Segmented electrode hall thruster with reduced plume

Details Segmented electrode hall thruster with reduced plume Segmented electrode hall thruster with reduced plume

2001-04-13

2004-08-17

Patel, Ashok (Department: 2879)

Electric lamp and discharge devices

With positive or negative ion acceleration

C313S361100, C313S362100, C313S363100, C315S501000

Reexamination Certificate

active

06777862

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention pertains generally to electric plasma thrusters and more particularly to Hall field thrusters, which are sometimes called Hall accelerators.
The Hall plasma accelerator is an electrical discharge device in which a plasma jet is accelerated by a combined operation of axial electric and magnetic fields applied in a coaxial channel. The conventional Hall thruster overcomes the current limitation inherent in ion diodes by using neutralized plasma, while at the same time employing radial magnetic fields strong enough to inhibit the electron flow, but not the ion flow. Thus, the space charge limitation is overcome, but the electron current does draw power. Hall thrusters are about 50% efficient. Hall accelerators do provide high jet velocities, in the range of 10 km/s to 20 km/s, with larger current densities, about 0.1 A/cm
2
, than can conventional ion sources.
Hall plasma thrusters for satellite station keeping were developed, studied and evaluated extensively for xenon gas propellant and jet velocities in the range of about 15 km/s, which requires a discharge voltage of about 300 V. Hall thrusters have been developed for input power levels in the general range of 0.5 kW to 10 kW. While all Hall thrusters retain the same basic design, the specific details of an optimized design of Hall accelerators vary with the nominal operating parameters, such as the working gas, the gas flow rate and the discharge voltage. The design parameters subject to variation include the channel geometry, the material, and the magnetic field distribution.
A. V. Zharinov and Yu. S. Popov, “Acceleration of plasma by a closed Hall current”, Sov. Phys. Tech. Phys. 12, 1967, pp. 208-211 describe ideas on ion acceleration in crossed electric and magnetic field, which date back to the 1950's. The first publications on Hall thrusters appeared in the United States in the 1960's, such as: G. R. Seikel and F. Reshotko, “Hall Current Ion Accelerator”, Bulletin of the American Physical Society, II (7) (1962) and C. O. Brown and E. A. Pinsley, “Further Experimental Investigations of Cesium Hall-Current Accelerator”, AIAA Journal, V.3, No 5, pp. 853-859, 1965.
Over the last thirty years, A. I. Morozov designed a series of high-efficiency Hall thrusters. See, for example, A. I. Morozov et al., “Effect of the Magnetic field on a Closed-Electron-Drift Accelerator”, Sov. Phys. Tech. Phys. 17(3), pp. 482-487 (1972), A. I. Morosov, “Physical Principles of Cosmic Jet Propulsion”, Atomizdat, Vol. 1, Moscow 1978, pp. 13-15, and A. I. Morozov and S. V. Lebedev, “Plasma Optics”, in Reviews of Plasma Physics, Ed. by M. A. Leontovich, V.8, New York-London (1980).
H. R. Kaufman, “Technology of Closed Drift Thrusters”, AIAA Journal Vol. 23 p. 71 (1983), reviews of the technology of Hall field thrusters, both in the context of other closed electron drift thrusters and in the context of other means of thrusting plasma. V. V. Zhurin et al., “Physics of Closed Drift Thrusters”, Plasma Sources Science Technology Vol. 8, p. R1 (1999), further reviews the physics and more recent developments in the technology of Hall thrusters.
What remains a challenge is to develop a Hall thruster able to operate efficiently with minimal plume divergence. What is a further challenge is to accomplish such operation with the same thruster in several parameter regimes, such as at different input powers or at varying output thrusts. A number of issues arise with such variable operation of Hall current accelerators. These issues include decreased thruster efficiency for low mass flow rate and for low discharge voltages. At lower mass flow rates, lower atomic density in the channel results in an increased ionization mean free path of propellant atoms. A longer ionization length reduces the ionization efficiency and increases ion losses in the channel. Moreover, an extended ionization region produces a spread of ion energies, including slow ions. These slow ions are particularly vulnerable to radial accelerations and so contribute importantly to the plume divergence. This is a crucial issue even for non-variable operation. A similar effect would be incurred through the use of not easily ionized gases.
The present invention comprises an improvement over the prior art cited above by providing for efficient operation, with decreased plume divergence, and with capability for variable operation. The present invention discloses means of accomplishing these objectives through the placement of segmented electrodes along the inner and outer channel walls with the electrode segments held at specific potentials that lead to the improved operation.
The present invention comprises an improvement as well as over the following prior art:
U.S. Pat. No. 4,862,032 (“End-Hall ion source”, Kaufman et al., Aug. 29, 1989) discloses specifically that the magnetic field strength decreases in the direction from the anode to the cathode. The disclosure of the above referenced patent is hereby incorporated by reference.
Other design suggestions are disclosed in U.S. Pat. No. 5,218,271 (“Plasma accelerator with closed electron drift”, V. V. Egorov et al., Jun. 8, 1993) which contemplates a curved outlet passage. The disclosure of the above referenced patent is hereby incorporated by reference. U.S. Pat. No. 5,359,258 (“Plasma accelerator with closed electron drift”, Arkhipov et al., Oct. 25, 1994) contemplates improvements in magnetic source design by adding internal and external magnetic screens made of magnetic permeable material between the discharge chamber and the internal and external sources of magnetic field. The disclosure of the above referenced patent is hereby incorporated by reference.
U.S. Pat. No. 5,475,354 (“Plasma accelerator of short length with closed electron drift”, Valentian et al., Dec. 12, 1995) contemplates a multiplicity of magnetic sources producing a region of concave magnetic field near the acceleration zone in order better to focus the ions. The disclosure of the above referenced patent is hereby incorporated by reference. U.S. Pat. No. 5,581,155 (“Plasma accelerator with closed electron drift”, Morozov, et al., Dec. 3, 1996) similarly contemplates specific design optimizations of the conventional Hall thruster design, through specific design of the magnetic field and through the introduction of a buffer chamber. The disclosure of the above referenced patent is hereby incorporated by reference.
U.S. Pat. No. 5,763,989 (“Closed drift ion source with improved magnetic field”, H. R. Kaufman Jun. 9, 1998) contemplates the use of a magnetically permeable insert in the closed drift region together with an effectively single source of magnetic field to facilitate the generation of a well-defined and localized magnetic field, while, at the same time, permitting the placement of that magnetic field source at a location well removed from the hot discharge region. The disclosure of the above referenced patent is hereby incorporated by reference. U.S. Pat. No. 6,075,321 (“Hall field plasma accelerator with an inner and outer anode”, V. J. Hruby, Jun. 13, 2000) contemplates an anode that can be part of either the inner or outer walls, rather than simply part of an inlet wall, but not a series of segmented electrodes for detailed control of the axial potential. The disclosure of the above referenced patent is hereby incorporated by reference.
U.S. Pat. No. 5,847,493 (“Hall effect plasma accelerator”, Yashnov et al., Dec. 8, 1998) proposes that the magnetic poles in an otherwise conventional Hall thruster be defined on bodies of material which are magnetically separate. The disclosure of the above referenced patent is hereby incorporated by reference.
U.S. Pat. No. 5,845,880 (“Hall effect plasma thruster”, Petrosov et al., Dec. 8, 1998) proposes a channel preferably flared outwardly at its open end so as to avoid erosion. The disclosure of the above referenced patent is hereby incorporated by reference.
The closest configuration in the literature to the present invention appears to be Russian Patent SU 1796777 A1 (

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