The interaction of intense fs laser pulses with thin foils that have an imposed deformation is compared with thick targets that develop bow shocks. Both target types yield good absorption. Up to 80% absorption is obtained for a 0.2µm thick, 15 times over-dense foil at 4 · 10 18 W/cm 2 . A value of 50% is obtained for a 4µm thick, 2 times over-dense thick target at 10 18 W/cm 2 . For comparable extension and curvature of the laser-plasma interfaces absorption levels in both targets become similar. In both absorption scales weakly with intensity and density. Energy transport in thin foils and thick targets, however, is different. 52.40.NkAbsorption of super-intense laser pulses in solids is based on collective mechanisms like the Brunel effect [1,2], anomalous skin effect [3,4], or j × B-heating [5] with continuous transitions from one to the other and wide overlaps among them. Particle-In-Cell (PIC) and Vlasov simulations in one dimension (1D) have shown that absorption varies between 5 − 15% at normal incidence to at most 60% at about 75• incidence for irradiances not exceeding several. Beyond this intensity emission of harmonics and effects of self-generated dc magnetic fields lead to a reduction of this maximum, as well as its shift towards smaller angles of incidence and , eventually, to the formation of secondary relative maxima in the angular absorption behaviour [7]. The signature of collective absorption is the generation of jets of fast electrons in the relativistic domain. Owing to E −3/2 energy scaling of the collision frequency, collisional absorption becomes inefficient at irradiances Iλ 2 ≥ 10 17 Wcm −2 µm 2 [8,9]. All beam photon conversion into fast electrons occurs over skin lengths l s ≈ c/ω p much less than a vacuum wavelength λ, simply because the free electron current induced by the laser field always tends to cancel the incident field [10]. If so, absorption is bounded to the thin critical layer. However, the question arises whether the geometry of the interaction region is a sensitive parameter for the degree of absorption. So far this problem has never been investigated systematically. The question to which degree absorption can increase in deformed targets and whether thin plasma layers already lead to good absorption is an interesting problem in itself, e.g. for better understanding the relevant interaction physics, as well as it is essential for applications. Three substantial applications in which good absorption is highly desirable are (i) the generation of collimated intense jets of energetic electrons, (ii) broad-band intense X-ray sources in thin foils (for instance for back-lighting), and (iii) the fast igniter scheme for ICF [11].The problems addressed can be reduced to the following three questions: (i) How does absorption change when target deformation is naturally present due to bow shocks and hole boring or when it is imposed as for corrugated targets? (ii) What is the energy current dynamics (efficiency into forward and lateral directions) in such targets? (iii) Is there a d...
A fast-ignitor scheme for inertial confinement fusion is proposed which works without hole boring. It is shown that a thermonuclear burn wave starts from the pellet corona when an adequate amount of energy (typically 10 kJ) is deposited in the critical layer by a petawatt laser ("coronal ignition"). Burn efficiencies as high as predicted for standard central spark ignition are achieved. In addition, the scheme is surprisingly insensitive to large deviations from spherical precompression symmetry. It may open a new prospect for direct drive.
In this paper, we want to discuss different hydrodynamic schemes for fast ignition and some of their basic effects. Relativistic hydrodynamics and a covariant generalized Ohm's law are presented. Matter perforation (hole boring) by an intense laser pulse is investigated by 2D numerical simulations and a simple formula for the perforation speed is derived. Furthermore, we study macroparticle acceleration by an intense laser beam and the possibility to provide an energetic and massive projectile for ballistic ignition. The dynamics of ignition of a precompressed D–T mixture is illustrated by numerical simulations in planar 2D geometry using direct laser irradiation and macroparticle impact as external drivers. Electron heat conduction is found to be responsible for an efficient burn wave propagation which prevents the hot gas bubble of burning fuel from deflagrating into vacuum.
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