Production of a short hydrogen gas pulse by laser-induced desorption from titanium hydride for investigation of transport processes in fusion plasmas

The investigation of particle transport in fusion plasmas requires short intense gas pulse to produce spatially and temporally localised density perturbations. Such gas pulses can be produced by laser induced desorption of hydrogen from titanium hydride. To study the properties of this method samples of TiH1.5 have been heated by 1 ms ruby laser with an energy content up to 10 J. With target spot diameters of 4.5 mm desorption rates of 2x1018 hydrogen atoms have been achieved. The duration of the gas puffs is closely coupled to the laser pulse. Computer simulations have shown that the desorption rate is probably reduced by surface effects. When heating the same target spot repeatedly the release of hydrogen is reduced drastically. This due to the depletion of the 100 μm thick heated region at the target surface which cannot be refilled by hydrogen from the bulk because of the low diffusion coefficient of hydrogen in titanium at temperatures below 200deg C. For practical applications it therefore seems to be necessary to change the target spot after each shot. With respect to radiation cooling of the plasma the titanium content of the hydrogen pulses is important. In our experiments we found a 1per mille contamination with titanium atoms mainly produced by evaporation from edges of cracks in the target surface. Such cracks inevitably occur during the target preparation. By proper conditioning of the target surface with short giant laser pulses the titanium contamination could be reduced by a factor of two without reduction of the hydrogen release. The results of our measurements show that with a 40 J laser system gas pulses suitable for transport studies can be produced. (orig.)