The research in the ICF domain with applications to the large scale laser facilities that are under construction on the site of CEA/CESTA, LIL, PETAL and LMJ, constitutes the main objective of the plasma group. This work is realized in the context of HiPER project, the direct drive scheme on the LMJ and the fast ignition on PETAL.
Reference target design for the HiPER project
Since 2007 the plasma group takes a significant contribution into the preparation and execution of the HiPER project. It is dedicated to the definition of an international facility for the inertial fusion energy driven by laser. The project was started in April 2008 and the group participates in three working packages : target design, laser plasma interaction experiment and fundamental physics. During the first year of the project, we studied the performances of the HiPER baseline target. We highlighted the importance of the radiative transport in the final fuel assembly, analysed the robustness and the stability of the target and we produced quantitative specifications that contribute to the technical specifications of the future facilities (in collaboration with S. Atzeni, Rome and J. Honrubia, Madrid).
Thermonuclear shock ignition
Shock ignition is a variant of the fast ignition scheme where a spherical target shell is first compressed by laser pulses with a low implosion velocity and then a small part of the central fuel is ignited by an additional laser pulse. In this case, the ignition energy is transmitted to the central part of the target by a strong converging shock produced by a laser pulse delivered at the end of the implosion stage.
The studies performed in 2008 and 2009 were focused on implementation of this scheme to the HiPER baseline target. We defined the laser pulse characteristics in terms of the laser power and the shock timing to achieve the ignition. It is shown that the power of 160 TW is sufficient to produce a thermonuclear gain of 80 and the shock launching window is as large as 250 ps. These performances are easily achievable on the LMJ or NIF class laser facilities. Preliminary studies show that asymmetry of the laser ignition pulse have a little effect on the ignition conditions.
The low mode perturbations due to laser irradiation non-uniformities were studied for the baseline HiPER target. 2D axisymmetric CHIC simulations of the perturbed target show that l = 12 mode remains dominant until the stagnation time. Safety factors have been defined to quantify targets sensitivity to low modes asymmetry and a target classification is established in the energy/power diagram. The high mode perturbations due to the target surface roughness have been studied by using the ablative Rayleigh-Taylor instability (ARTI) linear theory, the PERLE linear stability code simulations and the 2D simulations with the CHIC code showing an early nonlinear stage of the instability development. This nonlinear stage was delayed and more stable target design was found using an adiabat shaping scheme. We have also studied the dynamics and stability of moderated atomic number ablators (in collaboration with J. Sanz, Madrid). Double ablation front structures have been characterized for different materials. The analytical model confirmed by the simulations with the PERLE and CHIC codes, is showing a significant role of the radiative flux in the formation of such structures. The ARTI growth rates show a stabilisation at long wavelengths, which is very different from the one observed in classical ablators such as plastic.
Energy transport by energetic electrons
The success of fast ignition by the relativistic electron beam depends on the energy conversion efficiency from laser to electrons, on the electron transport and energy deposition in a strongly compressed matter. We have studied these issues in three experimental campaigns, carried out at the LULI and RAL laser facilities. The effect of the laser prepulse on the electron production was studied at LUIL. To study the transport efficiency, the targets have been compressed by shock waves in a planar geometry on the LULI2000 laser and in a convergent geometry at the RAL facility.