Ambipolar ion acceleration in a 20,000 G magnetic nozzle

Abstract

Summary form only given. An observed 50 eV argon ion energy is attributed to a measured axial plasma potential profile within the expanding magnetic nozzle region of a 35 kW helicon plasma source. The ion acceleration mechanism is identified as an ambipolar flow caused by expanding plasma that follows the fluid equations using the adiabatic definition of pressure, resulting in a maximum ion energy of several kTe. The helicon plasma source was operated with 50 to 150 mg/s argon propellant, and had a max and min magnetic field strength of 20,000 G. and 2 G respectively. The size scale and spatial location of the plasma potential structure of the expanding magnetic nozzle region appears to follow the size scale and spatial location of the expanding magnetic field along a 450 cm long section of the magnetic nozzle. The thickness of the potential structure was found to be 1e5 Debye lengths, many orders of magnitude larger than typical double layer structures. The background plasma density and neutral argon pressure were 1e9 cm-3 and 1e-5 Torr respectively. A large parameter scan of electron temperature and resulting ambipolar ion acceleration energies is presented.