Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

For more than half a century, researchers around the world have been engaged in attempts to achieve fusion ignition as a proof of principle of various fusion concepts. Following the Lawson criterion, an ignited plasma is one where the fusion heating power is high enough to overcome all the physical processes that cool the fusion plasma, creating a positive thermodynamic feedback loop with rapidly increasing temperature. In inertially confined fusion, ignition is a state where the fusion plasma can begin “burn propagation” into surrounding cold fuel, enabling the possibility of high energy gain. While “scientific breakeven” (i.e., unity target gain) has not yet been achieved (here target gain is 0.72, 1.37 MJ of fusion for 1.92 MJ of laser energy), this Letter reports the first controlled fusion experiment, using laser indirect drive, on the National Ignition Facility to produce capsule gain (here 5.8) and reach ignition by nine different formulations of the Lawson criterion.

Abstract:

A paradigm shift in the physics of laser-plasma interactions is approaching with the commissioning of multipetawatt laser facilities worldwide. Radiation reaction processes will result in the onset of electron-positron pair cascades and, with that, the absorption and partitioning of the incident laser energy, as well as the energy transport throughout the irradiated targets. To accurately quantify these effects, one must know the focused intensity on target in situ. In this work, a way of measuring the focused intensity on target is proposed based upon the ionization of xenon gas at low ambient pressure. The field ionization rates from two works [Phys. Rev. A 59, 569 (1999) and Phys. Rev. A 98, 043407 (2018)], where the latter rate has been derived using quantum mechanics, have been implemented in the particle-in-cell code SMILEI [Comput. Phys. Commun. 222, 351 (2018)]. A series of one- and two-dimensional simulations are compared and shown to reproduce the charge states without presenting visible differences when increasing the simulation dimensionality. They provide a way to accurately verify the intensity on target using in situ measurements.

Abstract:

The rapid irradiation and heating of matter can lead to material removal via a process known as ablation. While previous investigations have focused on ablation with optical and soft x-ray pulses, the process is not well understood for the high-energy x-rays delivered at current x-ray free electron laser facilities. In this paper, we use hybrid two-temperature model molecular dynamics simulations to determine the damage threshold and dynamics for tantalum foils under irradiation with x-rays in the range 1–5 keV. We report that damage occurs for foils with thickness 300 nm when heated to around 1.25 eV/atom. This damage results from the combined processes of melting and cavitation, finally resulting in the removal of material layers. The predictions of this study, in terms of the cavitation threshold and underlying dynamics, could guide interpretation of experiments as well as applications including development of beamline optics for free-electron lasers. We report consistency between cavitation and ablation behavior in isochoric heating experiments and spall processes in hydrodynamic compression and release experiments, confirming the primary modes of damage are mechanical in nature for the x-ray energies investigated.

Abstract:

The design of inertial confinement fusion (ICF) ignition targets requires radiation-hydrodynamics simulations with accurate models of the fundamental material properties (i.e., equation of state, opacity, and conductivity). Validation of these models are required via experimentation. A feasibility study of using spatially-integrated, spectrally-resolved, X-ray Thomson scattering (XRTS) measurements to diagnose the temperature, density, and ionization of the compressed DT shell of a cryogenic DT implosion at two-thirds convergence was conducted. Synthetic scattering spectra were generated using 1-D implosion simulations from the LILAC code that were post processed with the X-ray Scattering (XRS) model which is incorporated within SPECT3D. Analysis of two extreme adiabat capsule conditions showed that the plasma conditions for both compressed DT shells could be resolved.