Time-of-Flight detection of Al ions from laser produced plasma

Background: The interaction of highly intense laser pulses with solid targets covers a wide range of phenomena over several orders of magnitude in laser intensity. Time-of-flight measurements provide an accurate reconstruction of the ion energy spectra. We report on the observation of aluminium ions from a laser-plasma interaction by a scintillator-based detector with adjustable dynamic range. Methods: Data have been taken at a 30 fs, 200 TW pulsed Ti:Sapphire laser focused on aluminium foils with 1.8 and 12.5 μm thickness. A time-of-flight detector consisting in a plastic scintillator with fibre-optic coupling to a PMT has been mounted 50 cm behind the target. The PMT output pulses have been recorded on a fast oscilloscope. Results: After an initial peak caused by prompt X-rays and relativistic electrons, a second peak has been observed in the time-of-flight spectra after several microseconds. It corresponds to Al ions with energies of the order 1 keV. Quantitative spectra have been reconstructed. A PIC simulation of the plasma expansion gives an explanation for the acceleration of initially thermal ions to the observed energies. Discussion: At the given ion energies the detection mechanism, based on electrostatic charge transfer, is different to the usual ionisation process in scintillators. Additional tests have been performed to corroborate its working principle. At the nominal, focused laser intensity with femtosecond pulses, proton acceleration to MeV energies would be expected, but is excluded by our observations. This detailed study of the interaction kinematics indicates the formation of a cold plasma by a prepulse several picoseconds ahead of the peak intensity.