電子銃カソード面へのイオン流入

摘要

大出力電子銃の寿命を決定する主な要因が，イオン損傷にrn困るカソードの経年劣化であることはよく知られている．即rnち，電子ビームと真空槽内のガス分子との衝突により生じたrnガスイオンは，電子ビームの空間電荷に引き寄せられてビーrnム軸の周りを振動しながら，軸に沿ってより電位の低い側ヘrnドリフトし，最終的に「引き出し電圧」で加速され，高エネrnルギーでカソード面に衝突する．こうしたイオン衝撃によっrnて惹き起こされるカソードのダメージは，真空蒸着など比較rn的高い圧力で用いられる電子銃ばかりでなく，近年では加速rn器の電子源として超高真空で用いるスピン偏極電子銃においrnて深刻な問題となっている.%The flow of the residual gas ion generated by a collision with high-energy electron in an axisymmetric gun was numerically investigated. Since the lifetime of the gun is dominated by the cathode damage induced by the ion bombardment, it is of industrial importance to reveal how the ions impinge on the cathode and how the inflow can be repelled. Under the normal operating condition of the electron gun (40 kV, 6 A), we determined the inflow basin, i.e. the region where the gas ions were born there and finally reached to the cathode. The basin consisted of four separated zones characterized by the potential distribution on the beam axis. To eliminate the downstream zones, it was useful to append a positive voltage imposed electrode behind the anode. The voltage necessary for repelling the ion inflow was found to be 300 V being less than 1% of the electron acceleration voltage.