A small helicon source, 5 cm in diameter and 5 cm long, using a permanent magnet (PM) to create the DC magnetic field B, is investigated for its possible use as an ion spacecraft thruster. Such ambipolar thrusters do not require a separate electron source for neutralization. The discharge is placed in the far-field of the annular PM, where B is fairly uniform. The plasma is ejected into a large chamber, where the ion energy distribution is measured with a retarding-field energy analyzer. The resulting specific impulse is lower than that of Hall thrusters but can easily be increased to relevant values by applying to the endplate of the discharge a small voltage relative to spacecraft ground.

Helicon waves are waves in low-temperature, partially ionized plasmas in a dc magnetic field (B-field). The study of helicons involves both ion-neutral collisions and Larmor orbits, even when the B-field is uniform. Helicon discharges are ionized by helicon waves generated by a radiofrequency (RF) antenna. Interest in helicon discharges arose because of the high plasma densities they generate compared with other RF sources at comparable powers. The semiconductor industry has not taken advantage of this, even since the possible use of permanent magnets for the B-field has been demonstrated. Nonetheless, a large literature on helicons has evolved because of the numerous problems these discharges posed and the interesting physics found in their solutions.

A small helicon source using a permanent magnet has been tested for possible application as a spacecraft thruster. Ion energy distributions measured with a retarding-field ion analyzer show that ions are ejected with energies of ∼5 KT-e , in agreement with theory. The specific impulse can be increased by applying a positive bias to the endplate of the discharge.

The long-standing problem of plasma diffusion across a magnetic field (B-field) is reviewed, with emphasis on lowerature linear devices of finite length with the magnetic field aligned along an axis of symmetry. In these partially ionized plasmas, cross-field transport is dominated by ion-neutral collisions and can be treated simply with fluid equations. Nonetheless, electron confinement is complicated by sheath effects at the endplates, and these must be accounted for to get agreement with experiment.