Effect of a central ``squeeze'' potential on asymmetry-induced transport

We report on initial experiments measuring the radial particle flux produced when a ``squeeze'' voltage ($V_{sq}\sim\pm 1$~V) is applied to the center ring (S3) of our cylindrical Malmberg-Penning trap at the same time as the voltages producing our usual asymmetry potential $\phi_1(r)\cos{(kz)}\cos{(\omega t - l\theta)}$. Two results are of interest: 1) When a negative DC squeeze voltage is applied to S3, the flux produced by the asymmetry is reduced by a factor $e^{(V_{sq}/V_0)}$ where $V_0\approx 1.2$~V. Evidently, particles need to be able to transit the entire machine to produce transport. This is consistent with our transport model but the scale factor $V_0$ is much larger than expected. 2) When symmetric $\pm$ voltages are applied to the two azimuthally-divided halves of S3, DC or low-frequency voltages increase the radial flux while high-frequency voltages decrease it. In similar experiments, others\footnote{Daniel H.E. Dubin et al., Phys. Plasmas {\bf 19}, 056102 (2012).} have attributed such transport changes to induced chaotic particle orbits, but we note that the squeeze voltage itself produces transport and the resulting modification of the plasma may also be a factor in changing the observed flux. We have not yet found a way to distinguish between these two effects.