AVS 58th Annual International Symposium and Exhibition
    Plasma Science and Technology Division Wednesday Sessions
       Session PS-WeA

Invited Paper PS-WeA3
Study of Radio Frequency Breakdown Mechanisms in a Plasma Environment

Wednesday, November 2, 2011, 2:40 pm, Room 201

Session: Plasma Sources
Presenter: John Caughman, Oak Ridge National Laboratory
Authors: J.B.O. Caughman, Oak Ridge National Laboratory
R.H. Goulding, Oak Ridge National Laboratory
D.A. Rasmussen, Oak Ridge National Laboratory
C.H. Castano Giraldo, University of Illinois at Urbana Champaign
M. Aghazarian, University of Illinois at Urbana Champaign
E.H. Martin, North Carolina State University
S.C. Shannon, North Carolina State University
Correspondent: Click to Email
Radio frequency (RF) breakdown/arcing is a major power-limiting factor in antenna systems used for RF heating and current drive in magnetic fusion experiments and is also an issue for high voltage substrate operation in process plasmas. The factors that contribute to breakdown include gas pressure, gas type, magnetic field, materials, ultraviolet light, and local plasma density. The effects of these factors on RF breakdown are being studied in a resonant 1/4-wavelength section of vacuum transmission line terminated with an open circuit electrode structure with a well-defined electric field. A small plasma source is used to inject plasma into the high-field region of the electrodes. Changes in the electrical parameters, such as input impedance and the voltage at the electrodes, are being monitored to detect the breakdown events. Measurements of the light emission prior to and during an arc are also being made. For high vacuum conditions using copper electrodes, bright spots (unipolar arcs) appear on the electrode surfaces prior to a breakdown event. The voltage-current characteristic in this regime is consistent with Fowler-Nordheim field emission. An increase in the RF field results in an arc and a flash of light corresponding to copper line emission. Analysis of the electrode surfaces show large areas of melting and formation of micron-sized rounded protrusions, especially along the sharp edges of small scratches or at impurity inclusions on the surface. The maximum electric field that can be sustained without breakdown is on the order of 30-40 kV/mm for vacuum conditions, but this value is substantially reduced in the presence of plasma and magnetic field. An increase in the chamber pressure results in a decrease in the maximum RF electric field that can be sustained without breakdown as the pressure approaches a few mTorr. The breakdown event leads to formation of a plasma in the structure, and the addition of an external magnetic field causes the formation of a plasma at lower pressures. Ultra-violet light, with an energy greater that the work function of the electrode material, has been shown to induce electron emission from the surface and initiate multipactor discharges. In addition, we are using optical emission spectroscopy to determine the magnitude of the DC and RF electric fields near the electrode structure by utilizing the dynamic Stark effect. Experimental details and future research directions will be presented.