Simulation of positrons in a magnetic dipole trap

APEX (A Positron Electron eXperiment) aims to create an electron-positron pair plasma in a magnetic dipole trap. To achieve this goal, a highly efficient positron injection scheme is an essential prerequisite. The large parameter space (multiple electrodes and steering coils to manipulate the beam) and the limited diagnostic capabilities of the experiment demanded a numerical counterpart to further understand the processes occurring during injection as well as confinement. Using discrete electric fields, analytic formulas to calculate the magnetic fields and a variant of the leapfrog integrator as particle pusher, full trajectory simulations were conducted and were able to reproduce the experimental data. A simulation speed-up of two orders of magnitude was achieved, in comparison to the previously used SIMION trajectory simulator. Furthermore, the numerical energy loss of particles is negligible for the leapfrog method in contrast to the widely used 4^th-order Runge-Kutta algorithm. This made it possible to simulate long confinement of particles in the trap and estimate the major loss mechanisms. Possible future applications include also tests for adiabaticity and optimizations for the upcoming next stage of APEX with a levitating superconducting dipole coil.