Test Particle Transport due to Long Range Interactions

Abstract

Collisional transport of particles and energy across magnetic field lines is an active area of plasma theory and experiments, with fundamental questions as to the binary vs collective and short-range vs long-range nature of interparticle collisions. Classical Boltzmann theory [1] describes ion transport in terms of local collisions, with impact parameters less than the cyclotron radius, i.e., r& r c; but it has long been recognized that electron particle and heat transport may involve long-range collisions, with rce ,r& l D [2,3]. In Q machines, experiments with optical tagging of ions [4] have demonstrated classical collisional diffusion [5,6] and shear-induced convection [7]. In fusion plasmas, tritium diffusion and convection coefficients have been obtained from neutron emission, but direct comparison to theory is still ambiguous [8]. Measurements of bulk transport in pure electron plasma have shown enhanced viscosity thought to be due to longrange interactions [9]; and the present experiments are an attempt to quantify these interactions. Here, we measure cross-field test-particle transport in an unneutralized ion plasma, where the cyclotron radius is smaller than the Debye length, i.e., rc ,l D . The quiescent, near-thermal-equilibrium ion column is contained steady state in a Penning-Malmberg trap [10], with no net transport or particle loss. Ions are tagged by spin orientation, and the slow cross-field diffusion of these test particles is accurately measured using laser-induced fluorescence (LIF). This test particle diffusion has been measured over a wide range of density, temperature, and magnetic field, and is generally about ten times faster than predicted by the classical theory of velocity-scattering collisions. Measurements of velocity-space isotropization on the same ion plasmas agree closely with classical theory [11,12], indicating there is no uncertainty in this collision rate. The enhanced test particle diffusion scales essentially as B 22 , and is probably due to long-range “ E 3 B drift” collisions with rc ,r, l D [2,3]. These weak, long-range binary interactions between ions are ultimately limited by velocity-scattering collisions and by shear in the E 3 B rotation of the column.