1 - Nuclear Sciences and Applications - IAEA

IAEA-CN-50/A-II-3 117
These observations imply that the high frequency oscillations are sensitive to the
details of the peripheral plasma. The narrow frequency spectrum also suggests the
possibility of these oscillations being excited by a localized mode. Linear stability
analyses of ideal and resistive localized modes in the divertor configuration show that
the ideal interchange mode and the ideal ballooning mode are stable, because of the
strong shear stabilization effect. The resistive ballooning mode and the high-m
tearing mode are usually unstable but they have a broad frequency spectrum in the
normal profile of the plasma parameters. The resistive interchange mode is unstable
near the X-point because of the bad toroidal curvature effect in the case of the outer
X-point configuration.
High frequency magnetic oscillations (~ 10 kHz) have also been observed in
the case of lower X-point discharges, but these oscillations often disappear during the
NB heating phase. It is not clear by now whether these high frequency oscillations
in lower X-point discharges are the same as those that are observed in outer X-point
discharges. The resistive interchange mode is usually stable in the case of lower
X-point configurations, but is marginally unstable for the lower X-point configuration
of JT-60.
5. FREQUENT ELMs
In the case of lower X-point discharges, frequent ELMs have been observed in
plasmas with high NB heating power [1, 5], similar to high beta discharges or
hydrogen H-mode discharges in DIII-D [6]. Frequent bursts of Ua radiation and
soft X-ray signals near the peripheral region have been observed with bursts of
magnetic fluctuation. Figure 6 shows the regime of frequent ELMs in qeff - /3p
space, where qeff is the effective safety factor. It is found that frequent ELMs occur
in the regime of /3p/qeff s 0.25. A similar criterion (j3p/qeff > 0.17) for H-mode
transition is seen in D-III deuterium discharges [7].
In discharges without the frequent ELMs, a decrease in the oxygen lines and
an increase in the carbon lines in the later phases of NBI have been observed, as is
shown in Fig. 7. A similar behaviour is often seen in NB heated limiter discharges
[8]. One possibility of increase in the carbon lines in these discharges is the
increase in the heat flux onto the divertor plate or the limiter. In contrast to these
discharges, a different impurity behaviour has been observed in the discharges with
the frequent ELMs. The intensities of the carbon and Ti XX lines are reduced to a
small level as compared with those in the no-ELM discharges, as is shown in Fig. 7.
On the other hand, the emissivity of the O VIII line is higher than that in no-ELM
discharges and decreases during the later phase of ELMs. The emissivity of the
O VI line (1032 A), which corresponds to an oxygen influx, remains almost constant
in discharges with the frequent ELMs. These results suggest that the frequent ELMs
work as an impurity exhaust and the enhanced plasma-wall interactions due to
particles swept out by the ELMs affect the oxygen influx.