  Two-and-a-half-dimensional particle-in-cell plasma simulations are used to
study the particle energization in expanding magnetized electron-positron
plasmas with slab geometry. When the magnetized relativistic plasma with high
temperature (initial electron and positron temperature are
$k_{B}T_{e}=k_{B}T_{p}=5MeV$) is expanding into a vacuum, the electromagnetic
(EM) pulse with large amplitude is formed and the surface plasma particles are
efficiently accelerated in the forward direction owing to the energy conversion
from the EM field to the plasma particles. We find that the behavior of the
DRPA (Diamagnetic Relativistic Pulse Accelerator) depends strongly on the ratio
of the electron plasma frequency to the cyclotron frequency
$\omega_{pe}/\Omega_{e}$ and the initial plasma thickness. In the high
$\omega_{pe}/\Omega_{e}$ case, the EM pulse is rapidly damped and the plasma
diffuses uniformly without forming density peaks because the initial thermal
energy of the plasma is much larger than the field energy. On the contrary, in
the low $\omega_{pe}/\Omega_{e}$ case, the field energy becomes large enough to
energize all the plasma particles, which are confined in the EM pulse and
efficiently accelerated to ultrarelativistic energies. We also find that a
thicker initial plasma increases the maximum energy of the accelerated
particles.
