A powerful tool to quantitatively detect tiny amounts of 4He in a deuterium rich background for fusion research

The detection of tiny amounts of 4He in a deuterium rich atmosphere (as well as of 3He in a tritium background) is of great relevance in fusion research. One of the main concerns in a d-t fusion experiment is to prevent significant accumulation of 4He ashes in the core plasma, since this would result in a dilution of the d-t fuel and in a consequent reduction of the fusion power output. In order to control the impurity concentration in the core plasma and to sustain the fuel density, several active pumping methods for 4He (and other impurities) have been proposed, such as divertors or pump limiters. A quantitative analysis of the exhausts of such devices can provide a useful evaluation of the effectiveness of different methods and configurations, by comparing the related 4He content. Helium leak detection is a further major problem in fusion research. Vacuum tightness of the large torus vessel is indeed a rigorous requirement, in order to minimize the impurity content of the plasma. However, due to the strong deuterium release by the wall of the vacuum vessel, a conventional helium leak detector would not be able to discriminate a small amount of leaked 4He in such a high deuterium background. Determination of tritium activity by 3He in-growth method also requires a mass spectrometric analysis system capable to detect small changes of 3He content in a tritium background. A new method has been developed at ENEA Frascati, allowing quantitative detection of extremely small amounts of Helium isotopes (and other inert gases) in a deuterium rich atmosphere using a conventional (i.e. normal resolution) quadrupole mass spectrometer. This is accomplished by means of two non-evaporable getter (NEG) pumps, the first one operating at high temperature (300 divide 450degC) and the second one at room temperature, which effectively and quickly remove to a high degree all active components in the original gas mixture, with particular regard to hydrogen isotopes. Obviously, noble gases are preserved. The results of extensive tests carried out with such a facility are reported. A high-resolution quadrupole mass spectrometer was used during tests with 4He-D2 mixtures, just to provide evidence for the absence of deuterium in the final gaseous mixture. The minimum detectable peak ratio 4He/D2 has been found to be less than 10-6.