RevModPhys.85.751

Not cancel out thus short duration and high intensity

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Unformatted text preview: ty laser pulses favor electron heating because the hydrodynamic expansion does not have sufficient time to wash out sharp density gradients nor is it dominated by the strong ponderomotive force that steepens the density profile. At this point it is worth mentioning that in most highintensity experiments the main interaction pulse is preceded by prepulses4 which cause early plasma formation and expansion, so that the short-pulse interaction does not occur with a sharp-boundary, solid-density plasma. However, profile steepening at the critical surface is still effective; thus one may expect the interactions to still occur with a sharp density profile, having a lower density jump with respect to a solid interface. Occasionally, ‘‘preplasma’’ formation may also allow additional electron acceleration mechanisms to take place in the underdense plasma region (Esarey, Schroeder, and Leemans, 2009), possibly leading to electron energies much higher than given by Eq. (6). In more recent experiments, advanced pulse cleaning techniques allow minimizing prepulse effects (see Sec. III.E). 1. Heating models A popular electrostatic model of electron heating at a step-boundary plasma was proposed by Brunel (1987). In this model, electrons are dragged out of the surface of a perfect conductor by an oscillating ‘‘capacitor field,’’ extending on the vacuum side, and representing the P component of the incident plus reflected laser electric field. Electrons are considered to be ‘‘absorbed’’ when, after having performed about half of an oscillation on the vacuum side, they reenter the target, there delivering their energy, which is of the order of the oscillation energy in the external field.5 The model thus accounts in a simplified way for the pulsed generation (once per cycle) of hot electrons directed into the target and having an energy, roughly speaking, close to the ‘‘vacuum’’ value (6). This simple model is not self-consistent because, for instance, the capacitor field is assumed to vanish inside the target, implying the presence of a surface charge density. Nevertheless, following Mulser, Ruhl, and Steinmetz (2001) it is possible to provide a ‘‘minimal’’ 1D model, still in the capacitor approximation, where the electrostatic field is calculated self-consistently and an acceleration of electron bunches similar to that inferred by Brunel is apparent. We consider the electric field as the sum of the electrostatic and driver fields, e.g., Ex ¼ Ee þ Ed , where Ed ¼ ~ ~ Ed ðtÞ sin!0 t with Ed ðtÞ a suitable temporal envelope, a step-boundary density profile ni ¼ n0 ÂðxÞ (Z ¼ 1 for 4 In general the main pulse is preceded both by short pulses of duration similar to the main pulse and by a much longer pedestal due to amplified spontaneous emission. 5 This effect is also commonly referred to as ‘‘vacuum heating.’’ See Gibbon (2005b) for a discussion on the origin of the name. 756 Andrea Macchi, Marco Bor...
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This document was uploaded on 09/28/2013.

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