The ATHENA positron accumulator

A positron accumulator has been constructed for use in the ATHENA anti-hydrogen experiment in CERN. Employing a solid neon moderator plated on to a 50 mCi$^{22}$Na source, a low energy beam of $7 \times 10^6$ positrons/s is guided into a 0.14 T magnetic field where they are trapped and cooled down to room temperature using nitrogen as a buffer gas. Plasmas of up to $2 \times 10^8$ positrons in 450 s, with an FWHM of 4 mm after compressing with the rotating electrical wall technique have been observed. In order to transfer the plasma to the main ATHENA (3 T) magnet, where the recombination trap is situated, a transfer section has been constructed consisting of a valve and a pulsed magnet with a pumping restriction inside. This magnet pulses to 1.2 T during the transfer. Preliminary tests have yielded transfer efficiencies in the order of 50%.