Prof Peter Norreys FInstP;

Abstract:

Inertial confinement fusion fuel suffers increased X-ray radiation losses when carbon from the capsule ablator mixes into the hot-spot. Here we present one and two-dimensional ion VlasovFokker-Planck simulations that resolve hot-spot self heating in the presence a localised spike of carbon mix, totalling 1.9 % of the hot-spot mass. The mix region cools and contracts over tens of picoseconds, increasing its alpha particle stopping power and radiative losses. This makes a localised mix region more severe than an equal amount of uniformly distributed mix. There is also a purely kinetic effect that reduces fusion reactivity by several percent, since faster ions in the tail of the distribution are absorbed by the mix region. Radiative cooling and contraction of the spike induces fluid motion, causing neutron spectrum broadening. This artificially increases the inferred experimental ion temperatures and gives line of sight variations.

Abstract:

Low-density foams are of significant interest in inertial confinement fusion (ICF), with potential applications as fuel carriers, ablation layers, or as a hohlraum filling material. Despite their potential, the shock response of these materials remains poorly characterised, limiting the accuracy of hydrodynamic simulations. Here we report experimental measurements of the equation of state (EOS) for 90 mg/cm3 silica (SiO2) aerogel foam under laser-driven shock compression, conducted at the GEKKO XII laser facility. Shock pressures between 50 and 160 GPa were achieved, and the corresponding states were determined using standard impedance matching techniques with a quartz reference material. Initial measurements appeared to underestimate the foam shock velocity relative to predictions by the Quotidian Equation of State (QEOS) model. Experimental diagnostics indicated the presence of a vacuum gap between the reference material and the foam. The vacuum gaps were characterised, and one-dimensional radiation-hydrodynamic simulations were conducted to estimate their impact on the measured shock velocity. After applying simulation-based corrections, the experimental Hugoniot aligns closely with QEOS predictions, supporting the model’s applicability to low-density foams.

Abstract:

Bright high harmonic radiation from relativistically oscillating laser-plasmas offers a direct route to generating extreme electromagnetic fields. Theory shows that under optimised conditions the plasma medium can support strong spatiotemporal compression of laser energy into a Coherent Harmonic Focus (CHF), delivering intensity boosts many orders of magnitude above that of the incident driving laser pulse [1–4]. Although diffraction-limited performance [5] (spatial compression) and attosecond phase-locking [6] (temporal compression) have been demonstrated in the laboratory, efficient coupling of highly relativistic laser pulse energy into the emitted harmonic cone has not been realised to date. Here, conclusive evidence confirms that the relativistic laserplasma interaction can be tailored to deliver the maximum conversion efficiencies predicted from simulations. By fine-tuning the temporal profile of the driving laser pulse on femtosecond (fs, 10−15 s) timescales, energies > 9 mJ between the 12th and 47th harmonics (18 eV to 73 eV) are observed. These results are shown to be in excellent agreement with the theoretically expected efficiency dependence on harmonic order, indicating that optimal conditions have been achieved in the generation process. This is the important final element required to achieve the expected intensity boosts from a CHF in the laboratory. Although obtaining spatiotemporal compression and optimal efficiency simultaneously remains challenging, the path to realising extreme optical field strengths approaching the critical field of quantum electrodynamics (the Schwinger limit at > 1016V/m or > 1029 W cm−2 ) is now open, permitting all-optical studies of the quantum vacuum and drawing new horizons for intense attosecond science.