Effects of rapidly decaying plasmas on Langmuir probe measurements

Sheath motion, displacement current, and probe edge effects on temporal Langmuir probe (LP) ion saturation current measurements are investigated for a pulsed laser produced plasma during the period 100 ns→1000 ns after the turn on of the laser pulse. The plasma has a large volume (hundreds of cm3) and a high initial plasma density (ne>1013 cm−3). The sheath motion and edge effects are found to be very important, but the displacement current is found to be very small. We present both a quantitative correction for the effects of rapidly decaying plasmas on LP ion saturation current measurements and a validity condition for this method. The results are compared with the densities predicted from electron saturation currents, and the former are ⩽30% lower than the latter. The corrected probe measurements are utilized to determine the plasma recombination coefficient. It is found to be in good agreement with the results obtained by Stalder and Eckstrom [J. Appl. Phys. 72, 3917 (1992)] who utilized a microwave method.