Helium partial pressure measurement in a deuterium environment

Abstract

To study the removal of helium ash in present day, medium sized tokamaks, experiments with deuterium plasmas and helium puffs are conducted. Because D2 and He have almost the same mass, a spectroscopic method had been developed at TEXTOR where the He partial pressure in the exhaust line was measured by exciting the neutral gas in a Penning gauge and observing the characteristic emission lines of helium and deuterium. This method was limited to low pressures and concentrations because broad spectral lines from the deuterium molecules superimpose the He lines. By using a spectrometer and an intensified charge coupled device camera instead of optical filters and photomultipliers, it is now possible to record the interesting part of the spectrum with a time resolution of 20 ms. By subtracting the molecular spectrum of deuterium, helium concentrations from 2% on can be detected over about three orders of magnitude in pressure down to 2×10−6 mbar. He and D2 pressures are measured simultaneously. By another method the 58 nm resonance line of helium is observed and the crosstalk of the deuterium gas could be eliminated. Pressure measurements as low as 5×10−8 mbar and 1% He concentration are possible. A detection system utilizing a vacuum ultraviolet monochromator in combination with a channeltron has been tested.