Efficient shock drift acceleration in the collision of two asymmetric pair plasma shells

Abstract

The shock drift acceleration (SDA) process in the collision between two asymmetric relativistic pair plasma shells was investigated. It is found that the density ratio (nL/nR) of the two different plasma shells plays a crucial role in the SDA process and determines the efficiency of the SDA process. By increasing this parameter, the plasma bulk velocity and so as to the convection electric field in the downstream region can be much enhanced compared with the symmetric case, in which the densities in the two plasma shells are the same. As a result, the particles are efficiently accelerated by the large convection electric field and the efficiency of the SDA process is much increased as the density ratio is increased. Our particle-in-cell simulations demonstrate that the SDA efficiency can be improved by three to four times in the asymmetric case. In this way, more high energy particles could enter into the phase of the diffusive shock acceleration process.