Electron cyclotron resonance (ECR) enhanced diverging magnetic field for controlled particle flux in a microwave-excited plasma column – a numerical investigation

Abstract

The present work is an investigation of the effect of an externally applied diverging magnetic field on a surface microwave-sustained plasma column. Microwaves (2.45 GHz) are allowed to propagate through a tapered waveguide system containing a discharge tube made up of quartz. Argon gas flows down the tube from top to bottom maintaining a pressure of 1 Torr and a plasma is ignited within the tube owing to the surface microwave propagation. In the absence of a magnetic field, the plasma column exhibits discrete regions of overdense plasma near its center where the electric field of the incident microwaves is observed to be high. As the gas flows down the tube, the plasma density is also found to decrease and the resulting plasma profile is asymmetric about its length. However, in the presence of an axially applied diverging magnetic field , an axial force acts on the plasma, and the discrete overdense plasma regions are found to get symmetrically arranged along the plasma axis. Interesting results are observed when the diverging magnetic field includes a region of electron cyclotron resonance (ECR) corresponding to the microwave frequency. In the presence of an ECR, the electrons are expected to experience resonant heating by microwaves other than the direct heating by these waves. Under such conditions, the discharge dynamics are governed by the resonance mechanism, and the bright spots of overdense plasma regions get shifted to the ECR positions. As the magnetic field strength increases, the overdense plasma moves axially away from the center. These results are a clear indication of a magnetically controlled particle flux over a target and can be exploited in various material processing applications, particularly for surface cleaning applications in the semiconductor industries.