A study of exploding-pusher laser-induced implosions using X-ray backlighting

Abstract

1.3-keV X-ray backlighting of microballoons imploded with 0.8-TW, 50-ps Nd-glass-laser pulses has been performed early during the irradiation. Transmission measurements have been interpreted in terms of fast-electron energy deposition. Targets with the thinnest walls are shown to be pre-heated up to more than 200 eV.

Determination of energy transport parameters in flat, layered targets irradiated by picosecond Nd-laser pulses

“…On the other hand, suprathermal electrons attributed to resonance absorption [7], lead to target pre-heating. This effect has been deduced from hard-X-ray spectra [2,8], K a line radiation in multi-layered targets [9] or, more recently, from X-ray backlighting of coated glass microballoons [10]. Both thermal flux inhibition and fast-electron pre-heat are required in numerical simulations and scaling models for exploding-pusher experiments [1][2][3][4][5][6][7][8][9][10][11].…”

Laser implosion of microballoons: study of the transition from exploding-pusher to ablative regime

“…The experiments were performed with a pulse duration of 500 ps. Other results obtained at 50 and 200 ps and published elsewhere [7,35,45] will be referred to during the discussion. The characteristics of the more representative imploded targets, together with the nominal value of the laser irradiance on target 0i, are reported in Tables II and III.…”

“…Numerous studies [38] have shown that the classical thermal flux expression: q=-xVT e (5) where x « T | / 2 is not applicable when the characteristic length of the thermal gradient reaches the order of the electron-ion mean free path or when the flux value given by expression (5) is greater than the free-streaming theoretical limit: = 0 . 6 4 n e k T e m e 1 / 2 (6) To keep a local formulation, it is assumed that the effective electron heat flux is given by the usual harmonic mean formula: (7) where qL = fqFS> i-e -the limit flux is reduced below the classical free-streaming value by a factor f which can be adjusted from numerical simulation. Usually, f is kept constant and equal to 5 X 10~2.…”

“…First implosion experiments carried out by laboratories involved in laser inertial confinement were often performed with targets functioning in the so-called 'exploding-pusher' regime [2][3][4][5][6][7] which raised much interest for some fundamental and technical reasons:…”

Twenty-two years of laser-matter work at Centre d'Etudes de Limeil-Valenton (CEL-V)

“…The first experiments with a 1.06 jitm laser beam were directed at the exploding pusher regime, governed at this wavelength by suprathermal electrons, and its behaviour in the case of large irradiance defects. A direct measurement of shell heating was achieved by X-ray backlighting [20]. The transition to a more ablative regime in the laser range of 0.6 TW/500 ps was analysed with DT-filled microballoons coated with a plastic ablator of variable thickness [2.1 ]; the analysis was based on the evolution of preheat, hydrodynamic efficiency and density/ temperature performances (Fig.…”