The formation of the internal transport barrier (ITB) in reversed shear discharges was investigated in optimizing the reversed shear discharges. The role of tangential neutral beams is found to be very important for the formation of internal transport barrier. Co- and counter-tangential beams into core region and a counter-tangential beam into edge region were necessary for the ITB formation with good reproducibility. This suggests that the toroidal rotation and the heating power deposition are the key aspects of the ITB formation. Investigation of these effects is under way.
The deuterium pellet (4 mm diameter x 4 mm length, injection speed = 1.2~1.6 km/s) was injected into reversed shear plasmas to produce an H-mode edge by increasing edge beam deposition power. While the edge density was increased from 1x1019m-3 to 3x1019m-3 by the pellet., the electron density and temperature inside ITB was not changed even after the pellet injection. After all, the n=1 modes became unstable just after the pellet injection and no H-mode was produced.
High performance reversed shear experiments were conducted to push the equivalent QDT value during the three weeks just after the 16th IAEA meeting and accomplished the break-even condition of QDT>1 for the first time in JT-60U. In the first week, the plasma current was successfully increased up to 3 MA with a slightly enlarged plasma volume of 69 m3 and reached a high stored energy of 10.3 MJ. However, the neutron rate and QDT were not improved in spite of higher Ip probably because of off-axis beam deposition and increase in a ripple-induced beam ion loss. In the second week, it was difficult to maintain the reproducibly to form the internal transport barrier due to the lack of a co-directional beam unit tangentially looking at the center of the plasma. After the beam unit was fixed late in the week, the discharge optimization was made in progress, since the internal transport barrier became reproducible. In the third week, a stable reversed shear operation with a smaller volume of 58 m3 at the peak performance was found by optimizing the plasma configuration and beam injection scenarios during the current ramp-up phase. In addition, by increasing the main beam power, the dilution was reduced and the stability was improved so that the neutron emission rate was significantly increased. The plasma parameters for the best performance discharge are; Ip=2.8 MA, PNB=16 MW, Wdia=10.9 MJ, bN=1.9, q95=3.15, Sn=4.5x1016/s, nD(0)tETi (0)=7.8x1020m-3skeV. For this plasma, the equivalent QDT value was calculated by the TOPICS code to be 1.05 assuming deuterium beams into D and T plasmas with a mixture rate of 50% : 50%, in which the ratio of the thermal DT power to the total DT power reaches above 80%. The high performance reversed shear discharges were disruptively terminated by a fast beta collapse at which the qmin reached around 2.
During this campaign, remote analysis and participation in experiments were carried out using the data link system and TV conference system in collaboration with Princeton Plasma Physics Lab in U.S.A. The TRANSP code analysis was made in parallel showing a reasonable agreement with the above result. Detailed comparative analysis is ongoing.
BOUNDARY AND STEADY STATE OPERATION PHYSICS
Radiative divertor experiments by neon and deuterium gas puffing were carried out in the reversed shear plasmas at IP=1.2 MA, BT=3.0 T (qeff=5.6) and PNB=9 MW. Compatibility of the internal transport barrier (ITB) in reversed shear plasmas with the divertor MARFE was demonstrated. The reversed magnetic shear discharge with ITB was sustained for 1.8 s during the divertor MARFE. Full detachment in the divertor was obtained only in the reversed shear plasma with a high edge density. The total radiation loss from the divertor and the main plasma reached 8 MW, which was 80% of the total input power. The heat load to the divertor target was reduced by a factor of 10. The improved confinement inside the ITB terminated by a beta collapse.
In order to investigate the mechanism of confinement degradation in high density H-mode plasmas, profiles of ne, Te, Ti and VT (toroidal rotation velocity) in the edge plasma including SOL were measured in ELMy H-mode discharges with the line averaged electron density up to ~3.4x1019 m-3 at IP=1.2 MA, BT=2.1 T (qeff=4.3) and PNB=6 MW. The e-folding lengths of the density in the first and second SOL were not so changed though the electron density was increased in the main plasma. This result was different from one in L-mode discharges. The e-folding lengths were increased with increasing the electron density in L-mode discharges.