Note: Descriptions are shown in the official language in which they were submitted.
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00161/7005
WRM/sjd:8526Z
DISCHARGE DEVICE HAVING CATHODE WITH
MICRO HOLLOW ARRAY
Field of the Invention
This invention relates to gas discharge devices
and, more particularly, to gas discharge devices
which utilize a cathode having a micro hollow array.
Background of the Invention
The general concept of a discharge device which
utilizes a hollow cathode for increased current
capability is disclosed in the prior art. A hollow
cathode glow discharge utilizing a single, nearly
spherical hollow cathode is described by A.D. White
in Journal of Applied Physics, Vol. 30, No. 1, May
1959, pp. 711-719. The author reported a stable
discharge and negligible deterioration from
sputtering. The basic mechanisms contributing to
the hollow cathode effect are described by G.
Schaefer et al. in Physics and Applications of
Pseudosparks, Ed. by M.A. Gundersen and G. Schaefer,
Plenum Press, New York, 1990, pp. 55-76.
Measurements of the temporal development of hollow
cathode discharges are described by M.T. Ngo et al.
in IEEE Transactions on Plasma Science, Vol. 18, No.
3, June 1990, pp. 669-676.
A variety of hollow cathode structures for
fluorescent lamps have been disclosed in the prior
art. A directly-heated hollow cathode having an
interior coating of an emissive material is
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disclosed in U.S. Patent No. 4,523,125, issued
June 11, 1985 to Anderson. A shielded hollow cathode
for fluorescent lamps is disclosed in U.S. Patent No.
x,461,970, issued July 24, 1984 to Anderson. A
hollow electrode having an interior coating of an
emissive material that varies in thickness is
disclosed in U.S. Patent No. 2,847,605, issued August
12, 1958 to Byer. A short arc fluorescent lamp
having hollow cathode assemblies is disclosed in U.S.
Patent No. 4, 093, 893, issued June 6, 1978 to
Anderson. Cup shaped electrodes containing emissive
material for use in cold cathode fluorescent lams
are di jClOSed i=1 U.S. Pat~'nt No. 3, 90E, 27 i , i ssuc"d
September 16, 1975 to Aptt, Jr., and U.S. Pater_t No.
3,969,279, issued July 13, 1976 to Kern. A
fluorescent lamp wherein a filament is positioned
within a cylindrical shield is disclosed in U.S.
Patent No. 2,549,355, issued April 17, 1951 to
Winninghoff. Additional hollow cathode discharge
devices are disclosed in TJ.S. Patent No. 1,842,215,
issued January 19, 1932 to Thomas; U.S. Patent
No. 3,515,932, issued June 2, 1970 to King; U.S.
Patent No. 4,795,942, issued January 3, 1989 to
Yamasaki; U.S. Patent No. 3,390,297, issued June 25,
1968 to Vollmer; and U.S. Patent No. 3,383,541,
issued May 14, 1968 to Ferreira.
An electrical discharge electrode including a
plurality of tubes, which are filled with an electron
emissive material and embedded in a metallic matrix,
is disclosed in U.S. Patent No. 4,553,063, issued
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November 12, 1985 to Geibig et al.
A variety of different fluorescent lamp types
have been developed to meet different market
demands. In addition to conventional tubular
fluorescent lamps for office and commercial
applications, compact fluorescent lamps have been
developed as incandescent lamp replacements.
Subminiature fluorescent lamps have found
applications in displays and general illumination in
limited spaces.
Different fluorescent lamps may operate under
very different discharge conditions. The small size
of subminiature fluorescent lamps may not allow f or
hot cathode operation, thus requiring efficient cold
cathode emitters. The buffer gas pressure in
subminiature fluorescent lamps is often on the order
of 100 torr in order to limit electron loss to the
lamp wall. By contrast, conventional fluorescent
lamps typically utilize pressures on the order of
0.5-2.0 torn. Environmental concerns have
necessitated the investigation of lamp fill
materials other than mercury. In mercury-free
fluorescent lamps, radiation is often produced by
excimers of inert gases, such as neon, argon and
xenon. In order to form excimers, a gas pressure of
approximately 100 torn is requires. In subrniniature
fluorescent lamps utilizing cold cathodes, the
operating life may be limited by sputtering. In
addition, current limitations may restrict light
output. These trends have produced a need for
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improved cathode configurat ions.
The hollow cathode configurations disclosed in
the prior art are not suitable for use in
subminiature fluorescent lamps because of their
relatively large sizes and because of~the relatively
high pressures utilized in subminiature fluorescent
lamps.
Hollow cathodes have been studied in connection
with other applications, such as excitation sources
for gas lasers, ion sources plasma jets, electron
beams and plasma switches. In each case, a cathode
with a single, relatively large opening, or hollow,
has been studied at low (subtorr) pressure.
Summary of the Invention
According to the invention, a discharge device
for operation in a gas at a prescribed pressure
comprises a cathode and an anode spaced from the
cathode, and electrical means for coupling
electrical energy to the cathode and the anode. The
cathode comprises a conductor having a plurality of
micro hollows therein. Each of the micro hollows
has cross-sectional dimensions selected to support a
micro hollow discharge at the prescribed pressure.
Electrical energy is coupled to the cathode and the
anode at a voltage and current for producing micro
hallow discharges in each of the micro hollows in
the cathode.
Each of the micro hollows preferably has a
cross-sectior_al dimension that is on the order of
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the mean free path of electrons in the gas. Under
these conditions, electrons undergo oscillatory
motion within the micro hollows and produce
substantially higher currents than a planar
cathode. The micro hollow discharges operate
independently of each other.
The prescribed pressure for operation of the
discharge device is preferably in a range of about
0.1 torr to atmospheric pressure. The discharge
device may include a discharge chamber for
maintaining the prescribed pressure. When the
discharge device is operated at or near atmospheric
pressure in air, the discharge chamber may not be
required.
The discharge device may include a dielectric
layer between the cathode and the anode. The
dielectric layer is preferably disposed of a surface
of the cathode and is provided with openings aligned
with the micro hollows. The dielectric layer is
preferably utilized when the spacing between the
cathode and the anode is greater than about the mean
free path of electrons in the gas. The dielectric
layer ensures that a glow discharge between the
cathode and the anode terminates in the micro
hollows.
According to a first application of the
discharge device, a fluorescent lamp comprises a
sealed, light-transmissive tube containing a gas at
a prescribed pressure, and first and second
spaced-apart electrodes mounted within the tube.
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The first electrode comprises a conductor having a
plurality of micro hollows therein. Each of the
micro hollows has dimensions selected to support a
micro hollow discharge at the prescribed pressure.
The fluorescent lamp further includes electrical
means for-coupling electrical energy to the first
and second electrodes at a voltage and current f or
producing micro hollow discharges in each of the
micro hollows in the first electrode. The
fluorescent lamp preferably includes a phosphor
coating on the inside surface of the
light-transmissive tube. The phosphor coating emits
radiation having a prescribed spectrum in response
to radiation generated by the discharge between the
first and second electrodes. Each of the micro
hollows preferably has a cross-sectional dimension
that is on the order of the mean free path of
electrons in the gas.
For AC operation of the fluorescent lamp, the
second electrode preferably comprises a conductor
having a plurality of micro hollows therein. Each
of the micro hollows in the second electrode has
dimensions selected to produce a micro hollow
discharge at the prescribed pressure.
The fluorescent lamp preferably includes a
dielectric layer on the surface of each electrode.
Each dielectric layer has openings aligned with the
micro hollows.
In a second application of the discharge device,
a radiation source comprises a sea=ed discharge tube
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containing a gas at a prescribed pressure, first and
second spaced-apart electrodes mounted within the
discharge tube, and electrical means for coupling
electrical energy to the first and second
electrodes. At least one of the electrodes
comprises a conductor having a plurality of micro
hollows. Each of the micro hollows has dimensions
selected to produce a micro hollow discharge at the
prescribed pressure. In a preferred embodiment, the
radiation source is an excimer lamp wherein the gas
and the prescribed pressure are selected to emit
radiation in a wavelength range of about 80 to 200
manometers.
In a third application of the discharge device,
a laser for generating laser radiation at a
predetermined wavelength comprises a first mirror
that is substantially reflective at the
predetermined wavelength, a second mirror that is
partially reflective and partially transmissive at
the predetermined wavelength, a chamber for
enclosing a gas at a prescribed pressure between the
first and second mirrors, and a laser pumping device
positioned between the first and second mirrors.
The laser pumping device comprises a cathode having
at least one micro hollow therein, the micro hollow
having dimensions selected to produce a micro hollow
discharge at the prescribed pressure, an anode
spaced from the cathode and electrical means for
coupling electrical energy to the cathode and the
anode at a voltage and current for producing the
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micro hollow discharge in the micro hollow. The
laser pumping device provides an unobstructed
optical path along the optical axis between the
first and second mirrors. The cathode may include a
plurality of micro hollows and the anode may include
a plurality of openings aligned with the micro
hollows. In this case, each of the micro hollows
defines an optical axis between the first and second
mirrors for a generation of multiple laser beams at
the predetermined wavelength. Two or more of the
laser pumping devices may be disposed along the
optical axis between the first and second mirrors.
In a fourth application of the discharge device,
a light source comprises a sealed discharge chamber
containing a gas at a prescribed pressure, a cathode
mounted within the discharge chamber and an anode
spaced from the cathode. The cathode comprises a
conductor that defines an array of micro hollows.
Each of the micro hollows has a cross-sectional
dimension selected to support a micro hollow
discharge at the prescribed pressure and has an
axial dimension that is substantially less than the
cross-sectional dimension. The light source further
comprises electrical means for coupling electrical
energy to the cathode and the anode at a voltage and
current for producing micro hollow discharges in
each of the micro hollows in the cathode. The light
source is preferably configured as a thin, flat
light source.
The light source may be configured as a flat
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fluorescent light source, including a phosphor
coating on a light-transmissive portion of the
discharge chamber. The phosphor coating emits
radiation having a prescribed spectrum in response
to radiation generated' within the micro hollows.
In a preferred embodiment, the cathode of the
flat light source comprises a wire mesh including
spaced-apart conductors which define the micro
hollows. Alternatively, the cathode may comprise a
conductive pattern formed on a light-transmissive
substrate, the conductive pattern comprising a grid
of spaced-apart conductive lines.
In an additional application, the discharge
device of the present invention can be configured as
an electron source for generating multiple electron
beams. In a further application, the discharge
device is configured as an ion source for generating
multiple ion beams.
Brief Description of the Drawings
For a better understanding of the present
invention, reference is made to the accompanying
drawings, which are incorporated herein by reference
and in which:
FIG. 1 is a schematic diagram of a discharge
device in accordance with the present invention;
FIG. 2 is a graph of current as a function of
voltage for the discharge device, illustrating the
high glow mode and the low glow mode;
FIG. 3 illustrates an experimental setub for
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evaluation of the discharge device of the present
invention;
FIG. 4 is a graph of voltage as a function of
current for a cathode having a single hole, and an
anode-cathode separation of 2.5 centimeters;
FIG. 5 is a graph of voltage as a function of
current for a cathode having a single hole, and an
anode-cathode separation of 5 centimeters;
FIG. 6 is a graph of voltage as a function of
current for a cathode having four holes, and an
anode-cathode separation of 2.5 centimeters;
FIG. 7 is a graph of voltage as a function of
current for a cathode having four holes, and an
anode-cathode separation of 5 centimeters;
FIG. 8 is a graph of voltage as a function of
current for a cathode having eight holes, and an
anode-cathode separation cf 2.5 centimeters;
FIG. 9 is a graph of voltage as a function of
current for a cathode having eight holes, and an
anode-cathode separation of 5 centimeters;
FIG. 10A is a graph of voltage as a function of
current for a cathode having three holes and an
anode-cathode separation of 0.2 millimeter, for
pressures in the range of 1.5 torr to 6 torr and for
the low current glow mode;
FIG. lOB is a graph of current as a function of
pressure for the low current glow mode at 320 volts,
for three hole and one hole discharges;
FIG. 11 is a graph of voltage as a function of
current for a cathode having f our holes at a
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pressure of two torr, showing a transition to the
high current mode;
FIG. 12 is a simplified schematic diagram of a
subminiature fluorescent lamp in accordance with the
present invention;
FIG. 13 is an axial view of the cathode in the
subminiature fluorescent lamp of FIG. 12;
FIG. 14 is a schematic, cross-sectional diagram
of a gas laser using the discharge device of the
present invention for optical pumping;
FIG. 15 is an axial view of an array of micro
hollows;
FIG. 16 is a partial cross-sectional view of a
discharge device suitable for use as an excimer
light source in accordance with the present
invention;
FIG. 17 is a partial cross-sectional view of a
flat fluorescent light source in accordance with the
present invention;
FIG. 18 is a partial illustration of the mesh
cathode of FIG. 17; and
FIG. 19 is a partial cross-sectional view of an
alternate embodiment of the flat fluorescent light
source.
Detailed Description
A discharge device in accordance with the
present invention is shown schematically in FIG. 1.
The discharge device includes a cathode 10 and an
anode 12 mounted within a discharge chamber 14. The
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discharge chamber 14 is typically sealed and
contains a gas at a prescribed pressure, P. The
pressure P is typically in a range of about 0.1 torr
to atmospheric pressure. In some cases, the
discharge chamber 14 may have an opening to permit
gas flow or to permit passage of a charged particle
beam, as described below. In general, the discharge
chamber 14 maintains the pressure P between anode 10
and cathode 12 within a desired range. When the
discharge device is operated at atmospheric pressure
in air, the discharge chamber may be omitted. A
power source 18 connected to cathode 10 and anode 12
supplies electrical energy to the discharge device.
The cathode 10 comprises an electrically
conductive material having one or more holes,
referred to herein as micro hollows 20. Preferably,
cathode 10 includes a plurality of micro hollows 20
for increased current capability. The micro hollows
20 are formed in a surface 22 of cathode 10 which
may be flat or curved and which faces anode 12.
Each of the micro hollows 20 has a diameter, D, and
extends from surface 22 into cathode 10. As
described below, the diameter D of each of the micro
hollows 20 is selected to support a micro hollow
discharge at the prescribed operating pressure
within discharge chamber 14. The diameter D is
defined as the diameter of a cross-section of the
micro hollow in a plane parallel to surf ace 22 and
perpendicular to a longitudinal axis 24 of the micro
hollow 20. In some cases, the cross section of the
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micro hollow may not be circular. However, for ease
of understanding, reference is made herein to the
diameter D of the micro hollow. Where the cross
section is not circular, it will be understood that
the cross-sectional dimension is selected in the
manner described below to support a micro hollow
discharge. As shown in FIG. 1, the micro hollows 20
may be closed at one end. However, the micro
hollows can be open at both ends within the scope of
the present invention.
The shape of the micro hollows is not critical.
The micro hollows may, for example, be formed by
drilling, thus defining a generally cylindrical
shape, at least initially. The micro hollows
preferably have a circular cross section in a plane
parallel to surf ace 22 of cathode 10.
Alternatively, the cross section of the micro hollow
can be oval, square, rectangular or slit shaped. It
has been reported that the initial cylindrical shape
of the micro hollow transforms itself into a
spherical shape through sputtering and deposition.
In cases where the micro hollow does not have a
uniform diameter along the micro hollow axis 24, the
diameter D is defined at surface 22. The lifetime
of the micro hollow cathode is expected to be long
because of low cavity erosion, due to a balance of
sputtering and redeposition inside the micro
hollow.
The diameter D of each of the micro hollows 20
is selected to support a micro hollow discharge
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within each of the micro hollows 20. More
specifically, the diameter D is selected such that
the cathode fall region extending from the inner
wall of the micro hollow is on the order of the hole
radius. The cathode fall region is defined as a
region of increased electric field near the cathode
surface. The intensity and distribution of the
electric field is such that the ions accelerated
toward the cathode grin sufficient energy to provide
for emission of secondary electrons from the
cathode, which are needed for a self-sustained glow
discharge. The electrons emitted from the cathode
surface within the micro hollow are accelerated in
the cathode fall region toward the micro hollow axis
24. These electrons cross the axis and enter the
cathode fall region on the opposite side of the
axis, where they are reflected and accelerated
across the axis again. The oscillatory motion of
the so called "pendel" electrons allows them to
unload most of the energy gained in the cathode fall
region through exciting and ionizing collisions
inside the micro hollow before drifting to the
anode. The large ionization rate in a relatively
small volume causes a high plasma density on the
discharge axis inside the micro hollows 20 and
consequently a high current. A "micro hollow
discharge" occurs when electrons undergo oscillatory
motion within the micro hollows. As used herein,
the term."micro hollow" refers to a cathode hole
having a cross-sectional diameter D in a plane
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parallel to the cathode surface. The hole diameter
D times pressure P in the discharge chamber must be
in a range of 0.1 torr-centimeter to 10
torr-centimeters, depending on the gas type,
electrode material and~desired mode of operation
(high or low glow mode). In the discharge device of
FIG. 1, the current is found to be several orders of
magnitude greater than the current for a planar
cathode, and the voltage is lower.
The conditions for a micro hollow discharge as
described above are met when the hole diameter D is
on the order of the mean free path of the electrons
in the gas. The mean free path depends on the type
of gas and the gas pressure in the discharge chamber
14, and is approximately eaual to the dimension of
the cathode fall region. Optimum micro hollow
discharge conditions are obtained when the diameter
D of the micro hollows is about twice the mean free
path of electrons in the gas in the discharge
chamber. However, it will be understood that other
values of diameter D can be used within the scope of
the invention. Preferably, the diameter D is in a
range of about 0.1 to 10 times the mean free path of
electrons in the gas, but the diameter D is not
limited to this range.
The discharge parameters vary with the product
of pressure P times hole diameter D. The range of
P~D for which the micro hollow discharge is stable
for rare gases was found to be on the order of 0.1 to
torr-centimeters.
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It is believed that most of the micro hollow
discharge current is generated in a region of the
micro hollow wall that extends from the surface 22
of cathode 10 to a depth that is about three times
the diameter D of the micro hollow. Thus, little
additional current is obtained when the depth, L, of
the micro hollow is greater than about three times
the diameter D. However, a micro hollow discharge
occurs even when the depth L of the micro hollow is
smaller than the diameter D, with a reduction in
discharge current.
The number of micro hollows 20 is selected to
produce a desired total current at the operating
voltage. It has been found that the micro hollows
20 can be closely spaced without adversely affecting
the independent operation of the discharges.
Also shown in FIG. 1 is a dielectric layer 30 on
surface 22 of cathode 10. The dielectric layer 30
is required when the spacing, S, between cathode 10
and anode 12 is greater than about the mean free
path of electrons in the gas. When the spacing S
exceeds this value and the dielectric layer 30 is
not utilized, the glow discharge between cathode l0
and anode 12 may terminate on surface 22 of cathode 10,
rather than in the micro hollows 20. Preferably,
the dielectric layer 30 covers surface 22 and
surrounds micro hollows 20. The dielectric layer 30
can, for example, be a mica layer affixes to surface
22, or can be deposited on surface using known
deposition techniques. when the spacinc S is less
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than about the mean free path of electrons in the
gas, the dielectric layer 30 may be omitted.
The anode 12 can have any desired configuration
which permits an electric field to be established in
the vicinity of cathode l0. Preferably, the anode
12 is planar and has an area that is approximately
equal to the area of cathode 10, so that the spacing
S between cathode 10 and anode 12 is approximately
uniform over the area of surface 22. The planar
anode can optionally have holes aligned with the
micro hollows to provide a path for radiation
generated by the discharge, a gas flowing through
the micro hollows, or an electron or ion beam.
In cases where the power source I8 supplies an
AC voltage to the discharge device, the anode 12 can
be provided with micro hollows in the same manner as
cathode 10. To avoid confusion in the AC
configuration, cathode 10 is called "electrode 10",
and anode 12 is called "electrode 12". Electrode 10
functions as a cathode during those half cycles of
the AC voltage when electrode 12 is positive with
respect to electrode 10, and electrode 12 functions
as a cathode during those half cycles of the AC
voltage when electrode 10 is positive with respect
to electrode 12. By providing electrodes 10 and 12
with micro hollows as described above, micro hollow
discharges are obtained on both half cycles of the
AC voltage.
The gas in the discharge cha~rber 14 may, for
example, be an inert gas such as argon, neon or
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xenon. However, any desired gas can be utilized,
including mixtures of gases. As noted above, the
pressure within discharge chamber 14 is preferably
in a range of about 0.1 torn to atmospheric
pressure. A number of applications utilize
pressures in a range of about 0.1 torr to 200 torr.
The cathode 10 can be fabricated of any desired
conductive material. However, materials with a high
rate of secondary electron emission through ion
impact are preferred. Suitable materials of this
type include tungsten, barium oxide embedded in
tungsten, thoriated tungsten, molybdenum and
aluminum coated with oxygen. Materials, including
composite matt~rials, characterized by a low electron
work function are suitable as cathode materials.
Other suitable materials meeting these requirements
are known to those skilled in the art. In an
alternative configuration, the inside surfaces of
micro hollows 20 are coated with materials that have
high electron emissivity, and the remainder of
cathode 10 is fabricated of any desired conductive
material.
The discharge chamber 14 can have any desired
size and shape. Typically, the discharge chamber is
sealed to maintain pressure P in the discharge
region. The chamber 14 may be fabricated, at least
in part, of a material that transmits radiation
generated by the discharge. Thus, for example, the
discharge chamber 14 may be fabricated of a light-
transmissive material, such as glass or quartz, or
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may have a radiation-transmissive window. In other
embodiments, the discharge chamber 14 may be
conzigured such that gas at pressure P flows through
the discharge region.
The power source 18 may supply a DC voltage, a
pulsed voltage or an AC voltage to the discharge
device. For an AC voltage, a micro hollow discharge
occurs only on half cycles when the anode 12 is
positive with respect to the cathode 10, unless both
electrodes have a micro hollow configuration as
described above. The reauired voltage is typically
in a range of about 300 to 600 volts. The micro
hollow discharges have a posit-V2 JO~tage-current
(V-I) characteristic over a large range of currents
and voltages, which permits operation oz the micro
hollows in parallel without ballast resistors. The
micro hollow discharge has been observed to operate
at currents up to 200 to 500 milliamps per micro
hollow.
The micro hollow discharges have been observed to
have two glow modes, as illustrated in FIG. 2. In a
low glow mode 36 at relatively low voltage and
current levels, the plasma column is located on the
axis of the micro hollow and appears as a slight glow
in the micro hollow. In a high glow mode 38, the
plasma column fills almost the entire micro hollow
and appears as a very bright discharge in the micro
hollow. The high glow mode occurs at higher current
and voltage levels. The discharge switches abruptly
from the low glow mode to the high glow
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mode as the voltage is increased. In both modes,
the discharges are st able and do not influence each
other. Spectral measurements of the high glow mode
indicate the presence of spectral lines from the
cathode material, thereby suggesting increased
sputtering of the cathode material in the high glow
mode. Besides the gas ions, the metal ions of the
sputtered electrode material contribute to the
current flow and the secondary electron generation
at the cathode.
The high and low glow modes refer to the
discharges in the micro hollows 20. When the
cathode 10 and the anode 12 have a spacing S greater
than about the mean free path of electrons in the
gas, a glow discharge occurs in the region outside
the micro hollows 20 between cathode 10 and anode 12.
A set of experiments was performed to
investigate micro hollow cathode discharge in an
argon-mercury environment with single and multiple
cathode holes. A schematic diagram of the
experimental configuration is shown in FIG. 3. A
test chamber was defined by a glass tube 100 having
a length of 23 cm and a diameter of 4 cm. The ends
of glass tube 100 were sealed by stainless steel
blocks 102 and 1D4. A cathode 110 and an anode 112
located within the charr.ber could be varied in
spacing between 0.1 cm and 15 cm. Molybdenum
cathodes with 1, 4 and 8 holes of 0.7 mm diameter
and 2.1 mm depth were used. A Cober Model 605P High
Power Pulse Generator 116 was used to supply a 360
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microsecond pulse at 30 Hz to the cathode 110. The
voltage across the discharge was measured using a
Tektronix P-6015 100CX High Voltage Probe, and the
current across the load was measured using a
Tektronix AM503 currerit probe.
Different gas pressures, different cold spot
temperatures (mercury pressure), different electrode
separations and different numbers of micro hollows
were studied. FIG. 4 shows the voltage=current
(V-I) characteristics of a cathode having a single
micro hollow, kith 2.5 cm electrode separation, a
pressure of 3 torr of a mercury-argon mixture and
cold spot temperatures of 15°C and 25°C. A constant
voltage discharge was observed at the low currer_t
level, for example, less than 240 milliamps for T =
15°C and less than 250 milliamps for T = 25°C.
Positive V-I characteristics were obtained at a
higher current level. A higher cold spot
temperature promotes a lower current level when the
voltage is kept constant. At larger electrode
separation, that difference disappears, and the V-I
characteristics overlap. FIG. 5 shows the V-I
characteristics of a cathode having a single micro
hollow, with 5.0 cm electrode separation, a pressure
of 3 torr and cold spot temperatures of 15°C and
25°C. The threshold current for positive V-i
characteristics is higher for higher cold spot
temperatures, as shown in FIG. 5.
FIG. 6 shows the V-I characteristics of a
cathode having four micro hollows, with 2.5 cm
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separation between electrodes, a pressure of 3 torn
and cold spot temperatures of 15°C and 25°C. FIG. 7
shows the V-I characteristics of a cathode having
four micro hollows, with 5.0 cm separation between
electrodes, a pressure of 3 torr and cold spot
temperatures of 15°C and 25°C. A discharge with
four micro hollows demonstrated unstable conditions
at low current levels. In the current range below
300 milliamps at 2.5 em electrode separation and
below 350 milliamps at 5.0 cm electrode separation,
the four micro hollow cathode discharge switched
consecutively from the low glow mode into the high
glow mode. After all of the discharges operated in
the high glow mode, the V-I characteristic became
positive and stable. All four micro hollows were
then operatir_g in parallel with about equal light
intensity. The threshold current levels correspond
to 350 volts and 375 volts, respectively. The
current at 25°C was higher than at 15°C when the
voltage was maintained at a constant level.
FIG. 8 shows the V-I characteristics of a
cathode having eight micro hollows with 2.5 cm
electrodes separation, a pressure of 3 torn and cold
spot temperatures of 15°C and 25°C. FIG. 9 shows
the V-I characteristics of a cathode having eight
micro hollows with 5.o c:a electrodes separation, a
pressure of 3 torr and cold spot temperatures of
15°C and 25°C. The cathode having eight micro
hollows operated in a parallel and stable manner for
currents higher than 400 milliamps, corresponding to
CA 02565198 2006-11-08
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a voltage of 375 volts, at 2.5 cm separation (FIG. 8)
and for currents higher than 500 milliamps,
corresponding to a voltage of 350 to 375 volts, at
5.0 cm separation (FIG. 9), after all eight micro
hollows transferred into the high glow mode. The
currer_t level obtained with eight micro hollows is
only slightly higher than with four micro hollows.
For example, at 450 volts, tour micro hollows operate
at 700 milliamps at lSoC and 800 milliamps at 25°C,
while eight micro hollows operate at give 950
milliamps and 950 milliamps for temperatures of 15~C
and 25~C, respectively.
_'=mother set of experiments was performed with a
cathOd2 _aVing three hCl eS t0 St;~.dV par al le~'_
operation of micro hollow cathode discharge devices
in a situation where the anode-cathode distance was
less than the micro hollow diameter. Cathode holes
having a diameter of 0.7 mm and a depth of 2.1 mm
were drilled in a molybdenum disk. A molybdenum foil
12.7 micrometers thick with four 2 mm holes was used
as the anode. The anode and cathode were separated
by a 0.2 mm thick mica spacer. The voltage and
current were measured as described above in
connection with Figs. 4-9. FIG. 10A illustrates the
I-V characteristics of the three hole hollow cathode
discharge with pressures between 1.5 torr and 6 torn.
The discharge exhibited two modes of operation, the
first being a submilliamp unstable glow mode
indicated by the points below 1.0 milliamp in FTG_
10B, and the second being a low current glow
CA 02565198 2006-11-08
- 24 -
mode indicated by the points above 1.0 milliamp in
FIG. 10B. The discharge in the unstable glow mode
was a slight glow that occupied only the center of
the hole, and the discharge in the Iow current glow
mode occupied about half the hole. FIG. lOB
compares the current levels at a given voltage of
the three hole discharge with a one hole discharge.
Over the range of pressures shown in FIG. 10B, the
ratio of three hole current to one hole current is
about three, indicating the multiplication property
of the micro hollow cathode discharge.
In another set of experiments with four holes
with the same dimensions as above, the transition
between the low current glow mode and the high
current glow mode was observed. The high current'
glow mode was a very bright discharge that filled
most of the hole. The discharge started with a low
glow in each of the four holes, and as the voltage
applied to the discharge was increased, the
individual holes switched to the high glow mode.
FIG. 11 shows that a low glow mode was obtained at
400 volts across the discharge. The discharge
current increased linearly until 500 volts. Then,
one of the holes transferred into the high glow
mode, and the voltage decreased to 460 volts. The
discharge continued with one hole in high glow mode
until 580 volts was reached. At this point, a
second hole transferred into the high glow mode and
the voltage decreased to 500 volts. A third hole
did not transfer into the high glow mode until the
CA 02565198 2006-11-08
~',.
- 25 -
voltage reached 580 volts. At that point, the
voltage across the discharge dropped to 480 volts.
The fourth hole transferred to high glow mode when
the discharge voltage reached 540 volts. The
discharge voltage at this point decreased to 500
volts.
Spectra of the discharges were recorded at 3
torr argon pressure. A first spectrum was taken
with all of the holes in the low glow mode, and a
second was taken with three of the holes in the high
glow mode. The discharges contained molybdenum
lines when the high glow mode was present.
An application of the discharge device of the
present invention is shown in FIGS. 12 and 13. The
discharge device is configured as a fluorescent lamp
for generation of visible light. The fluorescent
lamp includes a first electrode 210 and a second
electrode 212 sealed within a light-transmissive
tube 214, which may be glass. The electrodes 210
and 212 are spaced apart and are preferably located
at or near opposite ends of light-transmissive tube
214. Electrical conductors 216 and 218 extend from
the exterior of light-transmissive tube 214 to
electrodes 210 and 212, respectively, and permit
connection of electrodes 210 and 212 to a source of
electrical energy (not shown). The
light-transmissive tube 214 defines a sealed chamber
that is maintained at a desired pressure during
operation. The conductors 216 and 218 extend
through vacuum feedthroughs, as known in the art.
CA 02565198 2006-11-08
,._ - 26 -
The light-transmissive tube 214 contains a fill
material for supporting a low pressure discharge
between electrodes 210 and 212. The fill material
is typically an inert gas, such as neon, argon or
xenon, and mercury vapor. Typically, the inside
surface of light-transmissive tube 214 is coated
with a phosphor material that emits visible light in
response to ultraviolet radiation generated by the
discharge within the tube. A variety of phosphor
materials are well known to those skilled in the art.
In the embodiment of FIGS. 12 and 13, the
electrode 210 comprises a generally disk-shaped
conductor. The electrode 210 preferably has a flat
surface 226 that f aces electrode 212 and has
sufficient thickness for formation of micro
hollows. An arrav of micro hollows 230 is formed in
the surface 226 of electrode 210. Each of the micro
hollows 230 comprises a hole having a prescribed
diameter that extends from surface 226 into
electrode 210. The diameter of each of the micro
hollows 230 depends on the type of gas and the
operating pressure within the discharge device. A
dielectric layer 228 is disposed on surf ace 226 of
electrode 210. The dielectric layer 228 surrounds
but does not cover micro hollows 230.
In the embodiment of FIGS. 12 and 13, the micro
hollows 230 are closed at one end. However, the
micro hollows can extend entirely through electrode
210 within the scope of the present invention. The
shape of the micro hollows is not critical. The
CA 02565198 2006-11-08
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micro hollows may, f or example, be formed by
drilling, thus defining a generally cylindrical
shape. The electrode 210 can be fabricated of any
conductive material, but is preferably fabricated of
a low work function material that has high electron
emlssivlty.
The diameter, D, of each of the micro hollows
230 is selected, depending on the operating
pressure, P, and the gas type within the light-
transmissive tube 214, to produce a micro hollow
discharge within each of the micro hollows 230. In
particular, the diameter D of each of the micro
hollows is preferably on the order of the mean free
path of electrons in the light-transmissive tube
214. For rare gases, this condition is met when the
product P~D is in a range of about 0.1 to 10,
where the pressure P is specified in torr and the
diameter D is specified in centimeters. The
operating pressure and the type of gas are usually
established by other design considerations, thus
setting an allowable range of diameters for the
micro hollows. Fluorescent lamps typically contain
argon and mercury vapor. Conventional fluorescent
lamps typically operate at pressures of 0.5 to 2.0
torr, whereas subminiature fluorescent lamps may
operate at pressures of 20 to 200 torr. By way of
example, for a subminiature fluorescent lamp having
a pressure of argon and mercury in the range of 20
to 200 torr, the micro hollows 230 preferably have
diameters in the range of 0.5 cm to less than 50
CA 02565198 2006-11-08
- 28 -
micrometers. The number of micro hollows 230 is
selected to produce a desired total discharge
current. Preferably, the micro hollows are
relatively uniformly distributed over surface 26,
and the surface 26 between micro hollows is covered
by dielectric layer 228.
In the fluorescent lamp, the radiation that
stimulates the phosphor coating on the
light-transmissive tube 214 is generated in the
positive column between electrodes 210 and 212. The
micro hollows function as a source of electrons, and
generation of radiation within the micro hollows is
not important. For this reason, the micro hollow
cathode is preferably operated in the low glow mode
for fluorescent lamp applications.
The electrode 210 is configured to function as a
cathode for emission of electrons when it is biased
negatively with respect to electrode 212. For
typical fluorescent lamp applications, the electrode
212 is fabricated with an array of micro hollows and
a dielectric layer in the same manner as electrode
210. In this configuration, an AC voltage is
applied between conductors 216 and 218. Electrode
210 functions as a cathode during those half cycles
of the AC voltage when electrode 212 is positive
with respect to electrode 210, and electrode 212
functions as a cathode during those half cycles of
the AC voltage when electrode 210 is positive with
respect to electrode 212.
In another embodiment, the electrode 212 is not
CA 02565198 2006-11-08
- 29 -
fabricated with an array of micro hollows and has a
conventional anode configuration. In this
embodiment, electrode 212 functions continuously as
an anode, and electrode 210 functions continuously
as a cathode. A DC voltage or a pulse train is
applied between conductors 216 and 218 in this
configuration.
The electrodes 210 and 212 are preferably
fabricated of a material with a high rate of
secondary emission through ion impact. Preferred
materials include tungsten, barium oxide embedded in
tungsten, thoriated tungsten and molybdenum.
Materials, including composite materials,
characterized by a low electron work function are
suitable as electrode materials.
A variety of different gases can be utilized in
~he fluorescent lamp of FIGS. 12 and 13. Preferred
gases include mercury vapor mixed with an inert gas,
such as argon or krypton, an inert gas, such as
neon, without mercury vapor, an excimer of an inert
gas, such as Xe2, vapors of sulfur or selenium,
and combinations thereof.
In an example, the fluorescent lamp shown in
FIGS. 12 and 13 is configured as a subminiature
fluorescent lamp. The light-transmissive tube 214
has an outside diameter of 7 mm, ar_d the spacing
between electrodes 210 and 212 is about 100 mm. The
tube 214 contains argon and mercury at a pressure of
about 100 torr. Each of the electrodes 210 and 212
has a diameter of about 5 mm and is provided with
CA 02565198 2006-11-08
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about 20 micro hollows 230. The micro hollows have
diameters of about 50 micrometers. The lamp is
expected to operate in the low glow mode at about
300 volts and a current of about 200 milliamps. In
another example, of the fluorescent lamp, the light
transmissive tube 214 is 100 mm long and has an
outside diameter of 3 mm. The tube 214 contains
argon and mercury at a pressure of about 50 torr.
Each of the electrodes 210 and 212 has a diameter of
about 1 mm and is provided with about 10 micro
hollows 230. The micro hollows have diameters of
about 50 micrometers. The lamp is expected to
operate in the low glow mode at about 400 volts and
a current of about 5 milliamps per micro hollow. In
general, the spacing between electrodes can vary
between 10 cm and loo cm, the pressure can vary
between 1 and 200 torr, the micro hollow diameters
can vary between 10 and 1000 micrometers, and the
number of micro hollows can vary from 5 to 50 in
order to achieve currents of 5 to 100 milliamps and
voltages of 2o to 500 volts. The range of selected
parameters provides discharge conditions with
minimum electrode sputtering, maximum light output
(10 to 1000 lumens) and extended life (500 to 5000
hours). This range is defined by subminiature
fluorescent Lamp conditions and applications.
The fluorescent lamp of FIGS. 12 and 13 has been
described in connection with subminiature
fluorescent lamps which have relatively small
dimensions and which operate at relatively high
CA 02565198 2006-11-08
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pressure. However, the cathode having an array of
micro hollows is not limited to application in
subminiature fluorescent lamps. The cathode having
an array of micro hollows can be used in any
fluorescent lamp where the operating pressure
permits an array of suitably dimensioned micro
hollows to obtain desired operating
characteristics. The size and number of micro
hollows is selected for a given operating pressure
and current requirement. Furthermore, the phosphor
coating on the light-transmissive tube can be
omitted when the discharge within the tube produces
a desired radiation spectrum. Different fill
materials can be utilized within the scope of the
present invention. Specifically, mercury free
fluorescent lamps can more easily be realized in the
micro hollow cathode array system, since the
expected electron energy distribution function is
enriched with the high energy electrons and
therefore promotes excitation of higher energy
levels of typical gases considered for mercury
replacement. Ionization is also enhanced in this
discharge arrangement. The cathode having an array
of micro hollows can optionally be heated to
increase electron emission further.
Another application of the discharge device of
the present invention is as an excimer lamp, which
generates far ultraviolet radiation, typically in
the wavelength range of 80-200 nanometers. The
excimer lamp can be used for water purification,
CA 02565198 2006-11-08
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pasteurization, waste treatment and surface
treatment of materials. The excimer lamp typically
operates at a relatively high pressure, on the order
of 100 torr or higher and contains a gas, such as
xenon or neon, that forms dimers at high pressures.
Other suitable gases include all other noble gases
and mixtures of noble gases with halogens. For
operation at pressures on the order of one
atmosphere, the micro hollows have diameters on the
order of 10 to I00 micrometers. The excimer lamp
can have any desired configuration such as, for
example, the discharge device shown in FIG. 1.
Alternatively, the excimer lamp may have a
configuration similar to the fluorescent lamp shown
in FIG. 12 or the flat light sources shown in FIGS.
17 and 19. Generally, since the part of the
discharge outside the micro hollows does not
contribute to the excimer radiation, it can be
eliminated, thus forming a flat light source with
the anode-cathode distance shorter than the electron
mean free path. All or a portion of the discharge
chamber is fabricated of a material, such as quartz,
that transmits ultraviolet radiation at the
wavelength generated within the discharge chamber.
This far ultraviolet radiation can be converted
inside the lamp into visible radiation by a
specially designed phosphor. Although the
efficiency of such a lamp is at present lower than
the efficiency of standard fluorescent lamps, this
environmentally friendly lamp fill makes it an
CA 02565198 2006-11-08
'-
- 33 -
attractive alternative.
The excimer lamp can also be implemented as a
micro hollow discharge array, as shown in FIG. 16.
A conductive cathode 400 is provided with an array
of micro hollows 402, 404, 406, etc., as described
above in connection with FIG. 1. An anode 410 is
spaced from cathode 400 by a dielectric layer 412.
A second dielectric layer 414 is formed on the
opposite surface of anode 410. The anode 410 may be
a thin metal film. The anode 410 and the
dielectric layers 412 and 414 may, for example, be
formed by sputter deposition on cathode 400. The
anode 410 and the dielectric layers 412 and 414 have
openings aligned with each of the micro hollows 402,
404, 406, etc.
A further application of the discharge device of
the present invention is as a miniature gas laser.
As discussed above, the increased current of hollow
cathode discharges compared to glow discharges
between planar parallel electrodes is believed to be
due to the high ionization rate of nonthermal
electrons, which oscillate between opposite cathode
surfaces inside the cathode hole. The high energy
electrons may be used for transverse pumping of
miniature gas lasers, operated at gas pressures of
up to one atmosphere. These miniature gas lasers,
which in size are almost comparable to semiconductor
lasers, may emit over a wide spectral range which
reaches into the ultraviolet. The hollow cathode
discharge pumped lasers are expected to be
CA 02565198 2006-11-08
.... - 3 4 -
efficient. The lifetime is expected to be long
because of low cavity erosion, due to a balance of
sputtering and redeposition in the cathode hole.
The radially accelerated, nonthermal electrons
in a cylindrical micro~hollow discharge unload most
of their energy close to the axis of the cathode
hole. This energy is close to the free fall energy
of the electrons, which corresponds to the value of
the applied voltage. For submillimeter micro
hollow discharges, the forward voltage is about 100
to 500 volts. An electron energy of tens up to
several hundred electron volts is optimum for
collisional ionization and excitation of atoms and
molecules. Most of the cross sections for
excitation peak at about this value. When the micro
hollow discharge is used f or laser pumping, the
highest rate of excitation of the upper laser state
is on the axis of the cathode hole, with a steep
decay toward the wall of the micro hollow. For
micro hollow discharges where the initially
cylindrical cathode hole may turn into a spherical
one due to sputtering and redeposition of electrode
material, the maximum energy deposition will occur
at the electron focal point rather than along a
focal line. In order to avoid this nonhomogenous
distribution, the cathode hole is shortened in
length to a dimension that is significantly smaller
than its diameter. For a 100 micrometer diameter, a
cathode hole length of about 25 micrometers is
suitable. Transverse pumping with micro hollow
CA 02565198 2006-11-08
- 35 -
cathode discharges provides a class of gas lasers
which are almost as compact as semiconductor lasers_
An additional advantage of these devices is the low
noise level of the laser intensity compared to that
of lasers pumped by conventional discharges. The
noise may be reduced by two orders of magnitude.
The helium-neon laser is particularly appropriate
for micro hollow discharge pumping, because
experimental results in capillary tubes with
diameters of approximately 1 mm show that optimum
gain is obtained when the pressure times distance
product is 0.36 tern-cm, a value close to the opt=mum
pressure times diameter product fcr micro hollow
cathode operation. The optimum relative pressures of
helium and neon depend or. the discharge diameter
only. For a helium-neon laser pumped with 100
micrometer diameter micro hollows, the optimum
pressure is 36 tcrr, with 32 torr of helium ar_d 4
tore of neon. The optimum power of this laser is
expected to be about 0.5 microwatt fer a 0.5 mm long,
100 micrometer diameter micro hollow cathode pumped
with continuous wave energy.
Micro hollow discharges are believed to be
ideally suited as pump sources for metal ion lasers,
with metals such as cadmium, silver, gold, lead and
others. Micro hollow discharges provide metal ions
through continuous sputtering, instead of thermal
processes, to create a sufficient metal vapor
pressure. Lasing from ultraviolet to near infrared
CA 02565198 2006-11-08
- 36 -
has been demonstrated with hollow cathode pumping in
various metal ion lasers.
The micro hollow cathode discharge device of the
invention may also be used for pumping of rare gas
ion lasers. The pressure times distance value for
rare gas ion lasers is close to the optimum pressure
times diameter value for micro hollow discharges. A
micro hollow cathode discharge with micro hollows
having diameters of 100 micrometers can pump a rare
gas laser operated close to atmospheric pressure.
Micro hollow cathode discharges may also be used as
pump SOUrc2s 'or n,~ trOge'? 1 aserS and rare gaS halide
exc_mer lasers.
__ cross sectional view o~ a s;_ngie micro '_-_ollow
discharge pumped min~~_ature gas 'user is shown_ in ~=G.
14. Micro hollow discharge elements 300, 302, and
304 are stacked along an optical axis 306 of the
laser. Different numbers of micro hollow discharge
elements can be utilized to provide desired laser
characteristics. The discharge elements 300, 302,
and 304 are positioned between a totally reflecting
mirror 310 and a partially reflecting mirror 312.
The partially reflecting mirror 312 permits
transmission of a laser beam 314 from the laser. The
reflection characteristics of mirrors 310 and 312 are
defined at the operating wavelength of the laser.
The discharge element 300 includes a cathode 320
and an anode 322 separated by a first dielectric
layer 324. A second dielectric layer 326 is formed
CA 02565198 2006-11-08
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on the opposite surface of anode 322. The cathode
320 is provided with a micro hollow 330 having a
diameter that is selected, based on the type of gas
and gas pressure in the discharge region, to support
a micro hollow discharge. For operation near
atmospheric pressure, the diameter of micro hollow
330 is preferably on the order of about 10
micrometers. As noted above, the depth of the micro
hollow 330 is preferably less than its diameter to
ensure relatively uniform pumping along optical axis
306. In a preferred embodiment, the cathode 320 has
a thickness on the order of 25 micrometers. The
anode 322 and the dielectric layers 324 and 326 have
openings that are aligned with the micro hollow 330
to provide an unobstructed path along axis 306. The
anode 322 and the dielectric layers 324, 326 can,
for example, be formed by sputtering on cathode
320. Discharge elements 302 and 304 have the same
structure as discharge element 300. The discharge
elements 300, 302, and 304 are attached to each
other with micro hollows 330 aligned to provide a
laminated discharge structure. As noted above, more
or fewer discharge elements can be utilized in the
miniature gas laser of FIG. 14.
An axial view of an array of micro hollows
configured as an array of miniature gas lasers is
shown in FIG. 15. The laser array includes array
elements 340, 342, 344, etc., each of which may be
constructed as shown in FIG. 14 and described
above. The laser array may have any desired number
CA 02565198 2006-11-08
- 38 -
or elements and may have a regular pattern of rows
and columns, or may have an irregular pattern. Each
discharge element of the laser array may be
constructed using conventional microlithography
techniaues. The discharge elements can be bonded
together to provide the laminated structure shown in
FiG. 14. The laser array generates multiple laser
beams.
In yet another application of the discharge
device of the present invention, the micro hollow
cathode discharge array is used as an electror~ source
or an ion source. ~s described above, electrons and
ions are generated within the micro hollows of the
ml cro hol low cathode. wi~~h r_ference to ~ ' G. 16,
electrons generated w_thin micro hollows 402, 404,
406, etc. are accelerated in a directior_ indicated by
arrow 420, and ions are accelerated in a direction
indicated by arrow 422_
In a further application of the discharge device
of the present invention, the micro hollow array is
utilized in a thin, flat light source, typically a
fluorescent light source. In this application, a
micro hollow cathode is made of a grid of conductors,
such as a wire mesh, having spacings which are
preferably in the submillimeter range. The cathode
is in close proximity to a planar anode. The flat
light source can, for example, be used for
backlighting of a display. The micro hollows are
formed as rings rather than cylindrical holes. The
micro hollows are implemented in
CA 02565198 2006-11-08
- 39 -
accordance with the invention as descrihed above,
but have small axial dimensions that are
substantially less than their cross-sectional
dimensions. The micro hollows may be, but are not
required to be, open at both.ends. Uniformity of
the discharge distribution in the micro hollows
depends on the gas type, gas pressure, applied
voltage and the mesh or grid size. While the light
source is described as being flat, it will be
understood that the components can be curved in a
desired contour, if desired.
Display systems which utilize liquid crystals
require some form of backlighting. This is
conventionally achieved by tubular fluorescent lamps
with optical elements such as reflectors,
collimators and diffusers. The discharge device of
the present invention is utilized by placing an
array of micro hollow discharges directly behind a
phosphor coating to achieve relatively homogeneous
illumination. The micro hollow cathode may consist
of a metal mesh with openings in the submillimeter
range placed between the phosphor coating and a
planar metal anode.
Because of the positive voltage-current
characteristics of micro hollow discharges, it is
possible to operate them in parallel. The micro
hollows do not necessarily have an extended depth in
the cathode material but may have the form of a
ring. Even the cylindrical shape of the r~~icro
hollows is not a precondition for micro hollow
CA 02565198 2006-11-08
- 40 -
cathode discharges. The micro hollow shape may be
quadratic, rectangular or beehive style. Thus.
metal meshes with openings in the submillimeter
range can be utilized in a micro hollow cathode
array. The anode can be a planar conductor
separated from the cathode by a distance which is
comparable to or smaller than the cross-sectional
dimensions~of the micro hollows.
A partial cross-sectional view of a flat light
source in accordance with the present invention is
shown in FIG. 17. A discharge chamber 500 includes
a light-transmissive wall 502 and a conductive wall
504. In the embodiment of FIG. 17, the light
transmissive wall 502 and the conductive wall 504
are planar, parallel, spaced-apart sheets and are
closely spaced. The light-transmissive wall 502 and
the conductive wall 504 are sealed around their
edges to define a sealed discharge volume. A
cathode is positioned in the discharge chamber
between light-transmissive wall 502 and conductive
wall 504. A phosphor coating 506 may be applied to
the inside surface of light-transmissive wall 502.
A gas at a prescribed pressure is sealed within the
discharge chamber 500.
In the embodiment of FIG. 17, the cathode is an
electrically-conductive mesh 508. The mesh 508
comprises a grid of spaced-apart wires or other
conductive strips which define a plurality of micro
hollows. More specifically, with reference to FIG.
18, wires 510, 512, 514 and 516 define a micro
CA 02565198 2006-11-08
!; _ ;:t
~T._ J
_ 41 _
hollow 520. The wires 510 and 5I2 are parallel to
each other and are perpendicular to wires 514 and
516. The micro hollow 520 in the example o~ FIG. I8
has a square cross-sectional shape with sides equal
to the spacing between the mesh wixes. The axial
depth of micro hollow 520 is defined by the
diameters of mesh wires 510, 512, 514 and 516. The
wires of the mesh 508 similarly define an array of
micro hollows, such as micro hollows 522, 524, 526,
etc. The spacing between adjacent micro hollows is
determined by the mesh wire diameter.
The mesh 508 is spaced from light-transmissive
wall 502 by a dielectric spacer 530 and is spaced
from conductive wall 504 by a dielectric spacer
532. It will be understood that dielectric spacers
530 and 532 may be located as required to maintain a
desired spacing of mesh 508 with respect to
light-transmissive wall 502 and conductive wall
504. The dielectric spacers 530 and 532 may, for
example, be in the form of elongated strips.
In operation, a voltage is applied between mesh
508, which functions as the cathode of the discharge
device, and conductive wall 504, which functions as
the anode. A micro hollow discharge is produced in
each of the micro hollows 520, 522, 524, 526, etc.
defined by the mesh 508. The radiation generated by
the micro hollow discharges stimulates emission of
visible light by phosphor coating 506. The light
emitted by the phosphor coating 506 passes through
light-transmissive wall 502 and appears as a
CA 02565198 2006-11-08
- 42 -
generally uniform planar light source.
In the light source of FIG. 17, the fill gas may
be a noble gas with mercury vapor, with dominant
emission in the ultraviolet. Other suitable gases
include inert gases, such as xenon, krypton and
argon, or their excimers which would emit
ultraviolet radiation, visible radiation or a
combination of visible and ultraviolet radiation.
The micro hollow cathode discharge enhances the high
energy tail in the electron energy distribution
function, allowing for more efficient excitation of
excimer states than conventional discharges.
Molecular gases, such as nitrogen, oxygen or air,
and sulfur or selenium vapors, and their mixtures
with inert gases, may be used in the flat light
source. The gas pressure depends on the diameter of
the cathode holes. For a mesh with 200 micrometer
openings and 50 micrometer wire diameter, the
pressure is preferably in a range of about 10 to 500
torr. The applied voltage is on the order of 400
volts DC or pulsed. Preferably, the mesh spacing is
in a range of 10-500 micrometers, which depends on
the gas and gas pressure. In cases where it is not
necessary to change the wavelength of the radiation
generated in the micro hollows, the phosphor coating
506 may be omitted.
F,xperiments were performed to study the gas
discharge between a planar electrode and a mesh
electrode for utilization as a flat light source.
The experimental setup included a vacuum chamber
CA 02565198 2006-11-08
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which included a planar anode made of tungsten
impregnated with barium, and a nickel mesh with
quadratic openings of 0.206 mm width separated by
0.044 mm wide metal bars, or strips, of 0.0014 mm
thickness. The spacing between the electrodes was
on the order of 0.15 mm, determined by a mica spacer
having an opening of about 2.5 mm. The gas was air
at a pressure of 37.5 torr. A voltage pulse with a
droop of about 100 over the entire duration of 0.4
milliseconds was applied to the electrodes, and the
current through the discharge was monitored with a
current viewing resistor. Simultaneously, the
discharge was observed by a CCD camera connected to
a magnifying system.
The results were as follows. At an applied
voltage of 384 volts and with the mesh biased
negatively, the current at the beginning of the
pulse was measured at 33 milliamps. The current
decayed to about half this value over the duration
of the voltage pulse, indicating a nonlinear
dependance of the current on the voltage.
Discharges developed in the mesh openings. Two
types of discharges were observed: a dim discharge
centered in the mesh openings in a majority of the
holes, and a bright, centered discharge in a small
number of holes. With an increasing number of
pulses, the dim discharges became brighter, and the
initially bright discharges lost intensity. The
current did not change significantly during the
transition phase from inhomogeneous to more
CA 02565198 2006-11-08
i ._
homogeneous light distribution. Continuous
operation in this mode leads to substantial erosion
of the mesh. In another experiment after more than
500,000 pules, the 0.0014 mm bar was completely
eroded at the position of the brightest discharge.
These results indicate that the flat light source
needs to be operated in a low glow mode to avoid
erosion. This also guarantees long lifetime and
preferable operation for lighting applications.
Experiments with reverse polarity (the planar
electrode 504 functioning as the cathode) showed a
homogeneous glow at a lower current of 16 milliamps
and reduced intensity at the same voltage and
.. pressure indicated above.
An alternate embodiment of the flat light source
is shown in FIG. 19. Like elements in FIGS. I7 and
19 have the same reference numerals. In the
embodiment of FIG. 19, a cathode 550 is formed as a
conductive pattern on a transparent substrate 552.
The cathode 550 includes a grid of spaced-apart
conductive lines 556, 558, 560, etc. which define
micro hollows 564, 566, etc. The conductive pattern
of cathode 550 can have any desired configuration
for defining a plurality of micro hollows. The
conductive pattern may formed using conventional
microlithography techniques. In the embodiment of
FIG. 19, the substrate 552 functions as a support
for the cathode 550, In other respects, the
discharge device of FIG 19 is similar to the
discharge device shown in FIG. 17 and described
CA 02565198 2006-11-08
._ - e~ 5 -
above.
The flat light sources of FIGS. I7-Z9 may have a
thickness on the order of one millimeter. As noted
above, the light sources shown in FIGS. 17-19 may be
flat or may have a desired curvature.
Generally, the devices of the present invention
can be operated in the low glow mode and the high
glow mode as described above. However, only the low
glow mode promises long lifetimes and operation
determined by the fill gas. In the high glow mode,
the lifetime is limited, and the electrode vapor
determines the characteristic of the discharge.
This may be desirable when metal vapor radiation is
required.
While there have been shown and described what
are at present considered the preferred embodiments
of the present invention, it will be obvious to
those skilled in the art that various changes and
modifications may be made therein without departing
from the scope of the invention as defined by the
appended claims.
