On the dynamics of a non-equilibrium Cu plasma produced by an excimer laser interaction with a solid

We report here on the expansion dynamics of a non-equilibrium plasma produced by an excimer laser interaction with a Cu solid target. Its characteristics were investigated in the fast and slow time regime by two Faraday cups of different diameter. The larger cup had an 8 cm diameter collector and was fixed along a drift tube at a distance of 20 cm from the plasma source; the smaller cup had a 3.3 cm diameter collector and was fixed transversally to the target at a distance of about 6 cm. During the experiments the target support signal was also recorded. The laser beam was focused onto the target and the spot dimensions were analysed by scanning it on the lens focal plane. An average power density on the target of 0.3 GW cm−2 was achieved with a 15 cm focal length lens. Using signals from the Faraday cups we obtained information on the overall plasma evolution in the slow and fast time regimes. Fitting the plasma current waveform by a ‘shifted’ Maxwell– Boltzmann distribution, a Knudsen-layer temperature of 5.3×105 K (∼50 eV) and a drift velocity of 5300 m s−1 resulted. The system efficiency in ablation yield and ion production was 0.235 μg pulse−1 and 5 × 1013 ion pulse−1, respectively.