Influence of Axial Magnetic Field on Angular Distribution of Charge and Energy of Laser Produced Slow and Fast Tungsten Ion Groups

Angle resolved measurements of laser produced ions not only provides an insight into the plasma dynamics but also important for many applications of laser produced plasma (LPP). We report the effect of the externally applied magnetic field on the angular distribution of laser produced tungsten (W) ions. Plasma plume was generated on the surface of a pure W target, placed inside a vacuum chamber, by focussing the nanosecond pulsed laser beam onto the target surface. A permanent magnet, with maximum field strength of 0.23 T, on the surface of W target, was installed behind the target. Angular distribution of W ion flux was measured with the help of an ion collector (IC), working in time of flight (TOF) configuration. Enhancement of ion charge state from 3+ to 4+ on application of magnetic field was confirmed by Ion energy analyser (IEA). Bimodal structure of W ion signals, representing slow and fast ion groups, was observed in the absence as well as in the presence of magnetic field (B). Combination of Shifted Maxwell Boltzmann and Gaussian function was fitted to bimodal tungsten ion signal for the first time which revealed that thermal and non-thermal nature of slow and fast ion groups is not limited to femtosecond laser produced plasma only. In the presence of a magnetic field, a two fold increase in total charge of the fast ion component, while no enhancement in the total charge of the slow ion component was observed. Angular distribution of ion flux was discussed under the framework of Anisimov model, which suggested the narrowing down angular spread of ion flux in the presence of magnetic field. No significant effect of magnetic field on peak energy of slow ion group was observed. Whilst energy of fast ion group, in the absence of magnetic field, was found higher as compared to the case of presence of magnetic field, especially in the angular range of 20 o to 70 o angle.