Collisionless Shocks, Implicit Moments and Laser Fusion Implosions

Abstract

As this year's C. K. Birdsall Award speaker, I will focus on technical challenges that have highlighted my career since 1964. In my earliest days, I studied kinetic effects in sound propagation. Particles can run ahead of fluid waves, sending forth a precursor signal. Later, I studied high altitude explosions in the earth's ionosphere that could lead to ion-acoustic, and magneto-sonic plasma shocks controlled by the earth's magnetic field. In 1972 breakeven with laser fusion seemed an imminent possibility — 8 years away. At Los Alamos I co-authored the first PRL on DT micro-balloon implosions driven by sharply rising laser pulses. Longer wavelength laser beams ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$10.6\mu$</tex> ) were found to produce copious hot electrons, drastically reducing implosion efficiency, and were then modeled by slow, explicit PIC codes. In 1981 with the new Implicit Moment Method I found that a co-set of fluid equations could be solved implicitly to generally predict net electron behavior using a time step far exceeding the electron Courant Condition. This greatly facilitated 2D studies of hot electrons, previously impeding straight-forward laser target implosions. In the later 80s the Sandia Laboratory and the Naval Research Laboratory were considering pulse power systems to implode targets. Energy flow to these targets was controlled by Plasma Opening Switches. We found that the Implicit Moment Method greatly facilitated the modeling of these switches. In the 90s we returned to making comprehensive studies of ICF target behavior with Los Alamos and Livermore target codes, supplemented by Implicit Moment hot electron modeling. Although I retired from Los Alamos in late 2005, I continued to explore implicit and ICF modeling issues, showing with coworkers in 2014 - at our Research Applications Corporation - that kinetics could be used to improve shock-viscosity modeling in ICF codes.