Transport analysis of the observed T_eprofile invariance in ASDEX

Abstract

The response of the electron temperature profiles to changes in heating profile is investigated analytically and by transport code simulations. A heat conduction model with local empirical scalings of the electron thermal diffusivity χe is used. It is found that the Te-profile responds rather insensitively, owing to double radial integration of the heating power density. By scanning the target density and injection energy of neutral beam heated ASDEX plasmas it is shown that only small profile changes in the conductive electron heat flux are achieved. In density scans these changes are further diminished by the convective power loss. The theoretically predicted variation in Te(r)/Te(0) is correspondingly weak and does not exceed the measured profile change. The observed Te profile invariance is thus produced by local transport models with a diffusive electron heat flux proportional to dTe/dr. Consequently, it is not necessary to introduce a χe(r) which adjusts to the profile of the conductive electron heat flux or which non-linearly depends on the electron temperature gradient.

Interval estimation of global H-mode energy confinement in ITER

“…The exponent of ρ * (r) for the normalized diffusivity χ(r) displayed in figure 8 in [49], varies, for ions as well as electrons, from roughly 2 at r/a = 0.3 to 0.5 at r/a = 0.8. The global confinement time depends primarily on the diffusivity in the outer region of the plasma, since τ E ∼ (nT (V /S))/(n a |∇T | a χ a ) where V /S ∼ af (κ, δ) and the volume averaged temperature T ∼ a 0 T (r)n(r)r dr/ a 0 n(r)r dr depends rather weakly (by a double radial integration) on the details of the profiles of heating power and diffusivity in the inner region of the plasma [54]- [56]. (In [60] by partial integration T has been rewritten as a normalized radial average of 1/χ(r) weighted by a function G(r), which depends on the density profile as well as the heat flux profile.…”

Test of Teprofile invariance by sensitivity studies

“…It was shown in Ref. [9] that the range of variation of qh(r) can be smaller than that of P&). In neutral beam heated ASDEX L-and H-mode plasmas the electron heat conductivity can be approximated by K, = n,(r)x,(r) = constl,, which yields…”