Fusion performance of reversed shear discharges with an L-mode edge has been significantly improved in a thermonuclear dominant regime with up to 2.8 MA of plasma current in the JT-60U tokamak. The core plasma energy is efficiently confined due to the existence of persistent internal transport barriers formed for both ions and electrons at a large minor radius of r͞a ϳ 0.7 near the boundary of the reversed shear region. In an assumed deuterium-tritium fuel, the peak fusion amplification factor defined for transient conditions involving the dW ͞dt term would be in excess of unity. [S0031-9007(97)04592-4] PACS numbers: 52.55.Fa, 52.55.PiThe reversed shear discharges are considered attractive for a steady state operation with a large bootstrap current fraction in tokamak reactors as proposed for SSTR [1] and ITER [2], since it would be possible to match the hollow current profile to a bootstrap current profile in a steady state. While the central magnetic shear in tokamak plasmas is naturally reversed during a sufficiently long discharge duration with a large bootstrap current fraction [3], the forced shear reversal operation by enhancing a skin current effect has become important for establishing a controlled approach to the steady state [4].In nuclear fusion research, critical conditions in which fusion power produced in plasmas is equal to loss power from the plasmas have been pursued as a crucial milestone ultimately towards the commercial use of thermonuclear fusion energy. In order to determine whether the reversed shear scenario is workable, it is crucially important to demonstrate the fusion-relevant performance, particularly in the thermonuclear fusion regime with the shear reversal operation. So far, however, most of the previous experiments addressing high fusion reactivity in tokamaks have been limited to a hot-ion regime with substantial beam-thermal reactions for deuterium plasmas in TFTR supershot [5], JET hot-ion H mode [6] and JT-60U high-b p H mode [7], and deuterium-tritium (D-T) plasmas in TFTR supershot [8]. Although fusion performance has been recently enhanced with strong profile and shaping control in deuterium reversed shear plasmas with an H-mode edge in DIII-D [9], the projected D-T fusion power is substantially below the loss power from the plasma. In the present paper, it is shown that fusion performance has been significantly improved in JT-60U for reversed shear discharges with an L-mode edge in a thermonuclear fusion regime, so that the transient fusion amplification factor defined as below would be in excess of unity.In JT-60U, the experimental campaign of the reversed shear discharges aiming at high fusion amplification factor ͑Q͒ has been intensively performed with D beams into D plasmas. The confinement properties for the reversed shear discharges created in JT-60U are characterized by (i) the significant reduction of heat and particle transport for electrons as well as ions around the internal transport barrier (ITB), (ii) a large extension of the enhanced confinement region up...
Emphases in recent JT-60 experiments are placed on 1)lower-hybrid (LH) current drive characteristics with a multi-junction type launcher and 2 ) the confinement study with combination of neutral beam injection, LH current drive and pellet injection. The new multi-junction LH launcher provides a 2 . 7 x 1 0 1 9 m -3 and Ip=1-1.7 MA.Volt-sec saving of -2volt-sec was demonstrated by 2 sec long, 1.6MW LHCD during the plasma current ramp of 0.4MA/s.A broad radial distribution of high energy electron current and -30% reduction in sawtooth inversion radius were obtained by high N I I (-2.5) LH injection. In order to fully suppress the sawtooth activity, low NII (-1.3) injection was found to be more effective, in which up to 1.8 sec sawtooth-free phase was obtained by 2MW LHCD for lOMW NB heating of Ip=I.SMA discharge, Improved energy confinement has been obtained with hydrogen pellet injection. Energy confinement time was enhanced up to 40% relative to usual gas fuelled discharges. The discharge has a strongly peaked electron density profile with ne(O)/
From June to October 1987, JT-60 achieved fusion product(ne(0) .r~*.Ti(Oj ) of 6 ~1 0 ' ~ m 3 . k e V . s with hydrogen plasma at plasma current of 2.8 to 3.1 MA with neutral beam power of -20 MW. The central electron density of 1 . 3 ~1 0 2 0 m-3 was obtained at plasma current of 3 MA with 13-20 MW neutral beam power and the confinement time reached 0.14-0.18 s. It is found that an offset linear scaling law like the Shimomura-Odajima scaling on confinement time will be able to reproduce experimental data better than that of the Goldston type scaling. With low beam energy injection, -40 keV, confinement degradation was found. Many short periods (0.05-0.1 s) of H-mode phase were found in outside X-point divertor discharges with NB or NB+RF(LH or IC) heating power above 16 MW. However, improvement in energy confinement time was limited to 10 %. The ballooning/interchange stability analysis were JT-60 TEAM also made for the outside X-point divertor equilibrium in connection with H-phase capability. Heating power of 9.5 MW and 1.9 MW was obtained by LHRF, ICRF injection, respectively. In combined LHRF and NB heating, the incremental energy confinement time of 0.064 s was obtained, which is the same level of that of NB heating only. In combined NB and on-axis ICRF heating of low ne discharge, an incremental energy confinement time of 0.21 s was obtained, which is three times as long as those of NB or ICRF heating only. It was also observed that high energy beam ions were accelerated by ICRF in the central region of the plasma.
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