Magnetically confined positrons forming positronium*

Recent experiments pinpoint charge exchange as dominant loss channel for magnetically confined positrons above threshold energies. A bunch of 10⁵ positrons is accumulated, cooled (=300meV), and E×B drift-injected (=7eV) into the dipole field of a permanent magnet trap. Once injected, the positrons are trapped through electrostatic reflection off the biased magnet. After one elastic collision (~4ms), their pitch angles are scattered and they are magnetically confined. A 21-BGO detector array situated 1cm from the confinement volume detects ~10,000 gammas per shot. The time evolution of the 511keV peak-to-valley ratio identifies Positronium (Ps) formation through charge exchange of the positrons with impurities in the ~5·10^-6 Pa vacuum as dominant loss mechanism during the first ~500ms of confinement. An increase in the positron kinetic energy, achieved by reducing the electrostatic bias, leads to an increase in charge exchange and associated triple coincidence detection. Inelastic collisions cool the positrons, enabling diagnosis through a ramp of the magnet bias to negative voltages pulling them into the loss cone. Once the positron population above the Ps-formation energy threshold is depleted, transport to the wall through elastic collisions becomes the dominant loss mechanism.