Electron heat transport in a steep temperature gradient

Abstract

Temporal and spatial measurements of electron heat transport are made in the University of California Davis AURORA device (J. H. Rogers, Ph.D. dissertation, University of California, Davis, 1987). In AURORA, a microwave pulse heats a region of underdense, collisional, plasma (n/ncr ≲1, where ncr =1.8×1010 cm−3 is the critical density, Te0 ≊0.15 eV, and the electron scattering mean free path λ⊥≳2 cm). In this region, strong thermal heating (Tc ≲0.7 eV) as well as suprathermal heating (Th≊3 eV) is observed. The strong heating results in a steep temperature gradient that violates the approximations of classical heat diffusion theory (LT/λ⊥≳3 for thermal electrons, where LT=Tc(∂Tc/∂z)−1 is the cold electron temperature scale length. The time evolution of the electron temperature profile is measured using Langmuir probes. The measured relaxation of the temperature gradient after the microwave pulse is compared to calculations using the Fokker–Planck International code [Phys. Rev. Lett. 49, 1936 (1982)] and the multigroup, flux-limited, target design code lasnex [Comm. Plasma Phys. 2, 51 (1975)]. The electron distribution function at the end of the microwave pulse is used as initial conditions for both codes. The Fokker–Planck calculations are found to agree very well with the measurements. However, the flux-limited diffusion calculations do not agree with the measurements for any value of the flux limiter.