Hall effect thruster with anode having magnetic field barrier

1. A Hall effect ion accelerator having an annular gas discharge channel including an exit end portion, discharge of gas through the exit end portion defining a downstream direction, said accelerator comprising:

an inner magnetic pole located at the inside of and encircled by the annular gas discharge area adjacent to the exit end portion;

an outer magnetic pole located at the outside of and encircling the annular gas discharge area adjacent to the exit end portion, the inner and outer magnetic poles being magnetically coupled by a first magnetic path;

a first magnetic field source associated with the first magnetic path for producing a generally radially extending magnetic field between the inner pole and the outer pole at the exit end portion of the gas discharge channel;

an anode located upstream of the exit end portion of the gas discharge channel;

a gas source for supplying an ionizable gas to the gas discharge channel for flow in a downstream direction toward the exit end portion;

an electron source for supplying free electrons for introduction through the exit end portion of the gas discharge channel in a generally upstream direction;

an electric field source for producing an electric field extending from the anode in a downstream direction toward the exit end portion of the gas discharge channel, interaction between the ionizable gas from the gas source and free electrons from the electron source providing ions accelerated in a downstream direction by the electric field;

a magnetic shunt of H-shape including an outer shell of magnetic material having a downstream end adjacent to the innermost part of the outer pole and extending upstream therefrom along an outer portion of the gas discharge channel, an inner shell of magnetic material having a downstream end adjacent to the outermost part of the inner pole and extending upstream therefrom along an inner portion of the gas discharge channel and a web located intermediate the upstream and downstream ends of the inner and outer shells, the web having portions extending inward from the outer shell and outward from the inner shell so as to magnetically couple the inner and outer shells behind the anode to define a second magnetic path; and

a second magnetic field source associated with the second magnetic path and generating magnetic flux there along sufficient to generate magnetic field lines extending generally axially in the gas discharge channel from a location adjacent to the anode and downstream toward the exit end portion of the gas discharge channel to enhance election mobility in an area between the anode and the exit end portion of the gas discharge channel.

2. The accelerator defined in claim 1 , in which the magnetic field in the gas discharge channel induced by the first magnetic field source and second magnetic field source transitions abruptly from a generally radial direction at the exit end portion to a generally axial direction upstream from the exit end portion.

3. The accelerator defined in claim 1 , in which the magnetic material of the second path is concentrated in an area upstream of the anode.

4. The accelerator defined in claim 1 , in which the web of the magnetic shunt includes a first long annular flange of magnetic material that extends outward from the inner shell with its outer end close to, but spaced from, the outer shell and a second long annular flange of magnetic material that extends inward from the outer shell with its inner end close to, but spaced from, the inner shell.

5. The accelerator defined in claim 4 , in which the first and second flanges overlap and are closely spaced for magnetic coupling.

6. The accelerator defined in claim 1 , in which the inner and outer magnetic poles have upstream ends magnetically coupled by a first thruster backplate, and the inner and outer shells of the H-shaped shunt have upstream ends connected by a separate backplate, spaced in a downstream direction from the first thruster backplate, such that the first and second backplates are not magnetically coupled.

7. The accelerator defined in claim 1 , in which the web portions extending inward from the outer shell and outward from the inner shell form a continuous intermediate annular plate extending between the inner and outer shells.

8. The accelerator defined in claim 1 , in which the inner and outer poles have upstream portions magnetically coupled by a thruster backplate, the shunt being constructed and arranged to be electrically insulated from the thruster backplate and to function as the anode of the accelerator.

9. The accelerator defined in claim 1 , in which the inner and outer poles have upstream portions magnetically coupled by a thruster backplate, the inner and outer shells having respective upstream ends magnetically coupled by the thruster backplate.

10. The accelerator defined in claim 1 , in which the second magnetic field source is located between the inner and outer shells at a location upstream of the web.