REVIEW AND PRESENT STATUS OF THE TEXTOR RADIATIVE IMPROVED (RI) MODE

From 1990, the boronized TEXTOR tokamak was characterized by an improv ed confinement (coined the 'I mode') at high power that was substantia lly better than the L mode, but densities had to be limited to (n) ove r bar(e0)/n(GR) less than or similar to 0.5-0.6, where n(GR) is the Gr eenwald density limit. With the injection of Ne, Si or Ar in order to increase the edge radiation and provided that gamma = P-rad/P-tot grea ter than or similar to 0.5, PNBI-co/P-tot greater than or similar to 0 .25 and (n) over bar(eo)/n(GR) greater than or similar to 0.75, a furt her 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 gamma can reach 0. 9, and the radiation is nearly isotropically distributed over the toru s wall. The RI mode is characterized by its ability to obtain simultan eously and stationarily high densities and high confinement. It is lin ked to a substantial lowering of edge n(e), T-e and T-i, a reduction i n particle transport and a peaking of the density profile. The RI-mode confinement scales on TEXTOR as tau(E) = ((n) over bar(e0)/n(GR))tau ITERH93-P and values up to (n) over bar(e0)/n(GR) approximate to 1.2 a re obtained. There is no detrimental concentration of the seeded impur ity at the centre of the plasma. Results of three different interpreta tive and modelling approaches are in agreement with the improved confi nement features; the preliminary indications are that ITG turbulence i s strongly reduced. The Z mode observed on ISX-B has a clear resemblan ce 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.