On temperature measurements in a non- equilibrium plasma

We report temperature and drift velocity measurements on the ion component of a plasma produced by the interaction of a UV laser beam with a Cu solid target. A XeCl excimer laser (308 nm wavelength, 20 ns pulse duration, 70 mJ pulse energy) was used, achieving a power density on the target of about 0.3 GW/cm2 after beam focusing. The diagnostics was performed by means of ion collector measurements. Suitable theoretical expressions were derived for fitting the recorded ion current, under the assumption of a “shifted” Maxwell-Boltzmann distribution for the ion velocity. Two specific Faraday cups were used, which sampled the plasma in two orthogonal directions. So, a longitudinal plume drift velocity value of 5300 m/s resulted. Surprisingly, two different temperature values were found: 5·105 K (~ 50 eV) and 5·104 K (~ 5 eV) for the “longitudinal” and “transverse” temperature, respectively.