PPPS-2013: A PIC-fluid hybrid algorithm for multi-scale simulations of laser-plasma interactions

Summary form only given. Fully electromagnetic, relativistic particle-in-cell (PIC) simulation of simulation of laser-plasma interactions has a forty-year history. The simulation of the fast ignition approach to laser fusion presents extreme challenges to traditional explicit algorithms because of the multi-scale nature of the plasma - the plasma density ranges from vacuum to near solid density. Here we describe a PIC-fluid hybrid algorithm that has proven successful in modeling laserplasma interactions in fast-ignition laser fusion.¹ In this hybrid algorithm a traditional fully electromagnetic, relativistic, explicit PIC method is used in the vacuum region and in the plasma up to a density much greater than the critical density for the laser, e.g., n_e/n_c~100. For higher plasma densities (as long as the plasma remains sufficiently cold and collisional) an Ohm´s law is used to determine the electric field in the plasma, and inclusion of the displacement current in Ampere´s law becomes optional, which eliminates light waves and electron plasma waves as justified by the high collisionality. This approach achieves efficiencies by eliminating requirements on resolving the electron Debye length, the skin depth and the plasma frequency in the high-density region. A particle representation of the plasma electrons and ions is retained throughout the simulation domain. An important aspect of this hybrid model is the inclusion of an accurate and efficient Coulomb collision operator that is compatible with unequally weighted particles, which can be achieved with a variant of the Takizuka-Abe (or Nanbu) collision algorithm extended to a grid-based model.