Transport through dissipative trapped electron mode and toroidal ion temperature gradient mode in TEXTOR

Abstract

A self-consistent transport code is used to evaluate how plasma confinement in tokamaks is influenced by the microturbulent fields excited by the dissipative trapped electron (DTE) instability. As shown previously, the saturation theory on which the code is based has been developed from first principles. The numerical results reproduce well the Neo-Alcator scaling law observed experimentally (for example in TEXTOR) in non-detached Ohmic discharges, the confinement degradation resulting when auxiliary heating is applied and a large number of other experimental observations. The potential impact of the toroidal ion temperature gradient (ηi) mode on energy confinement is assessed by estimating the ion thermal flux with the help of the mixing length approximation. The temperature and density profiles measured in TEXTOR (qa ≃ 2.45) are compared at either variable mean density or variable additional power, and their stability against DTE and ηi modes is checked; in the latter case, a new criterion is used, which is valid for arbitrary curvature. All profiles examined are marginally unstable for the two modes, essentially between the q = 1 and q = 2 magnetic surfaces. The code results and the stability analysis lead to the following conclusions and suggestions: (1) The DTE instability is sufficient to explain the anomalous heat transport in low density discharges (attached plasmas) with or without additional heating; the marginal instability for the DTE mode thus follows from heat flux constraints. (2) The observed marginal instability against the ηi mode must then follow from particle flux constraints. (3) The condition that both γηi and γDTE (γ = growth rate) must be small is the restriction which determines the profiles that correspond to the experimental conditions and which determines, to a large extent, profile consistency. (4) Finally, it is suggested that the deviation from Neo-Alcator scaling, the density limit and the phenomenon of plasma detachment are interrelated effects which arise at high densities, when the constraint on the electron heat flux becomes harder to satisfy.