric with housing 10, and mirror 18
and output coupler 20 are fixed at opposite ends of the
channel. Channel 16 is joined to a frustoconical member
22 having a plurality of equally spaced, cylindrical
passages or tubes 24,26 forme~ at an acute angle with the
axis of symmetry of the channel and serving as mixing
tubes 2~,26 for the gas streams. Main nozzles 28,30 are
fixed adjacent each respective mixing tube. Hollow
electrodes 32,34 are positioned within and concentric
with each respective main nozzle 28,30 and are connected
to a source of high voltage electrical power 50 through
conductors 56 to provide ths electric discharge for
lasing action.
A diffusing means provides a uniform plasma
flow in the laser channel 16 to provide a pressure rise
to balance the pressure drop from recirculating flow
through the laser channel 16. The diffusing means
pressurizes only a portion of the gaseous lasing medium
and delivers the pressurized portion into the laser
channel 16 at a preselected velocity substantially
greater than the velocity of the primary recirculating
gas. The diffusing means includes a plurality of
ej~ctors annularly distributed about the laser channel 16
and a pump 64 fluidicly connected between the primary gas
flow stream and the ejectors. Each ejector introduces a
secondary gas stream to effect diffusion, turbulence, a
pressure rise, and an increase in mass flow. Ballast for
the discharge streams may be provided by the device shown
in U.S. Patent 3,581,146 identified above. It has been
found that an exceptional and unusual characteristic of
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the ejector contributes significantly to the turbulence
and diffusion of the lasing gas streams. The ejectors
include the primary nozzle 28,30 and the associated
mixing tube 26,24. As the high velocity secondary gas
stream emerges from the ejector nozzle 28,30 it enters
the mixing tube 24,26, which is of a generally
cylindrical form, and the gas from the primary
recirculating stream is thus entrained by massive
collisions between gas molecu]es. Vortices result from
the collisions and these vortices blend the several
streams homogeneously. The angular velocities of the
vortices diminish as the blencled gases progress
downstream. An aerodynamic contour is formed at the exit
area of the mixing tube to develop a radial components of
flow to the blended gas streams. This yields a
homogeneous longitudinal flow in the laser channel with
uniform radial velocity distribution.
Figs. 2, 3, and 4 show the profile of the
mixing tubes 24,26 and, more specifically, the
aerodynamic contour formed at the exit area. As viewed
in section the tubes 24,26 terminate on one side in a
reverse ogee 29 comprising a first curve 31 blending into
a second curve 33. The first surface 31 is curved at a
first preselected radius in a direction extending
radially outward from the longitudinal center of the
mixing tube 24, and the second surface (33) is curved at
a second preselected radius in a direction radially
inward toward the longitudinal center of the mixing tube
24. The second curved surface 33 is positioned
intermediate the laser channel 1~ and the first curved
surface 31. A radius section 35, opposite the reverse
ogee 29, registers with the second curve. This
aerodynamic contour is material in the development of a
radial component in the gas stream as it exits the mixing
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tube. The reaction of the gas stream with the exit
contour results in an homogeneous gas flow stream in the
contour area and the laser channel with microturbulent,
uniform velocity distribution as shown by the flow arrows
and wave front profile. Unless the velocity distribution
is uniform in cross section, those regions which flow
more slowly will become preferentially heated, and these
hotter regions become less dense. This leads to an
optical disturbance of the laser beam, and can ultimately
produce an electric arc.
Laser channel 16 is interrupted as shown by
reference characters 40 and 42 to allow recirculation of
gas through the housing and into the mixing tubes 24,26,
as shown by the flow arrows. Hollow electrodes 44 and 46
are connected to electrical source 50 to complete the
discharge circuit. The arrows in Figs. 1, 2, 3, and 4
illustrate the gas streams flow. The primary stream is
the recirculating stream which flows through channel 16,
out interruptions 40 and 42, through the volume between
channel 16 and housing 10 and is entrained by the
ejectors which introduce a high velocity secondary gas
stream and vortices into th~ mixing tubes as seen in Fig.
3. A series of cylindrical finned heat exchangers, shown
generally by numeral 48, are positioned adjacent the
interruptions 40,42 to control the lasing gas
temperature. Helium under pressure in tanks 52 and 54 is
fed to electrodes 32,34,44,46 through piping 56,58.
A lasing gas bleed system for optimum
continuous and predictable results is provided to remove
impurities including, for example, oil, carbon monoxide,
and hydrocarbons from the lasing gas. The system
comprises an exhaust pump 60 connected to housing 10 by
pipe 62 and having another exhaust pipe 63 leading to an
appropriate storage vessel (not shown). A
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positive-displacement circulating pump 64 is connected to
housing 10 by pipe 66 and heat exchanger 68 substantially
as shown. Nitrogen gas and carbon dioxide gas under
pressure are held in tanks 70 and 72 and are connected to
piping 74 and thence to primary nozzles 32 and 34. A
catalytic converter 67 co~verts by chemical reaction any
CO formed in the system to CO2. An electric heating
system (not shown) is provided to maintain the gas
temperature at the proper level for the converter 67 to
function. A molecular sieve 69 is provided to prevent
the gas from being poisoned by impurities.
A plurality of annu~Lar magnets 80, shown as
permanent magnets, embrace laser channel 16 and are
longitudinally spaced thsreon for a purpose to be
explained.
Industrial Applicability
In the operation of the device the circulating
pump 64 establishes a pressure to create a high velocity
lasing gas flow through catalytic converter 67, heat
exchanger 68, piping 74, the annular array of primary
nozzles 28,30, and the annular array of mixing tubes
24,26 into the laser channel 16. Flow continues through
the laser channel 16, out the interruptions 40,42, around
the vanes 76,78, past khe heat exchanger 48, and back
into mixing tubes 24,26 in a recirculating mode flow
substantially as shown by the arrows~ The operation of
the ejector in mixing and diffusing the lasing gas
streams results in a uniform, large volume gaseous
discharge in the laser channel. Referring now to Figs. 2
and 3 it will be seen that very high velocity gas emerges
from each primary nozzle 28,30 and enters its respective
mixing tube 24,26. As represented in the figures a
relatively conical boundary layer of gas enters the
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mixing tube and flares outwardly in the tube as it
progresses down its length. The gas molecules from the
recirculating primary gas stream are entrained by the
boundary layer's mass velocity. Gas vortices result from
the collisions between gas stream molecules traveling at
different velocities and the vortices intimately mix and
blend the streams. The annular velocity of the vortices
diminish as the blended gas stream move downstream and
result in complete difEusion of the gas streams and a
homoyeneous and uniform gas mixture. In one example of
the system the cross sectional area ratio of each
respective mixing tube to the throat of each respective
primary nozzle is about 27.5 to 1. It is contemplated
that other ratios can be used without departing from the
scope of the invention; the stated ratio has been shown
- to be advantageous.
The laser beam output emerges through the
output coupler 20 substantially parallel to the primary
gas stream. By a rearrangement of the optical system,
the beam output can be made to emerge transverse to the
longitudinal axis of channel 16. This system is well and
truly shown and illustrated in Fig. 12 of U.S. Patent
3,795,838 identified above. In the present embodiment, a
horizontally distributed ejector pump array is
substituted for the nozzle block shown in Fig. 12 of the
patent.
In the closed cycle or recirculating lasing gas
mode, contamination of the lasing gas by oil, dust,
impurities, ozone, carkon monoxide or chemical reactions
within the system gives rise to serious attachment
instability problems and will affect system power output
and reliability by poisoning the gas. The catalytic
converter 67 and molecular sieve 69 prevent the gas from
being poisoned. Heat exchanger 68 dissipates heat and
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thereby cools the lasing gas to optimum temperature. ~n
exhaust pump 60 and its associated plumbing 10,62,63 is
provided to allow the composition of the lasing gas to be
monitored, as by taking periodic samples, so that the
optimum gas mixture can be maintained if necessary by
introducing fresh gas from tanks 52,54,70,72.
Referring now to Figs. 5 to 8 inclusive, it has
been found that in the design of an industrial laser
wherein continuity of operation and reliability are
material factors, the addition of a magnetic field which
interacts with the plasma flowstream insures a
homogeneous discharge plasma by inhibiting the growth of
thermal instabilities within the discharge. Thermal
instabilities are incipient arcs which align with the
flowstream and unless controlled prevent a laser from
delivering its full power output. If an arc develops it
could burn and destroy the laser optical system and will
short circuit the laser cavity. Thus, Figs. 5 to 8
illustrate various embodiments of a means surrounding the
laser channel 16 for generating magnetic fields to cross
couple thermal and attachment instabilities.
Fig. 5 shows the static magnetic field produced
by ~he permanent magnets 80 with poles opposed. In this
example, the interaction of the field with the flowstream
does no work and neither adds nor subtracts energy from
the system. The lines of force produce a static
resultant undulating magnetic field which rises and falls
in strength or intensity in the direction of plasma flow.
As the lasing gas flows it reacts with the undulating
magnetic field thus produced and changes the electron
flow pattern by giving the electrons a velocity component
at right angles to the flow, causing migration of charged
particles and mixing of hot and colder regions to prevent
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the growth of thermal instabilities. A more uniform
plasma results.
Fig. 6 shows another embodiment wherein a
static magnetic field is produced by D.C. excited
electromagnets 78. The field reacts with the lasing gas
as in the preceding example and thus changes the electron
- flow pattern to yield an homogeneous plasma. It follows,
however, that the magnetic field produced by the voltage
source 82 is variable in intensity and can be controlled
externally by controlling the applied D.C. voltage.
The embodiment shown in Fig. 7 has an elongate
electromagnet ~4 embracing a substantial segment of the
channel 16. When excited by a D.C. voltage source 83 an
axially uniform distributed magnetic field results which
prevents development of thermal instabilities by
distributing the magnetic lines of force in the direction
of the lasing gas flowstream. The action of this field
will prevent thermal instabilities by guiding the charged
particles in substantially parallel paths in the laser
channel.
The Fig. 8 embodiment shows an oscillator 86
which superimposes an A.C. field upon the D.C. field from
power supply 85. The interaction of the two energy
sources with the [distributed coil] electromagnet ~4
creates an undulating magnetic field at right angles to
the direction of lasing gas flow. The work to compress
and expand the field is s~pplied by the oscillator and
causes a radial direction of motion of charged particles
to homogenize the gas. To avoid standing wave-patterns,
the oscillator frequency must exceed the gas flow-through
time several fold.
The principles described above have been
embodied in a 5\kw laser. It has been found that the
level of plasma control obtained removes constraints on
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the laser geometry so that large volume cavities can be uniformly
filled with plasma, and one can thus design an optimum optical
system. Very large and sturdy optical mirrors can now be used/
resulting in a lower flux density and thus preventing warping and
destruction from heat. The technology eliminates the necessity
for very delicate, fragile, and expensive op-tical systems which
are not necessarily adaptable to industrial use.
It is expected that the use of plasma control to uni-
formly fill a large volume cavity will allow a 5/kw laser having
dimensions of approximately two feet by two feet by one foot,
thereby enabling use on a robot arm.
There has been described a high power laser gaseous
discharge system which has high reliability and exceptional beam
quality at high power, but at reduced size and weight. It will
be apparent to those skilled in the art that changes may be made
in the construction and arrangements without departing from the
scope of the invention as defined in the appended claims. Other
aspects, objects, and advantages of this invention can be obtained
from a study of the drawings, the disclosure and the appended
claims.
