Measurement of cross-magnetic-field heat transport due to long-range collisions

Cross-magnetic-field heat transport in a quiescent pure ion plasma is found to be diffusive and to be dominated by long-range “guiding center” collisions. In these long-range collisions, which occur in plasmas with Debye length ${\lambda }_D$ greater than cyclotron radius $r_c,$ particles with impact parameters $r_c{\mathrm{<\rho ⩽\lambda }}_D$ exchange parallel kinetic energy only. The resulting thermal diffusivity ${\chi }_L$ is independent of magnetic field B and plasma density $\mathrm{n.}$ The measured thermal diffusivity $\chi$ agrees within a factor of 2 with the long-range prediction ${\chi }_L{\text{=0.49nv̄b}}^2{\lambda }_D^2$ over a range of $1000$ in temperature, $50$ in density, and 4 in magnetic field. This thermal diffusivity is observed to be up to 100 times larger than classical diffusivity from short-range velocity-scattering collisions. These long-range collisions are typically dominant in unneutralized plasmas, and may also contribute to electron heat transport in neutral plasmas.