IUVSTA 15th International Vacuum Congress (IVC-15), AVS 48th International Symposium (AVS-48), 11th International Conference on Solid Surfaces (ICSS-11)
    Vacuum Science & Technology Thursday Sessions
       Session VST-ThA

Paper VST-ThA2
The Use of a Quadrupole Residual Gas Analyser to Automatically Verify the Purity of Tokamak Fuelling Gases

Thursday, November 1, 2001, 2:20 pm, Room 125

Session: Total & Partial Pressure Gauges & Their Calibration
Presenter: R.J.H. Pearce, EURATOM/UKAEA Fusion Association, UK
Authors: R.J.H. Pearce, EURATOM/UKAEA Fusion Association, UK
A. Henshaw, EURATOM/UKAEA Fusion Association, UK
J. Bruce, EURATOM/UKAEA Fusion Association, UK
S. Bryan, EURATOM/UKAEA Fusion Association, UK
Correspondent: Click to Email
It is essential that the gases injected into the JET experimental fusion tokamak be as requested and free from contamination. To give maximum flexibility a matrix architecture is used to handle the many different gases which can be introduced. In addition to problems of human error and contaminated gas bottles, the matrix architecture provides the potential for gases to become cross-contaminated either through valve leakage or due to control problems. A gas species verification system has been designed and commissioned to automatically confirm on-line the conformity of the gas being supplied to the torus. The system uses a quadrupole channeltron residual gas analyser (RGA) within a chamber, which is pumped by a turbomolecular drag pump. Under the control of the main control programme, samples of the module gases are leaked at regular intervals, through the controllable leak valve into the analysis chamber. Using the RGA the gas composition is computed in parts per million (ppm) of the 64 most relevant mass numbers. The analysis is corrected for offsets, background, and the mass positions. The mass spectrum is then compared with a reference gas spectrum. An acceptable tolerance on each mass number is defined for each reference spectrum. The system allows contamination of <1ppm to be detected. If a mass number falls out of the specified tolerance, an alarm message is communicated. The control of the gas checking is performed by the main matrix control program. The gas analysis is performed by a dedicated code written using a bespoke programming language designed for mass-spectrometry applications. Communication between these codes is performed through a SCADA database using dynamic data exchange (DDE). The system has been used to optimise the method of pumping and purging the matrix when changing gases. The results of tests, which have allowed the development of fast gas change cycles with little contamination or gas wastage, are presented.