A space-charge-sheath electric thruster.

Abstract

T method of electric propulsion discussed in this note consists of accelerating cesium, which has been ionized on the surface of a porous tungsten plug, in the space-charge sheath located between the ionizer and a plasma filled region, t The flow of the plasma electrons to the ionizer is inhibited by a magnetic field parallel to the ionizer surface. The magnetic-field strength (up to a maximum of 2500 gauss in the experiment) at the ionizer is such that the average electron-cyclotron radius is comparable to the length of the accelerating gap (of the order of 1 mm) between ionizer and plasma, whereas the ion motion is essentially unaffected by the magnetic field. More detailed considerations show that the accelerating gap represents a double space-charge sheath, with a positive net charge near the ionizer and a negative net charge near the plasma region. The device resembles in some respects the so-called Hallcurrent accelerators investigated by others. However, whereas the acceleration of the ions in Hall-current devices takes place in the interior of a quasi-neutral plasma over distances that are large compared with both a Debye length and with an electron-cyclotron radius, in the device described here, the ions are accelerated in a space-charge sheath. The sheath thickness is comparable to an electron-cyclotron radius. This distinction between the two types of devices is illustrated in Fig. 1. Since, in the Hall-current device, the plasma in the accelerating region is approximately charge neutral, the thrust density is not limited by space-charge effects. In the device discussed in this note, high thrust-densities are still possible despite space-charge limitations, provided the gap between the anode and the virtual cathode is sufficiently small. In Hall-current devices, energy and momentum transfer from the plasma to the walls is an important loss mechanism. This loss is essentially absent in the space-charge-sheath device, since the plasma beam is effectively confined by the inertia of the accelerated ions and is nowhere in contact with material boundaries. The remainder of this note is confined to a discussion of the ion and electron motion in the accelerating region of the device. The calculations are in some respects similar to known analyses of electron trajectories in magnetrons and crossed-field devices, but have added complexity due to the presence of positive ions whose motions in the combined electric and magnetic fields are treated self-consistently, together with those of the electrons.