INSTANTANEOUS MEASUREMENT OF HYDROGEN ISOTOPE RETENTION IN COLLECTOR PROBES IN THE SCRAPE-OFF LAYER OF A TOKAMAK BY PULSED LASER DESORPTION

The pulsed laser desorption technique is employed for the first time for the instantaneous analysis of hydrogen isotope fluxes in the scrape-off layer (SOL) of a tokamak. The laser desorption system developed for the TdeV tokamak is described. It has been used recently for the investigation of hydrogen isotope behavior in the SOL of TdeV during an isotope exchange experiment. A silicon probe is utilized for the collection of deuterium and hydrogen fluxes in both ion and electron drift directions. After exposure to a tokamak discharge, the hydrogen isotopes retained in the probe are analyzed in situ between two tokamak discharges by means of laser desorption. The measurements of the isotopic ratio D/(H+D) from hydrogen and deuterium retention in the Si collector probes show good agreement with neutral particle analysis, optical spectroscopy, mass spectrometry, and nuclear microanalysis. At the beginning of the experiment, the isotopic ratio decreases radially from the separatrix due to hydrogen recycling from the wall. As the changeover proceeds, the radial profile of the isotopic ratio becomes flat, showing evidence of the replacement of H with D in the wall. Hydrogen isotope retention is larger on the ion side than on the electron side. Measurements done during a plasma density scan show that the radial profile of retention changed with density due to the variation of the incident fluxes.