Speaker
Dr
Marija Vranich
Description
Particle-in-cell codes have been successfully employed to
model particle acceleration in laboratory (e. g. laser-wakefield
acceleration) and in space (e.g. in collisionless shocks). With
the advent of laser technology, one can experiment with very
intense fields, that would otherwise be available only in
astrophysical objects (e.g. in pulsars or magnetars). At
extreme intensities, new physics becomes relevant for
modelling laser-plasma interactions, which brings new
challenges for PIC development. For example, radiation can
be emitted at high frequencies that are not resolved by the
simulation grid. If a fraction of energy this radiation carries is
negligible, we can compute the output radiation spectra by
post-processing the particle trajectories or by using a real-
time diagnostics that does not interfere with the PIC loop
itself. However, if this radiation accounts for a large fraction
of the particle energy, one needs to correct the particle
momentum by including a classical description of radiation
reaction (e.g. Landau & Lifshitz equation of motion instead of
the Lorentz force). One can expect a correct post-processing
account of the emitted radiation only if the particle
trajectories themselves are correct and the emissivity
calculation includes radiation damping corrections. An even
greater computational challenge is modelling a quantum
regime of emission - when a particle can emit a single photon
that carries a large fraction of its energy. Such a photon is
treated as an additional particle species, which can propagate
through the simulation box and later decay into an electron-
positron pair. The new pairs re-accelerate in the laser field,
and they emit new photons. Repeated occurrence of this
process can induce a so-called “QED cascade”, that generates
an exponentially rising number of particles in the simulation
box. Macroparticle merging algorithm is then necessary to
keep the simulation load to a manageable level. We have
developed a merging scheme that resamples particles in the
simulation and preserves the particle distribution function. I
will discuss the implementation of the above-mentioned
computational developments in OSIRIS and show examples
of physical problems where they are essential.
Primary author
Dr
Marija Vranich
