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Sep - Nov 2001

High-Fusion-Performance Reversed Shear

A campaign for pushing up the fusion performance, the equivalent fusion gain (QDTeq), on JT-60U was performed during four weeks in October and November. We employed reversed shear plasmas with a high-elongation shape and the maximum toroidal field (~4.3 T at the plasma center). In 1998, we had obtained the record value of QDTeq (1.25) at 2.6 MA in these plasmas with large confinement improvement (H89 ~ 3) due to an internal transport barrier (ITB) and an L-mode edge. In order to improve the fusion performance, we tried (i) to reduce the impurity content by decreasing the wall temperature from 300°C to 150°C together with frequent boronization, (ii) to employ pellet injection for confinement improvement and/or impurity reduction, (iii) to combine an H-mode edge with the ITB, and (iv) to enlarge the radius of the qmin or ITB foot.

The neutron emission rate did not increase with the lower wall temperature, though impurity reduction was suggested by spectroscopic measurement. However, the highest attainable performance was not improved. Although H-mode transition was obtained by gas puffing, the pedestal pressure was not so high and significant improvement was not obtained. To enlarge the radius of the qmin or ITB foot, EC injection into plasmas with a limiter configuration at Ip < ~ 1 MA was employed instead of NB heating with a divertor configuration. Higher beta and larger radius of the ITB foot were obtained in the scenario. However, a collapse was encountered around Ip ~ 2.2 MA (qmin ~ 2.3).

The highest performance achieved in this campaign was QDTeq ~ 1.24 (preliminary) with the DD neutron emission rate of 4.57x1016 s-1, and it was almost similar to the previous record. In this discharge, compared with the previous discharge, the value of qmin at the collapse was smaller than the previous values and the time of the collapse was delayed by ~0.3 s. This resulted in a longer duration of high fusion performance; the duration with QDTeq > 0.8 reached 0.55 s. Another improved point was smaller dW/dt (time derivative of stored energy), which resulted in a higher value of the ratio of equivalent fusion power to absorbed NB power (~0.8). N-NB injection into high-current reversed shear plasmas was also attempted, and the highest value of QDD (6.0x10-3) was achieved due to significant beam-thermal reactions.

Data on ITB formation/degradation, MHD instabilities, H-mode transition, disruption characteristics, impurity generation/transport and so on were obtained in the high-fusion-performance plasmas.