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from April to June, 1998


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.


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.


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.