US3831052A

US3831052A - Hollow cathode gas discharge device

Info

Publication number: US3831052A
Application number: US00363904A
Authority: US
Inventors: R Knechtli
Original assignee: Hughes Aircraft Co
Current assignee: Raytheon Co
Priority date: 1973-05-25
Filing date: 1973-05-25
Publication date: 1974-08-20
Anticipated expiration: 1991-08-20
Also published as: JPS552902B2; DE2421907B2; FR2231099B1; JPS5022567A; DE2421907A1; GB1424658A; FR2231099A1

Abstract

The hollow cathode gas discharge device is configured with a maximized cathode-to-anode area ratio to operate in a lowpressure glow discharge mode to generate a plasme of adequate density from which electrons or ions can be extracted and accelerated. This permits the gas pressure to be kept low to avoid Paschen breakdown in the high voltage acceleration region. The invention herein described was made in the course of or under a Contract or subcontract thereunder with the Department of the Navy.

Description

Elite ttes Patet [191 Kneclitli 11] 3,31,52 Aug. 2%, 1974 HOLLOW CATHODE GAS DISCHGE DEVECE [75] Inventor: Ronald C. Knechtli, Woodland Hills,

Calif.

[73] Assignee: Hughes Aircraft Company, Culver City, Calif.

[22] Filed: May 25, 1973 [21] Appl. No.: 363,904

315/111 [51] Int. Cl. 1101] 7/24 [58] Field of Search 3l3/DIG. 8, 231, 74, 183, 313/191, 209, 210, 207, 187; 315/111 [56] References Cited UNITED STATES PATENTS 3,262,003 7/1966 Allen et a1. 313/187 3,411,035 11/1968 Necker et al. 313/187 r 85 g Vo OTHER PUBLICATIONS Low Pressure Glow Discharge by G. W. McClure, Applied Physic Letters, Vol. 2, No. 12, June 15, 1963.

Primary Examiner-Harold A. Dixon Attorney, Agent, or Firm-W. H. MacAllister; Allen A. Dicke, Jr.

[57] S CT The hollow cathode gas discharge device is configured with a maximized cathode-to-anode area ratio to operate in a low-pressure glow discharge mode to generate a plasme of adequate density from which electrons or ions can be extracted and accelerated. This permits the gas pressure to be kept low to avoid Paschen breakdown in the high voltage acceleration region.

The invention herein described was made in the course of or under a Contract or subcontract thereunder with the Department of the Navy.

9 Claims, 3 Drawing Figures Vg L gs PAIENIE AUBZO m4 SEW? l M 2 Fig. 5.

IGNITOR HOLLOW CATHODE GAS DISCHARGE DEVICE BACKGROUND This invention is directed to a hollow cathode gas discharge device having a low-pressure glow discharge plasma from which can be extracted electrons or ions for high voltage acceleration.

High energy electron and ion beams are used in a variety of equipment, such as irradiation equipment, TEA gas lasers, and ion thrustors. Various electron and ion sources are available. Glow discharge plasma contain both ions and electrons, are widely used for various purposes, and can be used for this purpose. However, when they are employed in equipment where accelerating voltages are required, differential pumping is usually needed to keep the gas pressure in the accelerating region low enough to prevent Paschen breakdowns in the beam under high accelerating voltages.

Among the prior art, attention is called to the thin wire anode, hollow cathode discharge described by G. W. McClure, AMERICAN PHYSICS LETTERS, Volume 2, No. 12, page 233, June 15, 1963.

SUMMARY In order to aid in the understanding of this invention, it can be stated in essentially summary form that it is directed to a hollow cathode gas discharge device which has a minimized internal gas pressure resulting from a maximized cathode-to-anode area ratio consistent with maintenance of discharge so that maximized accelerating voltages can be applied to an extracted beam without Paschen breakdwon and without need for differential pumping.

It is thus an object of this invention to provide a gas discharge device which provides a glow discharge gaseous plasma as a source for either ions or electrons. It is another object to provide a gas discharge device wherein plasma discharge occurs at a minimized pressure. It is another object to provide a hollow cathode gas discharge device with maximized cathode-to-anode area ratio so that a satisfactory glow discharge can be maintained at minimum gas pressure. It is a further object to provide an auxiliary ignition anode within a hollow cathode gas discharge device which operates at minimized pressure so that the glow discharge can be initiated.

Other objects and advantages of this invention will become apparent from a study of the following portion of the specification, the claims, and the attached drawmgs.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a hollow cathode gas discharge device, in accordance with this invention, as attached to a laser for ionization of laser gas by means of the high energy electron beam from the said hollow cathode gas discharge device.

FIG. 2 is an enlarged section, with parts broken away, taken generally along the line 22 of FIG. 1.

FIG. 3 is a transverse section through another embodiment of the hollow cathode gas discharge device of this invention showing the gas discharge device as an ion source.

DESCRIPTION In FIG. 1, the hollow cathode gas discharge device is a high energy electron source 10. It is shown as being attached to a laser cavity 12 for the ionization of the gas thereof. The laser cavity is shown as being in association with

laser mirrors

14 and 16. In FIG. 2, the laser cavity 12 contains sustainer discharge electrode 18, which is part of a conventional laser. The laser cavity 12 is the cavity of a gas laser, of which the TEA laser in U.S. Pat. No. 3,702,973 is an example. Furthermore, U.S. Pat. No. 3,577,096 discloses a gas laser, and U.S. Pat. No. 3,641,454 discloses an electron beam-ionzed gas laser to illustrate the fact that a gas laser can be electron beam-ionized. See also the article by A. J. Beaulieu in APPLIED PHYSICS LETTERS, Vol. 16, page 504, 1970. The entire disclosures of these patents and this article are incorporated herein in their entirety.

Laser cavity housing 12 has secured to the side thereof electron source housing 20. Housing 20 serves as a shell around the remainder of the electron source structure to serve as a vacuum envelope therefor. One side of housing 20 is wall 22, which is a common wall with laser cavity housing 12. Wall 22 has a thin foil section 24 which serves as an electron transmission window. Metals are suitable for these housings. The foil window section 24 is as thin as possible to permit electron passage with maximum freedom, but also to maintain the vacuum integrity of housing 20. Foil section 24 can be mechanically supported to aid in its support of the pressure differential between the interiors of the two housings. The pressure within electron source 10 is made independent of the ambient by means of housing 20, which is closed all the way around. It can be provided with a vacuum pump and/or gas supply means to maintain the interior of housing 20 at the appropriate pressure and with the proper gas atmosphere. The maintenance of as low a pressure within housing 20 as is consistent with the maintenance of a plasma discharge is a feature of this invention and is discussed in more detail below. The geometry of the various parts is related to the maintenance of the desirably low pressure. Hollow cathode 26 is mounted within housing 20 on suitable electrically insulative structural supports. Cathode 26 carries

webs

28 and 30 on which are mounted

insulators

31 and 33. On the inside surface of the insulator, toward the interior of cathode 26, is mounted perforated anode 32. On the other side of the insulators is mounted perforated control grid 34. The thin foil window section 24 is in line with and faces the perforated electrodes. Window section 24 is spaced from control grid 34 and is adapted to be connected as an electron accelerator electrode.

Cathode webs

36 and 38 extend inward over the exposed portion of the

insulators

32 and 34 and extend over the mounting edges of the perforated anode electrode 32 so that only the perforated section is visible from the interior of cathode 26. The effective cathode surface 40 has an area which extends around the interior of cathode 26 out to the facing edges of

cathode webs

36 and 38. The opening 42 between the facing edges of

cathode webs

36 and 38 is the effective anode area.

Webs

44 and 46 protect the outer surface of

insulators

32 and 34 by carrying the cathode potential around the outer, protected surfaces of these insulators. The presence and length of

cathode webs

36 and 38 is optional. They should extend at least over the exposed insulator if insulator protection is wanted, but they may cause beam focusing. Similarly,

webs

44 and 46 need extend only to the outer insulator faces, and may cause beam focusing if they extend as far as they are shown in FIG. 2.

The structure of electron source also includes an ignition electrode 48. Ignition electrode 48 is preferably in the form of a thin wire. It extends substantially through the center of the cathode space. When the cathode 26 is in the form of an elongated tube, as shown, the ignition electrode conveniently extends along the length of the structure.

Power sources are connected to provide the necessary currents for operation. Power source 50 is connected between cathode 26 and perforated anode electrode 32 to maintain the anode electrode positive with respect to cathode surface 40 to maintain the plasma of the glow discharge within the interior of the cathode. Voltage is in the order of 300 to 600 volts, and current is between about 10* to 1 amp per square centimeter of effective cathode area for the type of discharge desired. Ignition power supply 52 is connected between the cathode and ignition electrode 48. When ignition is desired, ignition power supply 52 provides a positive pulse on ignition electrode 48. A pulse in the order of 500 to 1,000 volts and in the order of l microsecond time duration is convenient. Control grid power supply 54 is connected between anode electrode 32 and control grid 34 to bias the control grid with respect to the anode electrode. The control grid can be made negative up to a voltage exceeding the discharge voltage of power supply 50 to cut off electron flow. When not employed as a turn-off device, the control grid power supply is usually operated so that control grid 34 is at a potential close to that of the anode electrode 32. It can be either positive or negative.

Accelerator power supply 56 is connected between anode electrode 32 and foil window section 24. The window section is made positive to accelerate the electrons. In accordance with this invention, accelerating voltages of in excess of 150 kilovolts can be achieved.

The hollow cathode electron gun of FIG. 2 is able to generate a plasma of adequate density, up to about 10 electrons and ions per cubic centimeter, from which electrons can conveniently be extracted and accelerated without causing Paschen breakdown in the high voltage acceleration region between control grid 34 and foil window section 24. To avoid such breakdown, the gas pressure of the discharge has to be kept relatively low. The pressure is typically below about 50 microns of mercury pressure column for helium and lower for other gases. The configuration of electron source 10 permits operation at such a low gas pressure, because most of the discharge volume is enclosed by the hollow cathode surface, because the anode area is kept much smaller than the cathode and because the anode area is essentially flush with the cathode area. The enclosure of the discharge volume by the hollow cathode surface leads to optimum utilization of the ions generated in the plasma, which substantially all fall back onto the cathode where they generate the secondary electrons needed to sustain the discharge. The second and third features, the small anode-to-cathode area ratio and the essentially flush configuration of the anode surface with respect to the cathode surface, are essential to permit sustaining the discharge down to low pressures and are further discussed below.

Presuming that the plasma discharge is established, the volume inside the hollow cathode, defined by the effective cathode surface area 40 and extending across between webs 36 ad 38, is filled with plasma which has potential close to that of the most positive electrode, the perforated anode electrode 32. This results from the fact that the electron mobility is much larger than the ion mobility. The discharge voltage therefor ap pears mostly across the cathode sheath which exists between the cathode surface and the plasma. The cathode sheath thickness is much smaller than the diameter of the cathode. This is typical of a cold cathode glow discharge. To sustain the discharge in steady state, the rate of ion generation has to equal the rate of ion loss. This condition determines both the lowest pressure at which the discharge can be sustained and the discharge voltage.

For the plasma densities found in cathode 26 when operating in the desired mode, typically up to about 10 ions per cubic centimeter, ion loss is due predominantly to the ion flux to the cathode. There is negligible ion loss due to volume recombination. The ions reaching the cathode are accelerated through the cathode sheath to an energy corresponding to the discharge voltage. As noted above, this is typically several hundred volts for the desired cold cathode glow mode discharge. Upon impact on the cathode, these ions produce secondary electrons. The secondary electrons, in turn, are accelerated through the cathode sheath to the full discharge voltage. In the low pressure regime, the electron mean-free path will be much longer than the distance between opposite cathode surfaces. Most of the accelerated electrons will, therefore, traverse the discharge volume, be reflected on the opposite cathode surface, and oscillate back and forth between opposite cathode surfaces in the hollow cathode volume until they eventually make an inelastic collision. Such inelastic collisions have a high probability of being ionizing collisions. The probability for an electron to reach the anode before having made such an ionizing collision increases with decreasing gas pressure, but decreases with decreasing anode area for a given cathode area. As a result, a small anode area is important to minimize lowest pressure at which the discharge can be sustained in this mode. Furthermore, it can be appreciated that this process can take place effectively only if the oscillating electrons are not intercepted by an anode surface. This is why a flush anode surface with respect to the cathode surface is important, while an anode surface intruding into the hollow cathode region would be detrimental.

In a particular structure, the effective cathode surface area was 250 square centimeters, and the opening which corresponds to the effective anode area was 30 square centimeter. The cathode material was stainless steel. The gas in the chamber was helium, and a wellcontrolled discharge could be sustained down to a helium gas pressure of less than 20 milli-Torr. This pressure is suitably low for a plasma cathode gas discharge device with high voltage electron or ion acceleration.

While the discharge configuration described above and illustrated in FIG. 2 is suitable for sustaining the discharge at the desired low gas pressure, it is inadequate to permit reliable ignition at this low pressure. This is due to the fact that the vacuum electric field existing between the cathode and an anode which is about flush with the cathode is quite unfavorable. Any initial electron present inside the hollow cathode will be rapidly focused toward the anode and will be collected before it has a chance to make an ionizing collision. Thus, the avalanche necessary to start the discharge cannot take place. This is a different situation than in the presence of the plasma because, in the presence of the plasma, the vacuum field does not exist. With the plasma discharge taking place, most of the electric field exists only in the cathode sheath region, and the electrons are not focused toward the anode. Thus, while the flush anode configuration is desirable for a number of applications, the ignition electrode 48 is provided for practical ignition.

The success of this ignition can be understood by realizing that, when the wire diameter is made thin enough, typically less than 1 millimeter in diameter, the probability for an electron which is accelerated toward the ignition anode electrode 48 being collected depends upon the initial azimuthal electron velocity. Under practical conditions, having the small diameter wire, such an initial electron under the influence of the vacuum field will be accelerated toward the ignition wire, but will have a high probability to miss it. Under these circumstances, it becomes trapped in an orbit around this wire until it makes an ionizing collision and initiates the avalanche required to ignite the hollow cathode discharge. Once the discharge is ignited, it can be readily transferred from the auxiliary ignition anode electrode 48 to perforated anode electrode 32. This can be simply accomplished by keeping the perforated anode electrode 32 at or above the discharge voltage and letting the ignition wire anode electrode 48 voltage fall below the discharge voltage once ignition has taken place.

It will be understood that, even with the thin wire anode ignition electrode 48, ignition is predicated upon the appearance of an initial electron to start an avalanche. In the least favorable case, the generation of this initial electrode will depend upon cosmic ray ionization. At a gas pressure below 50 microns, and with gas volumes having dimensions in the order of centimeters, the rate of generation of such electrons can be as low as on the order of 1 per second. This can result in a statistical time delay for ignition of the same order. This statistical time delay can be readily reduced to the order of microseconds or less by artificially producing the needed initial electrons. One means of doing this is to incorporate a low-intensity radioactive source in the discharge region.

Electrons are extracted from the plasma by means of the relative positive polarity of anode electrode 32. Electrons passing through the perforations of the anode electrode 32, which acts as an extraction grid, are first accelerated in substantially space-charge limited flow in the extraction and control region between

electrodes

32 and 34. They are further accelerated, once past control grid 34, by the high voltage accelerating field applied between window 24 and grid 34. It is seen that

electrodes

32 and 34 are at approximately the same potential. With a distance between

electrodes

24 and 34 in the order of 2.5 centimeters, and with helium as the gas in the space at a pressure of 50 milli-Torr or less, accelerating voltage in excess of 150 kilovolts has been applied without either Paschen or vacuum breakdown. The maximum accelerating voltage which can be applied between window 24 and grid 34 is determined by the conditions for both Paschen breakdown and vacuum breakdown. The vacuum breakdown voltage is essentially determined by the distance d between the

high voltage electrodes

24 and 34. For a voltage on the order of kilovolts, a practical minimum value for d is on the order of 2.5 centimeters. The Paschen breakdown voltage is determined by the value of the product p'd of the gas pressure p and the electrode spacing d. In the low pressure region of interest for the hollow cathode discharge electron (or ion) guns, the Paschen breakdown voltage increases with decreasing value of the product pd. For helium and the devices described herein, the Paschen breakdown voltage will exceed l50 kilovolts for values of p-d typically smaller than 0.4 Torr-centimeter. Now it is observed that increasing the vacuum breakdown requires an increase in the electrode spacing d, while maintaining the Paschen breakdown voltage at a selected value requires keeping the 1d product constant; hence, the need to increase d results in a need to decrease the gas pressure p. This is why the ability of the hollow cathode discharge described above to operate at low pressures is especially valuable for this application. A typical practical set of values is an acceleration voltage up to about 200 kilovolts for a maximum helium gas pressure of about 50 microns of mercury pressure column and an electrode spacing of about 4 centimeters. For helium gas pressure of about 30 microns, an extracted current density up to several amperes per square centimeter has been obtained with a stainless steel cathode, with a discharge voltage in the order of 300 to 500 volts, and an extraction grid current smaller than or of the same order of magnitude as the extracted electron current.

It is noted that the cross-sectional configuration of cathode 26 need not be cylindrical. A rectangular configuration is also satisfactory. The only key conditions to be satisfied are that theeffective anode area is much smaller than the effective hollow cathode area, that the cathode substantially enclose the plasma space, and that the anode be substantially flush with the cathode surface.

One application of the plasma cathode electron gun 10 is as an electron source for a gas laser with high energy electron ionization. In modern lasers of this type, large area high voltage electron guns are required. As compared to the thermionic cathode electron guns presently used for this application, the plasma cathode electron gun described above has the following advantages. Small leaks in the thin metal window can be tolerated, provided that the enclosure pressure is maintained at 10 to 10' Torr range. Thermionic cathodes require at least two orders of magnitude lower pressure. Furthermore, accidental loss'of vacuum would have no serious consequences with plasma cathodes; it is usually catastrophic with thermionic cathodes. Finally, the plasma cathode is not sensitive to electronegative impurity gases, in contrast to thermionic cathodes. The plasma cathode does not require heat-up time. The discharge in the plasma gun can be started within microseconds prior to the initiation of a high energy beam. The plasma cathode electrode gun structure can be maintained at much lower temperature than that of thermionic cathodes. No inherently delicate heater elements are required. The plasma cathode electron gun can readily be scaled to large size without major difficulties; i.e., no unwieldy heater power, no difficulties with structural rigidity, etc. The eventual cost of the plasma cathode gun is expected to be lower than that of a comparable large area thermionic cathode, due to its inherent structural simplicity and potentially greater reliability. The plasma cathode gun does not require power-consuming heater elements. The power required for the discharge in the plasma gun constitutes only a small fraction of the high energy electron beam power. In pulsed duty, the average power consumption can be lower than that of an equivalent thermionic cathode.

Another useful application of the plasma cathode electron gun described above is for industrial highenergy electron irradiation equipment. Electron irradiation is sometimes applied to polymer composition materials to cause polymerization and can be used for other chemical uses.

Referring to FIG. 3, ion source 60 is shown therein. Ion source 60 has a housing 62 which maintains a vacuum in the vacuum space 64 therein. Cathode 66 is mounted in and is insulated with respect to housing 64. Cathode 66 is the same as cathode 26. However, cathode 66 has an extraction grid 68 at the cathode potential. Pressure is maintained within the cathode space '70 at an appropriate value so that a plasma discharge can be initiated by ignition electrode 72 and maintained by discharge-sustaining anode 74, the latter being essentially flush with the cathode surface. For the same reasons as previously described, the pressure is maintained as low as practical within cathode space 70 consistent with the maintenance of a low pressure plasma discharge in cathode space 70. As ions drift from the plasma to extraction grid 68, they are accelerated by ion-accelerating grid 76 toward target 78.

Ignition pulse power supply 80 is connected between cathode 66 and ignition electrode 72 to produce a pulse which initiates the discharge. Discharge power supply 82 is connected between cathode 68 and discharge-sustaining anode 74 to maintain the previously described low pressure plasma glow discharge. Acceleration power supply 84 is connected between cathode 66 and accelerator grid 76. Target 78 is at about the same potential as accelerator grid 76, or is more negative, and thus is connected to the negative side of accelerator power supply 84 or to the negative side of a separate accelerating power supply 85.

Ion source 60 thus produces the low pressure ion and electron-generating discharge which was previously described, and ions can be extracted and accelerated from the plasma. The low pressure in space 64 again permits higher accelerating fields without Paschen breakdown. The extraction grid, and a control grid, if desired, can be best made according to the known techniques developed for electron bombardment and thrustors and ion sources. The key difference between the ion source 60 and the prior ion sources is the low pressure ion-generating discharge which is sustained by means of the hollow cathode structure and operating conditions described above, without the need for a magnetic field or a hot cathode.

One advantage in using a flush anode configuration to sustain the discharge rather than a thin wire such as is used for ignition is the easier cooling of the flush anode resulting in its ability to sustain a higher average discharge current. A higher average discharge current produces a higher plasma density and permits extraction of a higher average ion current. Another advantage of the configuration object of this invention over a thin wire anode is the fact that it results in a more uniform plasma density distribution, leading to a more uniform current density distribution for the extracted ion beam.

This invention having been described in its preferred embodiment, it is clear that it is susceptible to numerous modifications and embodiments within the ability of those skilled in the art and without the exercise of the inventive faculty. Accordingly, the scope of this invention is defined by the scope of the following claims.

What is claimed is: l. A hollow cathode plasma discharge device comprising:

walls defining a hollow cathode space, said walls comprising an anode wall and a cathode wall, said anode wall and said cathode wall together defining the exterior boundaries of a low-pressure glow discharge plasma, said anode wall being positioned so that it does not substantially intrude into the plasma, the ratio of anode area to cathode area being smaller than unity for low-pressure glow discharge plasma mode operation;

means for initiating plasma discharge interiorly of said space so that plasma discharge can be ignited by said initiation means and then transferred to said anode wall;

one of said walls being perforated so that particles can be extracted through said perforation into space exterior of said perforated wall;

an accelerator electrode positioned exteriorly of said hollow cathode space to accelerate particles passing out through said wall perforation; and

a vessel enclosing at least said perforation and said accelerator electrode to maintain pressure within said vessel and within said cathode at subatmospheric pressure to cause conditions between said perforation and said accelerator electrode to be outside the breakdown region of the Paschen curve for the particular gas.

2. A hollow cathode plasma discharge device comprising:

an auxiliary anode positioned interiorly of said space for initial ignition of the plasma discharge so that plasma discharge can be sustained by said anode wall;

one of said walls being perforated so that particles can be extracted through said perforation into space exterior of said perforated wall; an accelerator electrode positioned exteriorly of said hollow cathode space to accelerate particles passing out through said wall perforation; and

3. The device of claim 2 wherein an additional perforated electrode or conducting mesh is placed between said perforated anode and said accelerator electrode to serve as a control grid controlling the current of extracted electrons, the potential of said control grid being between the cathode potential and the potential of the accelerator electrode.

4. The device of claim 3 wherein said perforated wall is a perforated anode wall so that electrons are the particles extracted from the plasma.

5. The device of claim 3 wherein said vessel includes an electron transmissive window, and said electron transmissive window is said accelerator electrode so that electrons accelerated to said window are transmitted through said window into an atmosphere of arbitrary pressure and composition.

6. The device of claim 4 wherein said vessel includes an electron transmissive window, and said electron transmissive window is said accelerator electrode so that electrons accelerated to said window are transmitted through said window into an atmosphere of arbitrary pressure and composition.

7. The device of claim 3 wherein said perforated wall is a perforated cathode wall so that ions can be extracted from the plasma, said accelerator electrode being spaced from said perforated cathode wall and being connected to a negative electric voltage with respect to said cathode to accelerate ions from said perforated cathode wall, said vessel maintaining the space between said perforated cathode wall and said accelerator electrode in the non-breakdown region of the Paschen curve for the particular gas.

8. The device of claim 7 wherein a control grid consisting of a perforated electrode or conducting mesh is placed between said perforated cathode wall and said negative accelerator electrode to control the current of the extracted ion, the potential of said control grid being negative with respect to the cathode and the ion optical design of said control grid being such as to keep ion interception low.

9. The device of claim 7 wherein a control grid consisting of a perforated electrode or conducting mesh is placed between said perforated cathode wall and said negative accelerator electrode, the potential of said control grid being substantially equal to cathode poten tial or being positive with respect to cathode potential. l

Claims

1. A hollow cathode plasma discharge device comprising: walls defining a hollow cathode space, said walls comprising an anode wall and a cathode wall, said anode wall and said cathode wall together defining the exterior boundaries of a lowpressure glow discharge plasma, said anode wall being positioned so that it does not substantially intrude into the plasma, the ratio of anode area to cathode area being smaller than unity for low-pressure glow discharge plasma mode operation; means for initiating plasma discharge interiorly of said space so that plasma discharge can be ignited by said initiation means and then transferred to said anode wall; one of said walls being perforated so that particles can be extracted through said perforation into space exterior of said perforated wall; an accelerator electrode positioned exteriorly of said hollow cathode space to accelerate particles passing out through said wall perforation; and a vessel enclosing at least said perforation and said accelerator electrode to maintain pressure within said vessel and within said cathode at subatmospheric pressure to cause conditions between said perforation and said accelerator electrode to be outside the breakdown region of the Paschen curve for the particular gas.

2. A hollow cathode plasma discharge device comprising: walls defining a hollow cathode space, said walls comprising an anode wall and a cathode wall, said anode wall and said cathode wall together defining the exterior boundaries of a low-pressure glow discharge plasma, said anode wall being positioned so that it does not substantially intrude into the plasma, the ratio of anode area to cathode area being smaller than unity for low-pressure glow discharge plasma mode operation; an auxiliary anode positioned interiorly of said space for initial ignition of the plasma disCharge so that plasma discharge can be sustained by said anode wall; one of said walls being perforated so that particles can be extracted through said perforation into space exterior of said perforated wall; an accelerator electrode positioned exteriorly of said hollow cathode space to accelerate particles passing out through said wall perforation; and a vessel enclosing at least said perforation and said accelerator electrode to maintain pressure within said vessel and within said cathode at subatmospheric pressure to cause conditions between said perforation and said accelerator electrode to be outside the breakdown region of the Paschen curve for the particular gas.

9. The device of claim 7 wherein a control grid consisting of a perforated electrode or conducting mesh is placed between said perforated cathode wall and said negative accelerator electrode, the potential of said control grid being substantially equal to cathode potential or being positive with respect to cathode potential.