Plasma self-bias and ion acceleration in the madhex helicon source

Abstract

Summary form only given. Helicon and magnetized radio-frequency (RF) plasmas have been studied for some time. It is often claimed that the ion acceleration observed in these sources is due to double layers. Recently strong (160 V) time averaged self-bias has been observed in MadHeX <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> , resulting in a large potential difference between the plasma source and expansion regions <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , has piqued our interest to further examine and fully realize the ion acceleration process. The modified MadHeX experimental facility consists of a 120 cm long, 10 cm inner diameter Pyrex tube attached to a stainless steel expansion chamber, which is 60 cm long and 45 cm in diameter (expansion ratio R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</sub> = 4.5) with an axial magnetic field, variable up to 1.2 kG at the source region that can be operated in flat or nozzle field configurations. An 18 cm long, 12 cm diameter half-turn double-helix antenna is used to excite helicon waves in the source. A new double magnetic mirror and an additional magnetic coil placed between the transition region between plasma source and expansion chamber are used to increase plasma ionization rate and reduce ion-electron recombination and neutral reflux in the expansion region. The effect of RF power, magnetic field strength and gas flow rate on the plasma parameters including electron temperature, density, plasma potential and ion beam acceleration are explored by probe diagnostics (Langmuir probe, emissive probe and retarding potential analyzer) and non-invasive optical techniques (laser induced fluorescence and optical emission spectroscopy). The role of substantial RF fluctuations in the plasma potential and the upstream end-plate boundary condition are addressed. The effect of the electron energy distribution that may include substantial tails on plasma self-bias and the ion beam formation and acceleration is examined by optical emission spectroscopy and cross-checked with the results via using a retarding potential analyzer. Also, its effect on the ion energy distribution is verified by using argon 668 nm laser induced fluorescence.