Plasma Physics (physics.plasm-ph)

Abstract: Plasma simulations are powerful tools for understanding fundamental plasma science phenomena and for process optimization in applications. To ensure their quantitative accuracy, they must be validated against experiments. In this work, such an experimental validation is performed for a 1d3v particle-in-cell simulation complemented with the Monte Carlo treatment of collision processes of a capacitively coupled radio frequency plasma driven at 13.56 MHz and operated in neon gas. In a geometrically symmetric reactor the electron density in the discharge center and the spatio-temporal distribution of the electron impact excitation rate from the ground into the Ne 2p$_1$ state are measured by a microwave cutoff probe and phase resolved optical emission spectroscopy, respectively. The measurements are conducted for electrode gaps between 50 mm and 90 mm, neutral gas pressures between 20 mTorr and 50 mTorr, and peak-to-peak values of the driving voltage waveform between 250 V and 650 V. Simulations are performed under identical discharge conditions. In the simulations, various combinations of surface coefficients characterising the interactions of electrons and heavy particles with the anodized aluminium electrode surfaces are adopted. We find, that the simulations using a constant effective heavy particle induced secondary electron emission coefficient of 0.3 and a realistic electron-surface interaction model (which considers energy-dependent and material specific elastic and inelastic electron reflection, as well as the emission of true secondary electrons from the surface) yield results which are in good quantitative agreement with the experimental data.

Abstract: 2D and 3D numerical simulations with the adaptive mesh refinement Eulerian radiation-hydrocode RAGE are used to investigate hydrodynamic disruption of asymmetrically driven ICF implosions. A central aspect of this phenomenon is the connection between drive asymmetry and the generation of turbulence in the DT fuel. Long wavelength deviations from spherical symmetry in the pressure drive lead to the generation of coherent vortical structures in the DT gas and it is the three dimensional instability of these structures that in turn leads to turbulence and mix. RAGE simulations with spatial resolutions as high as 0.05 {\mu}m in 3D are presented to exhibit the detailed mechanisms of turbulence growth. These simulations suggest that the amplification of small initial surface imperfections by acceleration-induced instabilities is not the only important source of turbulent mix in ICF implosions as is commonly supposed. Rather, the three dimensional instability of coherent vortical structures induced in the gas by the asymmetries in the implosion are an additional important source of turbulent mixing in ICF, perhaps even the dominant source. The effect of convergence on the hydrodynamic disruption of an asymmetrically driven ICF capsule is also considered, demonstrating how higher convergence is expected to lead to greater hydrodynamic disruption by both large scale fingers of pusher material into the fuel as well as the formation of radially outgoing turbulent jets of fuel. Implications of these results for NIF ignition are discussed.