Electrostatic fields and charged particle acceleration in laser produced plasmas

Abstract

While the action of electrostatic double layers in the periphery of an expanding laser produced plasma has been discussed and treated many years ago, some new aspects pioneered recently by Alfven in the theory of cosmic plasmas, indicate the possibility of a new treatment. The thermally produced electrostatic double layer which has been re-derived for a homogeneous plasma shows that a strong upshift of ion energies (by the mass ratio) is possible, in agreement with experiments. The number of accelerated ions is many orders of magnitude smaller than observed at keV and MeV energies. The nonlinear force acceleration could explain the number and energy of the observed fast ions. It is shown, however, that electrostatic double layers can be generated which should produce super-fast ions. This paper presents a derivation of the spread double layers in the case of inhomogeneous plasmas. It is concluded that the hydrodynamically expected multi GeV heavy ions for 10TW laser pulses (generated by relativistic self-focussing and nonlinear forces) should produce super-fast ions up to the TeV range. Further conclusions are drawn from the electrostatically measured upshifted (by 300 keV) DT fusion alphas from laser compressed plasma. An analysis of alpha spectra attempts to distinguish between different models of the stopping power in the plasmas. The analysis preliminarily arrives at a preference for the collective model.

A new hydrodynamic analysis of double layers

“…The macroscopic hydrodynamic theory for the consequent description of the double layers and the generated electric fields required the use of the complete two fluids for electrons and ions including collisions, viscosity, equipartition of temperatures between ions and electrons, optical constants with the correct nonlinear dependence on the laser intensity (about an incorrect formula, see e.g., Duderstadt and Moses, 1983), and including the general expression of the nonlinear force apart from the thermokinetic force given by the gas dynamic pressure (Hora, 1969(Hora, , 1981(Hora, , 1985a. In one spatial dimension, the problem was then to solve the following seven quantities depending on the spatial coordinate x and the time t for given initial and boundary values: the density, temperature and velocity (in the x-direction) for electrons, the same for ions (ne, Te, ve, n_, T_, Vi) and the electric field E (in the x-direction) differing from the external electric and magnetic fields EL and He of the incident laser radiation.…”

“…Therefore the expression "inhomogeneity field" or "dynamic electric field" has been used. In the stationary approximation (8) the inhomogeneity field corresponds to the (thermionic) work function for the electrons that moved from the plasma interior to the vacuum (or an electrode) outside corresponding to the spread Debye sheath (Hora, 1983).…”

“…The resonance in this case is in the evanescent field region below the reflection point of the propagating radiation. When generalizing this derivation (Hora, 1979) to the case with collisions, the pole of the effective dielectric constant suddenly changes from minus infinity to a high positive value and the width of Denisov's resonance maximum can be directly calculated in a transparent way (Hora, 1981).…”

“…A special motivation forstudying thedouble layerin laser-produced plasma wasgivenfromthetheoryof the nonlinear force (Hora,1969(Hora, ,1974(Hora, ,1981Lindl andKaw, 1971;Peratt andWatterson, 1977;Peratt,1979). The electrodynamic, dielectrically caused acceleration of plasma by laser radiationis based ontheforceactingonthe highdensity electron gasin theplasma beingpushed orpulledandtheiongashastofollowthenbytheelectric fields generated between thetwofluids.Whentheessential properties ofthenonlinear forcewerederived fromthespace charge quasi-neutral plasma model (Hora,1969), thementioned fieldsweredisguised by thepresumptions of the model.However, theexistence of the fieldsof the description of the singleelectronmotion (Hora,1971)was evident,anda search wasoverdue sincethebeginning of the workonthenolinearforcein 1965.…”

Pellet fusion gain calculations modified by electric double layers and by spin polarized nuclei

“…The generalized gain calculations include then fuel depletion, and reheat by the stopping power of the charged fusion reaction products (alphas for DT) (Hora et al 1979) and a revised formulation of losses by bremsstrahlung. There are good reasons due to the alpha spectra from laser fusion pellets (Hofstadter 1979;Miley 1981) in favour of the collective model (Rag et al 1976), if the measured (Gazit et al 1979) upshift of the alphas by the electrostatic double layer on expansion is included correctly (Hora 1983). The model for the stopping power used in one series of calculations was the collective model (Ray et al 1976).…”