Compact plasma accelerator

Details Compact plasma accelerator Compact plasma accelerator

2002-08-08

2004-02-24

Lee, Wilson (Department: 2821)

Electric lamp and discharge devices: systems

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

Plasma generating

C315S111410

Reexamination Certificate

active

06696792

ABSTRACT:

ORIGIN OF THE INVENTION
The invention described herein was made by an employee of the United States Government and may be manufactured and used by or for the Government for Government purposes without the payment of any royalties thereon or therefor.
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a plasma generator and accelerator and, more particularly to a low power, compact plasma accelerator that can be used for satellite propulsion, drag reduction and station-keeping, or for ion plasma material processing in a vacuum.
BACKGROUND OF THE INVENTION
There is a need for a simple, low power, light-weight, compact, high specific impulse electric propulsion device to satisfy mission requirements for micro and nano-satellite class missions. Satisfying these requirements entails addressing the general problem of generating a sufficiently dense plasma within a relatively small volume and then accelerating it in a way that generates a net thrust reaction force in a desired linear direction. Known means for ion generation and propulsion generally require relatively large containment volumes in order to achieve reasonable ionization efficiencies, therefore new means are needed in order to achieve effective scaled-down propulsion devices.
Recent prior art electric propulsion devices and plasma accelerators are commonly some form of Hall effect thrusters (Hall accelerators or Hall engines). A conventional Hall effect thruster generally comprises an accelerating channel arranged along an axis with an anode and a propellant source at a first, generally closed, end of the channel, and a cathode (electron source) at a second, generally open, end of the channel. The cathode and anode establish an electric field with a gradient generally aligned with the axis of the channel. A system of magnets is arranged so that a magnetic field crosses the channel.
To continue the description of the Hall effect thruster, an exemplary thruster is presented comprising an annular accelerating channel extending circumferentially around the axis of the thruster and also extending in an axial direction from a closed end to an open end. The anode is usually located at the closed end of the channel, and the cathode is positioned outside the channel close to its open end. Means is provided for introducing a propellant, for example xenon gas, into the channel and this is often done through passages formed in the anode itself or close to the anode. A magnetic system applies a magnetic field in the radial direction across the channel and this causes electrons emitted from the cathode to move circumferentially around the channel. Some but not all of the electrons emitted from the cathode pass into the channel and are attracted down the electric field gradient towards the anode. The radial magnetic field deflects the electrons in a circumferential direction so that they move in a spiral trajectory, accumulating energy as they gradually drift towards the anode. In a region close to the anode the electrons, collide with atoms of the propellant, causing ionization. The resulting positively charged ions are accelerated by the electric field towards the open end of the channel, from which they are expelled at great velocity, thereby producing the desired thrust. Because the ions have a much greater mass than the electrons, they are not so readily influenced by the magnetic field and their direction of acceleration is therefore primarily axial rather than circumferential with respect to the channel. The ion stream is at least partially neutralized by those electrons from the cathode that do not pass into the channel.
Conventionally, the required radial magnetic field has been applied across the channel using an electromagnet having a yoke of magnetic material which defines poles on opposite sides of the channel, i.e. one radially inwardly with respect to the channel and the other radially outwardly with respect to the channel. An example is shown in European patent specification 0 463 408 which shows a magnetic yoke having a single cylindrical portion passing through the middle of the annular channel and carrying a single magnetizing coil; and a number of outer cylindrical members spaced around the outside of the accelerating channel and carrying their own outer coils. The inner and outer cylindrical members are bolted to a magnetic back plate so as to form a single magnetic yoke.
A recent example of the Hall effect thruster is disclosed in U.S. Pat. No. 5,847,493 (Yashnov, et al.; 1998) entitled “Hall Effect Plasma Accelerator”. The described invention in the U.S. Pat. No. 5,847,493 Patent comprises the use of magnets (permanent or preferably electric) wherein the magnetic poles are defined on bodies of material which are magnetically separate in order to allow greater freedom in selecting the dimensions of the thruster, particularly the length in the axial direction relative to the diameter of the accelerating channel.
U.S. Pat. No. 5,751,113 (Yashnov, et al.; 1998), discloses a closed electron drift Hall effect plasma accelerator with all magnetic sources located to the rear of the anode. It is stated that this makes it possible to provide a Hall effect accelerator with an optimum distribution of magnetic field inside the acceleration channel by means of a simpler and less heavy arrangement using a single source of magnetic field, such as a single coil or permanent magnet. As in all Hall effect thrusters, the magnetic field lines (
13
, as seen in
FIG. 2
) extend laterally across the accelerating channel (
1
) over the anode (
2
) and propellant gas source (
3
) located at the closed end of the channel (see FIG.
1
).
A problem common to the Hall effect thrusters is one of scaling its size. In general, it is difficult to scale down Hall effect thrusters appreciably because of the magnetic field requirements. In smaller engines, the large transverse magnetic fields required can hamper ion flow, thereby reducing the ion beam current. This is particularly problematic for such engines generating milliamp magnitude beams for micro-thruster applications, wherein small thrust to power ratios make Hall effect thrusters impractical for micro-satellite applications. Another scaling problem is that electromagnets do not scale well with size reduction because of heating issues and coil size required to achieve the desired field.
Hall effect thrusters generally employ hollow cathodes, and preferably employ electromagnets, thereby requiring fairly complicated, and thus heavier, control systems in order to control electromagnet current, gas flow in both the anode and the discharge electrode, and cathode discharge current. Adding to the problems of complexity and weight, the hollow cathode consumes propellant.
U.S. Pat. No. 6,075,321 (Hruby; 2000), discloses a Hall field plasma accelerator with an inner and outer anode, designed to deal with problems of wall heating and sputtering that are characteristic problems with Hall effect thrusters.
A non-Hall effect thruster is described by U.S. Pat. No. 4,937,456 (Grim, et al.; 1990), that discloses a dielectric coated ion thruster comprising a cathode chamber (
12
) from which free electrons flow into an attached ionization chamber (
14
) along with a flow of ionizable gas atoms. According to the abstract and to column
6
of the detailed description, the free electrons are accelerated by a positive potential applied to the interior surface of the ionization chamber, causing the electrons to collide with atoms of the gas with sufficient kinetic energy to create ions. The positively charged ions are accelerated toward a negatively charged perforated grid plate (
24
,
112
), pass through the grid plate, and exit in a focused beam, providing thrust in the opposite direction. A plurality of bar magnets (
20
,
22
,
108
,
110
) are arranged in a spaced apart circular array around the cathode chamber with a pole face of each of the magnets tangentially aligned with wall sections (
16
,
18
,
102
,
104
) of the ionization chamber. The bar magnets define an axial geodesic picke

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