1 - Nuclear Sciences and Applications - IAEA

IAEA-CN-50/A-II-l 93
stability limits:
swall at r = 1.5 a
(disruption)
0.7 0.8 0.9 1.0 1.1 1.2
/. (internal inductance)
v
FIG. 6. Predicted ideal kink mode stability versus /3W and (it compared to experimental data points.
Also shown is the trajectory of a high beta disruption discharge (60383).
conducting wall at a radius of 1.5 times the plasma's minor radius, but
unstable without the wall. Although the resistive vacuum vessel should
act as a conducting wall at the typical mode rotation frequency of a few
kHz, the stability calculations may be consistent with the fact that the
2/1 mode which precedes the disruption does not rotate. A model was
invoked for disruptions at the Doublet III beta limit [16] in which central
ballooning modes caused profile broadening, after which the additional
broadening due to a sawtooth fall triggered a 2/1 tearing or kink mode.
A similar mechanism may trigger the high beta disruptions in Dili—D.
In order to investigate the role of the current profile, stability calculations
were made for a two-parameter family of profiles which allowed
variation of beta and £i while go (safety factor on axis), boundary shape,
and total plasma current were kept fixed. The results are compared in
Fig. 6 to experimental data from equilibrium fits using magnetic data
only. The profiles for the stability calculations were consistent with these
equilibria. As £i decreases the maximum stable beta is predicted to increase,
until at low ti either a current driven stability limit is encountered
or an equilibrium cannot be found with this profile parameterization. In
experimental discharges, the internal inductance ii decreases as beta increases,
but the decrease is not rapid enough to avoid the kink instability.
In this plot the observed beta limit at /3N = 3.5 is above the calculated
limit without wall stabilization, and seems to agree well with the limit for
a conducting wall at 1.5 a. When more realistic profiles consistent with
kinetic pressure profile measurements are used, the predicted limit with
the wall at 1.5 a becomes somewhat larger.