## Current Assignment

Currently I am working as a post-doctoral fellow at Dept. of Physics, Umeå University, Sweden. So far here I have worked/working on problems like (a) wakefield generation in magnetized plasmas, (b) molecular dynamics of laser cluster interaction, (c) spatial auto-resonance acceleration, and (d) electron spin effects in plasmas.

(a) We consider wakefield generation in plasmas by electromagnetic pulses propagating perpendicular to a strong magnetic field, in the regime where the electron cyclotron frequency is equal to or larger than the plasma frequency. PIC simulations reveal that for moderate magnetic field strengths previous results are re-produced, and the wakefield wavenumber spectrum has a clear peak at the inverse skin depth. However, when the cyclotron frequency is significantly larger than the plasma frequency, the wakefield spectrum becomes broad-band, and simultaneously the loss rate of the driving pulse is much enhanced. A set of equations for the scalar and vector potentials reproducing these results are derived, using only the assumption of a weakly nonlinear interaction. (Phys. Rev. E, in press, 2011)

(b) Mainly there are three different approaches to understand the interaction dynamics of laser–cluster interaction, namely hydrodynamics, particle-in-cell (PIC) and molecular dynamics (MD). The earliest model developed to study the cluster interaction was based on hydrodynamic approach. Despite the success of fluid approach in predicting ionization, charge states, resonance absorption, the PIC, and MD models better suited to study cluster dynamics because clusters are barely a collection of few particles rather than a fluid. In view of this a molecular dynamic model is developed to study the laser interaction with the atomic clusters. The model is tested with the published results for deuterium, argon, and xenon clusters. The MD model is used to study the interaction dynamics of argon cluster driven by relatively long (~ 100 fs) laser pulses, and results are validated against the experimental findings. The ion energies of 200 keV are in good agreement with the experiments along with the asymmetry in ion energy distribution function (IEDF) in laser polarization direction, which is explained by the charge flipping model. The effect of cluster density on the energetics of the laser–cluster interaction is also being studied. One gets a qualitative understanding of the presence of optimum cluster size for maximum ion energies. It can be anticipated to have some correlation between cluster size and cluster density in the experimental scenario. Currently planning to include the neutron yield after D-D fusion in the code. (Phys. Plasmas, 18, 053102, 2011)

(c) The cyclotron auto-resonance acceleration of electrons in a stationary inhomogeneous magnetic fields is studied. Auto-resonance acceleration is a phenomenon in the combined steady state magnetic field and microwave fields. The fundamental principle is to maintain the cyclotron frequency with that of the microwave frequency, which can be attained by changing the strength of the magnetic field spatially so that the relativistic mass increase of the electron can be compensated. We have developed a simulation tool to study the spatial auto-resonance acceleration mechanisms and the results are benchmarked with previous studies (Phys. Rev. ST Accel. Beams, 11, 041302, 2008), excellent agreement is observed. The applicability of the model is limited because of the enhancement in the diamagnetic forces, which completely reflect the electrons out of the microwave cavity. In order to sustain the resonance condition for a longer time we are now thinking to incorporate the travelling EM waves instead of standing microwaves. Still work is under development in this direction.

(d) Recently our group has developed a theoretical model which include the spin-velocity correlation in MHD regime, its been suggested that in the MHD regime a single fluid electron model with spin correlations is equivalent to a model with spin-up and spin-down electrons constituting different fluids, but where the spin-velocity correlations are omitted. For the analytical calculations the degree of complexity is the same for single fluid and two fluid model. However, the two fluid model has a great advantage in case one would like to do Particle-In-Cell (PIC) simulations.

In view of this a 1D PIC code LPIC++ is modified to incorporate the inclusion of an extra species of particles. The code is also modified to incorporate the essential physics aspects related to the spin of the particles. This work is still in preliminary stage, but we do believe that this study would be of interest for astrophysics community.

(a) We consider wakefield generation in plasmas by electromagnetic pulses propagating perpendicular to a strong magnetic field, in the regime where the electron cyclotron frequency is equal to or larger than the plasma frequency. PIC simulations reveal that for moderate magnetic field strengths previous results are re-produced, and the wakefield wavenumber spectrum has a clear peak at the inverse skin depth. However, when the cyclotron frequency is significantly larger than the plasma frequency, the wakefield spectrum becomes broad-band, and simultaneously the loss rate of the driving pulse is much enhanced. A set of equations for the scalar and vector potentials reproducing these results are derived, using only the assumption of a weakly nonlinear interaction. (Phys. Rev. E, in press, 2011)

(b) Mainly there are three different approaches to understand the interaction dynamics of laser–cluster interaction, namely hydrodynamics, particle-in-cell (PIC) and molecular dynamics (MD). The earliest model developed to study the cluster interaction was based on hydrodynamic approach. Despite the success of fluid approach in predicting ionization, charge states, resonance absorption, the PIC, and MD models better suited to study cluster dynamics because clusters are barely a collection of few particles rather than a fluid. In view of this a molecular dynamic model is developed to study the laser interaction with the atomic clusters. The model is tested with the published results for deuterium, argon, and xenon clusters. The MD model is used to study the interaction dynamics of argon cluster driven by relatively long (~ 100 fs) laser pulses, and results are validated against the experimental findings. The ion energies of 200 keV are in good agreement with the experiments along with the asymmetry in ion energy distribution function (IEDF) in laser polarization direction, which is explained by the charge flipping model. The effect of cluster density on the energetics of the laser–cluster interaction is also being studied. One gets a qualitative understanding of the presence of optimum cluster size for maximum ion energies. It can be anticipated to have some correlation between cluster size and cluster density in the experimental scenario. Currently planning to include the neutron yield after D-D fusion in the code. (Phys. Plasmas, 18, 053102, 2011)

(c) The cyclotron auto-resonance acceleration of electrons in a stationary inhomogeneous magnetic fields is studied. Auto-resonance acceleration is a phenomenon in the combined steady state magnetic field and microwave fields. The fundamental principle is to maintain the cyclotron frequency with that of the microwave frequency, which can be attained by changing the strength of the magnetic field spatially so that the relativistic mass increase of the electron can be compensated. We have developed a simulation tool to study the spatial auto-resonance acceleration mechanisms and the results are benchmarked with previous studies (Phys. Rev. ST Accel. Beams, 11, 041302, 2008), excellent agreement is observed. The applicability of the model is limited because of the enhancement in the diamagnetic forces, which completely reflect the electrons out of the microwave cavity. In order to sustain the resonance condition for a longer time we are now thinking to incorporate the travelling EM waves instead of standing microwaves. Still work is under development in this direction.

(d) Recently our group has developed a theoretical model which include the spin-velocity correlation in MHD regime, its been suggested that in the MHD regime a single fluid electron model with spin correlations is equivalent to a model with spin-up and spin-down electrons constituting different fluids, but where the spin-velocity correlations are omitted. For the analytical calculations the degree of complexity is the same for single fluid and two fluid model. However, the two fluid model has a great advantage in case one would like to do Particle-In-Cell (PIC) simulations.

In view of this a 1D PIC code LPIC++ is modified to incorporate the inclusion of an extra species of particles. The code is also modified to incorporate the essential physics aspects related to the spin of the particles. This work is still in preliminary stage, but we do believe that this study would be of interest for astrophysics community.