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.
Edge shear flows and particle transport near the density limit of the HL-2A tokamak
