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 (fy) 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 (q a = 2.45) are compared at either variable mean density or variable additional power, and their stability against DTE and 7j ; 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 = l 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 JJ ; mode must then follow from particle flux constraints. (3) The condition that both y m and y DTE (y = 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.

A. A. Bagdasavow et al,, ibid.; K. B. Axom et al., ibid.; A. Gondhalekar et al., ibid.; M. Murakami et at., ibid.

The purpose of this paper is to demonstrate that the turbulence spectra and the electron heat fluxes derived from the theory of drift wave saturation presented recently are consistent with the discharge data from two tokamaks. It is shown that the calculated rapid increase in transport with increasing linear growth rate explains the observed relaxation of the profiles towards a weakly unstable state with respect to the trappedelectron instability. It is further suggested that the occurrence of sawtooth oscillations of the core can be interpreted in terms of drift instability quenching and anomalous heat flux clamping in the surrounding gradient layer. The high-density limit derived from this assumption agrees well with the observations.

A periodic enhancement of the microturbulence level by sawtooth relaxations has been detected by CO2 laser forward scattering in the TEXTOR tokamak. This feature is reproduced quantitatively by a heat transport code in which the anomalous electron transport coefficient is calculated self-consistently, following a theoretical model of the saturation of the dissipative trapped electron instability. The code also predicts a strong modulation of the heat flux throughout the plasma and a strong 'profile consistency' as demonstrated elsewhere by continuous temperature measurements. A simple interpretation of these results is given. The calculated global plasma parameters, such as the energy confinement time and the loop voltage, are in good agreement with the measured values.

The stability of ion Bernstein waves (frequency ω, wave number k⃗) in a plasma with two ion species pumped by a magneto-acoustic wave (ωp, k⃗p) which propagates perpendicularly to the static magnetic field is studied. The background plasma is assumed to be infinite, homogeneous and collision-free. If ωp > |ω| is properly chosen, ion Bernstein waves become unstable (decay instability, non-linear Landau instability) at rather low values of the pump electric field amplitude |E⃗D|. The instability is excited by the relative drift motion of different species induced by the pump wave. Assuming |k⃗⋅(D⃗a−D⃗b)|≪1 (D⃗a = displacement for particles of species a, a = e, 1, 2) the general non-linear dispersion relation is approximately solved in a deuterium-tritium plasma for two different k⃗: In case A (k⃗=k⃗⊥ ∥ k⃗p) only the relative ion motion D⃗d⋅D⃗t comes into play; this gives rise to decay instabilities which are only present in plasmas with two ion species. The instabilities of case B(k⊥ ⊥ kp, kz ≠ 0) caused mainly by the relative electron-ion drift motion are similar to those in plasmas with only one ion species. For given |E⃗p| the growth rates in both cases are equal in order of magnitude (for low values of ωp, case A is slightly more favourable); however, in case A the theory is valid up to considerably higher values of |E⃗p|. Some effects depending substantially on the presence of two ion species are discussed in detail.