1 Density Measurements During Ion Cyclotron Heating in

The response of ions during single-pass ion cyclotron resonance heating can be nonlinear [1]. The ion flow is accelerated due to the gradient along the axial magnetic field, and the electric and magnetic fields of the cyclotron-heating wave. This acceleration will produce a decrease in the plasma density around the resonance point depending on the incident flow velocity and the amount of power that is coupled into the ions at the resonance frequency. The primary application of this work is the acceleration of ions for electric propulsion systems. The experimental work for this paper was done on the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) that is being developed by Johnson Space Center. The measurements were taken using a 70 GHz density interferometer system. Density measurements taken during ion cyclotron heating show a clear and repeatable density drop of about 25 % for Helium plasma and about 75 % for Deuterium plasma. Initial calculations also show that the measured results agree well with theory. Nomenclature LHω = lower hybrid frequency ceω = electron gyrofrequency peω = electron plasma frequency Mi = ion mass Me = electron mass q = ion charge e = electron charge B = RF magnetic field vector Bo = DC magnetic field vector E = electric field vector V = velocity vector n = electron density c = speed of light ω = angular frequenc