Enhanced Particle Slowing from 1D Long-Range Like-Sign Collisions*

Recent theory [1] predicts enhanced collisional slowing rates for like-sign particle collisions in strong magnetic fields, in density and temperature regimes where binary Boltzmann collisions dominate over statistical Fokker-Planck collisions.  Here, the enhancement is from "long-range" collisions, with impact parameters greater than the cyclotron radius.  For protons (or anti-protons) at n~10^8/cm3 and B~3T, the enhancement is large for T<1.eV; and for Mg+ ions at n~10^7/cm3 studied here, the enhancement is large for T<10.meV.  Prior experiments have indirectly measured  Mg+ and MgH+ collisions causing damping of plasma waves well into the enhancement regime [2], obtaining damping rates consistent with enhanced inter-species collisional drag .  Two recent experimental campaigns have utilized LIF to directly measure Mg+ test-particle distributions f(v) colliding with warmer equilibria, with test-particle energies T>100.meV.  Here, the measurements are consistent with the predicted factor-of-two enhancement, but the (increasing) rates have not yet been measured into the strong enhancement regime. [1] D.H.E. Dubin, Phys Plasmas 21, 052108 (2014)  [2] M.Affolter et al, Phys. Rev. Lett 117, 155001 (2016)