Low power, linear geometry hall plasma source with an open...

Details Low power, linear geometry hall plasma source with an open... Low power, linear geometry hall plasma source with an open...

2000-06-28

2003-02-25

Wong, Don (Department: 2821)

Electric lamp and discharge devices: systems

Discharge device load with fluent material supply to the...

Plasma generating

C315S111810, C315S111410, C313S231310, C250S42300F

Reexamination Certificate

active

06525480

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to plasma sources. More particularly, it relates to linear geometry Hall plasma sources.
BACKGROUND ART
Hall discharge plasma accelerators have been considered for use in satellite propulsion since the early 1960's (see C. O. Brown and E. A. Pinsley, AIAA J. 3, 853, 1965, and G. S. Janes and R. S. Lowder, Phys. Fluids 9, 1115, 19966). In a Hall plasma source, a low-pressure discharge is sustained within a bounded dielectric channel in crossed electric and magnetic fields. Electrons emitted from a cathode external to the channel, or created by the ionization processes, drift along the channel towards the anode located at the channel base. The anode also serves as the source of neutral propellant particles (typically xenon atoms). A radial component to the magnetic field is designed to be a maximum near the channel exit, and in this region the electrons become highly magnetized, as the classical electron Hall parameter is much greater than unity.
FIG. 1A
is a cross-sectional schematic representation of a conventional Hall discharge plasma source
10
, having a coaxial geometry rotationally symmetric about an axis
104
.
FIG. 1B
is an axial view of conventional Hall discharge plasma source
10
taken along axis
104
of FIG.
1
A. Coaxial Hall discharge plasma source
10
has an annular channel
102
, within which surrounding solenoids
106
,
108
generate a radial magnetic field B. An electric field E is generated in an axial direction
122
within annular channel
102
between an anode
110
at the base
114
of annular channel
102
and a cathode
112
external to annular channel
102
. In this coaxial configuration, electrons are constrained to move in the azimuthal direction (indicated by arrows
152
) of a closed E×B drift, with cross-field drift providing the necessary electron current to sustain the discharge. As the electron Hall parameter (the product of the electron cyclotron frequency, &ohgr;
ce
=eB/m
e
, and the mean time between electron collisions &tgr;, where e is the electron charge, me is the electron mass, and B is the radial magnetic field), is much greater than unity, the Hall current density (the product of the electron charge e, the local number of electrons per unit volume n
e
, and the electron Hall drift velocity V
ed
=E/B, where E is the axial electric field) can be many orders of magnitude greater than the axial current density (the Hall current density divided by the Hall parameter). According to classical electron transport theory, electrons can circle annular channel
102
in the Hall (azimuthal) direction
152
many times before being captured at anode
110
. Anode
110
also serves as the source of neutral propellant particles
111
(typically xenon atoms). A coaxial geometry allows for a “closed” electron drift in the Hall direction, and uninterrupted Hall current. The region of electrons thus trapped acts as a volumetric zone of ionization
113
that in some devices may occupy only a small fraction of the overall channel depth D. Ions
120
are generated in the volumetric zone of ionization
113
by collisions of neutral propellant particles
111
with the trapped electrons. Ions
120
, substantially unaffected by magnetic field B because of their large inertia, are accelerated by electric field E resulting from the impeded electron flow (the resistance to the flow of electrons as a result of the applied magnetic field), producing thrust. Accelerated ions
120
recombine with available electrons in the region external to channel
102
to provide a source of high thrust neutral particles
121
. Very high ionization fractions and ion velocities can be generated with these discharges. Accordingly, due to their high efficiencies and high specific impulse (the resulting ion velocity divided by the gravity constant 9.8 m/s
2
), coaxial Hall discharge plasma sources
10
in the 1-5 kW power range are being evaluated as plasma thrusters for use on commercial, military, and research spacecraft (see F. S. Gulczinski and R. A. Spores, “Analysis of Hall-Effect Thrusters and Ion Engines for Orbit Transfer Missions,” AIAA-96-2973, 32
nd
Joint Propulsion Conference, Jul. 1-3, 1996, Lake Buena Vista, Fla.).
A precise theory is lacking for the mechanism of cross-field electron transport in Hall plasma thrusters. Early experiments on Hall plasma sources indicated that classical electron transport theory could not account for the measured “anomalous” axial (cross-field) electron current densities. Janes and Lowder (cited above) drew attention to the presence of density and electric field fluctuations within the channel of a Hall discharge, and first suggested that these plasma disturbances enhance the axial electron current. Indirect measurements of the “effective” Hall parameter as a result of these fluctuations were in agreement with the anomalous transport coefficient first identified by Bohm et al. (see D. Bohm, in The
Characteristics of Electrical Discharges in Macnetic Fields
, A. Guthrie and R. K. Wakerling, Eds., McGraw Hill, N.Y., 1949) which characterizes the process now widely recognized as “anomalous” Bohm diffusion (see F. F. Chen,
Plasma Physics and Controlled Fusion
, 2
nd
Edition, Plenum Press, NY, p. 193, 1985). The Bohm mechanism predicts an electron mobility that scales inversely with the magnetic field strength (as opposed to the classical B
−2
scaling), and an effective electron Hall parameter of about 16. At conditions typical of coaxial Hall plasma thrusters near the region where the magnetic field is strongest, the classical Hall parameter is about 500-1000. A value of 16 represents a significant enhancement in the cross-field drift, and indicates that the ratio of Hall current density to axial current density may be much less than that suggested by classical transport theory.
Whereas an enhanced electron current due to fluctuations is one possible mechanism for enhanced electron transport, the operation of modern Hall plasma thrusters seems to depend significantly on the properties of dielectric channel walls
116
(see Raitses et al., cited above). Previous researchers have proposed the possibility of an enhanced “near-wall conductivity” due to the “wall scattering” of electrons. Whereas it seems that precise knowledge of which mechanism is responsible for transport is necessary to properly scale a Hall discharge is lacking, it is shown below that either of these mechanisms exhibits the necessary dependency on discharge parameters to achieve a desired scaling in discharge size or power.
Modern coaxial Hall plasma thrusters
10
that operate in the 1-5 kW power range have been shown to operate with very high thrust efficiencies in the range of approximately 50%. These thrusters have annular acceleration channel diameters
2
R ranging from 50 to 280 mm. One feature common to these thrusters is that channel width W is approximately 15% of channel diameter
2
R, which itself is about twice the acceleration channel depth D. In scaling these discharges to operate at various power ranges, it is often desirable to preserve a geometrical relationship between channel width W, diameter
2
R, and depth D, although the physical basis for the commonly used geometrical relationships is not well understood.
In a coaxial Hall thruster
10
, the magnetic field B near a channel exit face
118
is sufficient to trap the electrons in an orbital cyclotron motion
130
, in a plane orthogonal to magnetic field B. The electron orbit radius r
e
(“Larmor radius”) is generally smaller than the electron mean free path &lgr; and the acceleration channel width W. In this way, the electrons are confined to the magnetized portion of the plasma discharge. The Larmor radius, being dependent on particle mass, is much larger for ions, which are substantially unaffected by the magnetic field. The electron Larmor radius, r
e
, scales as:
r
e
∼
T
e
1
/
2
B
.
(
1
)
Here B is the magnetic field strength and T
e
is the electron temperature. In the design of a low power (and he

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