JT-60U MONTHLY SUMMARY
from April to June, 1998
OPERATION AND CONFINEMENT PHYSICS
Throughout the 6 experimental weeks, we concentrated on the high performance
reversed magnetic shear experiments under the W-shaped pumped divertor and
successfully obtained the record value of the DT equivalent fusion gain
QDTeq=1.25 at the plasma current Ip=2.6MA, which exceeded the previous world
record of 1.14 obtained in JET in 1992 at Ip=3.1MA. This value also exceeded
the JT-60U's record value of 1.05 obtained by the reversed shear operation
at Ip=2.8MA in 1996 under the open divertor. The improvement from the 1996's
result was due to both ~10 % improvement in plasma purity and ~10% enhancement
of energy confinement time. In addition, a careful feedback control of neutron
production rate during the build-up phase of current and pressure profiles
with the internal transport barrier enabled a reliable control of the plasma
stability at a high pressure. The reproducibility of the discharges was
quite good; 15 discharges satisfying the break even (QDTeq>1) was obtained.
The main parameters of the best discharge are as follows
plasma current = 2.6MA, toroidal field=4.4T, plasma volume= 54 m3,
central deuteron density =4.8 x1019/m3, core ion temperature = 16.8keV,
energy confinement time = 1.07 sec, confinement enhancement factor (H-factor)=3.2,
normalized beta value = 1.53, QDTeq=1.25.
These results were presented in the 25th European Conference on Controlled
Fusion and Plasma Physics Prague, Czech, June 29 - July 2. To the high performance
reversed shear experiments, the international collaboration with the Princeton
Plasma Physics Laboratory contributed successfully including three participants
in the experiment and a TV conference treating the key issues of operation,
transport and stability.
In the experimental weeks, effects of externally applied helical perturbation
of magnetic field on halo current characteristics was also investigated.
In the second Joint Meeting of Fusion Energy in Tokyo, the detailed studies
on the neoclassical transport coefficients, formation physics of the internal
transport barriers and driving forces of the toroidal plasma rotation were
reported. In particular, for the latter two studies, contributions by Chinese
collaborators were significant.
CURRENT DRIVE AND HIGH ENERGY PARTICLE PHYSICS
After adjusting the phase controller of the lower hybrid (LH) multi-junction
launcher, the influence of phase velocities on a driven current profile
was examined by injecting LH waves with different phases. The internal plasma
inductance increases for larger phase velocities and saturates or decreases
for smaller phase velocities as expected. The emission profile of hard X-ray
was also consistent with previous results, showing peaked profile for larger
phase velocities and broad one for smaller phase velocities. The LHCD of
revered-shear plasma aiming to sustain a reversed-shear configuration with
an internal transport barrier is planned in a next campaign.
In the second Joint Meeting of Fusion Energy held in Tokyo on June 1 - 2,
recent results on current drive using NNB, NNB-driven TAE modes, evaluation
of multi-step ionization cross-section of NNB and ICRF heating of reversed
shear plasmas were presented. These new results of NNB experiments and the
collective Thomson scattering and gamma-ray diagnostics developed newly
for fast ion studies were presented in the International Congress of Plasma
Physics combined with 25th EPS Conference on Controlled Fusion and Plasma
Physics, Prague, Czech, June 29 - July 2.
DIVERTOR AND BOUNDARY PHYSICS
During the high QDT campaign by the reversed shear plasmas, several efforts was devoted to reduce impurity level and to improve neutron production.
It was found from the CXRS measurement that the oxygen level increased in the edge plasma as the main plasma approached to the inner wall as close as ~ 9 cm. Since flux expansion of the inboard scrape-off layer was larger than the outboard scrape-off layer by a factor of three, it was expected that sputtering of the oxygen in the SOL was responsible to the increase. In order to reduce the oxygen source in the wall, limiter OH discharges were fired during the morning session and frequently between the high QDT discharges. These clearing discharges was useful to reduce the oxygen level in the main plasma by 60 - 80 % and the increase in neutron production was obvious.
Seven papers was presented in the 13th PSI meeting in San Diego, May 14-17. The paper from JT-60U team included topics from the experiment with the W-shaped pumped divertor, such as helium exhaust, neutral pressure measurement, neutral transport, divertor X-point probes, chemical sputtering, pumping characteristics and radiative divertor.