1 - Nuclear Sciences and Applications - IAEA

266 TSUJ1 et a).
1. INTRODUCTION
JT-6O is a large tokamak with powerful auxiliary heating systems. It was
operated over a wide range of plasma parameters in 1987: 1 < Ip < 3.2 MA,
0.65 < ap < 0.92 m, and 5 X 10' 8 :£ n,. < 1.2 x 10 20 nr 3 . A combined heating
power of up to 30 MW was injected and the plasma stored energy was increased to
3 MJ [1, 2].
The improvements in confinement have been explored with beam heated hydrogen
plasmas by several methods; by current profile modification with LHCD [3-5],
by beam acceleration with ICRF [6, 7] and by H-mode transition with the outer
X-point [1, 2, 8], A new divertor coil was installed to produce a lower X-point configuration
at the beginning of 1988. Hence, JT-60 has become a unique device where
the effect of configurations, limiter and outer or lower X-point on the plasma confinement
can be examined with the same heating and diagnostic systems.
NB heating experiments in the lower X-point configuration (ap « 0.65 m,
K « 1.3) have been carried out with PMB up to 25 MW. A new regime of improved
divertor confinement (IDC) appears in the lower X-point configuration when the
heating power exceeds about 15 MW. The IDC regime is characterized by high particle
recycling and remote radiative cooling in the divertor region, which is required
to alleviate the heat load on the divertor plates of the fusion reactors.
2. IMPROVED CONFINEMENT REGIME WITH LOWER X-POINT
Discharge waveforms with and without improved divertor confinement are
compared in Fig. 1. The thick lines in the figure are traces of an improved
shot (E6950, Bt = 4.0 T) and the thin lines stand for an unimproved shot (E6956,
Bt = 2.7 T). The stored plasma energy of the former shot begins to rise around
6.1 s whereas that of the latter shots saturates early. Lowering the effective safety
factor, qeff, leads to an excitation of edge localized modes (ELMs), which prevent
the discharges from reaching the IDC regime. Frequent ELMs can be clearly seen
in the H° w trace.
The improvement is associated with an increase in the electron density and a
decrease in the radiation power from the core plasma. The central electron temperature
measured by ECE keeps rising with increasing density. Note that the Ha emission
from the divertor region is significantly enhanced during the improved phase.
Bolometric measurements indicate that radiation power both in the divertor and near
the X-point regions increases at these moments. These observations suggest that high
particle recycling takes place in the divertor region. Although the carbon impurity
lines tend to rise, the oxygen impurity lines emitted from the main plasma drop
during the improved phase. Visible bremsstrahlung measurement indicates that Zeff
is kept nearly constant throughout the improved phase. The IDC regime is different
from the H-mode since particle recycling never drops and there is no sharp transition.