Optimization of laser direct-drive implosions and the achievement of record performance on the OMEGA laser

By avoiding the conversion of laser light into x-rays, direct drive couples more energy to the capsule than indirect drive, leading to significantly higher energy gains (up to 6x) if the conditions for ignition are met. However, degradations from instabilities seeded and driven by the direct illumination constrain the parameter space available for achieving ignition and gain with direct drive.

Recent implosion experiments on OMEGA have led to record scaled Lawson parameters exceeding 0.9 in part due to two major advances: a novel target design optimizer that accounts for different degradation sources [1], and new measurements of hot spot mix and areal densities [2]. The optimizer uses a Bayesian optimization process for target specifications and laser pulse design parameters with a goal of maximizing the Lawson parameter, and is backed by predictive statistical model [3-5] trained on about 400 OMEGA DT-layered implosions and capable of predicting the outcome of implosion experiments with about 10% error in both yield and areal density. To inform the model at stability boundaries and more accurately measure and validate performance metrics, diagnostics for mix and areal density were developed. Two new mix diagnostics use either a measure of early emission onset with x-ray framing cameras[6], or time-gated spectrometers capable of identifying line emission from mid-Z dopants within the hot-spot region. A new measurement of the downscattered neutron ratio (DSR) was also implemented using neutron time of flight data [2]. This was particularly challenging given the large dynamic range required for this measurement when the areal densities are small. This measurement improved the solid angle coverage for inferring an accurate average value of the areal density.