Operation of the inductively coupled array (INCA) discharge as a para-array

Abstract

The inductively coupled array (INCA) discharge is a plasma source with a tailored electric field configuration that allows efficient stochastic heating of the electrons. The desired field configuration is realized by an array of small planar coils. This discharge configuration offers dense plasma production at pressures as low as 0.1 Pa and option for upscaling to square meter sizes. The theoretical works, which led to the development of the INCA discharge, propose two different electric field configurations—the ortho- and the para-configuration. The initial experimental work (P. Ahr et al 2018 Plasma Sources Sci. Technol. 27 105010) studied the ortho-configuration. Here we investigate the performance of the alternative configuration, the para-array, and compare it to the ortho-configuration. The results show similar behaviour of the electrical and of the plasma parameters. However, on first sight the ortho-array appears to perform better. The differences disappear when the power actually coupled to the plasma is considered. For both configurations the electron density follows the same linear trend with the power coupled into the plasma and the electron temperature remains nearly constant as prescribed by the energy balance equation of the electrons. The power coupling efficiency for both configurations increases with the plasma density, approaching 60% at a plasma density of 1 × 1017 m−3. No signs of saturation are observed. The variation of the electron distribution functions with the pressure shows that in the para-configuration the transition into the stochastic heating mode occurs at a pressure twice lower than that for the ortho-array in a very good agreement with the theoretical expectations. Based on the comparison of the discharge parameters and its performance, the advantages and the shortcomings of using one or the other configuration are outlined.

Electric field measurements on the INCA discharge

“…Addition-ally to the electric field configuration already described, called the ortho-array, another field configuration was proposed and discussed in the theoretical works on the INCA discharge -the so called para-array, where the phase of the current changes by 180°between the neighboring coils 8 . Recently, also this field structure was realized experimentally and it was shown its behavior is very similar to that of the ortho-array 12 . The differences are in good agreement to the theoretical predictions.…”