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 $(\frac{E_{\perp}}{B})$ is a so-called adiabatic invariant (AI), where $E_{\perp}$ is the energy in cyclotron motion in the direction perpendicular to magnetic field $(B)$. Adiabaticity requires the parameter $\gamma = \frac{2\pi}{\omega_{c}}\frac{v_{||}}{|B|}\frac{d|B|}{dz}$ to be $ << 1$, where $\omega_{c}$ is the cyclotron frequency and $v_{||}$ is the parallel positron velocity. For beam transport energies $\leq 30 eV$, 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 $(E_{\perp})$ beam distributions, while keeping the total energy conserved. Experimental results for a fixed source magnetic field show increases in perpendicular energy $(E_{\perp})$ with increased moderator bias in the range $50-80 V$ (i.e., where $\gamma \agt 1$). Implications of this observation for BGT-based beam systems will be discussed.