Pressure gauge filament for neutral gas density measurement using alternating current as source power

Abstract

In plasma fusion research the in-vessel neutral gas density is often measured using hot cathode ionisation gauges which are modified for the application in high magnetic fields and for a measurement range between 10−3Pa and 20Pa. To obtain sufficient electron emission, the cathode (filament) is heated to high temperatures in the range of 1800K by direct ohmic heating. To compensate for the induced Lorentz-forces, the filament must be relatively thick to provide sufficient mechanical stability which implicates increases of heating currents up to 20A.The heating current could be reduced by using a thinner filament in combination with alternating current with suitably chosen frequency to reduce mechanical stresses. The benefit of such a technique is besides saving of heating power, especially space-saving by applying thinner power supply cables. To estimate the suitability of such a solution a feasibility study by means of numerical methods has been carried out. The main subject of the investigation was the hot-filament for which alternating current has been used as power source. This paper provides first of all the main guidelines and features important in developing a pressure gauge filament heated by alternating current from the mechanical point of view.