Radiant energy – Ion generation – Field ionization type
Reexamination Certificate
1999-02-17
2001-04-10
Anderson, Bruce C. (Department: 2881)
Radiant energy
Ion generation
Field ionization type
C315S111210, C315S111810, C315S501000, C315S111610, C060S202000
Reexamination Certificate
active
06215124
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a system for “shaping” the magnetic and electric fields in an ion accelerator with closed drift of electrons, i.e., a system for controlling the contour of the magnetic and electric field lines and the strengths of the magnetic and electric fields in a direction longitudinally of the accelerator, particularly in the area of the ion exit end.
BACKGROUND OF THE INVENTION
Ion accelerators with closed electron drift, also known as “Hall effect thrusters” (HETs), have been used as a source of directed ions for plasma assisted manufacturing and for spacecraft propulsion. Representative space applications are: (1) orbit changes of spacecraft from one altitude or inclination to another; (2) atmospheric drag compensation; and (3) “stationkeeping” where propulsion is used to counteract the natural drift of orbital position due to effects such as solar wind and the passage of the moon. HETs generate thrust by supplying a propellant gas to an annular gas discharge area. Such area has a closed end which includes an anode and an open end through which the gas is discharged. Free electrons are introduced into the area of the exit end from a cathode. The electrons are induced to drift circumferentially in the annular discharge area by a generally radially extending magnetic field in combination with a longitudinal electric field. The electrons collide with the propellant gas atoms, creating ions which are accelerated outward due to the longitudinal electric field. Reaction force is thereby generated to propel the spacecraft.
It has long been known that the longitudinal gradient of magnetic flux strength has an important influence on operational parameters of HETs, such as the presence or absence of turbulent oscillations, interactions between the ion stream and walls of the thruster, beam focusing and/or divergence, and so on. Such effects have been studied for a long time. See, for example, Morozov et al., “Plasma Accelerator With Closed Electron Drift and Extended Acceleration Zone,”
Soviet Physics-Technical Physics
, Vol. 17, No. 1, pages 38-45 (July 1972); and Morozov et al., “Effect of the Magnetic Field on a Closed-Electron-Drift Accelerator,”
Soviet Physics-Technical Physics
, Vol. 17, No. 3, pages 482-487 (September 1972). The work of Professor Morozov and his colleagues has been generally accepted as establishing the benefits of providing a radial magnetic field with increasing strength from the anode toward the exit end of the accelerator. For example, H. R. Kaufman in his article “Technology of Closed-Drift Thrusters,”
AIAA Journal
, Vol. 23, No. 1, pages 78-87 (July 1983), characterizes the work of Morozov et al. as follows:
The efficiency of a long acceleration channel thus is improved by concentrating more of the total magnetic field near the exhaust plane, in effect making the channel shorter. Another interpretation, perhaps equivalent, is that ions produced in the upstream portion of a long channel have little chance of escape without striking the channel walls. Concentration of the magnetic field at the upstream end of the channel therefore should be expected to concentrate ion production further upstream, thereby decreasing the electrical efficiency.
Id. at 82-83. For experimental purposes, Morozov et al. achieved different profiles for the radial magnetic field by controlling the current to coils of separate electromagnets. For a given magnetic source (electromagnet or permanent magnets), other ways to affect the profile of the magnetic field are configuring the physical parameters of magnetic-permeable elements in the magnetic path (such as positioning and concentrating magnetic-permeable elements at the exit end of the accelerator), and by magnetic “screening” or shunts which can be interposed between the source(s) of the magnetic field and areas where less field strength is desired, such as near the anode. For example, in their paper titled “Effect of the Characteristics of a Magnetic Field on the Parameters of an Ion Current at the Output of an Accelerator with Closed Electron Drift,”
Sov. Phys. Tech. Phys.
, Vol. 26, No. 4 (April 1981), Gavryushin and Kim describe altering the longitudinal gradient of the magnetic field intensity by varying the degree of screening of the accelerator channel. Their conclusion was that magnetic field characteristics in the accelerator channel have a significant impact on the divergence of the ion plasma stream.
There does not appear to be any current dispute that the longitudinal gradient of magnetic field strength in HETs is important, and that it is desirable to concentrate or intensify the magnetic field at or adjacent to the exit plane as compared to the magnetic field strength farther upstream.
SUMMARY OF THE INVENTION
The present invention provides an improved system for magnetic flux shaping in an ion accelerator with closed electron drift (Hall effect thruster or HET). A specially designed magnetic shunt called a “flux bypass cage” is provided encircling the anode region and/or annular gas distribution area of the thruster at both the inside cylindrical wall and outside cylindrical wall. The circumferential sides of the flux bypass cage are connected behind the anode. Initially, the cage was formed by a solid walled, U-shaped cross section body of revolution, with the inner and outer sides encompassing substantially all of the anode region of the thruster. This construction was shown to be effective to steepen the axial gradient of the magnetic field strength and move the zone where ions are created downstream, as confirmed by measurement of the erosion profile of ceramic insulators adjacent to the exit end of the thruster. In accordance with one aspect of the present invention, however, the flux cage has large openings in the inner and outer circumferential sides. The open areas can constitute the major portion of both the outer and inner circumferential sides, hence the term “cage.” The flux bypass cage then resembles circumferentially spaced, longitudinally extending side bars connecting rings at the closed end (behind the anode) and rings at the exit end. With this construction, it has been found that desired profiles for the magnetic field can be achieved with substantially less total magnetic coercive force being required. Therefore, electromagnets can have fewer ampere-turns, as well as lighter cores and structural supports, and the reduction in weight lessens structural support requirements for the thruster itself. For systems using permanent magnets, smaller, lighter magnets can be used. Another feature of the cage design is that it gives the designer control over the shape of the magnetic field vectors in the ion discharge area. For example, a solid walled shunt can create lines of equipotential at steep angles relative to the centerline of the discharge area. The result is that the ion beam can be “over focused,” i.e., have ions at the inner and outer sides directed more toward the mid-channel centerline than is desired for greatest efficiency. Large open areas in the cage also permit radiative cooling of the thruster, reducing or eliminating the need for heavy thermal shunts to conduct heat away from the core of the thruster.
In another aspect of the invention, the magnet poles at the exit end of the HET are coated with insulative material, which further enhances the magnetic field shaping for greater efficiency and longer life. In another aspect of the invention, bias electrodes are added to the insulated magnetic pole faces. The electrodes can be conductive rings on the exposed surface of the insulated outer pole face and the exposed surface of the insulated inner pole face. The electrodes are biased to specific voltages, to assist in shaping the magnetic field and/or effect additional acceleration of ions.
In another aspect of the invention, the anode is formed with electrically conductive walls and a rear gas plenum having a porous outlet plate closely adjacent to the exit end of the thruster. The anode walls and/or porous part of the gas distribution sy
Anderson Bruce C.
Christensen O'Connor Johnson & Kindness PLLC
Primex Aerospace Company
Wells Nikita
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