Ion acceleration in the MadHex helicon source

Abstract

Summary form only given. The MadHex experimental system consists of a 150 cm long, 10 cm inner diameter Pyrex tube connected to a stainless steel expansion chamber 60 cm long and 45 cm in diameter (expansion ratio RE = 4.5) with an axial magnetic field, variable up to 1 kG at the source region that is operated in a nozzle configuration. A half-turn double helix antenna, 18 cm long and 12 cm diameter, is used to excite helicon waves in the source. Ion beams of energy up to E = 160 eV at 500 W RF power have been observed in a flowing argon helicon plasma formed in the expanding region of the helicon source using a magnetic nozzle (RM = 1.44). The effect of flow rate/pressure, RF power and magnetic field strength on the ion beam acceleration, plasma potential, electron density and temperature are explored. The ion energy distribution function (IEDF) is obtained by a two-grid Retarding Potential Analyzer (RPA). The plasma potential is determined by a floating emissive probe and the electron density and temperature are measured by both single and double probes. Additionally, the axial ion velocities and temperatures are observed via argon 668 nm Laser Induced Fluorescence (LIF). The measured density decrease of ~20 across the double layer in the magnetic expansion region does not fit a Boltzmann expansion but does agree with a conservation-of-flux calculation using the measured beam energy. The results of fast and slow argon ion beams at lower flow rates (~1.3 sccm) with different RF powers are reported in the transition between Pyrex and expanding exhaust regions.