Symmetric Set of Transport Coefficients for Collisional Magnetized Plasma.

Braginskii extended magnetohydrodynamics is used to model transport in collisional astrophysical and high energy density plasmas. We show that commonly used approximations to the α_{⊥} and ß_{⊥} transport coefficients [e.g., Epperlein and Haines, Phys. Fluids 29, 1029 (1986)PFLDAS0031-917110.1063/1.865901] have a subtle inaccuracy that causes significant artificial magnetic dissipation and discontinuities. This is because magnetic transport actually relies on ß_{∥}-ß_{⊥} and α_{⊥}-α_{∥}, rather than α_{⊥} and ß_{⊥} themselves. We provide fit functions that rectify this problem and thus resolve the discrepancies with kinetic simulations in the literature. When implemented in the gorgon code, they reduce the predicted density asymmetry amplitude at laser ablation fronts. Recognizing the importance of α_{⊥}-α_{∥} and ß_{∥}-ß_{⊥}, we recast the set of coefficients. This makes explicit the symmetry of the magnetic and thermal transport, as well as the symmetry of the coefficients themselves.

Magnetic field generation from composition gradients in inertial confinement fusion fuel.

Experimental asymmetries in fusion implosions can lead to magnetic field generation in the hot plasma core. For typical parameters, previous studies found that the magnetization Hall parameter, given by the product of the electron gyro-frequency and Coulomb collision time, can exceed one. This will affect the hydrodynamics through inhibition and deflection of the electron heat flux. The magnetic field source is the collisionless Biermann term, which arises from the Debye shielding potential in electron pressure gradients. We show that there is an additional source term due to the Z dependence of the Coulomb collision operator. If there are ion composition gradients, such as jets of carbon ablator mix entering the hot-spot, this source term can rapidly exceed the Biermann fields. In addition, the Biermann fields are enhanced due to the increased temperature gradients from carbon radiative cooling. With even stronger self-generated fields, heat loss to the carbon regions will be reduced, potentially reducing the negative effect of carbon mix. This article is part of a discussion meeting issue 'Prospects for high gain inertial fusion energy (part 1)'.

Kinetic simulations of fusion ignition with hot-spot ablator mix.

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 Vlasov-Fokker-Planck simulations that resolve hot-spot self-heating in the presence of a localized spike of carbon mix, totalling 1.9% of the hot-spot mass. The mix region cools and contracts over tens of picoseconds, increasing its α particle stopping power and radiative losses. This makes a localized 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.

Optimization of plasma amplifiers.

Plasma amplifiers offer a route to side-step limitations on chirped pulse amplification and generate laser pulses at the power frontier. They compress long pulses by transferring energy to a shorter pulse via the Raman or Brillouin instabilities. We present an extensive kinetic numerical study of the three-dimensional parameter space for the Raman case. Further particle-in-cell simulations find the optimal seed pulse parameters for experimentally relevant constraints. The high-efficiency self-similar behavior is observed only for seeds shorter than the linear Raman growth time. A test case similar to an upcoming experiment at the Laboratory for Laser Energetics is found to maintain good transverse coherence and high-energy efficiency. Effective compression of a 10kJ, nanosecond-long driver pulse is also demonstrated in a 15-cm-long amplifier.

Compression of X-ray Free Electron Laser Pulses to Attosecond Duration.

State of the art X-ray Free Electron Laser facilities currently provide the brightest X-ray pulses available, typically with mJ energy and several hundred femtosecond duration. Here we present one- and two-dimensional Particle-in-Cell simulations, utilising the process of stimulated Raman amplification, showing that these pulses are compressed to a temporally coherent, sub-femtosecond pulse at 8% efficiency. Pulses of this type may pave the way for routine time resolution of electrons in nm size potentials. Furthermore, evidence is presented that significant Landau damping and wave-breaking may be beneficial in distorting the rear of the interaction and further reducing the final pulse duration.