1 - Nuclear Sciences and Applications - IAEA

s.o
• .5
(a)
IAEA-CN-50/A-II-l 91
360
0 60 120 180 2*0 300 360
POLOIDAL ANGLE (DEGREES)
FIG. 5. Resistive mode calculation for n = 1 beta collapse (55791): (a) calculated perturbation of
poloidal magnetic flux; (b) phase and amplitude of Mirnov oscillations versus poloidal angle, comparing
measured (•) and calculated (o) values.
The low-n modes which cause beta saturation and collapse cannot
be attributed to ideal modes. Their growth rates are slow (typically
r ~ 10 4 TAifven) and they can persist unchanged for times approaching the
resistive time of the plasma. Ideal stability calculations using GATO [13]
(discussed further below) show that the point where the n = 4, 3, and 2
modes are first observed is usually far from the kink mode stability limit.
These low-n modes are most consistent with pressure driven resistive
instabilities. The three-dimensional resistive code CART—II [14] was used
to analyze a discharge similar to that of Fig. 4, at a time during the large
n = 1 oscillation. The nonlinear calculation predicts a predominantly
?n = 2/n = 1 instability, as shown in Fig. 5. The predicted mode structure
agrees well with soft X-ray and magnetic fluctuation measurements,
including the coupling to higher poloidal mode numbers which dominates
the structure along the inboard wall. The nonlinear calculation was halted
when the calculated value of Bg/Be at the outer wall was 1%, equal to
the measured value at the time when the n = 1 mode in the discharge of
Fig. 4 stopped rotating and the rate of energy loss accelerated. At this
point, the width of the 2/1 island was calculated to be 11 cm and flux
surfaces outside of the q = 2 surface had started to become stochastic.