1 - Nuclear Sciences and Applications - IAEA

536 PRATER et al.
added to 1.25 MW of neutral injection. With ECH alone no transition was
obtained and with neutral injection alone (D° —* D + ) the power threshold
was about 1.3 MW. These results, taken with the second harmonic heating
threshold of 0.75 MW, support a scaling of Pthreshold °c Bt. This scaling
appears to hold in DIII-D for neutral injection experiments as well.
4. STABILIZATION OF MHD ACTIVITY
MHD activity is known to reduce confinement or lead to disruptions
in tokamak plasmas. The effect of the internal disruption associated with
the sawtooth crash is to limit the attainable central electron temperature
and to increase the energy and particle transport in the central region of
the plasma. Activity with m = 2 is believed to lead to disruptions, and
in H-mode discharges the edge localized modes (ELMs) reduce confinement
and decrease the quality of the H-mode. Because of its localized
absorption, ECH can be used to affect these phenomena with relatively
little power [1].
In order to investigate the effect of the heating location on sawteeth
in DIII-D, a detailed scan of toroidal field was performed using the second
harmonic X-mode launched from the outside, with 0.85 MW of ECH
power. A summary of the results is displayed in Fig. 6. A maximum
was found in both the sawtooth period and amplitude when the ECH
resonance was placed near the sawtooth inversion surface, with the effect
sensitive to the resonance placement near that location. In this case, the
resonance surface was on the outboard side of the plasma center. A shift
of 3 cm in resonance location changed the sawtooth period by a factor
of three from 25 to 80 msec. In general the sawtooth period was still
evolving to longer periods when the 500 msec ECH pulse ended. The
length of the time scale for the sawtooth period to come to equilibrium
suggests that it is a result of a slow transport process such as a plasma
current redistribution and not an energy redistribution. We speculate
that the sawtooth period extension is due to an effect on the sawtooth
trigger mechanism. Support for this idea is found in the sharp peak in
the effect with the heating at the sawtooth inversion radius, where, for
example, the m = 1, n = 1 instability is thought to originate. Depending
on the form of the instability, changes in the local parameters or gradients
(current or pressure) can change the growth rates of the modes or saturate
them [14,15].
A second set of sawtooth modification experiments were performed
with higher power (1.1 MW) using the 0-mode outside launch system at
the fundamental frequency. In this case the sawtooth period was greater
than the 400 msec length of the if pulse when the ECH resonance was
placed near the sawtooth inversion surface and the line-averaged density
was below 1.2 x 10 19 m~ 3 . The sawtooth period was essentially unchanged
from the ohmic value when the heating was shifted 4 cm from
the sawtooth inversion toward either the center or the edge of the plasma.
The resistivity profiles calculated from the diagnostic measurements (visible
bremsstrahlung for Zeff, density and temperature profiles) became