1 - Nuclear Sciences and Applications - IAEA

674 SIMONEN et al.
MHD modes, observed in ISX-B [16] and Doublet III [17] at high 0P. As
shown in Fig. 4, no sawteeth are present in the central soft X-ray emission.
Edge localized modes (ELMs) similar to those observed at lower j3p are
seen on the Ha emission in the divertor region, and on the soft X—ray
signals near the edge of the discharge. Despite the large (3p, the moderately
large ratio of toroidal beta to I/aB, and the large fast ion population, the
only coherent MHD activity observed on the magnetic probes is associated
with the ELMs. Except during the ELM events, the amplitude of poloidal
magnetic field oscillations measured at the wall is at most 0.1% of the total
poloidal field.
Figure 5 shows EFIT magnetic analysis [18] of measurements from
41 flux loops distributed around the outside and 25 magnetic probes distributed
around the inside wall of the DIII-D vacuum vessel. Shownare
the flux surface equilibrium, toroidal current density and q profiles at two
times: (a) before beam injection and (b) in steady-state when /9p = 3.5.
This analysis is based on a third-degree polynomial for j versus magnetic
flux. The radial current profile exhibits (see Fig. 5b) a 0.2 m outward
Shafranov shift and an extreme outward current peaking which, together
with ej3p > 1, are indicators of possible second region stability.
The issue of whether these plasmas entered the second stable regime
revolves around the exact value of the axial safety factor qo. MHD equilibrium
analysis of external magnetic measurements for these elongated
plasmas can determine accurately the plasma shape, edge q (or q at the
95% flux surface, qgs), the poloidal beta /?p, the plasma internal inductance
ti [18], iand the magnetic axis shift. Information on q0 is only weakly
1000
-5.0 -2.5 0
PRESSURE GRADIENT P^
1 ST REGIME
RADIUS
NORMALIZED *
PLASMA CENTER
FIG. 6. A stability diagram for shot 57456 with q0 = 3 calculated with the CAMINO code. Here \
is the radial flux coordinate, and p^, and q^ are the pressure and safety factor gradients relative to $
The bold curve indicates that the plasma is at the entrance to the second stability region.