Experimental validation of single pass ion cyclotron resonance absorption in a high speed flowing plasma applied to the variable specific impulse magnetoplasma rocket (VASIMR).

Abstract

The topic of this thesis is the experimental characterization and analysis of single pass ion cyclotron resonance heating as applied to acceleration of ions for electric propulsion. The experimental work was done on the VX-10 experiment of the VASIMR (Variable Specific Impulse Magnetoplasma Rocket) concept. In ion cyclotron resonance heating (ICRH) a RF wave is launched into a magnetized plasma where it then accelerates the ions by increasing their rotational speed around the magnetic field lines. The electric field vector of the right hand component of the wave will rotate around the field lines with a frequency <italic>o_RF</italic> in the same direction as the ion's cyclotron motion about the field lines. Consequently, when <italic> o_RF</italic> &ap; <italic>o_ci</italic> (where <italic> o_ci</italic> is the ion's cyclotron frequency) the force from the electric field of the wave on the ions will result in a continuous rotational energy gain. The perpendicular velocity of the ions generated by ICRH is then converted into axial velocity by the decreasing gradient of the axial magnetic field at the exhaust of the propulsion system from conservation of the magnet moment. This increase in axial velocity is predicted to cause a decrease in density due to conservation of current in the plasma. In order to characterize this density drop during ion cyclotron heating, a single channel interferometer system was developed and implemented on the VX-10. Interferometer density measurements were taken at three different locations on the VX-10 experiment upstream and downstream of the ion acceleration zone. Measurements were made of the density drop in both Helium and Deuterium plasma discharges during ICRH under a variety of operating conditions including magnetic field profile, gas flow rate and ICRH power pulse timing, and ICRH power. A clear measurement of a density drop was observed downstream of the ion resonance zone characteristic of ion acceleration and measurement of little change in density upstream of the resonance zone where no acceleration was expected. Good agreement between the measured and predicted power scaling of ion acceleration due to ICRH was found. And experimental evidence that the shape of the magnetic field profile will influence ICRH acceleration as predicted is also presented and analyzed.