1 - Nuclear Sciences and Applications - IAEA

568 TANAKA et al.
Figures 3(a)-(i) show the temporal evolution of the plasma parameters in the
case of the STO suppression by LHCD. When PLH (=130 kW) is injected into an
OH plasma with Ip = 110 kA (a), the loop voltage VL decreases (b) and the EC
emission at f = 100 GHz increases (d). These data show the generation of LH driven
current carried by suprathermal tail electrons. The electron density increases continuously
from rie = 1.0 to 1.8 (X 10 13 cm" 3 ) at the end of the LH pulse (c). In
Figs 3(e)-(i), the detailed temporal evolution of MHD phenomena is shown. The
SXR signal along the central chord (g) shows that STO appearing in the OH plasma
are suppressed by LHCD. After the suppression, the MHD oscillation appears at
t = 52 ms and then STO with a large amplitude and a long period reappears at
56 ms. Weak MHD activity is observed on the MP signal at t = 52 ms (f). The
impurity line (O V) intensity decreases slightly at the beginning of the PLH injection
and then increases.
Figures 4(a)-(c) show the detailed temporal evolution of the SXR signals and
its spatial profiles in the case of complete stabilization and subsequent destabilization
of STO by LHCD. In the enlarged figures (b), showing successive crash phases of
STO in the LHCD plasma, the sawteeth crash very fast (t < 20 ps). Furthermore,
the two early sawteeth ((1) and (2)) accompany the precursor, while the two late sawteeth
((3) and (4)) accompany neither precursor nor successor. As is shown in
Fig. 4(c), the inversion radii of both STO appearing in the OH and LHCD plasmas
are nearly identical and rinv « 5 cm (rinv/a = 1/4), and the radial profile of the
MHD oscillation (m = 1) of the SXR signals has a peak near r = rinv. The results
suggest that there is no change in the location of the q = 1 surface between these two
OH and LHCD plasmas.
Figure 5 shows the temporal evolution of the contour curves of the SXR signal
intensity in the crash phase of STO, which is similar to (2) in Fig. 4(b). These figures
were reconstructed from the SXR intensities measured by three fan diode arrays
(60 SSB detectors) by means of computer tomography. These characters suggest the
formation of cold plasma STO bubbles as predicted by Wesson et al. [15].
4. CONCLUSIONS
The 2Qe EC driven current is generated in a target plasma with pre-existing
high energy tail electrons. The flat-top and ramp-up discharges are realized without
OH power. The current is carried by the EC heated high energy tail electrons. The
2fie ECCD efficiency is one order smaller than that of LHCD in WT-3.
The MHD instability (m = 2 mode) appearing in the high current OH discharge
can be suppressed by LHCD. The sawtooth oscillations in the OH discharge are
suppressed and then other sawteeth appear, following the m = 1 MHD oscillation
in the LHCD plasma. The latter sawteeth are not accompanied by precursors or
successors and have a long period and a large amplitude compared with those in the
OH discharge. Computer tomography of the SXR intensities shows the formation of
cold plasma STO bubbles similar to those of JET.