A Pirani gauge in a high-intensity proton accelerator

Abstract

We have developed a Pirani-type vacuum gauge for monitoring the back pressure of a turbo-molecular pump (TMP), which is employed as the main pump for the 3-GeV Rapid Cycling Synchrotron (RCS) at the Japan Proton Accelerator Complex (J-PARC).As vacuum components are subjected to high levels of radiation in these devices, the TMP and the vacuum gauge used for monitoring the TMP’s back pressure should have high reliability and a long life so as to minimize exposure to radiation during maintenance. Moreover, to ascertain the vacuum conditions of the ring and to monitor the back pressure, the gauge should preferably be capable of measuring pressures from 0.1 Pa to 103 Pa. To satisfy these requirements, the following measures were taken: All the components around the sensor head, including the connector and cables, were replaced with components that were guaranteed to perform satisfactorily even with a radiation dose greater than 30 MGy. Then, the semiconductor devices used in the measuring circuits were checked by using the four-point probe method and found to work satisfactorily at a distance of 150 m from the sensor. To measure higher pressures, the current control circuit was modified in such a way that the set value of the current increased in stages as the pressure increased. Further, it was ensured that the gauge showed high resistance to vibration and to an abrupt air intake as well as high radioactive resistance. The newly designed gauge head comprised a series of twelve Pt wires, each with a diameter of 100 μm. It was confirmed that pressures from 5 × 10−2 Pa to atmospheric pressure were measurable with an accuracy of less than 30%.This Pt Pirani gauge has shown good performance in monitoring the back pressure of the TMP at the J-PARC RCS, where it has been in use since Oct. 2009.