Collisionless shocks can be produced as a result of strong magnetic fields in a plasma flow, and therefore are common in many astrophysical systems. The Weibel instability is one candidate mechanism for the generation of su ciently strong fields to create a collisionless shock. Despite their crucial role in astrophysical systems, observation of the magnetic fields produced by Weibel instabilities in experiments has been challenging. Using a proton probe to directly image electromagnetic fields, we present evidence of Weibelgenerated magnetic fields that grow in opposing, initially unmagnetized plasma flows from laser-driven laboratory experiments. Three-dimensional particle-in-cell simulations reveal that the instability e ciently extracts energy from the plasma flows, and that the self-generated magnetic energy reaches a few percent of the total energy in the system. This result demonstrates an experimental platform suitable for the investigation of a wide range of astrophysical phenomena, including collisionless shock formation in supernova remnants, large-scale magnetic field amplification, and the radiation signature from gamma-ray bursts.The magnetic fields required for collisionless shock formation in astrophysical systems may either be initially present, for example in supernova remnants or young galaxies 1 , or they may be selfgenerated in systems such as gamma-ray bursts (GRBs; ref. 2). In the case of GRB outflows, the intense magnetic fields are greater than those which can be seeded by the GRB progenitor or produced by misaligned density and temperature gradients (the Biermannbattery effect) 3,4 . It has long been known that instabilities can generate strong magnetic fields, even in the absence of seed fields. Weibel considered the development of an electromagnetic instability driven by the electron velocity anisotropy in a background of resting ions 5 . The signature of the instability is a pattern of current filaments stretched along the axis of symmetry of the electron motion. The process is quite general, and subsequent work has shown that such instabilities can be excited in both non-relativistic and relativistic shocks. This general nature makes the Weibel instability common in astrophysical systems [6][7][8] . The instability provides a mechanism by which the electromagnetic turbulence associated with the formation of collisionless shocks is fed by the flow anisotropy of the protons (and ions) stochastically reflecting off of the shock 9-11 , and leading ultimately to strong particle acceleration in GRB's (ref. 12).

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A distinctive way of quantitatively imaging inertial fusion implosions has resulted in the characterization of two different types of electromagnetic configurations and in the measurement of the temporal evolution of capsule size and areal density. Radiography with a pulsed, monoenergetic, isotropic proton source reveals field structures through deflection of proton trajectories, and areal densities are quantified through the energy lost by protons while traversing the plasma. The two field structures consist of (i) many radial filaments with complex striations and bifurcations, permeating the entire field of view, of magnetic field magnitude 60 tesla and (ii) a coherent, centrally directed electric field of order 10 9 volts per meter, seen in proximity to the capsule surface. Although the mechanism for generating these fields is unclear, their effect on implosion dynamics is potentially consequential.

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High-resolution spectrometry of charged particles from inertial-confinement-fusion ͑ICF͒ experiments has become an important method of studying plasma conditions in laser-compressed capsules. In experiments at the 60-beam OMEGA laser facility ͓T. R. Boehly et al., Opt. Commun. 133, 495 ͑1997͔͒, utilizing capsules with D 2 , D 3 He, DT, or DTH fuel in a shell of plastic, glass, or D 2 ice, we now routinely make spectral measurements of primary fusion products ͑p, D, T, 3 He, ␣͒, secondary fusion products ͑p͒, ''knock-on'' particles ͑p, D, T͒ elastically scattered by primary neutrons, and ions from the shell. Use is made of several types of spectrometers that rely on detection and identification of particles with CR-39 nuclear track detectors in conjunction with magnets and/or special ranging filters. CR-39 is especially useful because of its insensitivity to electromagnetic noise and its ability to distinguish the types and energies of individual particles, as illustrated here by detailed calibrations of its response to 0.1-13.8 MeV protons from a Van de Graaff accelerator and to p, D, T, and ␣ from ICF experiments at OMEGA. A description of the spectrometers is accompanied by illustrations of their operating principles using data from OMEGA. Sample results and discussions illustrate the relationship of secondary-proton and knock-on spectra to capsule fuel and shell areal densities and radial compression ratios; the relationship of different primary fusion products to each other and to ion temperatures; the relationship of deviations from spherical symmetry in particle yields and energies to capsule structure; the acceleration of fusion products and the spectra of ions from the shell due to external fields; and other important physical characteristics of the laser-compressed capsules.

International audienc

Direct drive implosions of targets filled with different mixtures of D 2 and 3 He gas on the OMEGA laser system [T.R. Boehly et al., Opt. Commun. 133, 495 (1997)] have shown an unexpected scaling of experimental nuclear yields. At temperatures above a few eV, D 2 and 3 He gasses are fully ionized; and hydrodynamically-equivalent fuels with different ratios of D 2 and 3 He can be chosen to have the same mass density, total particle density and equation of state. Implosions with a 50-50 mixture of D: 3 He by atom consistently result in measured nuclear yields half of that anticipated by scaling from measured yields of implosions with pure D 2 and nearly pure 3 He. This observation is seen over a wide range of experimental configurations, including targets with a variety of shell thicknesses and fill pressures, simultaneously for two different nuclear yields (DD and D 3 He), as well as for shock and compression yields. A number of possible mechanisms to cause the scaling are considered, but no dominant mechanism has been identified. 1 also Visiting Senior Scientist at LLE.

Shock ignition is a two-step inertial confinement fusion concept where a strong shock wave is launched at the end of the laser pulse to ignite the compressed core of a low-velocity implosion. Initial shock-ignition technique experiments were performed at the OMEGA Laser Facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] using 40-μm-thick, 0.9-mm-diam, warm surrogate plastic shells filled with deuterium gas. The experiments showed a significant improvement in the performance of low-adiabat, low-velocity implosions compared to conventional “hot-spot” implosions. High areal densities with average values exceeding ∼0.2g∕cm2 and peak areal densities above 0.3g∕cm2 were measured, which is in good agreement with one-dimensional hydrodynamical simulation predictions. Shock-ignition technique implosions with cryogenic deuterium and deuterium-tritium ice shells produced areal densities close to the 1D prediction and achieved up to 12% of the predicted 1D fusion yield.