Temporally resolved target potential measurements in laser-target interactions

Rapid (100 ps) voltage oscillations have been observed in a capacitively coupled target irradiated with 3.5 ns pulses of Nd-YAG light at irradiances of between 5*1012 and 2*1015 W cm-2 on a 40 mu m spot. The measurements were made using a fast (40 ps rise-time) Pockels cell intercepting a diagnostic light pulse to a streak camera. The oscillations were found to depend on the spot size and the shape of the laser pulse. The oscillation frequencies found (up to 12 GHz) were too low to correspond to plasma instabilities. Double layers and plasma turbulence are proposed as the cause of the voltage oscillations.