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The role of self generated magnetic fields in the transport of a heat wave following a nanosecond laser irradiation of a solid target is investigated. Magnetic fields are expected to localize the electron carrying the heat flux but at the same time are affected in their evolution by the heat flux itself. We performed simultaneous measurements of heat wave propagation velocity within the target and magnetic fields developing on the target surface. These were compared to results obtained by numerical magneto-hydrodynamic modeling, including self-generated B fields. The comparison shows that longitudinal heat flow is overestimated in the simulations. Similarly, but most notably, the radial expansion of the magnetic fields is underestimated by the modeling. The two are likely linked, the more pronounced radial drift of B-fields induces a rotation of heat flux in the radial direction, and corresponding longitudinal heat flux inhibition. This suggests the need for improving present modeling of self-generated magnetic fields evolution in high power laser-matter interaction.
The use of laser-accelerated protons as a particle probe for the detection of electric fields in plasmas has led in recent years to a wealth of novel information regarding the ultrafast plasma dynamics following high intensity laser-matter interactions. The high spatial quality and short duration of these beams have been essential to this purpose. We will discuss some of the most recent results obtained with this diagnostic at the Rutherford Appleton Laboratory (UK) and at LULI - Ecole Polytechnique (France), also applied to conditions of interest to conventional Inertial Confinement Fusion. In particular, the technique has been used to measure electric fields responsible for proton acceleration from solid targets irradiated with ps pulses, magnetic fields formed by ns pulse irradiation of solid targets, and electric fields associated with the ponderomotive channelling of ps laser pulses in under-dense plasmas.
We consider the symmetry of cylindrical implosions of laser targets with parameters corresponding to experiments proposed for the LIL laser facility at Bordeaux: eight laser beams in octahedrical configuration, delivering a total of 50 kJ of 0.35 µm laser light in 5 ns, impinging on 1.26 mm diameter polystyrene cylindrical shells filled with deuterium at 30 bar and 5.35 mg cm−3; this configuration allows to place diagnostics along the symmetry axis to evaluate directly the uniformity of implosion. Numerical studies have been carried out by using the hydrodynamic computer codes MULTI and CHIC, including one-dimensional, and two-dimensional R–Z and R–θ simulations. Deuterium is compressed into a 1 mm long and 50 µm diameter filament, with density ranging from 2 to 6 g cm−3 and temperatures above 1000 eV. In spite of the reduced numbers of beams, a good symmetry can be achieved with a careful choice of the irradiation pattern. The heat transport smoothing between laser absorption zone and ablation layer plays a fundamental role in the attenuation of residual non-uniformities. Also, it has been found that the radiation transport determines the radial structure of the compressed filament.
In the previous design, the maximum drive radiation
temperature was 4 MK or 350 eV (Holstein 1996). Different
beam configurations gave roughly the same uniformity with
the NIF-size cavity. Our best configuration used four cones
of beams illuminating three rings. An integrated 2D simulation
pointed out that the symmetry was good enough to reach
a gain of ten. Two evolutions took place in the design
of our MJ laser. We moved from a capsule adapted to 4 MK
(L1000) to another one adapted to 3.5 MK (L1215) in order
to minimize the parametric instabilities (the cavity size
is almost the same). This new capsule also has a better
hydrostability according to the “classical modelling”
(Lindl 1995). The second evolution is a simplification
of the target chamber. We restricted ourselves to two major
configurations for indirect drive (two-ring and three-ring
configurations). Therefore, only three cones of beams are
necessary instead of five cones in the first design. Finally,
the number of holes in the chamber is 80 instead of 100.
The laser program developed at the Centre d'Etudes de Limeil-Valenton, Saint-Georges, France (CEL-V) is concentrated on a systematic investigation of indirect drive fusion; by comparison with direct drive, this process is expected to provide the required irradiation uniformity with relaxed constraints on laser beam quality. The main concerns are radiative transfer and preheat, hydrodynamic instabilities, and high-density X-ray driven implosions. Ablative implosion experiments have been conducted with the two beams at the Phebus facility (5 kJ, 1.3 ns, 0.35 μm). Symmetry was proved to be controlled by the casing structure, following scaling laws describing hohlraum physics. A compressed DT density ∼100 ρ0 (ρ0 liquid DT density) has been deduced from activation measurements. Different aspects of the soft X-ray transfer processes, and particularly of the ablation of a low-Z material, which drives the capsule implosion, are dealt with in detailed investigations. Reported here are results on X-ray reemission and penetration in several materials, and on induced hydrodynamics of accelerated foils. The laser energy required to reach fuel ignition conditions has been evaluated from numerical simulations as well as from analytical models, taking into account hohlraum physics, capsule implosion, hot spot formation, and burn propagation. Several crucial parameters have been drawn, the most important being the radiation temperature. A target gain in the order of 10 appears achievable with a 2-MJ laser.
Implosion experiments performed at Centre d'Etudes de Limeil-Valenton in the indirect drive scheme using the two-beams Nd:glass laser facility Phebus at the energy level = 6 kJ (blue light) are presented. A final density of compressed DT close to 100 ρ0 has been obtained; it has been deduced from radiochemistry of the activated silicon atoms in the pusher. The best irradiance uniformity on the microballoon was evaluated to = 15% rms. Phebus has also been equipped with an optical fiber oscillator to study the effect of a smoothing technique on coupling processes: It appeared that at 0·53 μm absorption efficiency is increased by =15–20%. With the eight-beams Octal laser, hydrodynamic instabilities development in accelerated planar targets has been investigated both for direct and indirect drives; the mixing zone detected at the light-heavy interface does not present visible bubble-and-spike like structures and is less developed in the indirect configuration. Atomic physics in laser plasmas is also deeply studied; a particular effort has been made on absorption spectroscopy, a powerful diagnostic of ionization dynamics in cold and dense plasmas. Experiments have been realized either in multilayered targets or using rear-side X-ray emission of thin Au foils to heat the samples. To reach fuel ignition conditions, more powerful lasers, in the range of megajoule, will be needed. Their design needs further technological developments to reduce the capital cost in $/W. At Limeil, we work mainly on high-damage threshold optical coatings, using the sol-gel process, high-quality, low-cost mirror fabrication, using the replica technics, and incoherent laser pulse generation for beam smoothing.
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