Abstract:
In accordance with one embodiment, the hollow cathode is comprised of a first tantalum tube, tantalum foil, and a second tantalum tube. The foil is in the form of a spiral winding around the outside of the first tube and is held in place by the second tube, which surrounds the foil. One end of the second tube is approximately flush with one end of the first tube. The other end of the second tube extends to a cathode support through which the working gas flows. To start the cathode, a flow of ionizable inert gas, usually argon, is initiated through the hollow cathode and out the open end of the first tube. An electrical discharge is then started between an external electrode and the first tube. When the first tube is heated to operating temperature, electrons are emitted from the open end of the first tube.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is based upon and claims benefit of Provisional Application No. 60/785,827 filed Mar. 25, 2006. 

   FIELD OF INVENTION 
   This invention relates generally to hollow cathodes, and more particularly it pertains to hollow cathodes used to emit electrons in industrial applications. 
   BACKGROUND ART 
   Hollow cathodes are used to emit electrons in a variety of industrial applications. As described in a chapter by Delcroix, et al., in Vol. 35 of  Advances in Electronics and Electron Physics  (L. Marton, ed.), Academic Press, New York (1974), beginning on page 87, there are both high and low pressure regimes for hollow-cathode operation. In the high-pressure regime, the background pressure (the pressure in the region surrounding the hollow cathode) approaches or exceeds 1 Torr (130 Pascals) and no internal flow of ionizable working gas is required for operation. In the low-pressure regime with a background pressure below 0.1 Torr, an internal flow of ionizable working gas is required for efficient operation. It is for operation in the low-pressure regime below 0.1 Torr, and usually below 0.01 Torr, that the present invention is intended. 
   An important industrial application of low-pressure hollow cathodes is for electron emission in ion sources. These ion sources are of both gridded and gridless types. The ions generated in gridded ion sources are accelerated electrostatically by the electric field between the grids. Gridded ion sources are described in an article by Kaufman, et al., in the  AIAA Journal , Vol. 20 (1982), beginning on page 745. The particular sources described in this article use a direct-current discharge to generate ions. It is also possible to use electrostatic ion acceleration with a radio-frequency discharge, in which case the only electron emitting requirement would be for a neutralizer cathode. 
   In gridless ion sources the ions are accelerated by the electric field generated by an electron current interacting with a substantial magnetic field in the discharge region, i.e., a magnetic field with sufficient strength to make the electron-cyclotron radius much smaller than the length of the discharge region to be crossed by the electrons. The closed-drift ion source is one type of gridless ion source and is described by Zhurin, et al., in an article in  Plasma Sources Science  &amp;  Technology , Vol. 8, beginning on page R1, while the end-Hall ion source is another type of gridless ion source and is described in U.S. Pat. No. 4,862,032—Kaufman, et al. 
   There are different types of low-pressure hollow cathodes. The simplest is a refractory-metal tube, usually of tantalum. This type is described in the review by Delcroix, et al., in the aforesaid chapter in Vol. 35 of  Advances in Electronics and Electron Physics . For hollow cathodes of the sizes, electron emissions, and gas flows of most interest herein, the use of this cathode type results in a high heat loss and a lifetime of only a few tens of hours, even when operating with clean inert working gas. With the working-gas contamination levels often encountered in industrial environments, the lifetime could be reduced to only several hours. 
   The lifetime of this type of cathode can be extended by the use of radiation shields, which reduces the heat loss, which in turn reduces the energy of bombarding ions within the hollow cathode—see U.S. Patent Application Publication 2004/0000853—Kaufman, et al. With the proper design of radiation shields, the lifetime with clean working gas can be extended to several hundred hours or more. With contaminated working gas, however, the lifetime could again be reduced to several hours. 
   Another type of hollow cathode has been developed for electric thrusters used in space propulsion and is described in a chapter by Kaufman in Vol. 36 of  Advances in Electronics and Electron Physics  (L. Marton, ed.), beginning on p. 265. The distinguishing feature of this type is an emissive insert that emits electrons at a lower temperature, and hence with a lower heat loss, than does the plain metal-tube of the type described above. The major advantage of this type is the long lifetime that is possible, of the order of 10,000 hours. The major disadvantage is the sensitivity of the supplemental emissive material to contamination. This emissive material requires “conditioning” before initial operation and is sensitive to atmospheric exposure after this conditioning. For example, barium carbonate is often used as the supplemental emissive material, which is heated during conditioning to become an oxide. If this emissive material is exposed to air after conditioning, the barium oxide combines with the water vapor in the air to become a hydroxide, which is much less effective as an emission material. Repeated exposure to air is not a problem in the space electric-propulsion application for which these cathodes were originally designed, but is much more serious in industrial applications. The combination of sensitivity to contamination and high fabrication costs make this type of hollow cathode a poor choice for most industrial applications. 
   What might be called a compromise of the two types of hollow cathodes has been used in industrial applications. In this type, an emissive insert is used, but this insert consists only of tantalum foil. The lifetime is not as long without a low-work-function emissive material such as barium carbonate, but the tantalum-foil insert is less sensitive to atmospheric exposure than an insert that depends on the addition of an emissive material. It should be mentioned that a purge of working gas is normally used for a hollow cathode after exposure to atmosphere and prior to operation. This purge removes most of the impurities from the atmosphere that are adsorbed on the hollow-cathode surfaces, unless they are chemically combined with hollow-cathode material—such as in the formation of barium hydroxide by the water vapor in the atmosphere. However, even with the reduced sensitivity to atmospheric exposure, this type of cathode is still sensitive to impurities (contamination) in the working gas. 
   Another example of possible hollow-cathode configurations is U.S. Pat. No. 5,587,093—Aston, which differs from other examples given above mostly by additional complexity. There is described a hollow cathode with both multiple radiation shields surrounding a tube through which the working gas is introduced and an emissive insert that is impregnated with an emissive material. Unlike other emissive inserts described herein, this one is directly heated by an electrical current passing through the insert. There are also intervening support structures between both the gas tube and the inner radiation shield and between the inner and outer radiation shields. The contamination-sensitive emissive material and the complicated structure both make it a poor choice for operation with contaminated working gas. 
   A hollow cathode for industrial applications should have an operating lifetime of at least several hundred hours and be insensitive to repeated exposures to atmosphere between periods of operation. The effect of frequent exposures to atmosphere can be minimized by keeping a flow of clean inert gas through the cathode during these exposures (purging). Shorter lifetimes than several hundred hours would be a problem because the time between maintenance in many industrial applications would then be limited by the cathode lifetime. While longer lifetimes might be of interest for industrial hollow cathodes, the time between maintenance would probably still be limited by other system components. In other words, the cost of a longer-lifetime hollow cathode, together with any special care and handling required, would have to be balanced against the replacement cost of a new hollow cathode of a simpler type. 
   The best tolerance to atmospheric exposure has been obtained by fabricating the hollow cathode entirely of refractory materials and avoiding the more reactive materials that are used to impregnate or coat an emissive insert. Atmospheric contamination is limited to the surface of refractory materials and is mostly removed by a purge of clean gas before operation. Tolerance to contamination in the working gas, which is usually argon, is a more serious problem. Contaminated working gas reaches the cathode when it is hot and is more likely to react with and/or be absorbed into refractory metals. This contamination results from the use of dirty gas tubing, leaky tubing connections, unsuitable gas regulators, and improper procedures such as opening a new gas bottle without first pumping down the trapped volume between the gas bottle and the regulator. The contaminants involved are usually some combination of oxygen, nitrogen, water vapor, and hydrocarbons. Compared to the use of a clean working gas, typically &gt;99.999% argon, such contamination can reduce the lifetime by a factor of ten or more. Controlling the purity of the working gas at all industrial locations is simply not practical. The approach taken herein has been to increase the tolerance of a hollow cathode to contamination in the working gas. 
   SUMMARY OF INVENTION 
   In light of the foregoing, it is a general object of the invention to provide a hollow cathode that is simple to fabricate and use, while having an operating life of at least several hundred hours using working gas contaminated with the typical impurities found in industrial applications. 
   Another general object of the invention is to provide a hollow cathode with an operating lifetime of at least several hundred hours that does not require conditioning before operation. 
   Yet another general object of the invention is to provide a hollow cathode, with an operating lifetime of at least several hundred hours, that does not degrade significantly due to atmospheric exposure between periods of operation. 
   A specific object of the invention is to provide a hollow cathode with an operating lifetime of at least several hundred hours that does not incorporate a supplemental emissive material. 
   Another specific object of the invention is to provide a hollow cathode that has a lifetime of at least several hundred hours while using a robust metallic part as the emissive surface. 
   Still another specific object of the invention is to provide a hollow cathode that minimizes thermal losses by not having a continuous thermal conduction path between the dense internal plasma and the cooler cathode support. 
   Yet still another specific object of the invention is to provide a hollow cathode that resists failure to contain the working gas by having a compressed laminar structure, resistant to cracking or leaking, in that part of the hollow cathode that is most likely to absorb and react with contaminants in the working gas. 
   A still further specific object of the invention is to provide a hollow cathode with an operating lifetime of at least several hundred hours that does not require a metallic resistive heater for starting. 
   In accordance with one embodiment of the present invention, the hollow cathode is comprised of a first tantalum tube, tantalum foil, and a second tantalum tube. The first tantalum tube has a diameter that is smaller than that of the second tube. The first tantalum tube is the electron emitter. The foil is in the form of a spiral winding, wrapped around the outside of the first tube, and comprises a plurality of radiation shields (the plurality comprising at least about ten shields, preferably twenty or more). The second tantalum tube surrounds both the first tube and the radiation shields, with one end of the second tube approximately flush with one end of the first tube. The second tube extends to a cathode support through which the working gas flows and to which the other end of the second tube is attached. The radiation shields are compressed between the large and small tantalum tubes, holding the shields in place inside the outer tube, and holding the first tantalum tube in place inside the radiation shields. This construction forces most of the working gas to flow through the first tube. To start the hollow cathode, a flow of ionizable inert gas, usually argon, is initiated through the hollow cathode and out the open end of the first tube. An electrical discharge is then started between an external electrode and the first tube, ionizing some of the molecules of the ionizable gas and forming an electrically conductive plasma that extends from the external electrode back into the open end of the first tube. When the first tube is heated to operating temperature, electrons are emitted from the open end of the first tube and conducted away from it by the plasma. 

   
     DESCRIPTION OF FIGURES 
     Features of the present invention which are believed to be patentable are set forth with particularity in the appended claims. The organization and manner of operation of the invention, together with further objectives and advantages thereof, may be understood by reference to the following descriptions of specific embodiments thereof taken in connection with the accompanying drawings, in the several figures of which like reference numerals identify like elements and in which: 
       FIG. 1  is a prior-art hollow-cathode assembly; 
       FIG. 2  shows a cross section of the prior-art hollow-cathode assembly of  FIG. 1 ; 
       FIG. 3  shows a prior-art electrical circuit diagram of a hollow cathode; 
       FIG. 4  shows a cross section of another prior-art hollow cathode; 
       FIG. 5  shows a cross section of yet another prior-art hollow cathode; 
       FIG. 6  shows a cross section of a still another prior-art hollow cathode; 
       FIG. 7  shows a cross section of yet still another prior-art hollow cathode; 
       FIG. 8  shows a cross section of a prior-art hollow-cathode assembly incorporating the hollow cathode shown in  FIG. 6 ; 
       FIG. 9  shows a cross section of another prior-art hollow-cathode assembly incorporating the hollow cathode shown in  FIG. 7 ; 
       FIG. 10  shows temperature distributions over the length of a hollow cathode; 
       FIG. 11  is a cross section of an embodiment of the present hollow-cathode invention; 
       FIG. 11   a  is a cross section of another embodiment of the present hollow-cathode invention; 
       FIG. 12  shows a hollow-cathode assembly incorporating an embodiment of the present invention shown in  FIG. 11 ; 
       FIG. 12   a  shows an electrical circuit diagram of a hollow cathode incorporating an embodiment of the present invention shown in  FIG. 11 ; 
       FIG. 13  is a gas feed system for a hollow cathode; 
       FIG. 14  is a gas feed system for a hollow cathode modified to introduce contamination into the working gas; 
       FIG. 15  is a cross section of yet another embodiment of the present invention; and 
       FIG. 16  is a cross section of still another embodiment of the present invention. 
   

   DESCRIPTION OF PRIOR ART 
   Referring to  FIG. 1 , there is shown prior-art hollow-cathode assembly  10  of the type described by Delcroix, et al., in the aforesaid chapter in Vol. 35 of  Advances in Electronics and Electron Physics . The hollow cathode is tube  11 , which has a circular cross section and is fabricated of a refractory metal. Possible refractory metals include molybdenum, niobium, rhenium, tantalum, tungsten, or alloys of these metals, with tantalum the most common choice. Carbon is a refractory material that has also been used and is considered either a metal or nonmetal, depending on the particular field of study. It is considered a metal for the discussion herein. Cathode holder  12  supports hollow cathode  11 , as well as conducting ionizable working gas  13  which is supplied to the cathode holder through feed tube  14 . Igniter/keeper electrode  15  is located near open end  16  of hollow cathode  11 . Further from open end  16  is anode  17 . Hollow-cathode assembly  10  operates in surrounding volume  18 . 
   A cross section of the prior-art hollow-cathode assembly of  FIG. 1  is shown in  FIG. 2 . The operation of interest herein is what Delcroix, et al., refer to as a hollow cathode arc (HCA), with the potential difference between the anode and cathode ≦50 V. Further, it is in the low-pressure regime in which the background pressure (the pressure in surrounding volume  18 ) is ≦0.1 Torr (≦13 Pascals). It is apparent to one skilled in the art that this low operating pressure also requires the use of a vacuum pump and a vacuum chamber enclosing volume  18 , both of which are not shown in  FIG. 1  or  2 . 
   To obtain normal operation (≦50 V) in the low-pressure regime, it is necessary to supply a sufficient flow of ionizable working gas  13  to the hollow cathode so that the pressure in volume  16 A, within and near open end  16  of cathode  11 , is of the order of one Torr (133 Pascals). In operation, there is an electrical discharge between cathode  11  and either or both of igniter/keeper electrode  15  and anode  17 . This discharge generates electrons and ions by ionization of atoms or molecules of the working gas. Some of the ions are carried with the flow of working gas and, together with the emitted electrons form a conductive plasma that extends from volume  16 A inside cathode  11  to the igniter/keeper electrode and the anode. 
   Electrons created by the ionization of atoms or molecules of the ionizable working gas constitute some of the electron emission from the hollow cathode, but a major part of this emission comes from surface  16 B inside the open end of the hollow cathode. This emission includes secondary electrons from ion bombardment, as well as enhanced emission due to high electric fields, but is primarily thermionic in nature. A thermionic-emission temperature is required for surface  16 B for this emission to take place. 
   The thermionic-emission temperature near the open end is maintained primarily by ion bombardment. The electrical conductivity of the plasma extending from the cathode to the anode is high enough that most of the discharge voltage appears between the plasma and the cathode. If the emission is low, the discharge voltage rises, increasing the energy of the ions bombarding surface  16 B, thereby increasing the surface temperature. Conversely, if the emission is high, the discharge voltage decreases, decreasing the energy of the ions bombarding that surface, thereby decreasing that surface temperature. In this manner, controlling to a given emission results in the discharge voltage varying to maintain the emission surface within a narrow temperature range. In addition, thermionic electron emission varies extremely rapidly with emitter temperature, which means that a wide range of electron emissions corresponds to a narrow range of emission-surface temperatures. The net result is that, for a given emission-surface material, there will be a narrow range of emitter temperature for a wide range of operating conditions and configurations. For tantalum, that narrow temperature range is near 2400-2500 K. 
   The ions bombarding surface  16 B also cause erosion, thereby limiting the lifetime of hollow cathode  11 . To reduce the erosion and increase the lifetime, it is necessary to reduce the discharge voltage. To maintain the temperature of surface  16 B in the 2400-2500 K operating range while, at the same time, reducing the discharge voltage, it is necessary to decrease the heat loss that is offset by the energy of the bombarding ions. The heat loss consists primarily of radiation from the hot surfaces and conduction in the continuous support paths from these hot surfaces to colder bodies, such as along hollow-cathode tube  11  extending from hot surface  16 B to colder support  12 . Those skilled in the art will recognize that electron emission and the heating of the working gas also constitute heat loss mechanisms for hot surface  16 B, but should also recognize that the magnitudes of these heat losses are small compared to the radiation and conduction losses. 
   Referring to  FIG. 3 , there is shown prior-art electrical circuit diagram  20  for hollow cathode  21 . Igniter/keeper power supply  23  provides a positive potential to the igniter/keeper electrode  15  relative to cathode  21 . Note that cathode  21  may be prior-art hollow cathode  11  or some other hollow cathode. When electrode  15  is functioning as an igniter, a high voltage of at least several hundred volts and usually approximately 1 kV is supplied by power supply  23  to initiate the discharge. The requirement for a voltage of at least several hundred volts results from the need to generate an electrical breakdown in the ionizable working gas. This breakdown results from imposing a voltage greater than the Paschen-law minimum, which varies with the working gas used but ranges from about 400-600 V. If there is also a need to heat the cathode to an operating temperature, the voltage is usually in the range of 600-1500 V, or approximately 1 kV. After the discharge is started, a sustaining keeper discharge of ≦50 V and ≧1 A can be used. Electrode  15  and power supply  23  can thus act as igniter and igniter power supply, keeper and keeper supply, or both. 
   Still referring to  FIG. 3 , discharge power supply  24  provides a positive potential to anode  17  relative to hollow cathode  21 , causing a discharge current to the anode which consists primarily of electrons emitted by hollow cathode  21  and arriving at the anode. In normal operation the discharge is ≦50 V. Delcroix, et al., also give an electron emission current of several amperes or more for normal operation, but minimum emissions of 1-2 A have been found by others. This difference in minimum emission (the total current to both ignitor/keeper  15  and anode  17 ) is attributed to the larger hollow-cathode exit openings used by Delcroix, et al. Delcroix, et al., typically used apertures several millimeters in diameter, compared to the approximately 1 millimeter exit diameter used by those finding lower minimum emissions. 
   Power supply  24  may also incorporate a high-voltage starting circuit of at least several hundred volts and usually approximately 1 kV. If there is such a starting circuit incorporated in power supply  24 , ignitor/keeper electrode  15  and igniter/keeper power supply  23  could be omitted. Anode  17  is shown in cross section as being made of metal, which is often the case. The anode may also be the entire vacuum chamber, instead of an electrode within it. When used with an ion source, the anode may be the quasi-neutral plasma of an ion beam, i.e., not a metallic electrode. 
   Heater power supply  26  energizes resistive heater  27  to bring hollow cathode  21  to operating temperature. This power supply may be of either the direct or alternating current type. When a metallic resistive heater is used, radiation shields may surround the resistive heater to reduce the electrical power required for the hollow cathode to reach operating temperature. If the cathode is heated to operating temperature by igniter/keeper supply  23 , power supply  26  and resistive heater  27  could be omitted. 
   Different ground connections may be used. The surrounding vacuum chamber is typically defined as ground potential and is often, but not always, at earth ground. If the cathode is at the potential of the surrounding vacuum chamber, the ground connection would be as shown by ground  28 . If the anode is the surrounding vacuum chamber, the ground connection would be as shown by ground  29 . In the latter case, electrical isolation would be required in the gas line which, far from the cathode, would also be at ground potential. The techniques for such electrical isolation are well known to those skilled in the art and are not pertinent to the present invention. 
   The preceding description of the electrical circuit diagram of  FIG. 3  should make clear that a variety of electrical circuit options are possible. Regardless of the particular options selected, the electrical circuit must initiate the discharge from the hollow cathode, with the heating of the hollow cathode to operating temperature provided either prior to the initiation of discharge or during that initiation. If the heating is prior to the initiation of the discharge, a maximum of several hundred Volts will usually be sufficient for this initiation, rather than the previously mentioned approximately 1 kV. Following the initiation of the discharge, a normal discharge is sustained at ≦50 V. This sustained discharge can be directly to the anode, or it can be to a keeper electrode. In the latter case, a pre-existing discharge to the keeper can provide rapid initiation of a normal discharge to an anode, without a large potential being applied to that anode. In this sense, the keeper discharge “keeps” the cathode ready for normal operation. 
   The simple tubular cathode of Delcroix, et al., has a limited lifetime, typically a few tens of hours in the sizes and operating conditions of interest for ion sources. Delcroix, et al., do not discuss the effect of working gas on lifetime, but the use of an inert gas such as argon, krypton, or xenon would be required to reach even this limited lifetime. A reactive gas such as oxygen or nitrogen would result in much shorter lifetimes. Nitrogen is considered inert in many applications, but is reactive in the environment of an electrical discharge. 
   As a measure of tubular-cathode lifetime at operating conditions of interest, a tantalum tube 1.57 mm in outside diameter and 38 mm long, with a wall thickness of 0.38 mm was operated with a clean argon gas flow of 10 sccm (standard cubic centimeters per minute). The igniter/keeper current was 1.5 A (power supply  23  in  FIG. 3 ) and the emission was 5 A (power supply  24  in  FIG. 3 ), giving a total electron emission of 6.5 A. The pressure in surrounding volume  18  was less than 0.001 Torr. A cathode assembly with an enclosed ignitor/keeper was used, similar to that to be discussed in connection with  FIGS. 8 and 9 . This hollow cathode was operated with an ion source that was generating an ion beam. The ion beam and surrounding plasma constituted the anode for the discharge. The most direct measurement of the discharge voltage was the voltage of the keeper supply (power supply  23 ), which was 16-17 V over most of the life test. Operation was periodically interrupted and the cathode exposed to atmosphere for wear measurements. The limit in lifetime was reached when the cathode could not be restarted at a gas flow of about 40 sccm (four times the operating gas flow). The operating lifetime was about 60±20 hours for the simple tubular cathode at these conditions. While such a lifetime may be adequate for some applications, it is far too short for the electron emission functions of many industrial ion sources. On the other hand, exposure to atmosphere had no significant adverse effect on the simple tubular cathode. While an adsorbed layer of impurities would be expected from exposure to atmosphere, this layer is thin and would be mostly removed during the purge of clean working gas used after exposure to atmosphere and prior to operation. 
   The use of radiation shields is discussed by Delcroix, et al., in the aforesaid chapter in Vol. 35 of  Advances in Electronics and Electron Physics . The use of two cylindrical radiation shields is shown in the figure on page 147 and the discussion on pages 145-146 therein to result in a drop in discharge voltage from about 44 V to about 35 V. While Delcroix, et al., find this drop worth noting, there is no indication of a possible effect on lifetime. On pages 147-148 therein, the total radiation from an unshielded cathode is estimated at 15-20% of the total discharge power. While this result is also worth noting, there is again no indication of a possible qualitative effect on lifetime that can be obtained by reducing radiation losses. 
   To obtain a lifetime for the double-shielded configuration described above, a 1.57-mm-diameter, 38-mm-long hollow cathode (similar to that described previously) was operated with two concentric cylindrical tantalum shields having outside diameters of 9.5 mm and 3.18 mm. The thicknesses of these shields were approximately the same 0.38-mm thickness as the tantalum tube. Using the same operating conditions as were used for the simple tantalum tube hollow cathode, the initial keeper voltage was 13-14 V, significantly lower than the 16-17 V obtained with the simple tubular cathode and qualitatively in agreement with the reduced operating voltage described by Delcroix, et al. However, the keeper voltage increased more rapidly than was observed with the simple tubular cathode and there was no significant increase in operating lifetime over that cathode. The rapid degradation of simple radiation shields, with only several shields and no texturing of those shields, has been observed before. This degradation is believed due to the welding together of the shields, providing a direct thermal conduction path through those shields. 
   Referring to  FIG. 4 , there is shown a cross section of another prior-art hollow cathode, the space-propulsion hollow cathode described by Kaufman in the aforesaid chapter in Vol. 36 of  Advances in Electronics and Electron Physics . Cathode  30  has a cathode body that is comprised of tantalum tube  31 A with a circular cross section that is electron-beam welded to tungsten tip  31 B. Inside the tantalum tube and also part of the hollow cathode is a spiral wound tantalum-foil insert  32 . The tantalum foil from which the insert is fabricated is 0.013 mm thick. The foil in this insert was coated with a low-work-function, low-temperature emissive material, barium carbonate, which becomes barium oxide during initial heating or conditioning of the cathode. Outside the tantalum tube and also part of the hollow cathode is resistive heater  27  imbedded in flame-sprayed alumina  33 . Igniter/keeper  15  is spaced from the open end of the cathode and has an annular shape. 
   Hollow cathode  30  is brought to approximately operating temperature when resistive heater  27  is energized by a heater power supply (see power supply  26  in  FIG. 3 ). With a flow of ionizable working gas (mercury vapor in this case), a discharge is initiated by a positive voltage of several hundred volts on igniter/keeper electrode  15  relative to cathode body  31 A/ 31 B. This discharge is then sustained by a 1-2 A current to igniter/keeper electrode  15 . The electron emission is through opening  34 , which is reduced in diameter from the inside diameter of tantalum tube  31 A. The electrons that pass through the aperture come from volume  35  adjacent to the aperture, and are believed to mostly originate from internal insert surface  36  adjacent to volume  35 . The lower cathode tip temperature (1400-1500 K) of this cathode type compared to that of the configuration in  FIGS. 1 and 2  is attributed to the lower work function of the oxide-coated insert. 
   As described by Nakanishi, et al., in an article in  Journal of Spacecraft and Rockets , Vol. 11, beginning on page 560, operating lifetimes of the order of 10,000 hours have been demonstrated with the type of hollow cathode shown in  FIG. 4 . Much of this increased lifetime can be attributed to the lower operating temperature, and the reduced energy of bombarding ions that is sufficient to maintain this reduced temperature. However, exposure to atmosphere rapidly degraded the electron emission characteristics of the emission material—see Zuccaro in  AIAA Paper  73-1140, 1973. This degradation was not observed with storage in either an inert gas (argon) or a vacuum. 
   The heat losses of the prior-art hollow cathode shown in  FIG. 4  are again by radiation and conduction, but the heat loss paths are more complicated than those for the hollow-cathode shown in  FIGS. 1 and 2  because of the more complicated construction. The heating of the emissive surface is again by ion bombardment from the conductive plasma that extends back into the hollow cathode. The emissive surface is insert surface  36  and the ion bombardment is from ions coming from the conductive plasma that extends back into volume  35 . Insert  32  consists of a spiral winding of tantalum foil, where the layers of foil serve as radiation shields for heat flow in the radial direction. Ultimately, the heat flow into insert  32  by ion bombardment must leave by radiation to tantalum tube  31 A and tungsten tip  31 B, and from there by conduction to the cathode support (not shown in  FIG. 4 ). (Those skilled in the art of vacuum technology will recognize that simple contact between insert  32  and surrounding tube  31 A does not result in significant thermal conduction between the two and the heat transfer is primarily by radiation.) However, there is another major heat loss path. The electrically conductive plasma is most dense in volume  35  and the volume in opening  34 , becoming less dense outside of tip  31 B where the current density of emitted electrons decreases. The surface inside opening  34 , surface  37 , therefore receives ion-bombardment heating in an amount comparable to that of emissive surface  36 , and this heat can be conducted through tip  31 B and tube  31 A to the cathode support. The dual paths for heat loss (through both the insert and the tip) presumably increase the discharge voltage required for maintaining emissive surface  36  at emissive temperature, but are not a serious problem because the operating temperature for the emissive surface is so low (1400-1500 K). 
   The use of electrode  15  as a keeper electrode permitted electron emission to be available for the subsequent initiation of ion-source operation without having to make that initiation simultaneous with starting the hollow cathode. For example, it was desirable to have the neutralizer hollow cathode ready to emit electrons before an ion beam is initially accelerated, and not to generate an unneutralized ion beam with the attendant high accelerator-grid impingement while the neutralizer hollow cathode was started. 
   Referring to  FIG. 5 , there is shown yet another prior-art hollow cathode, a space-propulsion hollow cathode described by Zuccaro in the aforementioned  AIAA Paper  73-1140, 1973. Hollow cathode  40  differs from the one shown in  FIG. 4  in having porous-tungsten insert  42  in place of spiral-wound foil insert  32 . The pores of the porous tungsten are impregnated with an emissive material, barium carbonate. Another difference is that resistive heater  27  is enclosed in swaged composite structure  43  consisting of outer metal tube  44 , resistive heater  27 , and insulator  45  between the two. 
   The operation of hollow cathode  40  is similar in all important aspects to that of hollow cathode  30  described in connection with  FIG. 4 , including both the long life and the degradation of the emission material due to exposure to atmosphere. The function and performance of the spiral-wound foil insert are generally similar to those of the porous-tungsten insert, with both serving as long-duration dispensers of emissive material. Porous-nickel inserts impregnated with emissive material have been used elsewhere with similar results. Reliability of resistive heater  27  has been an recurrent problem with both designs shown in  FIGS. 4 and 5 . The space-propulsion hollow cathodes shown in  FIGS. 4 and 5  are from publications that are several decades old. However, more recent space-propulsion hollow cathodes are similar, as shown by U.S. Pat. No. 6,380,685—Patterson, et al. The heat loss paths for the hollow cathode shown in  FIG. 5  are also similar to those for  FIG. 4 , starting with emissive surface  46  and surface  37  inside opening  34 . There is the minor difference that there are no internal radiation shields in insert  42 . Again, the dual paths for heat loss are not a serious problem because the operating temperature for the emissive surface is so low (1400-1500 K). 
   Referring to  FIG. 6 , there is shown a cross section of still another prior-art hollow cathode. Hollow cathode  50  is the compromise mentioned in the Background Art section and has been marketed as the HCES 1000 and HCES 5000 by Commonwealth Scientific Corporation and more recently by Veeco Instruments Inc. The cathode body is comprised of tantalum tube  31 A′ having a circular cross section and tip  31 B′ and is formed by swaging a tantalum tube to a small diameter at the open end. Although the cathode body is fabricated in a different manner than the cathode bodies of prior-art hollow cathodes  30  and  40 , the functions of all three are the same. Tantalum-foil insert  52  is generally similar to insert  32  in  FIG. 4 , except that insert  52  is not coated with emissive material. The tantalum foil used for this insert is textured (with a large plurality of small dents or wrinkles) to minimize layer-to-layer contact. The igniter/keeper is comprised of cylindrical wall  15 A and apertured end  15 B, and is of an enclosed design. The enclosed ignitor/keeper will be described further in connection with  FIG. 8 . 
   The lack of an additional emissive material on the spiral wound tantalum-foil insert  52  of hollow cathode  50  has both adverse and beneficial effects when compared to hollow cathodes  30  and  40  that incorporate emissive material. The operating lifetime is reduced from thousands of hours to several hundred hours, but is still adequate for most industrial applications when operating on clean working gas. The adverse effect of atmospheric exposure is also reduced. With no emissive material to degrade with atmospheric exposure, the cathode performance degradation is also less severe. Repeated exposure of the foil insert to atmosphere, however, still results in embrittlement and flaking of the foil insert, with the flakes eventually plugging the central passage in the insert through which the ionizable working gas flows. The embrittlement and flaking is believed due primarily to adsorbed layers of water vapor accumulated during atmospheric exposure on the extended surface area of the spiral-wound foil insert. As the result of the layered structure of this foil insert, much of this water vapor (or other atmospheric contaminants) is not removed during purging, and is present to react chemically with the tantalum foil as it heats up to operating temperature. There can also be a failure of tantalum tube  31 A′ at approximately the axial location indicated by the dashed line F shown in  FIG. 6 . This failure can be due to the formation of cracks in tube  31 A′ that permit much of the working gas to escape before reaching opening  34 , thus preventing either the starting or the normal operation of the hollow cathode. The failure can also be more dramatic in that tube  31 A′ completely separates at that location. This type of failure is discussed further near the end of this section. 
   The mechanisms and paths for heat loss in the prior art hollow-cathode of  FIG. 6  are similar to those in  FIG. 4 , but the large reduction in lifetime is attributed mostly to the increased discharge voltage and erosion that results from the higher operating temperature, 2400-2500 K versus 1400-1500 K. Because of this large reduction in lifetime, the conductive heat loss path from surface  37  through tip  31 B′ and tube  31 A′, that does not contribute directly to the heating of emissive surface  56  is a more serious concern. 
   Referring to  FIG. 7 , there is shown a cross section of yet still another prior-art hollow cathode. Hollow cathode  60  comprises a hollow tantalum tube  61  having a circular cross section and inner and outer radiation shields  62 A and  62 B. Radiation shields  62 A and  62 B each comprise a plurality of shields constructed with spiral, multiple-turn windings of tantalum foil, wound external to the hollow cathode tube  61 . Radiation shields  62 A and radiation shields  62 B are adjacent to each other and to tube  61 , without the presence of intervening support structure between either any of the radiation shields or between tube  61  and any of the shields. The term “adjacent” as used herein means immediately preceding or following. “Support structure” refers to support from a source exterior to radiation shields  62 A and  62 B and tube  61 . Textured tantalum foil is used to fabricate radiation shield  62 B in order to minimize layer-to-layer contact of the radiation shields. The effect of this texturing is to increase the average thickness of a heat-shield layer by a factor of several over the original 0.013-mm thickness of the foil. More details on the dimensions and construction of this hollow cathode can be found in the aforementioned U.S. Patent Application Publication 2004/0000853—Kaufman, et al. An enclosed ignitor/keeper with cylindrical wall  15 A and apertured end  15 B is also shown in  FIG. 7 . The electrons that pass through open end  64  of tube  61  come from volume  65  near the aperture, and mostly originate from internal tube surface  66  adjacent to volume  65 . Except that longer lifetime is obtained through more efficient thermal control, the starting and operation of hollow cathode  60  is similar to that of hollow cathode  10 . An important failure mode is a failure of tantalum tube  61  at approximately the axial location indicated by the dashed line F shown in  FIG. 7 . This failure is due to the formation of cracks in tube  61  that permit much of the working gas to escape before reaching opening  64 , thus preventing the starting or normal operation of the hollow cathode. Similar to hollow cathode  50 , the failure can also be more dramatic in that tube  61  completely separates at that location. This type of failure is also discussed further near the end of this section. 
   There can also be a question of whether a continuous spiral winding of tantalum foil, such as shown in insert  52  of  FIG. 6  or radiation shields  62 A and  62 B in  FIG. 7 , is a thermally conductive path or a plurality of radiation shields. For the several millimeter diameters of the windings and the 0.013-mm thickness of the foil, the radiation heat transfer from layer-to-layer at temperatures near 2400 K is much greater than the conductive heat transfer along the length of the spiral. Such a spiral winding of foil therefore performs more as a plurality of radiation heat shields than it does as a spiral conductive heat path, and is assumed to be a plurality of heat shields herein. This is in addition to the obvious distinction that the construction comprises multiple layers in approximately the circumferential direction, as opposed to a simpler and more substantial path in a radial direction. 
   The enclosed ignitor/keeper can be better understood by reference to  FIG. 8 , where hollow cathode  50  is incorporated in hollow-cathode assembly  70 . Hollow cathode  50  is assembled within main body  71 , one end of which forms igniter/keeper cylindrical wall  15 A. Apertured end  15 B is a separate part that is held in contact with cylindrical wall  15 A by screw fitting  72 . Main body  71 , cylindrical wall  15 A, and apertured end  15 B enclose volume  73 . Cathode holder  12  in this design is a union fitting between tantalum tube  31 A′ and gas feed tube  14 . Cathode holder  12  is separated from and positioned relative to main body  71  by insulators  74 . Cathode holder  12  and insulators  74  are held in position in main body  71  by screw fitting  75 . Volume  76  adjacent to cathode holder  12  is vented to surrounding volume  18  by vent hole  77 . From a functional viewpoint, an enclosed ignitor/keeper is defined as one in which most of the ionizable working gas from the hollow cathode must pass through the ignitor/keeper aperture ( 78  in  FIG. 8 ). In contrast, an ordinary or non-enclosed ignitor/keeper permits much or most of the ionizable working gas to flow around the outside of the ignitor/keeper (see igniter/keeper  15  in  FIG. 1 ,  4 , or  5 ). 
   The discharge with an enclosed ignitor/keeper of the type shown in  FIG. 8  can be started by applying a positive potential of approximately 1 kV to main body  71  (including igniter/keeper  15 A/ 15 B) relative to cathode  50 . The ionizable working gas enters volume  73  through cathode opening  34  and leaves through igniter/keeper aperture  78 , so that the pressure in volume  73  is intermediate of the pressure in cathode opening  34  and surrounding volume  18 . Because of the intermediate pressure in volume  73 , the starting discharge is concentrated in this volume, thereby heating hollow cathode  50  to approximately operating temperature while starting the discharge. That is, a discharge between cathode  50  and igniter/keeper  15 A/ 15 B is the heating means to bring cathode  50  to operating temperature. After the discharge is started to the igniter/keeper, the current to the igniter/keeper is maintained at about 1.5 A, which corresponds to a cathode-keeper voltage ≦50 V and is usually in the 20-30 V range. 
   The electrical circuit diagram for operating cathode assembly  50  is similar to that shown in  FIG. 3 , with hollow-cathode assembly  50  replacing hollow cathode  21  and igniter/keeper  15 A/ 15 B replacing ignitor/keeper  15 . Because the cathode heating is provided by igniter/keeper power supply  23 , power supply  26  and resistive heater  27  are not required. Operation is completed by using discharge power supply  24  to cause the electron emission to the anode. (The anode is  17  in  FIG. 3  and is not shown in  FIG. 8 .) 
   Referring to  FIG. 9 , there is shown hollow-cathode assembly  80 , which differs from hollow-cathode assembly  70  primarily in using hollow cathode  60  instead of hollow cathode  50 . Hollow cathode  60  is assembled within main body  71 , one end of which forms igniter/keeper cylindrical wall  15 A. Apertured end  15 B is a separate part that is held in contact with cylindrical wall  15 A by retainer  81 , which in turn is held in position by washers  82 , screws  83 , and nuts  84 . Main body  71 , cylindrical wall  15 A, and apertured end  15 B enclose volume  73 . Cathode holder  12  is a union fitting between tantalum tube  61  and gas feed tube  14  and provides a support means for tantalum tube  61 . Cathode holder  12  is separated from and positioned relative to main body  71  by insulators  74 . Cathode holder  12  and insulators  74  are held in position in main body  71  by retainer  85 , which in turn is held in position by washers  86 , screws  87 , and nuts  88 . Volume  76  adjacent to cathode holder  12  is vented to surrounding volume  18  by vent hole  77 . Startup and operation is similar to that described in connection with  FIG. 8 . 
   To summarize the prior art of hollow cathodes, the simple tubular hollow cathode of Delcroix, et al., withstands exposure to atmosphere very well, but it has a very short lifetime. The space electric-propulsion hollow cathodes, with an insert coated or impregnated with emissive material, can have extremely long lifetimes, but cannot withstand repeated exposure to atmosphere. The compromise hollow cathode with a spiral-wound foil insert that has no additional emissive material has an acceptable lifetime if the number of exposures to atmosphere is limited. With repeated exposures, the foil insert also fails. 
   The hollow cathodes shown in  FIGS. 6 and 7  are both capable of reaching lifetimes that are adequate for most industrial applications. In addition, they are both constructed of refractory materials and are not subject to the more severe effects of repeated atmospheric exposure that occur with the use of more reactive emissive materials—see discussions of  FIGS. 4 and 6 . However, the hollow cathodes shown in  FIGS. 6 and 7  both show shortcomings when operated with contaminated working gas. In addition to severe flaking of the tantalum-foil insert of cathode  50  with repeated atmospheric exposure, cathodes  50  and  60  both show rapid structural degradation when operated with contaminated working gas. This structural degradation was similar for both cathodes and consisted of either the formation of cracks in the tantalum tubes ( 31 A′ in  FIGS. 6 and 61  in  FIG. 7 ) or complete separation of those tubes. What was most surprising was that this structural damage in both cathodes was confined to narrow regions—near dashed lines F in  FIGS. 6 and 7 . 
   A review of literature was made to find a possible explanation for the extremely localized damage due to impurities. The absorption of contaminants in “getters” was studied in vacuum tube technology, where the removal of these contaminants was necessary for the proper operation of the vacuum tubes. As described by Spangenberg in the book entitled  Vacuum Tubes , McGraw-Hill Book Company, New York (1948), beginning on page 809, tungsten, molybdenum, and tantalum, the most common materials for hollow cathodes, have all been used as getters. Information from Spangenberg in the aforementioned book,  Vacuum Tubes , and Dushman in the book entitled  Scientific Foundations of Vacuum Technique , John Wiley &amp; Sons, New York (1962), beginning on page 624, can be summarized. Most of the absorption and/or reaction of getter materials with reactive gases takes place over only a narrow temperature range. Below this range, the adsorption and reaction rates are small and the amounts of gases adsorbed or reacted are therefore small. Above this range, the high temperature of the getter material drives the gases out of it. For tantalum, the effective range for gettering is about 700-1200 C. Several reactions are involved. Oxygen and nitrogen can react with the getter to form oxides and nitrides. Water and hydrocarbons can dissociate to form oxides and carbides. The hydrogen from the dissociation can be directly absorbed into the getter. The formation of the oxides, nitrides, and carbides in the getter material will change its physical dimensions, reduce ductility, and introduce stresses. The absorption of hydrogen can cause embrittlement. These processes explain the formation or cracks in, or rupture of, the tantalum hollow-cathode tubes, while the narrow temperature range for these processes to take place explains the compact physical location for the damage. 
   The temperature distribution of 38-mm long tantalum tube  61  of hollow cathode  60  was calculated and presented in the aforementioned U.S. Patent Application Publication 2004/0000853—Kaufman, et al., for both no radiation shielding and a reduction in radiated heat loss of 90 percent. These two thermal conditions were believed to bracket the actual temperature distribution and their average value at the location of maximum damage was about 1200 C, which is the upper end of the gettering range given for tantalum. The gettering literature of Spangenberg and Dushman thus agrees with the nature of the damage to hollow cathodes  50  and  60  that resulted from the use of contaminated working gas. In the case of hollow cathode  60 , it was also possible to find agreement for the location. 
   It may be noted that hollow cathodes  30  and  40  did not exhibit failures of the gas confining tubes as described above. But that lack of failure was only due to the more rapid failure of the reactive emissive materials in inserts  32  and  42 . Without these emissive materials, those cathodes were unable to operate in the temperature range of 1400-1500 K for which they were designed. 
   DESCRIPTION OF PREFERRED EMBODIMENTS 
   Referring to  FIG. 11 , there is shown an embodiment of the present invention. Hollow cathode  90  comprises refractory-metal first tube  91 , which is surrounded by plurality of refractory-metal radiation shields  92 , which in turn is surrounded by refractory-metal second tube  93 . A radiation shield is defined herein as a single layer that circumferentially encloses the hollow-cathode tube. As described in the prior art, this definition is consistent with radiation heat transfer from layer-to-layer being much greater than conductive heat transfer along a spiral winding for the dimensions, temperatures, and foil used. A plurality of shields is therefore conveniently constructed as a spiral, multiple-turn winding of refractory-metal foil, or a plurality of such windings. In order to minimize the layer-to-layer contact between shields in a spiral winding, the metal foil may be textured before winding. The foil can textured by pressing it against a rough or corrugated surface, which imparts a similar shape to the foil. 
   Shields  92  end approximately flush at the two ends of first tube  91 , that is, approximately in the planes of these two ends. One end of second tube  93  is also approximately flush at the corresponding end of the first tube, that is, approximately in the plane of that end. Radiation shields  92  are compressed between first tube  91  and second tube  93 . In  FIG. 11  this compression is accomplished by swaging second tube  93  to a smaller diameter at two axial locations indicated by dashed lines S. This swaging of second tube  93  compresses radiation shields  92  between it and first tube  91 , as well as preventing the leakage of gas around the first tube. The texturing of the foil of which the radiation shields are fabricated permits considerable reduction in the outer diameter where the swaging occurs without significantly degrading the radiation shielding effectiveness. The compression could have been accomplished by expanding the first tube. It could also be accomplished by using a conically tapered surface on the outside of the first tube and/or the inside of the second tube so that sliding the parts into position accomplished the compression. An enclosed ignitor/keeper with cylindrical wall  15 A and apertured end  15 B is also shown in  FIG. 11 . 
   First tube  91 , radiation shields  92 , and second tube  93  are adjacent to each other without the presence of intervening support structure between any of the adjacent radiation shields, between the first tube and the inner radiation shield, or between the outer radiation shield and the second tube. The term “adjacent” as used herein means immediately preceding or following. “Support structure” refers to support from a structural member other than radiation shields  92 , first tube  91 , and second tube  93 . Refractory material (e.g. in the form of particulates) could be included between adjacent radiation shields, or between the inner shield and first tube  91 , or between the outer shield and second tube  93 , and serve the same function as texturing. The presence of such refractory material is not considered to be intervening support structure in this invention because it does not connect to a structural member other than the first and second tubes and the radiation shields. 
   First tube  91  should be attached to radiation shields  92 . This can be done by spot welds of the inner end of the spiral winding that is radiation shields  92  to first tube  91 . No similar attachment was required where radiation shields  92  contact second tube  93 , presumably because of both the larger contact area at this location and the lower temperature. 
   The operation is generally similar to other hollow cathodes. There is a discharge between hollow cathode  90  and enclosed ignitor/keeper  15 A/ 15 B and or an external cathode (not shown in  FIG. 11 ). This discharge generates electrons and ions by ionization of atoms or molecules of the working gas. Some of the ions are carried with the flow of working gas and, together with the emitted electrons form a conductive plasma that extends from volume  95  inside open end  94  of cathode  90  to igniter/keeper  15 A/ 15 B and the anode. The electrical conductivity of this plasma permits the operation with an anode-cathode (or ignitor/keeper-cathode) voltage of &lt;50 V and consistent with a long operating lifetime. The electrons that pass through open end  94  come from volume  95  near the open end, and mostly originate from internal tube surface  96  adjacent to volume  95 . 
   The uniqueness of hollow cathode  90  is in the absence of a continuous piece of refractory metal extending from the open end of the hollow cathode to the cathode support, which confines the working gas, and is subject to failure in the confining function when exposed to high levels of contamination in the working gas. Prior-art examples of such a continuous piece of refractory metal are hollow-cathode tube  11  in  FIGS. 1 and 2 , tip  31 B and tube  31 A which are electron-beam welded into one continuous piece in  FIGS. 4 and 5 , tip  31 B′ and tube  31 A′ which are a continuous piece of tantalum in  FIG. 6 , and tube  61  in  FIG. 7 . This absence has two important benefits. One is the reduction of heat loss by removing a major thermal conduction path for this loss, which permits operation at a lower discharge voltage and has a beneficial effect on lifetime. The other important benefit is to reduce the effect of contamination in the working gas. The first tube is near the electron emission temperature and is above the critical temperature range for absorbing or reacting with contaminants. The large tube is much closer to the support temperature and is below this critical temperature range. The temperature of some of the radiation shields will fall in the critical temperature range. The absorption of or reaction with contaminants near the critical temperature range will cause distortion or fracture of some of the radiation-shield layers. But the compression between layers will hold fractured or distorted pieces in place, while the length of the microscopic passages between layers will effectively seal the space between the first tube and the second tube and force almost all of the working gas through the first tube. In this manner hollow cathode  90  is more resistant than prior-art hollow cathodes to containment failures for the working gas as a result of contamination in that working gas. 
   Referring to  FIG. 11   a , there is shown another embodiment of the present invention, hollow cathode  90 ′. Hollow cathode  90 ′ differs from hollow cathode  90  in  FIG. 11  only in the construction of the first tube and the plurality of radiation shields. First tube  91 ′ and plurality of radiation shields  92 ′ are fabricated from one continuous piece of refractory-metal foil. The portion of the foil used to make first tube  91 ′ is not textured, so that the density of this portion approximates the density of solid metal. The transition from the smooth foil of first tube  91 ′ to the textured foil of radiation shields  92 ′ provides the attachment between the two. Although the absence of texturing was used to make the first tube have a density significantly greater than the surrounding heat shields, such a density difference could have been generated with a difference in the tension of the foil while winding the first tube and the radiation shields. 
   In  FIG. 12 , hollow cathode  90  is incorporated in hollow-cathode assembly  100 . Hollow cathode  90  is assembled within main body  71 , one end of which forms igniter/keeper cylindrical wall  15 A. Apertured end  15 B is a separate part that is held in contact with cylindrical wall  15 A by retainer  81 , which in turn is held in position by washers  82 , screws  83 , and nuts  84 . Main body  71 , cylindrical wall  15 A, and apertured end  15 B enclose volume  73 . Cathode holder  12  is a union fitting between second tube  93  and feed tube  14  and provides a support means for second tube  91 . Cathode holder  12  is separated from and positioned relative to main body  71  by insulators  74 . Cathode holder  12  and insulators  74  are held in position in main body  71  by retainer  85 , which in turn is held in position by washers  86 , screws  87 , and nuts  88 . Volume  76  adjacent to cathode holder  12  is vented to surrounding volume  18  by vent hole  77 . 
   The starting and operation of hollow cathode  90  and hollow-cathode assembly  100  is similar to that described for hollow cathodes  50  and  60  and hollow-cathode assemblies  70  and  80 . The electrical circuit diagram is shown in  FIG. 12   a  and is similar to that shown in  FIG. 3 , except that heater power supply  26  and resistive heater  27  are not required and hollow cathode  90  replaces hollow cathode  21 . 
   Tantalum is the most common hollow-cathode material because it withstands high operating temperatures and is easily formed or machined. Tungsten has also been used and provides a higher temperature capability with a generally higher fabrication cost. Molybdenum is easily machined, but has less temperature capability than tantalum. Carbon, considered a metal for the discussion herein, also provides higher temperature capability but with decreased strength. Hollow cathodes have been made of refractory metals such as these, as well as alloys of two or more metals. 
   DEMONSTRATION OF RESISTANCE TO CONTAMINATION 
   Tests were carried out to demonstrate the improved capability of a hollow cathode constructed in accord with this invention to withstand the adverse effects of contaminated working gas. To provide realistic and reproducible contaminated working gas, a gas feed system was modified. A typical gas feed system is shown in  FIG. 13 . Feed system  110  is comprised of gas bottle  111 , gas-bottle valve  112 , gas regulator  113 , first gas line  114  connecting the gas regulator and gas flow controller  115  (often called a mass flow controller), second gas line  116  connecting the gas flow controller and gas feedthrough  117 , which introduces the gas to vacuum chamber  118 . Although it is not shown in  FIG. 13 , the gas flow is conducted to a hollow cathode inside the vacuum chamber. 
   Some of the usual sources of contamination are: using a gas regulator that is not intended for high-purity applications, using gas lines that have not been thoroughly cleaned, and not making leak-tight connections between the gas lines and the gas regulator, gas flow controller, and feedthrough. Stainless-steel tubing is preferred for the gas lines, but an internal residue left from its fabrication can contaminate the gas flowing through it unless it is cleaned thoroughly. Polymer tubing is a less acceptable choice for a gas line, in that even when clean, its more porous structure can result in water vapor and hydrocarbon contamination of the gas flowing through it. The connections at the ends of second gas line  116  are more frequently a source of contamination than those of first gas line  114  because the gas in the second gas line is usually below atmospheric pressure during operation, so that the atmosphere can leak into the gas line. In comparison, the pressure in first gas line  114  is usually at or above atmospheric pressure. The connections inside the vacuum chamber are usually not a problem because the pressure inside the vacuum chamber is usually less than that in the gas tubing. The replacement of gas bottles is a common source of contamination. If the regulator is attached to a new gas bottle and then opened without pumping down the gas line, the trapped atmosphere between the regulator and the new gas bottle will mix with the clean gas in the bottle (typically &gt;99.999 percent purity) and contaminate it. The proper procedure is to connect the gas bottle to the gas regulator, pump down the vacuum chamber to operating pressure, fully open both the gas flow controller and gas regulator, and continue to operate the vacuum pumps until the vacuum chamber reaches its normal base pressure. Then, with the volume between the gas bottle and the gas regulator pumped to a low pressure by the vacuum chamber, close the gas regulator and open the valve on the gas bottle. An additional purge is then required to remove the adsorbed contaminants from atmospheric exposure on the inside of the gas lines and the gas flow controller. 
   The procedure used to introduce a controlled level of contamination into the working gas can be explained with reference to  FIG. 14 . The only change in gas feed system  120  compared to that of feed system  110  is the replacement of first gas line  114 , which was constructed of clean stainless steel tubing, with modified first gas line  114 A, which was comprised of 30 meters of 6.35-mm-diameter nylon tubing. A normal gas purge was used before operating a hollow cathode, so that the contamination consisted of a thin layer of atmospheric contaminants (usually oxygen, nitrogen, water vapor, and some hydrocarbons from the laboratory background) adsorbed on the surface of the nylon tubing plus similar contaminants absorbed into the nylon. There was probably some additional hydrocarbon in the form of residual plasticizer in the nylon. To make sure that the nylon tubing did not gradually become cleaner, the nylon tubing was re-exposed to the atmosphere whenever a new hollow cathode was tested or whenever the operating time after the previous atmospheric exposure exceeded 48 hours, whichever came first. It should be emphasized that this contamination test is a severe one. In the absence of contamination and with only occasional exposure to atmosphere, the typical lifetime of either hollow cathode  50  or  60  was of the order of 1000 hr. Previous operation had shown that 20-30 cm of polymer tubing in an otherwise clean gas line was sufficient to dramatically reduce this lifetime. By using 30 meters of polymer tubing, a very high level of contamination was being introduced. 
   A failure was defined in either of two ways. Either emission could not be sustained or the hollow cathode could not be restarted. For operating times less than 48 hours, the failures were all of the first type. For operating times longer than 48 hours, the failure was an inability to restart the hollow cathode after operation was stopped to expose the nylon tube to atmosphere. The maximum argon flow used for starting was 100 sccm. Visual appearance of the hollow cathode was not a consideration in defining a failure. 
   The first test was of hollow cathode  60  shown in  FIG. 7  and described in the aforementioned U.S. Patent Application Publication 2004/0000853—Kaufman, et al. The tantalum tube of this hollow cathode was 1.57 mm in outside diameter and 38 mm long, with a wall thickness of 0.38 mm. It was operated with an argon gas flow of 10 sccm (standard cubic centimeters per minute), a keeper current of 1.5 A, and an emission of 5 A. Several tests were made with the working gas contaminated as described above, resulting in lifetimes of 1-5 hours before failing. Although these lifetimes were shorter than were found in actual industrial applications, presumably due to a higher level of contamination, the appearance of the failures was indistinguishable from that of prior failures found in industrial applications. This similarity in appearance means that the effects of the test impurities are similar to the effects in industrial applications. Using the same number of radiation shields, but increasing the tube diameter to 3.18 millimeters and the wall thickness to 1.17 mm increased the lifetime to 8 hours. Apparently more material in the tantalum tube increased the time to failure, without changing the failure process. 
   A test was also made of the prior-art hollow cathode shown in  FIG. 6 . The outside diameter of tantalum tube  31 A′ was 6.4 mm for this hollow cathode with a wall thickness of 0.5 mm, and the lifetime was increased to 144 hours. The longer lifetime for this hollow cathode was felt to be due in part to the larger tube diameter and the greater amount of material available to absorb contamination. However, at the end of the test, cracks were nearly continuous around the body of the hollow cathode near dashed line F in  FIG. 6 . 
   The invention described herein was also tested using the configuration shown in  FIG. 11   a . The first (tantalum) tube had an outside diameter of approximately 1.6 mm, while the inside diameter was approximately 0.8 mm. The axial length of the first tube and radiation shields was 25 mm. Because the small tube was constructed of tantalum foil, these diameters are less precise than those for solid tubing. The radiation shields were wound to a diameter just small enough to fit inside the second (tantalum) tube, which had a outside diameter of 6.4 mm, a wall thickness of 0.5 mm, and a length of 64 mm. The lifetime of this hollow cathode was 240 hours. From the severe nature of this test, a lifetime of 240 hours with such a high level of contamination should translate into useful lifetimes of at least several hundred hours at more realistic levels of contamination. Even though the lifetime was longer with the configuration of  FIG. 11   a , the cracks in the 6.4 mm tube were much less extensive at the end of test than the corresponding cracks in the configuration of  FIG. 6 . This result indicated that the outer tube of the former operated at a lower temperature and had less of a gettering effect than the outer tube of the latter. 
   DESCRIPTION OF ALTERNATE EMBODIMENTS 
   Referring to  FIG. 15 , there is shown another embodiment of the present invention. Hollow cathode  130  differs from hollow cathode  90  in having first tube  91  divided into two pieces  91 A and  91 B. Depending on the operating conditions and hollow-cathode dimensions, such a change could reduce thermal losses. Also shown in  FIG. 15  is an extended region of swaging, instead of the more localized swaging of  FIG. 11 . 
   Referring to  FIG. 16 , there is shown yet another embodiment of the present invention. Hollow cathode  140  differs from hollow cathode  90  (in addition to the difference in swaging) in having small tube  91 C extend beyond the ends of radiation shields  92  and large tube  91 . Such a change in the small tube can reduce the thermal efficiency slightly in that more area of the small tube can radiate directly to the surroundings instead of being shielded by the radiation shields. But the extension can also increase the ease of starting a discharge. 
   Other changes should be evident to those skilled in the art. Tubes with circular cross sections and generally cylindrical configurations are typical in hollow cathodes. Tubes with circular cross sections were used in tests of the configurations shown in  FIGS. 11 and 11   a , and are reasonable to assume for those of  FIGS. 15 and 16 . It should be apparent that tubes with other cross sections, such as triangular, square, rectangular, or elliptical are possible, with the radiation shields accommodating the tubing shape. In a similar manner, radiation shields are assumed to be comprised of spiral windings of thin material. The radiation shields could also be comprised of many turns of fine refractory filament or wire, or they may be comprised of concentric cylinders instead of a spiral winding of foil. 
   Different lengths of tubing and radiation shields could also be used. The configuration of this invention used in the contamination test had an axial length for the first (inner) tube of about 16 times the outside diameter of that tube. Longer lengths could probably be used, but would tend to increase the heat loss and decrease lifetime. Experience with a variety of hollow cathodes has shown that the internal erosion typically extends back inside the tube for a length equal to several outside diameters of that tube, so the minimum length of the inner tube should be equal to about 4-5 outside diameters of that tube. The inside diameter of the first tube should be roughly half of its outside diameter. Larger inside diameters can be used, but will reduce the amount of material available for erosion, hence reduce the lifetime. Smaller inside diameters can be used, but are more likely to fail due to closing up completely. The length of the shields must also be considered relative to the diameter of the second (outer) tube. If the shields are too short, less than about equal to the diameter of the second tube, it would be difficult to keep them in place while they are being compressed between the first and second tubes. That is, they would tend to move back into the second tube, or out the end of it. In general, the flush ending of the second tube with one end of the radiation shields is preferred. Extending this tube beyond the radiation shields can make starting more difficult, while ending it before the end of the radiation shields can degrade the structural integrity of the hollow cathode by not fully supporting the radiation shields. 
   The number of radiation shields can also be varied. Simple one-dimensional analysis will show that the radiation heat loss will vary approximately as 1/N, where N is the number of heat shields. It would therefore be expected that about 10 or more heat shields would be required to obtain most of the beneficial effects of heat shields. In practice, there is a tendency of heat shields to weld together when operated for a long time at very high temperatures, thereby providing an increasingly direct path for heat conduction. (This is probably the failure mode for the simple heat shields suggested by Delcroix, et al, in the aforesaid chapter in Vol. 35 of  Advances in Electronics and Electron Physics .) Texturing of the heat-shield material tends to slow this welding process, but for high heat-shield efficiency over long operating lifetimes, 20, 30, or even more heat shields are preferred. 
   While particular embodiments of the present invention have been shown and described, and various alternatives have been suggested, it will be obvious to those of ordinary skill in the art that changes and modifications may be made without departing from the invention in its broadest aspects. Therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of that which is patentable.