1 - Nuclear Sciences and Applications - IAEA

IAEA-CN-50/A-HI-2 167
near the first stability boundary. These preliminary results suggest that the pressure
gradients are lower than would be required by the bifurcation model of
Bishop [17] . Nevertheless they indicate that the edge pressure gradients in the
H-mode are sufficiently steep that ideal ballooning stability becomes relevant.
Furthermore the current density parameter A is found to be consistent with the
critical value needed for coalescence.
3.5 Divertor plasma
The plasma in the vicinity of the X-point target plates (divertor plasma) has
been investigated with an array of eight Langmuir probes mounted in the target
plates (see inset in Fig. 6), with a reciprocating Langmuir probe (at R=3.25 m),
and with a 2-dimensional view of the radiation from the divertor region.
Fig. 6 shows profiles of density, temperature and ion saturation current in front
of the target plates for a 4.6 MA discharge (OH and H-phase with PNBI=14 MW).
Although a single-null discharge, the inner divertor plasma is colder and denser
than the outer. More power is carried to the outer divertor, suggesting that the
main source of power flow into the scrape-off layer is in the vicinity of the outer
mid^plane, ie, closer to the outer divertor target. The unequal temperatures lead
to thermo-electric currents in the scrape-off layer [18 l Current densities
g 10 5 AnT 2 have been measured during the H-phase (see Fig. 6) corresponding
to more than 10% of the mean current density in the discharge. The scrape-off
layer current flows from outer to inner divertor and returns through the target
plates, thus flowing in the same direction as the plasma current.
The power estimated to be carried by plasma to the target plates increases
from 0.8MW in the ohmic phase to 1.9MW at the end of the H-phase. For most
of the discharge, i.e. L and H-phases, this is about 15% of (Ptotal-dWdia/df)
but about 25% during the ohmic phase.
Profiles with the reciprocating probe in the scrape-off layer agree reasonably
well with those at the divertor target. Preliminary results for two or three pulses
show that the radial electric field, just inside the separatrix, Er = - (d Vp/asma)/dr
is negative in the ohmic phase and about + 5 kVnT 1 with H-mode. This result,
if substantiated, would agree with the prediction of Itoh and Itoh [19] .
The collisionality of the divertor plasma may have a bearing on whether an
H-mode can be achieved. Two aspects of collisionality are considered: Coulomb
collisions with electrons, and ionisation of neutral particles in the divertor plasma.
For Coulomb collisions, the Coulomb mean free path, Xee, is compared with
the distance Ln in the divertor measured along a field line from the target to
the X-point. With the X-point typically 0.10m from the target and a pitch angle
of the field lines of ~ 1°, Ln«6m. Fig.7 shows probe measurements of Te
plotted against ne on a logarithmic scale. The straight line represents Xee = 6 m: