Permanent Magnet Helicon Source for Ion Propulsion

Helicon sources have been proposed by at least two groups for generating ions for space propulsion: the Helicon Double Layer Thruster (HDLT) concept at the Australian National University and the Variable Specific Impulse Magnetohydrodynamic Rocket (VASIMR) concept at the Johnson Space Center in Houston. These sources normally require a large electromagnet and power supply to produce the magnetic field. At this stage of research, emphasis has been on the plasma density and ion current that can be produced, but not much on the weight, size, impulse, and gas efficiency of the thruster. This paper concerns the source itself and shows that great savings in size and weight can be obtained by using specially designed permanent magnets (PMs). This PM helicon design, originally developed for plasma processing of large substrates, is extended here for ion thrusters of both the HDLT and VASIMR types. Measured downstream densities are on the order of 10¹² cm^-3 , which should yield much higher ion currents than reported so far. The design principles have been checked experimentally, showing that the predictions of the theory and computations are reliable. The details of two new designs are given here to serve as examples to stimulate further research on the use of such sources as thrusters.