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Fusion Plasma Research


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June 1996


Vertical position (Zj) control has been demonstrated to avoid VDE during Ip-quench phase. By this time, the coefficients of the statistical method using real-time calculation of plasma position were determined by the least square method only for steady state database. The Zj for real-time control was largely different from plasma position during Ip-quench, mainly due to eddy current effect. The coefficients of the statistical method were modified based on both steady state and Ip-quench databases to improve the accuracy of Zj of real-time calculation during Ip-quench. After the modification, the Zj was successfully controlled and VDE was avoided during Ip-quench at density limit disruptions. No halo current was observed as a result of VDE avoidance.


High performance experiment with a reversed shear configuration has been conducted by increasing the plasma current from 2.1 MA up to 2.5 MA at 4 T. The fusion performance was found to be significantly improved with the plasma current in which the transport barriers placed near the qmin were sustained during the current ramp-up phase. The nD(0)tETi(0) and equivalent QDT values reached 6x1020 m-3skeV and 0.6, respectively, at 2.4 MA with 22 MW beam injection where the stored energy and neutron emission rate were 9.6 MJ and 4x1016/s, respectively. The enhanced core confinement achieved with high ne(0) of 7.8x1019 m-3 and relatively low Ti(0) of 21 keV lead to a reactor relevant plasma dominated by thermal fusion reactivities. The longest energy confinement time of 0.87 s was attained at 2.5 MA with less power of 15 MW injection. While the reversed shear configuration demonstrated excellent and attractive confinement properties, the recent triton burn-up experiment exhibited 14 MeV neutron production a half of the normal shear configuration. The observed confinement degradation for tritons is probably attributed to weak poloidal magnetic field in the core region. In contrast to reversed shear discharges, most of tritons are well confined for normal shear discharges.


In the June operation period, the emphasis was placed on Core Physics research. During this period, line emission profile from helium atoms was measured to study helium behavior. He I(667.8 nm) array, using optical fibers and optical interference filters, was newly installed, which cover the main plasma. He I profile in the divertor zone was measured by a 60-channel optical fiber array. It was found that He I intensity profiles in the vicinity of the strike points during the MARFE became broader than that before starting MARFE in NB heated discharges. The observed helium behavior in main and divertor plasma will be analyzed by using DEGAS code.