Self-focusing and nonlinear acceleration process in laser produced plasma

Abstract

Ruby laser intensities exceeding/* -101' W/cm 2 create a predominant acceleration of dense plasma due to nonlinear collisionless interaction resulting mainly from collective effects. Recoil causes confinement of the plasma interior in the form of a superlinearly increased radiation pressure. Similar nonlinear forces produce selffocusing in plasmas at a threshold laser power of only 10' to 10' W. The resulting filaments have intensities/*, from which their diameter can be determined in agreement with measurements of Korobkin and Alcock. These high intensities should allow some observed properties of laser produced plasmas (keV ions, linear increase of the ion charge) to be interpreted on the basis of the nonlinear acceleration described. IntroductionWhen intense laser radiation interacts with dense materials, e.g. gases or solids, many complicated processes occur. After absorption of the radiation, heating, evaporation, and ionisation of the material, the plasma is subjected to an expansion process. The energy of the light is transferred at least initially by a thermalisation process into the plasma, where electron collisions cause an inverse bremsstrahlung mechanism. Besides this process there is direct interaction between the laser light and the plasma owing to collective effects of the electrons. This leads to acceleration of the plasma due to a nonlinear force. The mechanism can be described as a collisionless nonlinear absorption process.Section 2 presents the essential properties of the acceleration process and shows that the nonlinear force is a special part of Landau and Lifshitz's formulation [1] of the ponderomotive force if the collective effects are essential. The mechanism can still be described non-relativistically, and the relativisitic change of the plasma frequency I-2] is still negligible. The properties of the force are described according to a modification of a preceding derivation I-3].Section 3 describes some analytic integrations of the nonlinear force, resulting in the momentum being transferred to the plasma surface region, which is accelerated by a pressure exceeding the radiation pressure by a factor up to more than one hundred. The resulting ion energy increases superlinearly relative to the laser intensity if the electrons have a higher energy due to coherent oscillation than to random (thermal) motion, this being the case at intensities around I* ~ 1014 W/cm 2 for ruby or neodymium lasers.Section 4 confirms the predominance of the nonlinear force over the gasdynamical (thermokinetic) forces at laser intensities 1 > I*. This threshold is also the intensity at which a nonlinear decrease of the collision frequency starts, causing a decrease of the collision induced absorption constant at higher or increasing intensity L This behaviour was deduced on the

Air breakdown characteristics within a TEA laser cavity

“…It is striking that each of these sparks migrates into the focus away from the incoming radiation at ~3 • l0 G cm/s -1. This implies that the plasma density exceeds the critical value where radiation pressure dominates [2,7]. The lack of movement of the first/creakdown may be attributable to this radiation pressure confining effect.…”

Plasmas at High Temperature and Density

“…Using this. The absorption constant and collision frequency V ei (2.35) is then calculated to [146] (6.58) Very simply, this means that we have to use an effective temperature T* 57) instead of the temperature T tb representing the mean energy of random thermal motion only.…”

“…The formula (8.84) for plane waves at perpendicular incidence is rewritten (9.3) The comparison of the nonlinear force with the thermokinetic force would be 179 simply the comparison of the potentials nekTwith that value of the nonlinear force before spatial differentiation in Eq. Apart from the mentioned numerical calculation ofthe nonlinear threshold intensities 1* [146], a more direct estimation can be used by an extension of the calculation of Steinhauer and Ahlstrom [180]. This integration constant is determined by the fact that for a constant dielectric constant there has to be no net nonlinear force if the electromagnetic waves have a· temporally constant amplitude.…”

“…(9.3) where, however, an appropriate integration constant has to be added. !-------· £ Electron Temperature Tth reV} Figure 9.2 Minimum intensity I* for neodymium glass laser radiation to create larger nonlinear forces f NL than thermokinetic forces with an undefined exponential function for the collision induced attenuation [146]. The value of neKT must be compared, therefore, with the excess of E2 +H 2 over its vacuum value.…”

Propagation characters of Gaussian laser beams in collisionless plasma: Effect of plasma temperature

“…In the present paper, we take into account the ponderomotive nonlinearity, 32 considering the cold collisionless plasma with the temperature of electrons under 10 eV. The focusing=defocusing propagation characters of Gaussian laser beam in terms of plasma temperature are studied, herein, the P 0 ; q 0 ð Þplane, dimensionless beam width parameter f n ð Þ, and plasma density profile n=n 0 are discussed.…”