Anomalous skin effect and collisionless power dissipation in inductively coupled discharges

Abstract

In this article we present an experimental study of the electron electrodynamics in an inductively coupled argon discharge. The discharge is configured in a re-entrant geometry and operates in the stochastic heating regime at pressures below 10 mTorr. The radial distribution of the induced rf electric field E and current density J were determined for a wide range of plasma parameters in argon gas from the measurement of the radial distribution of the magnetic field components and its spatial derivatives. The results show an anomalous skin effect at low pressure and high plasma densities that is characterized by a nonmonotonic spatial decay of the electromagnetic field E and current density J along with phase reversal and bifurcation of E and J and negative power absorption regions. These features are interpreted to be a result of nonlocal electrodynamics due to the electron thermal motion (which causes spatial dispersion in the conductivity). The electron thermal motion in the inhomogeneous rf induced electric field induces a phase randomization that leads to collisionless heating. The relative ratio of ohmic collisional heating to collisionless heating is estimated by comparing the total and collisional (deduced from an estimation of the electron-neutral collision frequency) power fluxes absorbed by the plasma electrons. This shows that collisionless heating dominates ohmic heating for pressures below 5 m Torr. These results are compared with the previously published work of Godyak et al. [V. A. Godyak, R. B. Piejak, and B. M. Alexandrovich, Phys. Rev. Lett. 80, 3264 (1998)] and show a surprisingly good agreement (at constant gas pressure and plasma density) despite the differing chamber geometry.