Turbomolecular pumps (TMPs) are used for not only rough pumping but also evacuation during beam operation. The backing pressure of the TMPs is always monitored with Pirani gauges. 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 micrometer in a diameter is adopted as the filament instead of Pt wire. However, the surface of W wire as received is sometimes unstable because of the contamination, roughness, oxidized layer and so on. As annealing the W wire in a high vacuum is expected to make the stable surface through the evaporation of the contamination, we have investigated the change in electrical resistance of W wire. As a result, we have found that the annealing at 1500 K for 10 min is effective to obtain the stable filament. Thus the annealing process for getting stable filament has been established.
The control unit for the gauge has been 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 than that with constant temperature method, because the change in the output voltage so drastically decreases increasing the pressure over than about 100 Pa. As we have to measure the pressure up to about 1000 Pa precisely in order to monitor the backing pressure of TMPs, we have carried out the experiment about the change in the output as a function of the current. And, we have confirmed that the higher pressure can be measured when the higher current is passed. For example, we can measure the pressure less than 20 Pa 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. We have been now under examination of prototype controller.
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