The evolution of a bunch of 10⁴-10⁵ positrons in a dipole trap: efficient injection, toroidal homogenization, collisional pitch angle scattering, and radial diffusion to the wall*

The evolution of a positron bunch injected into a permanent magnet dipole trap is measured with increased spatial and temporal resolution thanks to a 100x improvement in the number of positrons injected and the solid angle coverage as well as the time resolution of the annihilation gamma detector array. A buffer-gas trap (BGT) accumulates positrons generated by the AIST Linac source and extracts 10⁴-10⁵ positrons as a 6 eV pulse which is efficiently ExB-drift injected into the dipole field. A 22-detector array placed in reentrant ports 1cm from the electrode wall detects ~1000 annihilation gamma-rays per shot. FPGA processing timestamps detections to 24ns accuracy and identifies ~100 coincident lines of response per shot. The efficiency of injection is characterized by the absence of a prompt annihilation signal. The toroidal homogenization of the positron pulse during the first 100 mus is characterized by pile-up annihilation signals generated by reapplying the injection potentials and dumping all remaining positrons onto the walls. The pitch angle of the positrons determines if there are trapped through magnetic mirroring or electrostatically due to the bias of the permanent magnet. The rate of pitch angle scattering due to collisional processes can be characterized by dumps of the electrostatically confined positrons onto the magnet when its potential is grounded. The diffusion rate and confinement time are characterized by recording individual annihilation events as they occur.