Langmuir Probe Measurements of Nd-YAG Laser-Produced Copper Plasmas

The plasma was produced by focusing Nd-YAG laser pulses of 1064-nm wavelength and 10-ns duration onto a copper target. A time-resolving plane Langmuir probe was used to record the electron and ion currents during the plasma expansion in vacuum. The Langmuir probe theory was used to determine the temporal variation of the plasma potential, electron temperature, and ion density. At the time of maximum ion current, the estimated plasma potential, electron temperature, and ion density were 2.42 V, 3.28 eV, and 2.30 × 10¹² cm^-3, respectively. In addition, a time-of-flight signal was used to determine the ion density and velocity of the various ion components as a function of laser irradiance and probe angle w.r.t. target surface normal. For the range of laser irradiance, 4-13 × 10⁸ W/cm², we found an ion density in the range of 6.88 × 10¹⁰-2.64 × 10¹² cm^-3 and an ion velocity in the range of 2.2-4.6 × 10⁶ cm s^-1 at the time of maximum ion current. In particular, the velocity of the fastest plume component was most affected by the irradiance. The estimated threshold irradiance for the onset of plasma was 3.95 × 10⁸ W/cm². The experimental scaling laws for the variation of ion velocity and ion charge with the laser irradiance and probe angle were proposed.