並列タイトル等Characteristics of voltage holding and light emission on the accelerator of JT-60U N-NBI ion source
一般注記JT-60負イオン源の長パルス化及び高出力化を図るうえで問題となっている500keV加速部の耐電圧性能について調べた。設計加速電圧500kVに対してビーム加速を伴わない(無負荷)場合、最大455kVであった。この低い耐電圧性能の原因を調べるために、電圧印加時に負イオン源の内部(真空側)で発生する光の強度を光電子増倍管で測定した結果、耐電圧と真空側の光強度との間に強い相関関係があることがわかった。さらに分光器で光の波長を測定した結果、光の波長は420nmのピークを中心とした広い領域に分布した。この際、水素,炭素等のガス放電で発生する輝線スペクトルは観察されなかったことから、発光は負イオン源の絶縁管で使用しているFRPへの電子衝突が起因していると類推できる。この結果に加えて、時間応答性の高い高速データ収集系で内部発光及び外部の球ギャップスイッチ放電光の発生タイミングを詳細に測定した結果、内部で放電破壊が発生すると球ギャップが放電破壊を起こすこと、一度球ギャップが動作すると高電圧の再印加時に、設定破壊電圧より低い電圧で球ギャップが絶縁破壊を起こし、耐電圧を制限してしまうことがわかった。
Voltage holding capability of the 500 kV accelerator in the JT-60 negative ion source that is one of the key issues for high performance of the JT-60 negative-ion-based NBI system was investigated. The achieved voltage holding capabilities with and without the beam acceleration were 400 kV and 455 kV, respectively. To understand a poor voltage holding capability of the negative ion source, correlation between the voltage holding capability and the light emitted inside the ion source was carefully examined. The acceleration voltage was stably applied at $<$ 400kV, where the light intensity was almost zero. Increasing the acceleration voltage beyond 400 kV, the voltage holding become very unstable where the light intensity increases in proportion to the acceleration voltage. The spectroscopy measurement showed that the light spectrum was a broad wavelength of 360 - 500 nm peaked at 420 nm. There was no line spectrum due to the gas discharge such as hydrogen, oxygen, carbon. From these results, it is seemed that the origin of the light emission is a cathode luminescence from the FRP (Fiberglass Reinforced Plastic) insulator in JT-60 negative ion source due to the electron impact. Moreover, breakdown phenomena at inside and outside of the ion source were examined by using photo-multipliers with fast data acquisition system. When the breakdown occurred inside the ion source, the breakdowns sequentially occurred at the spark gap switches outside of the ion source, which protect the FRP insulator from the flashover on its surface. Once the spark gap was turned on after the breakdown inside the ion source, the breakdowns at the spark gap occurred at lower voltage than the normal set value when the high voltage was applied again after $\sim$70 ms interval. This result indicates that the voltage holding capability was limited by the spark gap switches in this operational sequence.
一次資料へのリンクURL/JAEA-Technology-2007-027.pdf （fulltext）
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