Ion acceleration and non-Maxwellian electron distributions in a low collisionality, high power helicon plasma source

Ion acceleration through plasma double layer and non-Maxwellian two temperature electron distributions have been observed in Madison Helicon Experiment (MadHeX) operated in high RF power (\textgreater 1000 W) and low Ar pressure (0.17 mtorr) inductive mode. By applying Optical Emission Spectroscopy (OES)[1] cross-checked with an RF-compensated Langmuir probe (at 13.56 MHz and its second and third harmonics), the fast (\textgreater 80 eV), untrapped electrons downstream of the double layer have a higher temperature of 13 eV than the trapped bulk electrons upstream with a temperature of 4 eV[2]. The reduction of plasma potential and density observed in the double layer region require an upstream temperature ten times the measured 4 eV if occurring via Boltzmann ambipolar expansion. The hot tail electrons of the non-Maxwellian electron distribution affect the formation and the potential drop of the double layer region. The mechanism behind this has been explored via several non-invasive plasma diagnostics tools. The OES measured electron temperatures and densities are also cross-checked with Atomic Data and Analysis Structure (ADAS) and a millimeter wave interferometer respectively. The IEDF is measured by a four-grid RPA and also cross-checked with argon 668 nm Laser Induced Fluorescence (LIF). An emissive probe has been used to measure the plasma potential. [1] J. B. Boffard, R. O. Jung, C. C. Lin, A. E. Wendt, Plasma Sources Sci. Technol. 19, 065001 (2010) [2] Y.-T. Sung, Y. Li, J. E. Scharer, Phys. Plasmas 22, 034503 (2015)