Implementation basically corresponds to the

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Unformatted text preview: neering, elementary model of plasma simulation formulated by Dawson (1962). Rev. Mod. Phys., Vol. 85, No. 2, April–June 2013 FIG. 5. Numerical solution of the electrostatic ‘‘plasma sheet’’ model based on Eq. (10) plus the exchange of initial position for crossing plasma sheets (see text for details). The trajectories of a limited number of sheets (1 over 20) in the (x; t) plane are shown. The driver field has the profile of an evanescent wave with peak amplitude 0:5me !c=e in vacuum and a sin2 ðt=2Þ rising front with  ¼ 5 T where T ¼ 2=!. A density ne =nc ¼ ð!p =!Þ2 ¼ 5 is assumed. 1985). By considering the driver capacitor field as a model for the magnetic force component, the related electron dynamics may still be described using the above outlined models, but with two significant differences. First, to lowest order the magnetic force oscillates at 2!, thus leading to the generation of hot electron bunches twice per laser period. Second, the oscillating component perpendicular to the surface vanishes for circular polarization (and normal incidence), so that hot electron generation might be strongly suppressed under such conditions. In fact, the vector potential representing a plane, elliptically polarized field may be written as AðxÞ ^ ^ Aðx; tÞ ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðy cos!t þ z sin!tÞ 1 þ 2 (11) with 0  1. Using B ¼ r  A and p? ¼ eA=c for the transverse momentum of electrons, the Àeðv  B=cÞ force can be written as   v e2 @x A2 ðxÞ 1 À 2 ^ 1þ cos2!t ; (12) Àe  B ¼ Àx c 4me c2 1 þ 2 showing that the oscillating component vanishes for circular polarization ( ¼ 1).7 The integral over x of ne fpx , where fpx ¼ hfx i ¼ hÀeðv  B=cÞx i is the steady ponderomotive force density on electrons, equals the total radiation pressure on the target surface. For circular polarization and normal incidence we thus expect radiation pressure to push the target while electron heating is quenched. These conditions have been investigated in order to optimize radiation pressure acceleration of ions versus other mechanisms driven by hot electrons; see Sec. IV.A. 7 A more detailed analysis shows that electron heating is quenched when the parameter  exceeds some threshold value; see Rykovanov et al. (2008) and Macchi, Liseikina et al. (2009). Andrea Macchi, Marco Borghesi, and Matteo Passoni: Ion acceleration by superintense laser-plasma . . . 757 2. Simulations, multidimensional effects, and simple estimates A more quantitative description of laser absorption and hot electron generation requires numerical simulations. To address electromagnetic effects in his model Brunel (1988) performed two-dimensional (2D) PIC simulations in a plane wave, oblique incidence geometry. Several later studies using 1D simulations with the ‘‘boosted frame’’ technique (Bourdier, 1983) are summarized and reviewed by Gibbon et al. (1999). The absorption degree of a P-polarized laser pulse is quit...
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This document was uploaded on 09/28/2013.

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