Speaker
Ola Embreus
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P5.4011.pdf
Dynamics of positrons during relativistic electron runaway
O. Embréus1 , K. Richards1 , G. Papp2 L. Hesslow1 , M. Hoppe1 , T. Fülöp1
1 Department of Physics, Chalmers University of Technology, SE-41296 Göteborg , Sweden
2 Max-Planck-Institute for Plasma Physics, D-85748 Garching, Germany
Sufficiently strong electric fields in plasmas can accelerate charged particles to relativistic en-
ergies via the runaway mechanism. In this contribution we describe the dynamics of positrons
that are created during a runaway avalanche, and calculate the fraction of created positrons
that become runaway accelerated. We find a sensitive electric-field dependence that is unlike
the electron runaway growth rate due to the fact that positrons are born anisotropically in the
direction opposite to their direction of acceleration.
For runaway in systems larger than a pair-production mean-free path, we derive a threshold
electric field above which the direct pair production in collisions will dominate the pair produc-
tion due to photons produced in hard X-ray emission, which is traditionally the main positron
producing mechanism. This pair-production threshold field is found to be of the order of tens of
avalanche threshold fields.
We present analytical and numerical solutions of the positron kinetic equation with a strong
constant electric field, illustrating similarities and differences between the runaway dynamics of
positrons and electrons. The numerical study provides the rate coefficients of created positrons
that become runaway accelerated or thermalized. These are used to estimate the amount of
annihilation radiation that will be detected during tokamak runaway scenarios as a function of
background parameters, which can be compared to the amount of hard X-rays near 511 keV
emitted by the runaway electron population. We find that the signal-to-noise ratio becomes
worse when the plasma charge increases, whereas the total photon count increases. This tradeoff
can be important in scenario development for positron detection experiments, which may be
useful for assessing the validity of present models for runaway acceleration during disruptions.
