Investigation of number density, temperature, and kinetic energy of nanosecond laser-induced Zr plasma species for self-generated electric and magnetic fields in axial expansion of plume

Abstract

The number density, temperature, and kinetic energy of laser-induced Zr plasma species have been evaluated experimentally under ultra-high vacuum conditions as a function of 1 to 4 cm axial distances of the plume at laser irradiances ranging from 4.5 to 11.7 G W c m − 2 . Zr plasma parameters have been evaluated using time-of-flight (ToF) measurements by using a Faraday cup (FC) as a diagnostic technique. The amplitudes of the signals as well as the number density and temperature are strongly dependent upon laser irradiance and axial distances. With increasing laser irradiance the laser-induced plasma (LIP) parameters show an increasing trend. The increasing trend of the plasma parameters along with increasing laser irradiance is attributed to an enhanced energy deposition per atom, which is responsible for an enhanced ablation rate, surface temperature, and ablation/shock pressures. However, a decreasing trend is obtained with increasing axial distances, which is explainable based on adiabatic expansion, cooling, and recombination losses. The LIP electron and ion number densities vary from 3.13 × 1 0 13 to 6.81 × 1 0 13 c m − 3 and 1.84 × 1 0 13 to 4.41 × 1 0 13 c m − 3 , respectively. The electron temperature varies from 1.54 to 125 eV, whereas the kinetic energy of the ions varies from 1.2 to 34 keV. The maxima of Zr plasma parameters are achieved away from the target surface. In order to correlate the charge particle distribution and their energies with self-generated electric and magnetic fields (SGEMFs), the measurements have been performed by employing electric and magnetic probes, respectively. The signal profiles of the SGEMFs reveal the quadrupolar distribution, which is attributed to two oppositely generated dipoles whose contribution is strongly dependent upon axial distances and laser irradiances. The strength of both SGEMFs shows an increasing trend with increasing laser irradiance, and a decreasing trend is obtained with increasing probe-target distance. For overall plume axial expansion, the LIP electric and magnetic fields vary from 30 to 3140 V/m and 0.2 to 0.4 T, respectively. Similar trends of the SGEMFs and the axial gradients of the electron-ion number density confirm the quadrupolar structure of plasma, which corresponds to the electron-ion charge separation. This charge separation is due to the spatially evolved number density and temperature gradients in the plasma. Their correlation and optimization will be helpful for using LIP as a powerful source of electric and magnetic fields in ion acceleration and inertial confinement fusion.