The design of LMJ focal spots for indirect drive experiments

Abstract

Abstract. LMJ is a 240 high power laser beam facility for achieving laser matter interaction experiments, high energy density science, including the demonstration of fusion ignition through Inertial Confinement. The Laser Integration Line (LIL) facility is currently a 4-beam prototype for LMJ. The intensity I 0 at the focal spot centre drives hydrodynamic and plasma instabilities and the intensity in the wings must be low to go through the laser entrance Hohlraum. A simple model has been developed to compute the LMJ focal spot. The model gives the intensity at the centre as a function of the focal spot area at 3% of the maximum. IntroductionThe Laser Integration Line (LIL) facility is currently a 4-beam prototype for LMJ. The purpose of LIL and of these experiments is to show that the previously untested features of the LMJ laser design will perform as projected at LMJ scale and that the laser is therefore ready for final engineering design. LIL Quad beam waist was recorded for various pulse durations, smoothing techniques and for a wide range of laser intensities up to LMJ-nominal ones [1]. Now, LIL quad has been commissioned to the center of the target chamber and the first plasma experiments are being made.LMJ focal spot requirements are : elliptic shape, almost flat top high-order super-gaussian envelope, spot size, intensity at the center. In fact, it is important to know precisely the intensity I 0 at the focal spot center (to control hydrodynamic and plasma instabilities) and the intensity at the edges of the focal spot (to go through the laser entrance Hohlraum). For LMJ focal spots, it has been decided that this level will be 3 % of I 0 .

Progress on LMJ targets for ignition

“…LPIs may be enhanced in these small targets and their control has to be one of the major requirements of the design process. Two key parameters are the laser intensity and the electronic density inside the hohlraum; these factors can be optimized by playing on the size of the laser focal spot [12] and on the composition and density of the gas filling the hohlraum. Linear growth rates of the Raman (SRS) and Brillouin (SBS) instabilities are evaluated by post-processing the 2D FCI2 simulations with our code PIRANAH, as shown in figure 4.…”