Energy-balance equation and enhanced collisional plasma diffusion

Abstract

An exact energy-balance equation is derived from the first-order moments of the ion and electron transport equations. In this case the particle loss of a stationary plasma through an isobaric surface is expressed as the sum of several terms each of which contains a specific force (E-field, collisional friction, anisotropic pressure including inertial terms). The equation provides a general frame of interpretation for any stationary diffusion process or loss mechanism of a plasma. In particular, it is shown that Galeev-Sagdeev diffusion in an axisymmetric torus cannot be explained by the friction term η j2 or the Joule term E⃗⋅J⃗, but comes about by pressure anisotropy. This result rests on the assumption that the perpendicular resistivity η⊥ is not altered in order of magnitude by the presence of trapped particles, an assumption that is plausible on account of recent work by Hinton and Oberman. For Tokamak-like parameters the effective pressure anisotropies required for explaining Galeev-Sagdeev diffusion are very small, of the order of 10−5 to 10−6.