Temperature-gradient instability induced by conducting end walls

Abstract

A new rapidly growing electron-temperature-gradient instability is found for a plasma in contact with a conducting wall. The linear instability analysis is presented and speculations are given for its nonlinear consequences. This instability illustrates that conducting walls can produce effects that are detrimental to plasma confinement. This mode is of importance in open-ended systems such as mirror machines and relevant to the edge of tokamaks where field lines are open and are connected to limiters or divertors and astrophysical plasmas like the ones of the flux tubes in a solar atmosphere, with the footpoints on the photospheric level.

Rotational stability of plasma blobs

“…The sheath term on the rhs yields the drive term for the BRT electron-temperature-gradient instability in the eikonal limit. 26 However, we will show that the sheath term turns out to have a net stabilizing effect in the low mode number limit of interest here.…”

“…Physically, the condition S α << 1 implies that the blob spin time around its axis (τ s ~ Ω −1 ) is much shorter than the time for its vorticity to dissipate by parallel loss In the eikonal or high-m limit (k r ~ k θ = m/r >> 1/a), one can show that Eq. (33) yields the following well-known local dispersion relation 26,29 …”

“…In the limit of rapid spin, the curvature-driven modes are suppressed 22 and but we show here that new rotational modes arise which can go unstable through a combination of centrifugal force, Coriolis, Kelvin-Helmholtz and rotational shear effects. When the sheath conductivity is included in the analysis, the stability equation also includes the Berk-Ryutov-Tsidulko (BRT) ∇T e -driven instabilities 26 in the eikonal limit. In the low mode number limit of interest here, the sheath term turns out to have a net stabilizing effect.…”

Drift waves and transport

“…The Xu et al (1995) simulations were subsequently extended to include both a core plasma with magnetic shear at separatrix (xϭ0), and a scrape-off layer with magnetic field lines ending on conducting end plates. In these simulations of the Berk et al (1991) configuration of a separatrix outside of which fields are terminated on conducting plates is the important divertor configuration in fusion reactors. The turbulent diffusivity on the open field lines with ␣ eff used in Eq.…”

Final Report for DE-FG02-96ER54370

“…Recombination occurred at the divertor, and the recombined neutral particles would then make their way back into the core plasma, where ionization would occur. Recent experimental data from a wide variety of magnetically confined devices, however, paints a different picture, suggesting that a majority of the recycling actually occurs at the wall of the main chamber [1][2][3] as opposed to at the divertor. The implications of these data are twofold.…”