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 lD greater than cyclotron radius rc , particles with impact parameters rc,r<lD exchange parallel kinetic energy only. The resulting thermal diffusivity xL is independent of magnetic field B and plasma density n. The measured thermal diffusivity x agrees within a factor of 2 with the long-range prediction xL50.49n v̄b lD 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. © 2000 American Institute of Physics. @S1070-664X~00!93205-1#