1 - Nuclear Sciences and Applications - IAEA

538 PRATER et al.
2.0
1.5
1.0
0.5
/
L-MODE
0.0
i
900 msec
OHMIC
. i ..
130 150 170 190
R (m)
1500 msec
H-MODE
s^~ ELM-FREE
k 1320 msec
^K^. H-MODE
FIG. 7. Electron temperature profiles from ECE for the discharge of Fig. 5.
for the ballooning mode fails to show that the plasma has approached the
second stability region, although the edge pressure gradient is clearly
higher when the ELMs are suppressed [16].
More recent discharges using outside launch 0-mode have shown
similar effects. For the discharge of Fig. 5, 1.25 MW of neutral injection
power failed to attain the H-mode, but when 0.9 MW of ECH was added
the transition started promptly. Following the transition to H-mode, the
density began rising, although large frequent ELMs were present. After
100 msec the ELMs disappear, for a period of 200 msec. Discharges
without the ECH do not show this ELM-free behavior. What may be happening
is that as the density rises during the early H-mode, the plasma
enters the density regime for which the first pass absorption is rapidly
dropping [Fig. l(a) and (b)]. The power balance measurements imply
that the ECH power which is not absorbed at the first pass is apparently
deposited in the plasma nearer the edge. This edge absorption may be
similar to that of the earlier experiment in which the resonance was placed
near the plasma edge. The end of the ELM-free period may be due to a
gradual increase of the edge pressure gradient into the ballooning unstable
region, or due to increasingly poor absorption of the ECH even at the
plasma edge because of the increasing density.
Energy confinement improves dramatically during the ELM-free period.
Figure 5 shows that there is little improvement in re after the
H—mode transition, presumably because of the giant ELMs. When the
ELMs disappear confinement improves dramatically, becoming comparable
to ohmic confinement, but with 5 times the plasma energy.
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