Pirani vacuum gauge for J-PARC 3GeV synchrotron vacuum system

Kuroiwa, Masahide*; Matsumoto, Nobuhiko*; Fujii, Susumu*; Ogiwara, Norio; Arai, Hideyuki*

In J-PARC 3GeV synchrotron vacuum system, turbomolecular pumps (TMPs) are used for evacuation during beam operation. The backing pressure of the TMPs is always monitored with Pirani gauges. It is necessary for the gauge head to have high toughness against the vibration and abrupt air inlet etc. Thus W wire 50 micrometer in a diameter is adopted as the filament instead of Pt wire. Then we have found that the annealing at 1500 K for 10 min is effective to obtain the stable filament. Thus the gauge head with high toughness has been developed. The control unit for the gauge has been also newly designed, as the unit should be placed far from the gauge head which is located in a high level of radiation and electrical noise. We have adopted the combination of the constant current method with four-point probe method. Thus, with these techniques the output has been confirmed experimentally to be independent of the cable length, and less influenced by the noise. Generally the constant current method is not so suitable for measuring the higher pressure, however, we have confirmed that, for example, the pressure less than 20 Pa can be measured at 50 mA, and the pressure ranging from 20 Pa to 1000 Pa at 90 mA, respectively. Thus the current is controlled in such a way that the set value is increased with the increasing pressure in several stages.