Inductively coupled array (INCA) discharge

Abstract

Recently a novel concept for collisionless electron heating and plasma generation at low pressures was proposed theoretically (U Czarnetzki and Kh Tarnev, Phys. Plasmas 21 (2014) 123508). It is based on periodically structured vortex fields which produce certain electron resonances in velocity space. A more detailed investigation of the underlying theory is presented in a companion paper (U Czarnetzki, submitted to Plasma Sources Sci. Technol. (2018), arXiv:1806.00505).Here, the new concept is experimentally realized for the first time by the INCA (INductively Coupled Array) discharge. The periodic vortex fields are produced by an array of small planar coils. It is shown that the array can be scaled up to arbitrary dimensions while keeping its electrical characteristics. Stable operation at pressures around and below 1 Pa is demonstrated. The power coupling efficiency is characterized and an increase in the efficiency is observed with decreasing pressure. The spatial homogeneity of the discharge is investigated and the behaviour of the plasma parameters with power and pressure are presented. Linear scaling of the plasma density with power and pressure, typical for conventional inductive discharges, is observed. Most notably, the plasma potential and the corresponding mean ion energy show clear evidence for the presence of super energetic electrons, attributed to stochastic heating. In the stochastic heating mode, the electron distribution function becomes approximately Maxwellian but with increasing pressure it turns to the characteristic local distribution function known from classical inductive discharges. Large area processing or thrusters are possible applications for this new plasma source.

Describing local and non-local electron heating by the Fokker–Planck equation

“…Tanskov et al 2015). Obviously, the absolute densities scale linearly with the power coupled to the plasma as expected (Lieberman and Lichtenberg 2005;Ahr et al 2018). The linearity follows from the simple argument that (a) for stationarity, power input and losses must be balanced and (b) all losses are proportional to the electron density.…”

“…However, it should be rather straightforward to apply the same general procedure also to other heating scenarios, e.g. in the INCA discharge (Ahr et al 2018) or in ECR discharges (Lieberman and Lichtenberg 2005). The detailed form of the coupling functions will likely be different but the general structure of the heating operator and the structure of the global kinetic model will remain the same.…”

“…Further, the small temporal oscillation of the homogeneous distribution is of second order only and can be neglected for the heating aspect. Note, however, that advantage is taken of the residual small oscillation at 2 0 for instance in the so-called RF modulation spectroscopy, where tiny oscillations of the excitation rate and the related optical emission of the order of 1% are detected Tsankov and Czarnetzki 2011;Ahr et al 2018). This optical diagnostic technique is an alternative to the so-called B-dot probes, more commonly used for the measurement of the field structure in inductively coupled plasmas (e.g.…”

Electric field measurements on the INCA discharge

“…A more detailed description of the discharge setup can be found elsewhere 9,10 . Here only a brief description is give.…”

“…The individual coils are connected with each other through copper stripes. The previous studies 9,10 have shown that for a matching to be possible, the coil wiring has to be separated into three parallel branches, each containing 12 coils connected in series.…”