Modelling of a dipolar microwave plasma sustained by electron cyclotron resonance

Abstract

Multi-dipolar plasmas are sustained in large-volume chambers by a network of antennas located at the wall. Each antenna consists of a permanent magnet, trapping electrons in an axisymmetric dipole field, and a microwave applicator, heating the trapped electrons by cyclotron resonance (ECR). This paper presents a two-dimensional self-consistent model of a plasma sustained by one such antenna. The microwave fields and power absorption are calculated from the Maxwell equations coupled with a local electron momentum equation by an adaptation of the finite difference time domain method. The plasma is described by fluid equations for magnetized electrons and inertial ions, where quasi-neutrality is imposed through a semi-implicit numerical method based on Poisson's equation, which also yields the sheath potentials. Steady-state model results for argon show that below the critical plasma density (7.4×1016 m−3) the microwave power is absorbed in a narrow region all along the ECR surface around the end of the antenna; beyond this density the main absorption occurs near the plasma edge. Although the electron temperature varies considerably across the magnetic field lines, the plasma potential is nearly uniform all around the antenna and is controlled by the maximum electron temperature.