1 - Nuclear Sciences and Applications - IAEA

IAEA-CN-50/A-II-4 123
FIG. 1. Hugill plot of explored parameters with typical discharge trajectories.
which often occur when the plasma current begins to fall can be divided into two
categories, A and B, according to the perturbation range in the plasma. Category A
disruption causes obvious changes in Ip, while category B disruptions do not affect
Ip and only show up in Vp fluctuations. B disruptions can take place alone, but
sometimes they develop into A disruptions. In the operational regime, there are two
MHD activity areas. As is shown in Fig. 1, soft disruption occurs when the discharge
trajectory in the Hugill plot crosses area I during the current decay phase (trajectory
a), or when the plasma current ramps up to area II with a lower rising rate
(trajectory b). However, between these two areas, there appears to be a narrow
window (5.8 < n,,R/BT < 6.6 x 10 18 nr 2 -T" 1 ), through which discharges can pass
pass into the low qa value region (trajectory c). By controlling the current rising rate
carefully, stable discharges at qa = 2.5 can be obtained by passing through this
window.
2.3. Sawtooth-like relaxation
For category B disruptions, frequently regular relaxation phenomena similar to
sawtooth oscillations occur. These events that are observed on soft X-ray signals have
slowly rising and fast falling periods, and the modulation of the central soft X-ray
signal is weaker; it resembles a subordinate oscillation in a compound sawtooth. The
inversion radius of the relaxation is much larger than the radius of the q = 1 surface,
but cannot be determined by the present array of X-ray diodes, although occasionally
inverted signals are observed [3]. Figure 2 shows the soft X-ray signals for sawtoothlike
relaxation and the response of the m = 2 frequency and amplitude to it. Both