Proposed Experiment to Measure Test Particle Diffusion in a Correlated Nonneutral Plasma

We have measured cross-magnetic-field test particle diffusion D due to long range collisions in plasmas with r_c< λ_D and 0.05 eV<T<3 eV. We propose to extend these measurements to lower temperature, where ions become correlated. Magnesium ion plasmas are routinely cooled down to T= 5x10^-6eV resulting in a correlation parameter Γ=(e²/aT)~10. We have measured collisional equipartition rates in this regime, and observe a Salpeter enhancement of 10⁹x over the rate neglecting correlations [1]. For a diffusion measurement, laser cooling with a standard cyclic transition is used to optically pump all ions into one electronic spin state. The inner radial part of the plasma can then be tagged by reversing the spins using direct optical pumping. These “spin reversed” test particles can be followed as they diffuse across the magnetic field, using a weak probe tuned to a cyclic transition. We intend to test theory [2] for Γ>1 predicting decreasing D as T decreases, and a transition from D~ B^-2 to D~B^-1 .

[1] F. Anderegg et al., PRL, 102, 185001 (2009).

[2] T. Ott and M. Bonitz, PRL, 107, 135003 (2011); S.D. Baalrud and J. Daligaut, Phys. Rev. E 96, 043202 (2017).