Review and present status of the TEXTOR radiative improved (RI) mode

Abstract

From 1990, the boronized TEXTOR tokamak was characterized by an improved confinement (coined the ‘I mode’) at high power that was substantially better than the L mode, but densities had to be limited to n[bar]e0/nGR[lsim ]0.5–0.6, where nGR is the Greenwald density limit. With the injection of Ne, Si or Ar in order to increase the edge radiation and provided that γ=Prad/Ptot[greater, similar]0.5, PNBI-co/Ptot[greater, similar]0.25 and n[bar]e0/nGR[greater, similar]0.75, a further improved confinement called the radiative improved mode (RI mode) was discovered in 1993 on TEXTOR, a tokamak of intermediate size, and confirmed on TEXTOR-94. The radiated power fraction γ can reach 0.9, and the radiation is nearly isotropically distributed over the torus wall. The RI mode is characterized by its ability to obtain simultaneously and stationarily high densities and high confinement. It is linked to a substantial lowering of edge ne, Te and Ti, a reduction in particle transport and a peaking of the density profile. The RI-mode confinement scales on TEXTOR as τE= (n[bar]e0/nGR)τITERH93-P and values up to n[bar]e0/nGR≈1.2 are obtained. There is no detrimental concentration of the seeded impurity at the centre of the plasma. Results of three different interpretative and modelling approaches are in agreement with the improved confinement features; the preliminary indications are that ITG turbulence is strongly reduced. The Z mode observed on ISX-B has a clear resemblance to the RI mode. The very favourable features of the RI mode justify efforts in trying to establish it on larger machines to verify if the present scaling then holds.

Turbulent transport and turbulence in radiative I mode plasmas in TEXTOR-94

“…The turbulent particle flux convected power is 20% that of the radiated power in L mode conditions (37% radiated power) and drops to 1% of the radiated power in RI mode conditions (95% radiated power); thus the particle confinement does not play a critical direct role in the energy exhaust. Not only τ p but also τ E shows a sharp increase with radiated fraction [48], showing a good correlation between the increase in energy and particle confinement and indicating that the mechanism responsible for the enhancement affects both the particle and the energy confinement. The τ E /τ p ratio exhibits a tendency to increase with density in RI mode discharges, as shown in Fig.…”

Improved confinement regime in the current ramp down phase of Frascati Tokamak Upgrade plasmas

“…confinement [3]. The application of radiofrequency (RF) heating during the current ramp up phase of a plasma produces a reverse shear (RS) in the Q profile and an increase in confinement [4].…”

Kinetic ion gradient instability: bifurcation properties of a simple, kinetic ηi-unstable slab model

“…Other important issues are magnetic shear as well as flow shear. In general, it is found that the addition of non-locality tends to stabilize ITG modes (Hahm and Tang 1989;Hong and Horton 1990;Hong et al 1991;Bai et al 1998;Vandenplas et al 1998;Waltz et al 1998;Tokar et al 1999). In view of the potentially negative effect that the ITG instability might have on core confinement, it is important to note that effects not considered in the usual analysis often tend to stabilize the ion-temperature-gradient mode.…”