Energetics measurements of silver halfraum targets at the National Ignition Facility

Abstract

The energetics of novel silver halfraum targets are presented from laser experiments at the National Ignition Facility (NIF). Four beams from the NIF laser were used to heat the halfraum targets with ∼10 kJ of energy in a 1 ns square laser pulse. The silver halfraum targets were spheres 2 mm in diameter with an 800 μm laser entrance hole (LEH). Targets with different spherical wall thicknesses (8–16 μm) were characterized. The energetics and the laser coupling to the targets were determined using the NIF X-ray (i.e. Dante and FFLEX spectrometers) and optical backscatter diagnostics (NBI and FABS). The energy coupled into the targets was 0.85–0.88 of the total laser energy with a defocused laser spot of 400 μm in diameter and no spatial smoothing of the beams with phase plates. The coupling increased to 0.92 when 400 μm spot size phase plates were used to smooth each of the four lasers beams. The energy losses from the targets were through X-ray radiation and backscatter from laser plasma instabilities (SBS and SRS) from the LEH. As expected the different wall thickness had different levels of burn through emission. The thickest walled target (∼15.9 μm) had very low radiative losses through the target wall. The thinnest walled targets (∼8 μm) radiated about 0.2 of the input energy into X-ray emission.