J-PARC 3 GeV シンクロトロソ用ピラニ真空計の開発

摘要

J-PARC RCSにおいてTMPの背圧監視に使用しているrnピラニ真空計の測定下限が0.1Paから3Pa程度に上昇するrnという変化が起きている．これに対し，今回，Pt線を用いrnた測定子の開発および制御方法の見直しを行い，十分実用にrnたえる真空計を実現した．また，本真空計をRCSに実装しrnて，運転開始から2200h経過後も，順調に稼働しているこrnとを確認した．%The back pressure of Turbo-Molecular Pumps (TMPs) is constantly monitored using Pirani gauges at J-PARC (Japan Proton Accelerator Complex) RCS (3-GeV Rapid Cycling Synchrotron) where they are used not only in rough pumping but also evacuations during beam operations. The gauge head needs to be very resistant to vibration and abrupt air inlet etc. in minimizing exposure to radiation during maintenance and hence a 50 μm in diameter W wire was adopted as the filament. This type of Pirani gauge has worked well in monitoring the back pressure of the TMP but it has become difficult to measure the low pressure of less than several Pa with the gauge, which may have been due to changes in the emissivity of the W surface.rnAn attempt was therefore made to develop a gauge head made of Pt wire in allowing pressures as low as 0.1 Pa to be measured. Platinum is one of the best possible materials to use because it is very stable against oxidization. However, ordinary Pt gauge heads are rather weak when it comes to vibrations and abrupt air inlet due to its low tensile strength. In order to improve its toughness the filament was composed of twelve 100 μm in diameter Pt wires that were 65 mm long, resulting in it being capable of enduring a force of 25 N. All the wires were welded in series on metal poles in two separate glass plates, with the poles being electrically insulated. This resulted in the filament, 78 cm long and about 10 Ω at room temperature, being containable in a 5 cm in diameter and 10 cm long cylindrical envelope.rnThe output from the gauge head was then examined as a function of pressure under constant current as the plan was for it to be controlled using the constant current method. Confirmation then took place that the pressures of 0.1 Pa up to 10~3 Pa were measurable with the gauge using current control in such way that the set value increased with pressure increases in three stages.