Proton ring trapping in a magnetic mirror

A rotating beam of 430 keV protons was trapped in a 1.5 m long magnetic mirror using a fast gated coil and was studied for up to 4 μs. The protons were detected using magnetic probes, Faraday cups, and heat-sensitive witness targets. The ring was injected through a magnetic cusp where about 90% of its total energy was converted to rotational energy resulting in an axial velocity of about 3 m/μs. The inside and outside diameters of the ring were typically 10 cm and 28 cm, respectively. The rings were propagated in puffed and static hydrogen with pressures as high as 300 mtorr for the purpose of space-charge neutralization. Eight passes through the length of the mirror were observed before the rings became undetectable. The measured proton inventory in the ring was typically 2 x 1015 protons on the first pass and 2 x 1013 on the eighth and final pass. Particle losses are attributed to charge-exchange and transmission of particles through the end mirrors. Mirror losses due to: (1) particles emerging from the cusp with too much residual parallel energy, (2) the upstream mirror being weaker than the downstream mirror, and (3) the pitch-angle of some particles might have changed during propagation, possibly due to asymmetric space-charge regions or instabilities