Discharge at the pillbox window for an LHRF launcher

The discharge at a pillowbox-type waveguide vacuum window for a lower hybrid resonant frequency (LHRF) launcher at 2 GHz is studied experimentally. A discharge is observed near the window at a pressure below 10/sup -2/ Pa. The discharge has little or no effect on the RF-transmission waveform; however, the temperature rise of the ceramic due to the discharge reaches about 100 times higher than that in the no-discharge case. Moreover, the breakdown of gas in the waveguide is enhanced by the discharge. The discharge can be suppressed by sufficient RF conditioning and/or by coating the window with materials having a low secondary electron emission ratio such as carbon or TiN. These results are accurately explained by the theory of single-surface multipactoring discharge induced by an electric field perpendicular to the ceramic and the space charge.>