Measurements of neutral depletion effects and techniques for initiating low pressure, high power helicon plasmas

Abstract

Summary form only given. A flowing argon helicon plasma is formed in a 10 cm diameter, 1.5 m long Pyrex chamber with an axial magnetic field in nozzle or flat configuration, variable up to 1 kG in the source region. Experimental upgrades have allowed for operation at high, pulsed RF powers (up to 10 kW at 13.56 MHz) and low flow rates and pressures (as low as 1 seem and 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> Torr). Neutral-collisional plasma exists upstream of the half-turn, double helix antenna and neutral-collisionless plasma exists downstream, leading to bulk plasma acceleration due to reduced neutral drag. Calculated variation of the RF frequency (from 12 MHz to 15 MHz) during the 5 ms pulse allows for low (< 3%) reflected powers during the gas breakdown and the approach to and formation of the steady state plasma. Microwave interferometry (105 GHz), collisional radiative spectroscopic codes and diamagnetic loops are used to measure electron density and temperature. An initial transient high-density peak (> 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sup> /cc) is observed followed by the development of a neutrally depleted steady state plasma (> 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> /cc). Neutral depletion is observed along with substantial plasma acceleration. We also present results from AntenaII and MAXEB 1 and 2-D finite difference modeling of the helicon wave coupling from the antenna in collisional and collisionless wave regimes to determine the spatial absorption profile of the wave and compare with experimental observations. A static magnetic field threshold for discharge initiation is seen at low flow rates, where discharges will not start above a certain magnetic field value that depends on RF power and flow rate. This threshold is a consequence of the multipactor effect, which is the dominant mechanism for breakdown when the electron-neutral collisional mean free path is longer than the system dimensions. A magnetic field ramping technique for starting discharges at low flow rates is described.