Patent Application
20090134804
1. Field of the Invention
The present invention is directed generally to a closed drift Hall type accelerator in a vacuum space.
2. Description of related art
The present invention attempts to achieve a high energy ion beam source which is superior to existing accelerators in beam density and energy. Hall Current accelerators operate without space charge limitations as are present in conventional ion beam accelerators, which use electrostatic lens (grids) to extract ions from a neutral plasma (gridded ion sources). Such devices can have very good optics. That is they can have a very well defined velocity vector and energy spread.
In Hall accelerators ion acceleration is achieved by providing a voltage potential between an anode associated with a neutral gas source and a cathode in the general vicinity of the beam exit. Electrons from the cathode migrate through the Hall effect fields, which pick up the electrons in E×B azimuthal orbits, restraining their axial transit. This allows electrons to accumulate in the Hall magnetic fields. The neutral gas from the anode structure encounters the counter-streaming electrons and becomes electron impact ionized, thereby forming ions that are accelerated by the electric field between the anode and the cathode. The ions are never separated from the electrons as in electrostatic extraction. An accelerating electric gradient is established between a virtual cathode formed by the electrons captured in Hall effect orbits and the anode. Closed drift Hall effect accelerators are generally defined as having ion gyro-radii much greater than the acceleration channel width and electron gyro-radii generally somewhat less than the channel width. Thus the ions go through minor azimuthal deviation in the acceleration Hall effect field. Most Hall effect devices have been of the single stage type, where the neutral gas is introduced into the ionization and acceleration region from within the Hall effect magnetic field structure. Therefore the ions are accelerated only through the exit cusp field of the solenoid. Transport through the cusp field generally imparts azimuthal momentum to the ions. Two stage accelerators have been studied in an attempt to reduce the divergence of the beam because the azimuthal thrust imparted to the ions in the first cusp field is countered by the second cusp field which is in the opposite direction and therefore imparts opposite azimuthal thrust to the ions. Two stage accelerators contain inner and outer pole pieces that generate the two alternately directed radial cusp fields.
To inject ions into the end of a solenoid, they must be transported across the cusp (fringe) magnetic field. Axial ion injection has been most frequently attempted by means of magnetically insulated pulsed type ion diodes. The ions generated by such devices are then typically transported across a full cusp, not a half cusp. A full cusp exists between two oppositely aligned solenoids. Ion transport across a full cusp results in large canonical angular momentum orbits that do not approach the magnetic field axis but rather encircle the axis. The prior art ion injection methods are limited by issues of ion trapping as well as space charge neutralization. Generally trapping schemes involve fast magnetic field ramping or a change in the charge to mass ratio by molecular disassociation. Prior art methods for space charge neutralization typically involve the pre-introduction of background plasma because electrons are stripped from the beam as the ions transport across the field.
The present invention is distinguished from previous methods of cross field transport (cusp injection) into a solenoid field in that the present invention actively accelerates ions across the cusp field, utilizing the cusp field itself as a Hall effect ion acceleration field. Because the accelerator operates on the Hall effect principle electrons are present everywhere, mitigating space charge issues, both in the cusp and in the solenoid. Additionally, since the ions are accelerated into the field they will be decelerated if they attempt to escape, and ideally returned by the initial process. Hall acceleration occurs across the cusp field. When the electron source cathode is located outside of the solenoid, beyond the distal (exit) cusp, then both cusp fields serve as Hall effect fields and the ions are accelerated out of the distal end of the solenoid. The azimuthal momentum imparted by the first (entrance) cusp field is countered by the second (exit) cusp field, returning the original axial trajectory to the ions. The ions will have gained energy by acceleration across both fields.
The present invention is further distinguished from the prior art in that the ion gyro-radii is generally equal to the width of the acceleration channel. The ions are made to undergo a generally 90 degree angle as they pass through the Hall effect field. The initial axial trajectory is bent into azimuthal during the entrance acceleration process. Within the solenoid the ion orbit should have very low canonical angular momentum. An ion with low canonical angular momentum, passes close to or crosses the magnetic field axis. This much greater ion bending is made possible because the inner pole piece has been removed and the ions are free to gyro-rotate too the axis of the magnetic field once they have entered the solenoid.
In a present embodiment the ions should have low, desirably zero, values of canonical angular momentum. An ion that transports across one end cusp of a solenoid can have zero canonical angular momentum if the ion was originally traveling parallel to the magnetic field axis. Zero canonical angular momentum orbits are characterized by having contact with the magnetic field axis once each orbit. The apparatus is preferably designed such that the ions exit the solenoid at a point in the solenoid where the ions are on axis. Zero canonical angular momentum ions will exit the solenoid with maximum axial momentum, and minimum radial momentum. Thus a well collimated beam emerges and the initial annular distribution of ions is combined into a small cross section central axis ion beam. Hall Current accelerators are of annular design because of the need for radial magnetic field lines to restrain the electrons in closed drift Hall Current orbits capable of producing the accelerating electric field gradient. The Hall effect electrons also provide ionization to the neutral gas.
The present invention is further distinguished from the prior art in that an annular gas valve is implemented such that the gas entering the radial Hall effect cusp magnetic field is a cylindrical sheath of collimated neutral gas, thereby minimizing any non-axial momentum particles. Said neutral gas particles encounter the radial Hall effect field traveling parallel to the axis but not on axis, as is required for a Hall accelerator. The annular well collimated axial neutral gas sheath is formed into a stream of spiraling ions in the solenoid.
The present invention is further distinguished from the prior art by its ability to combine the annular geometry Hall effect beam into a single central beam in the exit cusp field. This is possible because there is no inner pole piece. The beam collimation is determined by the collimation of injected neutral gas.
The following references illustrate the prior art with regard to Hall Current accelerators. Raitses et.al. U.S. Pat. No. 6,448,721 reveals an example of the progression of Hall accelerators from the common annular design towards a reduction of the inner electrode with their cylindrical geometry Hall accelerator, the design references a single stage Hall accelerator. Fisch et.al U.S. Pat. No. 6,777,862 discloses a segmented electrode Hall thruster with reduced plume, addressing the importance of reducing plume divergence. Mahoney et.al. U.S. Pat. Nos. 5,973,447 and 6,086,962 discloses a gridless Hall effect ion source for the vacuum processing of materials. I. P. Zubkov et.al. in OPTIMIZATION OF A HIGH CURRENT ION ACCELERATOR, Soviet Physics—Technical Physics, Vol. 17, No. 2 October 1972, reveals how a two stage Hall accelerator has an appreciably reduced angular beam divergence. Kornfeld et.al. in U.S. Pat. Nos. 6,523,338 and 7,075,095 discloses plasma accelerators using multi-acceleration stages. Cann U.S. Pat. Nos. 3,309,873 and 3,243,954 discloses a plasma accelerator utilizing a Laval type nozzle and Cann U.S. Pat. No. 3,388,291 discloses an annular array of multiple collimating Anode gas sources, but not an annular structure. Kapetanakos U.S. Pat. No. 4,293,794 reveals a method of pulsed full cusp cross field transport of ions into a solenoid field, with a half cusp beam exit. Maglich U.S. Pat. No. 4,788,024 reveals a high energy, low current injection that attempts to achieve zero canonical angular momentum orbits. J. R. Pierce U.S. Pat. No. 2,847,607 discloses a beam focusing apparatus. Erwin Becker U.S. Pat. No. 3,628,342 reveals a method for fluid gas separation utilizing an annular nozzle. Kaufman et. al. US2002/0163289 A1 discloses a single stage Hall effect accelerator. C. E. Berry U.S. Pat. No. 2,672,560 reveals an annular ionization chamber. Jassby et. al. reveals a particle beam injection system. Wells U.S. Pat. No. 4,267,488 discloses a system for forming and compressing plasma in axis encircling ringlike toroidal plasma vortex structures. H. C. cole in A HIGH CURRENT HALL ACCELERATOR, Nuclear Fusion, 10, 1970 reveals a high current two stage Hall accelerator. I. P. Zubkov in EXPERIMENTAL STUDY OF A TWO-LENS ACCELERATOR, Soviet Physics—Technical Physics, Vol. 15, No. 11, May 1971 discloses a high current closed drift Hall accelerator.
In general, the invention is directed toward an ion accelerator comprising a collimating gas source and an anode external of a solenoid for producing a magnetic field and an electron source cathode beyond the proximal (entrance) cusp, contained in a vacuum space. An embodiment has a proximal entrance end with an annular vacuum gap adapted for a half cusp radial magnetic field in the vicinity of the annular vacuum gap. The annular collimating gas source is adapted for directing gas past an anode electron extraction apparatus associated with a source of electric power. The collimated gas is directed into the annular vacuum gap wherein a half cusp radial magnetic field forms a Hall effect field in conjunction with said anode and a cathode electron source. An annular ion beam is formed from the neutral gas in the vicinity of the Hall effect electrons. The ion beam is accelerated into the solenoid by the electric field established between the anode and the cathode.
In an embodiment ions exit the distal end of the solenoid through the distal exit end cusp field. Since the ions are accelerated by a Hall Effect Current, space charge issues are mitigated by the Hall effect electrons. The ions exit at a radius and with a collimation that is length and phase coherent dependent.
Intermediate collimating gas throats may be introduced between the gas source and the entrance cusp field.
Intermediate pre-ionization Hall fields of much lower magnetic field, similar to prior art Hall accelerator fields, may be implemented between the Solenoid proximal entrance cusp and the gas source. Said intermediate Hall effect fields may be created by a pair of permanent magnets or by sets of coil windings producing a generally radial magnetic field across the annular cylinder defined by the gas sheath. Where permanent magnets are implemented they would have opposite (attractive) radial fields facing one another. Where electromagnet coils are implemented they incorporate radial pairs that carry currents such that a generally and predominantly radial magnetic field is present between the coil pairs in the general vicinity of the gas sheath channel produced by the collimation nozzles. Typically such magnetic field structures incorporate hi-mu yoke structures.
Intermediate electrodes may also be implemented in the vacuum space between the cathode and the Anode. They may be associated with said intermediate Hall field magnets or they may be associated with the collimation throats or separate from either or both, either running along the inner aspect of the gas sheath or on the outer aspect or a combination of the two.
An electrical bias placed between a pair of intermediate electrodes implemented on opposite sides of the gas sheath and in conjunction with an intermediate magnet Hall effect field, can serve to separate the Anode from the gas source by sweeping the electrons out of the channel prior to encountering the gas source. This novel composition of elements makes possible the separation of the Anode and the neutral gas source structure. Thereby protecting the gas source structure from bombardment by the counter-streaming Hall effect electron current. This useful feature is herein referred to as a gas free anode.
Because the gas provided to the accelerator is collimated and therefore restrained in its trajectory to only the channel of acceleration the intermediate Hall field and acceleration electrodes may be suspended in the free space within the vacuum vessel. This is not possible in a typical Hall effect accelerator where the coaxial channel must direct the gas flow which in the prior art is not collimated and therefore is traveling generally in many directions.
Characteristics of the ion orbits and trajectories are established by the following system parameters:
There has thus been described a novel closed drift Hall type accelerator. It is important to note that many configurations can be constructed from the ideas presented. The foregoing disclosure and description of the invention is illustrative and explanatory thereof and thus, nothing in the specification should be imported to limit the scope of the claims. Also, the scope of the invention is not intended to be limited to those embodiments described and includes equivalents thereto. It would be recognized by one skilled in the art the following claims would encompass a number of embodiments of the invention disclosed and claimed herein.
