No abstract

Charging of multiple spherical dust grains located in presheath and sheath regions of an rf discharge has been studied using a three-dimensional particle-particle-particle-mesh (P3M) code. First, dust charge, potential, and ion drag force on two dust particles for various interparticle separations are computed. It is found that for dust separations larger than the shielding length the dust parameters for the two dust particles match with the single particle values. As the dust separation is equal to or less than the shielding length, the transverse component of ion force increases which is due to dynamic shielding effect caused by neighboring dust particle. However, dust charge, potential, and ion drag are found not to be affected considerably. Further, dust charge and potential on an arrangement of nine dust particles are computed. The dust charge and potential do not differ much from the single particle values for the presheath. However the dust charges of multiple dust particles in the sheath are much less negative compared to the single dust case which is shown to be due to ion focusing.

A two-dimensional axis symmetric hydrodynamic model was developed to investigate nanosecond laser induced plasma and shock wave dynamics in ambient air over the input laser energies of 50–150 mJ and time scales from 25 ns to 8 μs. The formation of localized hot spots during laser energy deposition, asymmetric spatio-temporal evolution, rolling, and splitting of the plasma observed in the simulations were in good agreement with the experimental results. The formed plasma was observed to have two regions: the hot plasma core and the plasma outer region. The asymmetric expansion was due to the variation in the thermodynamic variables along the laser propagation and radial directions. The rolling of the plasma was observed to take place in the core region where very high temperatures exist. Similarly, the splitting of the plasma was observed to take place in the core region between the localized hot spots that causes the hydrodynamic instabilities. The rolling and splitting times were observed to vary with the input laser energy deposited. The plasma expansion was observed to be asymmetric for all the simulated time scales considered, whereas the shock wave evolution was observed to transfer from asymmetric to symmetric expansion. Finally, the simulated temporal evolution of the electron number density, temperature of the hot core plasma, and the temperature evolution across the shock front after the detachment from the plasma were presented over the time scales 25 ns–8 μs for different input laser pulse energies.

Ablative plasma and a shock wave (SW) in ambient air were experimentally produced using Nd:YAG laser pulses of ∼7 ns width and a wavelength of 532 nm. The numerical simulations of the experiments were performed using a two-dimensional axis-symmetric radiation-hydrodynamics code. The numerical approach to simulate the experimental observations was not straightforward due to the complex behavior of the laser-air interaction and the associated processes, such as plasma formation and SW evolution, that occur concurrently. Hence, the modeling was attempted based on the combination of two laser absorption coefficients and two equations-of-state (EOSs). One form of absorption coefficient was taken from Zel'dovich and Raizer [Physics of Shock Waves and High-Temperature Hydrodynamic Phenomena (Dover Publications/Academic Press Inc., New York, 2012)], which is the sum of photoionization and inverse bremsstrahlung (IB) due to electron-ion collisions, and the other was taken from DeMichelis [IEEE J. Quantum Electron. 5(4), 188 (1969)] that considers the IB due to electron-ion and electron-neutral collisions. Similarly, the two EOSs, namely the ideal gas EOS and the chemical equilibrium application [S. Gordon and B. J. McBride, NASA Ref. Publ. 1311, 1 (1994)] EOS, are considered. The simulated results obtained using four models were compared with each other and with the experimental observations. These models enabled understanding the transient behavior of the laser-induced air plasma and the SW evolution. The results showed that the absorption coefficient and the EOS play a key role in modeling the dynamics of air plasma and SW. We present the results of this study and the models which validate the experimental results the best in terms of the asymmetric plasma expansion, formation of hot spots, plasma splitting and rolling, SW external dynamics such as the transition from a tear-drop to a spherical shape, and shock front velocity.