Laser-plasma interactions and hot electron generation in inertial confinement fusion
Monday, August 29, 2016
We have studied laser plasma instabilities (LPI) that can generate hot electrons in direct drive ICF under a range of laser intensities relevant to both the conventional hot-spot ignition and shock ignition. We find that ion density modulations can turn convective LPI to absolute ones in the region below the quarter critical density (nc/4). In this region, our fluid simulations show that when a sinusoidal density modulation is superimposed on a linear density profile, convective two-plasmon decay (TPD) and stimulated Raman scattering (SRS) instabilities can become absolutely unstable under realistic direct-drive ICF conditions. Analysis of a three-wave model with a two-slope density profile shows that a sufficiently large change of the density gradient in a linear density profile can turn convective instabilities into absolute ones. An analytical expression is given for the threshold of the gradient change, which depends on the convective gain only. Growth rates for the absolute modes are also obtained. The threshold and growth rates from the two-slope profile are found to approximate those under sinusoidal modulations. These results explain the origin of the TPD modes below the nc/4 surface that in previous research were found to be critical to hot electron generation.
We have also studied the influence of LPI and hot electrons on the hydrodynamic evolution of ICF targets. Combing PIC and hydrodynamics simulations, we study the LPI and hydro evolution of coronal plasmas in an OMEGA EP long-scale-length experiment with planar targets. Plasma and laser conditions are first obtained in a DRACO hydro simulation with only inverse-bremsstrahlung absorption. Using these conditions, an OSIRIS PIC simulation is performed to study laser absorption and hot-electron generation caused by LPI near the nc/4 region. The obtained information from the PIC simulation is subsequently coupled back to another DRACO simulation to examine how the LPI affect the overall hydrodynamics. The results show that the LPI-induced laser absorption can increase the electron temperature due to local heating by plasma waves. But it does not significantly change the density scale length in the corona because the portion of the energy carried by the forward-going hot electrons is still deposited beyond the nc/4 region.