Radially selective inward transport of positrons in a Penning–Malmberg trap

Abstract

The anharmonic component of the electric field of a Penning–Malmberg trap is exploited to manipulate a subset of the radial (r) distribution of trapped positrons, using a dipole field made to rotate about the long-axis (z) of the trap. This ‘rotating wall’ technique (RW) induces inward transport at frequencies associated with the motion of trapped particles, although similarly it causes heating. The motional frequencies vary spatially within a non-ideal trap, thus resonant interaction with the rotating field may be restricted to a region selected to lie away from the trap centre, thereby forming a pseudo-potential barrier and reducing losses due to both heating and expansion. We demonstrate this effect for improved accumulation of positrons and further outline a technique to achieve strong compression with low RW amplitudes by chirping the drive frequency.