1 - Nuclear Sciences and Applications - IAEA

IAEA-CN-50/A-VII-4 397
the electron temperature falls to less than 50% of its initial value in ~ 1 ms. This
precedes or is concurrent with a fast positive current spike, which is sometimes
followed by a slow current quench. Estimates of the electron thermal diffusivity
during the transport phase give values of order 120 m^/s. Some major
disruptions are found to have no visible precursors, although locked modes (or
extremely fast growing modes) would not normally be detectable. Occasionally
low amplitude mode structure (m/n = 1/1 in core, coupled to m/n = 5/1 or 6/1 at
edge) is observed preceding the electron temperature decay. Figure 1 shows such
a disruption at high p on TFTR. It is typical of those observed at high beta
poloidal, where the discharge is stable until about 1 ms before the disruption,
when a mode appears with a growth time 1.2 MA ) or lower q,
sawteeth or bursts of m/n = 1/1 activity are observed. These modes appear as
m = 1 on the SXR diodes, but as higher m on the Mirnov coils due to mode
coupling. The rise in stored energy and neutron emission slows at these bursts,
indicating that the modes are affecting the plasma center and thus are harmful
at present levels; in addition enhanced losses of fast ions [3] and high energy
tritons are observed [4]. From the experimental data on'Mirnov coils or soft xrays,
no direct evidence of ballooning modes (i.e. n > 2 ) has been found, even
when the profiles are close to the high-n ideal MHD ballooning threshold. It
should be noted that TFTR L-mode and ohmic discharges are generally free of
coherent low-m MHD activity.
4. SAWTOOTH STABILIZATION
Stabilization of the sawtooth oscillation has been observed in the supershot
regime for up to 2 s [5]. Transport analysis calculations using the TRANSP
code [6] indicate that the stabilization is due to q0 rising above 1. The calculated
increase in q0 for supershots is due in part to the presence of non-ohmic current
drive which tends to broaden the q profile [7]. Figure 2 shows the onset of sawtooth
stabilization for a) co- only, b) balanced and c) counter-injection on TFTR. It is
clear from figure 2 that co- only and balanced are more effective at sawtooth
stabilization than counter-injection. These results coincide with the TRANSP
calculation which shows qo rising with co- and balanced neutral beam
injection, but staying constant with counter-injection.