1 - Nuclear Sciences and Applications - IAEA

IAEA-CN-50/A-H-2 107
lowermost inboard passive plates. In these early experiments, usually the open
divertor geometry has been used with neutral beam heating.
In general, clear transitions to the H-mode are observed in PBX-M
discharges which have the separatrix either inside or very near the surface of
the limiters on the passive stabilizers. These are confirmed by a sudden drop
in the Ha emission in both the divertor and main plasma regions, a
simultaneous rise in the electron density, and a sudden sharpening of the edge
plasma in visible television pictures of the discharge. For example, Fig. 7
shows the waveforms from several diagnostics for a beam heated discharge
exhibiting the H-mode transition in PBX-M. At t =475msec, the characteristic
drop in Ha occurs, and persists for some 90 msec until the injected power is
lowered. At the same time, rapid increases in the line averaged density and
central chord intensities of both the visible bremsstrahlung and soft X-ray
signals. The transition often occurs about 2 msec after a sawtooth crash, but a
sawtooth crash is not necessary to trigger an H-mode transition. With
indentations of 15% and the plasma limited mainly by the magnetic separatrix,
the threshold absorbed power for transition to the H-mode is as low as
1.1MW, considerably below the approximately 2MW threshold required in the
predecessor PBX device.
The confinement properties of these H-mode plasmas are similar to those
observed in PBX with an open divertor geometry. That is, the improvement in
the particle confinement is always evident while the improvement in the energy
confinement is often marginal. This is presumably due to both the facts that^to
date, these discharges have been formed with only the open divertor
configuration and that the working gas was H+ with DO beam species. For
several discharges with Pjn: ranging from 1.1 to 2.5 MW, the total energy
confinement time averages about 1.2 times the typical L-Mode values given by
Kaye-Goldston scaling. With further control over the separatrix location,
operation in the closed divertor geometry, and DO->D + injection, it is
reasonable to expect improvement in this enhancement factor to the 1.5-2.0
range. At these powers levels, there was no indication of impurity
accumulation in the plasma core during the H-phase, unlike the experience with
PBX, where H-mode plasmas typically exhibited strong accumulation of both
low-Z and high-Z impurities.
Changes in the MHD activity across the H-mode transition were observed
by the poloidal and toroidal Mirnov coil arrays. An increase in broad-band
activity (30-200kHz) is observed across the transition on the plasma midplane,
while this activity decreased in the region of the X-point, as seen in Fig. 8. Of
particular interest is the virtual absence of the edge relaxation perturbations
which give rise to the Ha spikes often observed in H-mode discharges.
Whether the absence of these edge modes reflects a stabilizing influence of the
nearby conducting wall or is a reflection of the moderate quality of these H-