Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

Wetted-foam layers are of significant interest for inertial confinement fusion capsules, due to the control they provide over the convergence ratio of the implosion, and the opportunity this affords to minimize hydrodynamic instability growth. However, the equation of state (EOS) for fusion relevant foams is not well characterized, and many simulations rely on modelling such foams as a homogeneous medium with the foam average density. To address this question, an experiment was performed using the the VULCAN Nd:glass laser at the Central Laser Facility. The aim was to measure the principal Hugoniot of TMPTA plastic foams at 260 mg/cm3 , corresponding to the density of liquid DT-wetted-foam layers, and their ‘hydrodynamic equivalent’ capsules. A VISAR was used to obtain the shock velocity of both the foam and an α-quartz reference layer, while streaked optical pyrometry provided the temperature of the shocked material. The measurements confirm that, for the pressure range accessed, this material can indeed be well described using the equation of state of the homogeneous medium at the foam density.

Abstract:

Wetted-foam layers are of significant interest for inertial-confinement-fusion capsules, due to the control they provide over the convergence ratio of the implosion and the opportunity this affords to minimize hydrodynamic instability growth. However, the equation of state for fusion-relevant foams are not well characterized, and many simulations rely on modeling such foams as a homogeneous medium with the foam average density. To address this issue, an experiment was performed using the VULCAN Nd:glass laser at the Central Laser Facility. The aim was to measure the principal Hugoniot of TMPTA plastic foams at 260 mg/cm³, corresponding to the density of liquid DT-wetted-foam layers, and their “hydrodynamic equivalent” capsules. A VISAR was used to obtain the shock velocity of both the foam and an α-quartz reference layer, while streaked optical pyrometry provided the temperature of the shocked material. The measurements confirm that, for the 20–120 GPa pressure range accessed, this material can indeed be well described using the equation of state of the homogeneous medium at the foam density.

Abstract:

We investigate the growth of ion density perturbations in large-amplitude linear laser wakefields via two-dimensional particle-in-cell simulations. Growth rates and wave numbers are found to be consistent with a longitudinal strong-field modulational instability (SFMI). We examine the transverse dependence of the instability for a Gaussian wakefield envelope and show that growth rates and wavenumbers can be maximised off-axis. On-axis growth rates are found to decrease with increasing ion mass or electron temperature. These results are in close agreement with the dispersion relation of a Langmuir wave with energy density that is large compared to the plasma thermal energy density. The implications for wakefield accelerators, in particular multi-pulse schemes, are discussed.