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Anomalous time of flight behavior of fast ions in laser produced aluminum plasma

In this work, dynamics of multi-charged ions emitted from an aluminum plasma produced by Q switched Nd: Yag laser is studied using time of flight (TOF) measurements from Langmuir Probe (LP) and spectroscopy (STOF) under Ar ambient of 0.02 mbar. The temporal evolution of multi-charged ions, background neutrals and ions is systematically studied for varying laser intensities. The temporal evolution shows all the species have double peak structure for all the laser intensities considered in the study. The fast peak is sharp whereas the slow peak is broad similar to that observed in previous studies. Moreover, higher charged ions have higher velocity, indicating acceleration from the transient electric field produced at the very initial temporal stages of expansion. Interestingly, the fast peak gets delayed, whereas the slow peak advances in time with increased laser intensity, which has not been reported in earlier studies. The observations point towards the possible role of ambipolar electric fields in the unexpected observed behavior of the TOF profiles.