J-PARC 3GeVシンクロトロン用ピラニ真空計の開発

The Development of a Pirani vacuum gauge with a platinum wire in the J-PARC 3-GeV rapid cycling synchrotron

荻原 徳男; 引地 裕輔; 吉成 洋治*

Ogiwara, Norio; Hikichi, Yusuke; Yoshinari, Yoji*

J-PARC RCSにおいてTMPの背圧監視に使用しているピラニ真空計の測定下限が0.1Paから2Pa程度に上昇するという変化が起きている。これに対し、今回、Pt線を用いた測定子の開発及び制御方法の見直しを行い、十分実用にたえる真空計を実現した。また、本真空計をRCSに実装して、運転開始から750h経過後も、順調に稼働していることを確認した。本真空計の特徴は、以下のとおりである。(1)フィラメントには、酸化等による感度変化を起こしにくい材料として、Ptを用いた。さらに、直径100m長さ60mm程度のPt線を12本直列に接続した構造とし、測定に有効な電気抵抗(室温時10)を確保しつつ機械強度を著しく向上させた。(2)本真空計の制御には、定電流切換え方式を用いた。出力変化の比較的大きい圧力範囲が狭いという定電流特性に対し、電流値を3段階に切り替えて大きな出力変化のあるところをつなぎ合わせ、0.1から1000Paまでの圧力範囲を精度よく測定できるものとした。(3)本ピラニ真空計の応答時間は30秒程度である。また、測定範囲は、0.05Paから大気圧までと広範囲である。

The backing pressure of the turbomolecular pumps (TMPs) is always monitored with Pirani gauges in J-PARC (Japan Proton Accelerator Research Complex) RCS (3-GeV rapid cycling synchrotron), where TMPs are used not only for rough pumping but also for evacuation during beam operation. To minimize radiation exposure during maintenance, it is necessary for the gauge head to have high toughness against the vibration and abrupt air inlet etc. Thus W wire 50 m in diameter was adopted as the filament. This type of Pirani gauge has been working well for monitoring the backing pressure of the TMP. However, it has become difficult to measure the low pressure less than several Pa with this gauge. This may be due to the change in emissivity for W surface. Thus we attempted to develop the gauge head made of Pt wire for measuring the pressure as low as 0.1 Pa. Platinum is one of the best materials, because it is very stable against oxidization. However, the ordinary Pt gauge head is rather weak against the vibration and abrupt air inlet due to its low tensile strength. In order to improve the toughness the filament was composed of 12 Pt wires 100 m in diameter and 65 mm long, which lead to the endurance against the force of 25 N. All the wires are welded in series on the metal poles in two separate glass plates. Each pole is electrically insulated. As a result, the filament, which is 78 cm long and has about 10 at room temperature, is contained in the cylindrical envelope 5 cm in diameter and 10 cm long. Then we examined the output from this gauge head as a function of pressure under constant current, because this gauge was planned to be controlled by the constant current method. Finally, we confirmed that the pressure from 0.1 Pa up to 10 Pa is well measured by this gauge, employing the current control in such a way that the set value increases with the pressure increase in three stages.