A model for the transition to the radiatively improved (RI) mode triggered in tokamaks by seeding of impurities is proposed. This model takes into account that with increasing plasma effective charge the growth rate of the toroidal ion temperature gradient (ITG) instability, considered nowadays as the dominant source of anomalous energy losses in low-confinement (L) mode, decreases. As a result the plasma density profile peaks due to an inward convection generated by trapped electron turbulence. This completely quenches ITG induced transport and a bifurcation to the RI mode occurs. Conditions necessary for the L-RI transition are investigated.

In a tokamak plasma the maximum achievable density is limited. A too high density will result in a violent end of a discharge. Two types of density limit disruption can be distinguished: (a) impure and moderately heated discharges, if the radiative power exceeds the input power, (b) clean, auxiliary heated discharges, where the Greenwald limit is encountered. It has been found that in TEXTOR-94 these two density limits differ by the radiative instability in the plasma boundary, which preceeds the disruption. A symmetric radiative mantle and a detachment are observed prior to the first type, while the Greenwald limit has a MARFE precursor. Control of the impurity content, edge and recycling properties prevents the growth of the MARFE and makes it possible to exceed the Greenwald limit in TEXTOR-94 by more than a factor of 2. High densities have been obtained by means of normal gas feed. Maximum central densities of ne(0) = 1.3 × 1020 m-3 have been obtained. The maximum achievable density scales with the input power and plasma current. Non-disruptive discharges, with a stationary (t > 25 τE) density a factor of 1.93 above the Greenwald limit have been produced in L mode. The radiative losses and impurity concentration have been maintained at a relatively low level during the entire high density phase.

AbstraeL A numerical onedimensional radial timedependent madel of particle and energy transport in tokamak plasmas taking lhe interaction of background and impurity particles thmugh the radiation losses of energy and ion dilution is developed. Detachment in ohmic discharges caused by an increase in the elearan density is modelled and the effect of particle and energy transport on detachment conditions is shulied. Investigation of dynamic changes in plasma parameters during detachment pmvoked by an increase in impurity influx is performed. The effect of the nature of heat mute on the stabilization of the radiating layer is elucidated. The influence of neoclassical Wnsport and chargeexchange with hydrogen nenhals on the impurity &havioUr in 'attached' and 'detached' plasmas is discussed.

The characteristics of the toroidal ion temperature gradient (ITG) instability, considered as the main source of anomalous transport in the low (L) confinement mode of tokamaks, are analysed for the conditions of the radiatively improved (RI) mode triggered by seeding of impurities. Based on experimental profiles from TEXTOR-94 we deduce that the ITG-induced turbulence is quenched in the RI-mode in a large part of the plasma radius. Under those conditions the dissipative trapped electron (DTE) instability dominates the transport. The peaking of the electron density, one of the most remarkable features of the L-RI transition, is explained by the difference in contributions of ITG and DTE unstable modes to the diffusive and convective components of the electron flow. The shearing of the plasma toroidal velocity observed in the RI-modes with unbalanced neutral injection is successfully modelled by taking into account both the anomalous plasma viscosity due to ITG-turbulence and predictions of the revisited neoclassical theory.

No abstract