Influence of magnetic non-adiabaticity on a solid-Ne-moderated positron beam energy distributions

High quality, trap-based positron beams typically operate in the regime in which particle transport is adiabatic. In this regime, the quantity (E_⊥/B) is a so-called adiabatic invariant (AI), where E_⊥ is the energy in cyclotron motion in the direction perpendicular to magnetic field (B) . Adiabaticity requires the parameter γ =2/π ω_cv_|/| | B | d/| B | dz to be < < 1 , where ω_c is the cyclotron frequency and v_{| |} is the parallel positron velocity. For beam transport energies <= 30 eV , invariance holds quite well for our trap-based beam from the buffer gas trap (BGT) to the test-gas cell. However, for larger transport energies, breaking of AI is observed at both ends of the beam tube between solid-Ne moderator and BGT, due to low B and strong field gradients. This influences the parallel (E_{| |}) and perpendicular energy (E_⊥) beam distributions, while keeping the total energy conserved. Experimental results for a fixed source magnetic field show increases in perpendicular energy (E_⊥) with increased moderator bias in the range 50 - 80 V (i.e., where γ > 1). Implications of this observation for BGT-based beam systems will be discussed.

Work supported by NSF Grant PHY-1702230.