Particle-in-cell modeling of potential gradients in expanding helicon plasmas using XOOPIC

Summary form only given. The particle-in-cell (PIC) code XOOPIC is used to model expanding, low-density (n_e~10⁹-10¹⁰ cm^-3, T_e~5-10 eV) argon plasmas in an effort to further understand the formation of potential gradients in the region of expansion. There is experimental evidence that a self-bias voltage forms between the plasma in the grounded expansion chamber and that in the insulating source region in the Madison helicon experiment. In this experiment, low-density argon plasma is created in a system typically used to create helicon discharges. The RF antenna capacitively couples to the low-density plasma, which results in large-amplitude plasma potential fluctuations at the RF frequency (13.56 MHz). These fluctuations lead to an increased initial flux of electrons from the source region to the expansion region, resulting in the measured self-bias voltage and corresponding gradient in time-averaged plasma potential between the upstream and downstream chambers. Plasma ions are then accelerated by this gradient. This phenomenon is being modeled in XOOPIC in an effort to better understand the evolution of the plasma potential and electron energy distribution (EEDF).