DEUTERIUM-TRITIUM CONCENTRATION MEASUREMENTS IN THE DIVERTOR OF A TOKAMAK VIA A MODIFIED PENNING GAUGE

The measurement oi the relative concentrations of hydrogen, deuterium, tritium and helium is an important task in the nuclear fusion researc h area. Control of the deuterium-tritium (D-T) isotropic ratio and lim iting the helium ash content in a fusion plasma are the key to optimiz ing the fuel burn in a fusion reactor such as ITER. A diagnostic techn ique has been developed to measure the D-T isotopic ratio in the diver tor of a tokamak with a Penning vacuum gauge. The Penning discharge pr ovides a source of electrons to excite the neutral deuterium and triti um in the pumping duct. Subsequently, the visible light from the hydro gen isotopes is collected in an optical fibre bundle, transferred away from the tokamak into a low radiation background area and detected in a high resolution Czerny-Turner spectrometer, equipped with a fast CC D (charge-coupled device) camera for optical detection. The intensity of the observed line emission (D-alpha, 6561.03 Angstrom; T-alpha, 656 0.44 Angstrom) is directly proportional to the partial pressure of eac h gas found in the divertor. The line intensity of each isotope is cal ibrated as a function of pressure. The ratio of the line intensities t hus provides a direct measurement of the D-T isotopic ratio. The lower limit for the determination of the D-T isotopic ratio is about 0.5%. This system is applicable for the pressure range from 10(-5) mbar to a few times 10(-2) mbar. (C) 1997 Elsevier Science S.A.