INVESTIGATION OF LASER BLOW-OFF ATOMIC-BEAMS BY ELECTRON-IMPACT EXCITATION

Electron impact excitation in the edge region of the MT-1 tokamak was used to measure the velocity distribution of a sodium laser blow-off beam in a single shot. The different dependencies of the propagation velocity of the blow-off neutral beam on the laser fluence at different sodium layer thicknesses reveal different regimes of the blow-off process. In the case of thick targets laser ablation is the main accelerating mechanism giving a upsilon approximately I1/3 dependence. Thin targets, however, burn through during the laser pulse, and simple thermal heating process results in upsilon approximately I1/2 in good agreement with the experiments.