Time-resolved measurements of hydrogen and deuterium fluxes in the ASDEX plasma boundary

Abstract

Hydrogen and deuterium fluxes parallel to the toroidal magnetic field were measured in the plasma boundary of ASDEX using graphite collector probes. Time resolution of the order of 100 ms was obtained by rotating the cylindrical probes behind an aperture during the discharge. The trapped amount of hydrogen was determined by subsequent thermal desorption, in the analysis of deuterium the D( 3He, p) 4 He nuclear reaction was used. Both methods yield quantitative results. Measurements were done for limiter and divertor discharges in the range of 4 to 20 cm outside the limiter or separatrix. The time distributions show a maximum flux at the beginning and at the end of the discharge. The relatively lower flux during the plateau phase of the discharge is in the range of 10 15 to 2 × 10 17 cm −2/s , depending on the radial probe position, the maximum values are higher by a factor of 5 to 50. During neutral hydrogen injection an additional maximum can be observed. The radial 1/e- decay length is about 0.9 cm in front and 0.4 cm behind the fixed limiter. The results are compared with independent measurements in ASDEX and other plasma machines.