Abstract:
A closed drift ion source which includes a channel having an open end, a closed end, and an input port for an ionizable gas. A first magnetic pole is disposed on the open end of the channel and extends therefrom in a first direction. A second magnetic pole disposed on the open end of the channel and extends therefrom in a second direction, where the first direction is opposite to the second direction. The distal ends of the first magnetic pole and the second magnetic pole define a gap comprising the opening in the first end. An anode is disposed within the channel. A primary magnetic field line is disposed between the first magnetic pole and the second magnetic pole, where that primary magnetic field line has a mirror field greater than 2.

Description:
CROSS REFERENCE TO RELATED CASES 
     This application is a Continuation-In-Part application claiming priority from a U.S. application having Ser. No. 10/411,024, filed Apr. 10, 2003, now U.S. Pat. No. 6,919,672. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to closed drift ion sources and to closed drift type ion thrusters. More particularly, it includes embodiments that extend the life and efficiency of these devices. 
     BACKGROUND OF THE INVENTION 
     Closed drift ion sources have been known since Russian ion thrusters for satellite propulsion were reported in the 1960&#39;s. These prior art devices suffer from problems of sputter erosion of the closed drift side walls, loss of energetic electrons to the side walls, and poor beam collimation out of the source. 
     Side wall erosion has deleterious effects on ion source performance including:
         The source wall inserts, magnetic poles, or other plasma exposed surfaces must be routinely replaced. Where replacement is not possible in space thruster applications, wall erosion is eventually catastrophic. In these applications, thrusters are rated in thousands of hours of life with some 2,000–10,000 hours being the published life expectancies.   Ion sputtering of the side walls contaminates industrial ion source processes with the sputtered atoms. In many applications, this precludes these ion sources as potential process tools.   Sputtering of the side walls raises the source wall temperature. This can be a severe problem in space based applications where heat must be dissipated by radiation. The high temperatures experienced by the side walls requires special, expensive materials.   Ions striking the side walls do not exit the source, reducing source efficiency. (Efficiency is the ion current and energy relative to the power supply discharge current and voltage.)   In ion sources operated in the diffuse mode, erosion is particularly problematic if not ruinous. In the diffuse mode, the source is operated at sufficiently high pressure and power to create a neutral, conductive plasma in the gap between the poles. Operating in this mode, the plasma density is dramatically increased, and the electric fields change significantly, increasing ion bombardment of the pole pieces or side walls.       

     Other problems generally recognized with prior art ion sources include:
         Loss of high energy electrons to the side walls. This especially affects extended acceleration channel type ion sources. Side wall losses of electrons capable of ionizing the propellant gas results in loss of efficiency and side wall heating   Beam spreading outside the source. Here, the ion beam produced leaves the source in a spread cosine distribution rather than the preferred collimated output.       

     There are two basic types of closed drift ion sources for which many variations have been offered. The two types are anode layer and extended acceleration channel. Prior art examples for each type of source are described below. 
       FIG. 1  is a section view of prior art linear anode layer type ion source  100 . Additional description of this prior art device can be found in Capps, Nathan, et al., Advanced Energy Industries, Inc.  Application note: Ion Source Applications: Si Doped DLC,  and in Advanced Energy Industries, Inc.  Application note: Industrial ion sources and their application for DLC coating,  which are hereby incorporated by reference. 
     Such a prior art source  100  can either be annular or stretched out to lengths beyond three meters, the confined Hall current design enables extendibility similar to a planar magnetron.  FIG. 1  shows the magnetic field lines as calculated and mapped by a two-dimensional magnetic field software program. The field in the gap  120  is created by back shunt  110 , permanent magnet  130 , and pole pieces  140  and  150 . Electrically, poles  140 ,  150  and shunt  110  are connected to ground, and anode  102  is connected to the positive terminal of a high voltage power supply. 
     As those skilled in the art will appreciate, the anode  102  in a closed drift ion source is disposed a distance from the gap  120  between the poles  140  and  150 , where that distance exceeds the Larmor radius of the captured electrons. As those skilled in the art will further appreciate, the width of the gap  120  is adjusted to maintain a magnetic field of sufficient strength to magnetize electrons and to allow a plasma to exist therein. 
     Referring to  FIGS. 1 and 1A , in prior art device  100 , the half bevel shaped poles  140  and  150  produce a magnetic fields with the strongest magnetic field line, described herein as the “primary field line,” emanating from the flat, gap facing pole surfaces  142  and  152 . The magnetic configuration and pole shapes of this prior art device, calculated using a Ceramic  8  ferrite type magnet  130 , results in a primary field line  170  having a magnetic field strength of 682 Gauss at first end  172  on surface  152 , 542 Gauss at second end  176  on surface  142  of outer pole  140 , and a minimum strength of 445 Gauss at location  174 . As those skilled in the art will appreciate, use of other magnetic materials will change the relative strengths of the field lines but will not substantially change the relative location of the primary line or ratio between surface and gap fields. 
     By “primary field line,” Applicant means the field line having the least curvature and the strongest field strength in the gap. As the bloom of the field in the gap is viewed, the primary field line is the centerline of the bloom. Field lines to both sides of the primary field line are concave, i.e. curved, and face this field line. 
     As the magnetic field lines leave the high permeability poles  140  and  150 , enter the “air” gap  120 , and travel toward the center of the gap, the magnetic field strength lessens. Visually, this is seen as field lines spreading out in the gap. The result of this effect is a magnetic mirror. By “magnetic mirror,” Applicant means the “reflection” of electrons as an electron moves from a region of weaker field to a stronger field. 
     Applicant has discovered that the mirror ratio is an important aspect of closed drift ion source magnetic design. By “mirror ratio,” Applicant means the ratio of the strong field strength at an end of the field line to the minimum field strength along that field line. For example, in source  100 , using calculated field strengths of the primary field line  170  from first end  176  to location  174 , the magnetic mirror ratio is 1.22. From second end  172  to location  174  the magnetic mirror ratio is 1.53. Therefore, the minimum mirror ratio for source  100  is 1.22. 
     In addition, the ratio of the magnetic strengths at the end of the primary field line indicates whether that primary field line is substantially symmetric or asymmetric. By “substantially symmetric,” Applicant means an end-to-end ratio of magnetic strengths of between about 0.94 to about 1.06. For prior art device  100 , the ratio of the magnetic field strengths at locations  172  and  176  is about 1.26 indicating an asymmetric mirror field existing between the pole portions. 
     Applicant has found that a minimum mirror ratio greater than  2  in combination with an end to end ratio of between 0.94 and 1.06 to be optimal. The magnetic pole design of device  100 , however, produces weak magnetic mirror fields in gap  120 . The result is that when a plasma is disposed in gap area  120 , electrons are not strongly focused into the center of the gap. This results in substantial sputtering of the poles  140  and  150  and lower source efficiency. 
     Pole sputtering is exaggerated when the source is operated in the diffuse mode. This mode is entered when the plasma is dense enough to become electrically neutral. When this occurs, the electric fields change from a gradient field from the cathode poles  140  and  150  in gap  120  to anode  102  to a field dropping from the cathode poles across the dark space to the plasma and from the plasma to the anode. The diffuse mode is entered when a combination of higher process gas pressure and high discharge power produces a bright glow in the gap region. The diffuse mode is visually quite different from the collimated mode making the modes easy to distinguish by eye. In the diffuse mode, sputtering of the poles is increased due to the higher concentration of ions in the gap and the large voltage drop between the plasma and cathode pole surfaces. 
     Sputtering of the poles contaminates the substrate with sputtered material, causes wear of the cathode poles requiring their regular replacement, adds appreciably to the heat load the source must handle, and makes the source less energy efficient. 
     In contrast to this prior art device, Applicant&#39;s device creates a strong magnetic mirror field in the gap along the primary field line. Such a strong magnetic mirror has dramatic benefits for source operation. Without this focusing mirror field, not only are the poles eroded more rapidly, but the lack of the mirror field focusing effect causes the ion source to produce a broader, less collimated beam. 
     In addition, prior art device  100  includes a single central magnet. The resulting magnetic field is not symmetrical across gap  120  with one magnetic mirror being stronger than the other. As will be described below, symmetrical magnetic mirrors can be created with strong mirror fields along the central field line to focus the plasma in the center of the gap and optimize magnetic mirror repulsion from the poles. 
       FIGS. 2 and 2A  show a section view of prior art anode layer ion source  200 . Device  200  includes shunt  210 , magnet  230 , poles  240  and  250 , and anode  202 . An analysis of this pole design shows that the primary field line emanates from the flat faces  242  and  252  of poles  240  and  250 , respectively, rather than from the pointed portions  241 / 251 . 
     Magnetic field line  270  comprises the primary field line in this prior art embodiment. Field line  270  has a magnetic field strength of 683 Gauss at first end  272  on surface  252 , 580 Gauss at location  276  on second end  242 , and 373 Gauss at location  274  on field line  270 . Location  274  comprises the portion of field line  270  having the minimum magnetic field strength. Dividing the magnetic field strength at end  272  by the magnetic field strength at location  274  gives a mirror ratio of 1.83. The magnetic mirror formed between  276  and  274  is 1.55. Therefore the minimum mirror ratio is 1.55. Dividing the strength at end  272  by the strength at end  276  gives a ratio of about 1.17 thereby indicating an asymmetric mirror field existing between the pole elements. 
       FIGS. 3 and 3A  show prior art anode layer source  300  as depicted in  FIG. 3  in the publication ‘High Current Density Anode Layer Ion Sources’ by J. Keem, Society of Vacuum Coaters 44 th  Annual Technical Conference Proceedings. Device  300  includes permanent magnets  331  and  332 , in combination with pole portions  340  and  350 , and anode  302 . Field line  370  comprises the primary field line produced by device  300 . Field line  370  has a magnetic field strength of 1013 Gauss at first end  372  on surface  352 , 954 Gauss at second end  376  on surface  362 , and a minimum strength of 565 Gauss at location  374  on field line  370 . Therefore, the minimum mirror ratio for the primary field line for device  300  is 1.69. 
       FIG. 4A  shows a second type of ion source sometimes referred to as an extended acceleration channel type. Extended acceleration channel type ion source  400  is typical of prior art ion thruster propulsion devices. U.S. Pat. No. 5,892,329, in the name of Arkhipov et al., and U.S. Pat. No. 5,945,781, in the name of Valentian, describe such sources. Extended acceleration channel sources are commonly used in space thruster applications but can be adapted for industrial use also. 
       FIG. 4A  shows the magnetic field lines produced by extended acceleration channel source  400 . In this source, magnetic poles  440  and  450  are electrically floating. An electron source  480  serves as the cathode with anode  402  located inside ceramic isolator  490 . Anode  402  is positioned at the bottom of channel  422  such that electrons must pass through magnetic fields crossing gap  420  to reach anode  402 . 
     It is known that the ceramic side walls of an extended acceleration channel source, such as source  400 , tend to be eroded by ion bombardment. Because prior art device  400  separates the magnetic poles  440  and  450  from the channel with the insulating ceramic  490 , and because device  400  does not optimize the pole shapes, a strong magnetic focusing mirror radial field is not created in the channel. 
     Prior art device  400  produces a primary field line  470  having a magnetic field strength of 1011 Gauss at  472  on the inner surface of insulator  490 , 883 Gauss at  476  on inner surface of insulator  490 , and a minimum magnetic field strength of 687 Gauss at location  474 . This being the case, the minimum magnetic mirror ratio along the primary field line for device  400  is 1.29. The result of a weak mirror field is:
         Electrons, accelerated into the magnetic field in the channel by the electric field, are trapped by the magnetic field. Without a containing radial magnetic mirror field, these energetic electrons move along the field lines and can be absorbed by the side walls. Loss of high energy electrons to the walls lowers source ionization efficiency and heats the side walls.   Ambipolar diffusion causes the side walls to be charged negatively, and ions are attracted to the side walls.   The lack of radial electron focusing results in electron distribution across the full channel width. Ions then are created across the full width producing a wider, less collimated beam and added likelihood of ions hitting the side wall.   Only the ions created in the center of the channel experience the electric field pushing them perpendicularly out of the source. Without strong electron focusing, fewer are created in the center.       

       FIG. 4B  is a section view of ion source  900  described in U.S. Pat. No. 5,763,989 in the name of Kaufmann. Ion source  900  includes poles  940  and  950 , in combination with anode  902 , in further combination with a magnetic screen shunt similar to that taught in U.S. Pat. No. 5,892,329 in the name of Arkhipov, except the Kaufman shunt is arranged to allow a single permanent magnet to be used. This shunt technique produces a limited focusing effect in the acceleration channel that potentially results in reduced wall losses and less wall erosion. 
     While producing a mirror field at one side of the gap, the flat pole faces produce a weak mirror field in the center of the gap. Device  900  produces a primary field line having a magnetic strength of 600 Gauss at first end  972 , 550 Gauss at second end  976 , and a minimum magnetic field strength of 400 Gauss at location  974 . Therefore, the minimum mirror ratio for device  900  along the central primary field line  970  is 1.4. 
     U.S. Pat. No. 4,277,304 in the name of Horiike et al. teaches an ion source and ion etching process. Horiike et al. teach an arrangement for what is termed a grid-less ion source. The ion beam is created by two cathode surfaces with a magnetic field passing between the two surfaces The cathode surfaces and magnetic field are shaped into a racetrack to provide an endless Hall current confinement zone. An anode is disposed on one side of the racetrack magnetic field loop. This arrangement produces an ejection of ions from the side opposite the anode. Other prior art devices implemented electromagnets to create the magnetic field between the cathode surfaces. Horiike et al. teach the use of permanent magnets and a flat facing pole shape. 
     U.S. Pat. No. 5,359,258 to Arkhipov et al. teaches a closed drift ion accelerator wherein side wall erosion is reportedly lessened by lowering the amount of magnetic field in the acceleration channel by shunting the field with permeable screens. The idea is to move the containment of electrons from the central channel area out closer to the opening. The screens also shape the magnetic field to provide an amount of focusing of the plasma that helps to reduce side wall erosion. According to Arkhipov et al., the focusing effect allows making the channel walls thicker so the source lasts longer too. 
     Arkhipov et al. nowhere teaches shaping the magnetic poles to produce a strong radial mirror magnetic field in the gap and, more particularly, to produce that strong mirror field along the primary field line. As shown in  FIG. 4A , when the poles are separated from the channel by an insulator, the mirror ratio along the primary field line is less than 2. 
     U.S. Pat. No. 5,838,120 in the name of Semenkin et al. describes an anode layer source comprising a magnetically permeable anode to shape the magnetic field. The use of a magnetic shunt to remove radial, poorly mirrored magnetic field from the central channel, and moving the anode closer to the exit end, may reduce wall erosion. This prior art device, however, only provides marginal improvements. Semenkin et al. nowhere teaches shaping of the magnetic field to produce a strong, focusing mirror field along the primary field line. The device taught by Semenkin et al. results in electrons that are largely free to move along magnetic field lines and, in this case, recombine at the walls. 
     U.S. Pat. No. 6,215,124 in the name of King discloses a multistage ion accelerator with closed electron drift. In this device, the life and efficiency of the thruster is improved by shunting the magnetic field away from the central accelerator channel region and moving the B max  field line toward the open end. When this is done, the region of wall erosion moves farther toward the opening, extending the life of the thruster. While use of thin pole pieces could generate a mirror field of some strength, the poles are distanced from the channel by inserts. The result is a weak magnetic mirror field at the exit end with the accompanying negative results. 
     SUMMARY OF THE INVENTION 
     Applicant&#39;s invention includes a closed drift ion source for generating an accelerated ion beam having an annular or otherwise closed loop discharge region into which ionizable gas is introduced with an anode located at one longitudinal end of said region, the other end open to allow ion flow out of the discharge region. A first magnetic pole is located radially inward from the discharge region. A second magnetic pole is located radially outward from the region. These poles create a strong magnetic mirror field in the discharge region with the mirror field approximately centered on the primary magnetic field line between the said two poles and where the magnetic mirror has a minimum mirror ratio greater than 2. 
     Applicant&#39;s invention further includes a closed drift ion source for generating an accelerated ion beam having an annular or otherwise closed loop discharge region into which ionizable gas is introduced with an anode located at one longitudinal end of the region and the other end open to allow ion flow out of the discharge region. A first magnetic pole is located radially inward from said region, a second magnetic pole is located radially outward of said region and the poles are shaped to a point including beveled, non-orthogonal surfaces on both the internal and external pole surfaces. 
     Applicant&#39;s invention further includes a method to focus a plasma. Applicant&#39;s method provides an ionizable gas and introduces that ionizable gas into Applicant&#39;s closed drift ion source comprising a first magnetic pole and a second magnetic pole separated by a gap. Applicant&#39;s method produces a primary magnetic field line disposed between the first magnetic pole and the second magnetic pole, wherein that primary magnetic field line has a mirror field greater than 2. Applicant&#39;s method forms in the gap a plasma from the ionizable gas. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a section view of a prior art anode layer ion source device; 
         FIG. 1A  is a detail view of one gap region of the device of  FIG. 1 ; 
         FIG. 2  is a section view of a prior art anode layer ion source; 
         FIG. 2A  is a detail view of one gap region of the device of  FIG. 2 ; 
         FIG. 3  is a section view of yet another anode layer ion source; 
         FIG. 3A  is a detail view of one gap region of the device of  FIG. 3 ; 
         FIG. 4A  is a section view of a prior art extended acceleration channel closed drift ion source; 
         FIG. 4B  is a section view of the source in U.S. Pat. No. 5,763,989; 
         FIG. 5  is a section view of one embodiment of Applicant&#39;s ion source. 
         FIG. 6  shows a section view of one embodiment of Applicant&#39;s ion source implementing an extended acceleration channel; 
         FIG. 7  shows a section view of one half of a symmetrical anode layer type source implementing the Applicant&#39;s inventive method; 
         FIG. 8  shows one embodiment of Applicant&#39;s closed loop ion source with a wide pointed pole gap; and 
         FIG. 9  shows plasma containment using Applicant&#39;s ion source. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     While the prior art has recognized the problems of existing ion source technology, Applicant&#39;s improvements described herein address these prior art problems. Referring to the illustrations, like numerals correspond to like parts depicted in the figures. The invention will be described as embodied various ion source devices to contain, focus, and direct a plasma formed from one or more ionizable gases. The introduction of such one or more ionizable gases into an ion source device, and the formation and ignition of such a plasma is known to one of ordinary skill in the art. This being the case, for purposes of simplicity  FIGS. 5 ,  6 ,  7 ,  8 , and  9 , do not show an input for one or more ionizable gases or a plasma formed therefrom. 
       FIG. 5  is a section view of a closed drift ion source showing the magnetic fields of the preferred embodiment. The magnetic field across gap  520  is created by magnet shunt  510 , magnets  531  and  532 , pole pieces  540  and  550  and magnetic screen  590 . In this source, magnet shunt  510 , poles  540  and  550  and screen  590  are connected to the cathode. Anode  502  is inside the body of the source. The anode is positioned to cut electron trapping magnetic field lines. This arrangement is termed an anode layer ion source as are the sources shown in  FIGS. 1 ,  1 A,  2 ,  2 A,  3 ,  3 A, and  4 B. 
     This preferred embodiment uses a single, strong, symmetrical magnetic mirror field in gap  520  between poles  540  and  550 . In this case, the strong mirror field is created by the pointed shape of magnetic poles  540  and  550  and by shunts  580 ,  582  and  590 . The pointed shape concentrates the magnetic field from magnets  531  and  532  to create a large magnetic mirror field across the gap  520 . The shunts  580 ,  582  and  590  tend to accentuate the mirror field while also pulling magnetic field away to eliminate low mirror field lines. The result is a single, strong magnetic mirror field across gap  520 . 
     An analysis of the field strengths in this configuration show a primary field line  570  having a magnetic field strength of 5141 Gauss at end  572  disposed on central pole  550  and 4848 Gauss on second end  576  disposed on outer pole  560 . In the center of the gap  520  at position  574 , the primary field line has a minimum magnetic field strength of 1487 Gauss. This results in a mirror field ratio from  572  to  574  of 3.5 and a ratio from  576  to  574  of 3.3. Therefore the minimum magnetic mirror ratio for device  500  is in excess of 3:1. (These field strengths were obtained using Ceramic 8 magnets and carbon steel poles and shunt. The materials and absolute magnitudes are not critical. Rather, it is the relative magnitudes from the pole surface to the gap center along the central field line that is important. For instance, rare earth magnets could be used along with vanadium permador pole material to increase the magnitudes.) The strong mirror field produces a focusing effect on electrons trapped in the field. Instead of ranging between the containing pole surfaces, they are concentrated in the central gap region. 
     Not only is a strong mirror field important, but reducing regions of weak mirror fields where ionization occurs is also helpful. This is accomplished using two techniques in  FIG. 5 . First, magnetic shunt  590  pulls magnetic field from pole regions of weaker magnetic field, and, second, anode  502  is positioned to remove electrons from weaker magnetic field regions. Both these methods are effective in preventing high energy electrons from being trapped in regions of weak magnetic mirror fields. Magnetic shunts  580  and  582  have a reduced roll in accomplishing this. Because less electric field penetrates through the gap  520 , high energy electrons are less prevalent outside the source and less ionization occurs. However, if the gap width is increased, more E field moves outside the gap, and eliminating weak mirror fields outside the source becomes more important. 
     Note also that the magnet design and pole structure creates a relatively symmetrical magnetic mirror field between the two poles. As electrons gyrate along field lines, they are trapped into the center by both poles. In several prior art sources, a single magnet is used in the center region. As was shown in the analysis of these sources, this produces an unsymmetrical magnetic field in the gap. If a strong magnetic mirror on one pole is not matched along that field line by a similarly strong mirror field at the opposed pole, the mirror field is wasted. Electrons will be pushed away from the mirror pole and will escape to the wall of the poor mirror pole. Therefore, symmetrical strong mirror magnetic fields opposed to each other along the same primary field line is an important aspect of an improved ion source. Analyzing the magnetic fields in  FIG. 5 , the ratio of magnetic strengths at the poles, i.e. at ends  572  and  574 , is 1.06 showing a substantially symmetrical mirror field disposed within gap  520 . 
     Creating a single strong mirror field in the containment region and minimizing weak mirror fields has several benefits: 
     The high energy electrons are confined radially by the mirror field. Instead of only the longitudinal v X B confinement, radial confinement limits electron “conductance” to further compact and condense the electrons into the center of the gap. This produces a higher electron “pressure” in the central region improving efficiency of the source. 
     More ionization occurs in the center of the gap away from the pole surfaces. In this central region, the electric field tends to push the ions out of the source rather than toward the cathode poles. This further improves efficiency and reduces pole erosion. 
     In sources with insulating poles and weak mirror magnetic fields, a significant portion of electrons are lost to the walls without accomplishing ionization. With a strong mirror field, many electrons are reflected back as they approach the side wall. The stronger the mirror field, the larger the percentage of reflected electrons and the higher the source efficiency. 
     By minimizing regions of weak mirror field, pole erosion is reduced and source efficiency is increased. In regions of weak mirror field, electrons can more freely range between the containing surfaces. As ions are produced from electron collisions wherever high energy electrons are, ions are created more evenly throughout the physical containment region. When ions are created close to a side wall, they are more likely to “see” the side wall and be accelerated to it. Ion bombardment of the side walls causes side wall erosion and reduces source efficiency. 
     A strong mirror field in the gap also reduces source heating. Source heating is caused by both high energy electron wall losses and ion wall bombardment. The preferred embodiment reduces both of these. 
     By focusing electrons in the center of the gap and concentrating ionization there, more ions are ejected perpendicular to the racetrack closed loop. This results in a more efficient ion thruster or industrial ion source. 
     The preferred embodiment is also effective when these sources are operated in the plasma or diffuse mode. In the standard “ion beam” or collimated mode, the electric fields are not altered by a conductive plasma in the gap. This mode is maintained by operating at low pressures (˜less than 1 mTorr) or at lower powers. In the diffuse mode, sufficient plasma develops in the gap to produce a conductive plasma region and change the electric fields. This mode is often avoided because the earlier stated problems of source heating and side wall erosion are exacerbated. Focusing the plasma into the center of a single, strong mirror field helps to reduce pole erosion and increase efficiency in the diffuse mode. As in the collimated mode, the mirror field tends to confine electrons into the center of the gap. This confines the plasma toward the center producing the benefits as stated above. 
     Ions can also be affected by the preferred embodiment. When magnetic field strengths approach or exceed 1000 G, ions in the gap can become magnetized. That is, the radius of gyration of the ions is less than the size of the magnetic field. When magnetized, ions are also affected by a strong magnetic mirror field in the gap and, like electrons, are focused into the center of the gap. 
     Other important aspects of the preferred embodiment are: 
     The poles are shaped to focus the magnetic field to create a strong mirror at the pole. By shaping the high permeability poles, the magnetic field emanating from the pole can be made significantly stronger. This is an important design aspect that has been overlooked by prior art. As shown in  FIG. 5 , as the poles neck down toward the gap, the magnetic field tends to try to stay in the pole material. This progressively compresses the field and results in a strong mirror field at the end of the pole. Steel is used in the preferred embodiments shown because it has a relatively high permeability and high saturation level; it is inexpensive and easy to machine. More esoteric materials are available that are more permeable and saturate at higher levels than steels. Other magnet materials such as rare earth magnets, soft ferrite magnets or electromagnets can also be implemented. The material selection and choice of magnets will vary with the application, and the appropriate design will be evident to one skilled in the art. 
     Note: While water cooling is not shown in the figures, it is often required in industrial applications where high powers and continuous usage is the norm. One option is to gun drill the poles and directly flow water through them. In this case, a magnetic stainless steel such as grade  416  is a good choice. It does not corrode easily, is machinable, and has decent magnetic properties. 
     The regions  572  and  576  on the poles can be either sharp or rounded. A 0.03 inch radius is given to the poles in  FIG. 5 . While sharper points can provide higher surface magnetic fields and a larger central field mirror effect, the mirror effect is concentrated in a smaller region, enlarging the weaker mirror regions. Using a radius as shown produces a larger strong mirror field region. Also, magnetic saturation tends to lower the local sharp point effect reducing the effectiveness of sharply pointed poles. 
     The poles can take on a variety of shapes while still being in accordance with the preferred embodiment. For instance, the poles can be made from thin sheet metal or a combination of several metal sheets or plates. 
       FIG. 6  shows a section view of an extended acceleration channel ion source of a preferred embodiment. Again, a strong magnetic mirror field is produced in gap region  620  by magnetic shunt  610 , magnets  631  and  632  and poles  640  and  650 . Magnetic shunt  690  is extended downward to allow anode  602  to be placed further from the magnetic field. In this source, the magnetic poles are not connected to the source power supply. (They can be connected to a second bias supply if desired.) Electrons are supplied by source  606 . External magnetic shunts  680  and  682  reduce the external magnetic fields and help to concentrate the mirror field in the gap  620 . In this source, electrons leaving the emission source  606  are trapped in the gap by the magnetic field. By eliminating regions of weaker mirror fields, the circuit resistance is concentrated in the strong mirror region, and the voltage drop between the cathode  606  and anode  602  takes place wholly in this region. Again, high energy electrons are “focused” both longitudinally and radially into the center of the gap  620 , and a greater majority of the ions are produced in the center. All the benefits stated above are achieved with this source. 
       FIG. 7  shows a section view of one half of a symmetrical anode layer type source implementing a preferred embodiment. Magnetic field strengths at different locations are indicated to show that the magnetic field is concentrated effectively at the pointed pole regions  772  and  776  producing a minimum mirror field in the gap  720  in excess of 2:1. The values also show that further away from the pole points, the magnetic field strength diminishes quickly, and the mirror field becomes weaker. The magnetic field in gap  720  of source  700  is produced by steel back shunt  710 , ceramic magnets  731  and  732  and steel poles  740  and  750 . At pole end  742  the magnetic field strength is  4320  gauss. At pole end  752  the field is 4530 gauss. In the center  774  of gap  720  along primary field line  770  the field is 1420 gauss. This produces a minimum magnetic mirror of 3:1. The mirror field of source  700  is also relatively symmetrical with a symmetry ratio between poles  752  and  742  of 1.05. Away from the rounded pole end  742  on beveled surface  744  the magnetic field strength at  782  is 1320 gauss. Across the gap on field line  780  the field at  786  is 1520 gauss. At the center  784  of line  780  the field strength is 1040 gauss. Therefore, away from the pointed pole the mirror magnetic field is weaker, with a minimum ratio of 1.3:1. Rather than eliminating the weaker field regions with magnetic shunts as in sources  500  and  600 , in ion source  700  the anode  702  is placed to cut these weaker mirror field lines. In this position, the anode serves to collect electrons and eliminate ionization in the region of weak mirror field. In this source the magnetic poles  740  and  750  are connected to the cathode electrode. Non-magnetic housing  760  is also connected to the cathode. Housing  760  serves to present anode  702  with a uniform dark space. Insulators supporting anode  702  are not shown and are well know in the art. In this arrangement, the electric field is largely contained within the body of the source so the magnetic field lines external to the gap  720  have less affect on operation. 
     Note that the poles  740  and  750  of ion source  700  are shaped with beveled, sloping surfaces on both the internal  744 / 754  and external  743 / 753  sides. These bevels taper toward distill ends  742  and  752 . By shaping the poles accordingly, the primary field line  770  is readily made to emanate from the pole ends  742  and  752 . If the poles are beveled on only one side as shown in  FIGS. 3 and 3A , the primary field line does not emanate from the pole ends. Also, by beveling both inner and outer surfaces toward a point, the magnetic field is concentrated toward the point to help create a strong magnetic mirror field. Note that the point can be sharp or include a radius as described earlier. 
     In order to position anode  702  close to poles  740  and  750  to cut weak mirror magnetic field lines  780 , the top surface of anode  702  is raised and includes beveled surfaces  703  and  704 . By shaping the anode, anode  702  can be raised up between beveled poles  740  and  750 . 
     The term beveled is defined as a surface that is not orthogonal to the ion beam line  790 . For instance, beveled pole internal  744 / 754  and external  743 / 753  surfaces are non orthogonal to the ion beam  790  emanating out of source  700 . The term ‘internal’ is defined as the side of the pole ( 740 / 750 ) facing the anode  702 . The term ‘external’ is defined as the pole surface facing toward the process chamber and substrate. In prior art closed drift ion sources, most often the poles are of a rectangular shape, orthogonal to the beam line as in the prior art sources shown in  FIG. 4A and 4B . In some prior art sources (reference  FIG. 1 ,  1 A,  2 ,  2 A,  3  and  3 A) one surface is flat and orthogonal while the other is beveled. In the Applicants preferred embodiment both the inner and outer pole surfaces include at least one non-orthogonal beveled surface. This beveled, pointed pole structure can be constructed from a single pole piece or other methods such as stacking strips of metal to create a pointed pole. If stacks of ferromagnetic metal strips are used, the bevels will be stepped. While steps of excessive height are not preferred, stepped sloping poles remain within the inventive method. Pointed poles  740  and  750  may also be shaped using a large radius or some other curved shape. In experimentation, a simple radius without pointing the pole is not optimum and does not concentrate the magnetic field as well as a beveled, pointed pole. A compound pointed pole using sloping curves would however perform very well. This is not done due to the increased manufacturing difficulty. 
       FIG. 8  shows a detail view of one side of Applicant&#39;s closed loop ion source  800  having a wider gap between the magnetic poles. Analysis of the field strengths existing in device  800  shows that by widening the gap, the minimum magnetic mirror field ratio along the primary field line is increased. Primary field line  870  has a strength of 3535 Gauss at first end  872  disposed on surface  842 , a strength of 3535 Gauss at second end  876  disposed on surface  852 , and a minimum field strength of 685 Gauss at location  874 . 
     Location  874  is substantially equidistant between surface  842  and surface  852 . The minimum mirror field ratio of primary field line  870  is greater than 5:1. Primary field strength line  870  has an end-to-end ratio of 1 showing a symmetrical mirror field. 
     Formula (1) expresses the fraction, in percent, of trapped electrons to the mirror field ratio.
 
Fraction (%)=(1−( B   min   /B   max )) 1/2   (1)
 
Using device  800  with a mirror ratio of 5:1, the fraction of trapped electrons is about 89%.
 
       FIG. 9  diagrams another aspect of plasma containment relating to the inventive method. In this view, a conductive plasma  901  is shown in the gap  920 . The point of note is that while the plasma  901  is conductive, all regions of the plasma are not equally conductive. This is due to the changing magnetic fields within the plasma. Axially, the plasma “current” impedance is greater in the central region where the magnetic field is greatest. The larger impedance is due to the smaller gyro-radius in this region and the reduced electron mobility. Radially, with a strong magnetic mirror field achieved by the preferred pointed pole embodiment, the impedance of the plasma is greater closer to the poles. Changes in impedance, like current in a wire, results in associated voltage drops and therefore, while the plasma may be considered conductive, the voltage within the plasma varies. For instance, at the poles, since the impedance due to the mirror magnetic field is higher for electrons, fewer electrons will “flow” toward the poles. This leads to electron depletion near the pole and a more positive voltage near the pole within the plasma. The voltage reaches a steady state when enough electrons are attracted to region to balance the positive bias. The result is beneficial to ion source efficiency. The more positive voltage near the poles causes ions to be repelled back toward the center of the plasma. Axially, a similar effect is at work that produces a higher voltage in the center with the peak voltage at the magnetic field primary line. Here, the higher voltage pushes ions out of the central region. The combined effect is to produce a gradient field toward regions of lower magnetic field strength. With a strong magnetic mirror field present in the gap, this produces a beneficial focusing effect out of the source. 
     Applicant&#39;s ion sources, reduce the rate of erosion of the acceleration channel and/or pole surface material. As a result, several benefits are realized. For example, the life of the source is extended, less heat is generated in the source, the source is made more efficient, and less sputtered, contaminating material is ejected from the source. In addition, Applicant&#39;s ion sources collimate the ion beam exiting the source to produce a more focused, useful energy beam. 
     Applicant&#39;s ion sources reduce the wall losses of energetic electrons, particularly those capable of ionizing the source fuel. This further increases the efficiency of the source and reduces source heating. In addition, Applicant&#39;s ion sources improve the operation of extended acceleration channel ion sources and space based ion thrusters. 
     Applicant&#39;s ion sources further improve the operation of short acceleration channel sources termed anode layer sources, and improve the operation of anode layer type sources operated as plasma sources in the diffuse high current, low voltage mode. 
     While the preferred embodiments of the present invention have been illustrated in detail, it should be apparent that modifications and adaptations to those embodiments may occur to one skilled in the art without departing from the scope of the present invention as set forth in the following claims.