International audienceELECTRONS in a plasma undergo collective wave-like oscillations near the plasma frequency. These plasma waves can have a range of wavelengths and hence a range of phase velocities1. Of particular note are relativistic plasma waves2,3, for which the phase velocity approaches the speed of light; the longitudinal electric field associated with such waves can be extremely large, and can be used to accelerate electrons (either injected externally or supplied by the plasma) to high energies over very short distances2á¤-4. The maximum electric field, and hence maximum acceleration rate, that can be obtained in this way is determined by the maximum amplitude of oscillation that can be supported by the plasma5á¤-8. When this limit is reached, the plasma wave is said to ᤘbreakᤙ. Here we report observations of relativistic plasma waves driven to breaking point by the Raman forward-scattering instability9,10 induced by short, high-intensity laser pulses. The onset of wave-breaking is indicated by a sudden increase in both the number and maximum energy (up to 44 MeV) of accelerated plasma electrons, as well as by the loss of coherence of laser light scattered from the plasma wave

We have demonstrated efficient coupling of 0.35 p, m laser light for radiation production in inertial confinement fusion (ICF) cavity targets. Temperatures of 270 eV are measured in cavities used for implosions and 300 eV in smaller cavities, significantly extending the temperature range attained in the laboratory to those required for high-gain indirect drive ICF. High-contrast, shaped drive pulses required for implosion experiments have also been demonstrated for the first time. Low levels of scattered light and fast electrons are observed, indicating that plasma instability production is not significant.PACS numbers: 52.50.Jm, 52.40.Nk, 52.70.La Inertial confinement fusion (ICF) uses high powered laser or particle beams to compress and heat capsules containing fusion fuel with the goal of producing thermonuclear energy [1,2]. One proposed method for ICF is x-ray drive where high powered beams heat high-Z cavities, or Hohlraums, converting the driver energy to x rays which implode the capsule [3]. Present indirect drive target designs predict ignition, and gain can be attained with a 1-2 MJ laser for radiation drive temperatures on the order of 300 eV [4]. In this Letter we report experiments using the Nova laser that demonstrate efficient cavity heating with 0.35 p, m light to the temperatures required for these ignition target designs. Radiation temperatures in excess of 270 eV have been obtained in cavities used for implosions [5], while 300 eV temperatures have been obtained in smaller cavities. These radiation cavities are the highest thermal sources measured in the laboratory. The temperature scaling is consistent with a simple power balance model successfully used to model previous experiments at lower temperatures [6,7], extending its proven range of validity. We have demonstrated that shaped radiation drive pulses required to control shock preheat can easily be attained by varying the incident laser power. Laserplasma instabilities [8] that could reduce coupling efficiency and produce superthermal electrons appear not to be significant. Fast electrons are low, typically less than a few percent, indicating superthermal electron preheat is small. In addition to high density implosion experiments [9,10], these cavities have been used for a variety of radiation heating experiments including hydrodynamic instability studies of radiatively accelerated material both in planar [11,12] and convergent systems [13] and opacity experiments of radiatively heated material [14].

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Nearly 10 years of Nova [E. M. Campbell, Laser Part. Beams 9, 209 (1991)] experiments and analysis have lead to a relatively detailed quantitative and qualitative understanding of radiation drive in laser-heated hohlraums. Our most successful quantitative modeling tool is two-dimensional (2-D) LASNEX numerical simulations [G. B. Zimmerman and W. L. Kruer, Comments Plasma Phys. Controlled Fusion 2, 51 (1975)]. Analysis of the simulations provides us with insight into the physics of hohlraum drive. In particular we find hohlraum radiation conversion efficiency becomes quite high with longer pulses as the accumulated, high-Z blow-off plasma begins to radiate. Extensive Nova experiments corroborate our quantitative and qualitative understanding.