The 500-keV negative-ion based neutral beam injector for JT-60U started operation in 1996. The availability of the N-NBI system has been improved gradually through modifying ion source and optimizing its operation parameters. Recently, the extension of the pulse duration up to 30 sec has been intended to study quasi-steady state plasma on JT-60U. The most serious issue is to reduce the heat load on the grids for long pulse operation. Two modifications have been proposed to reduce the heat load. One is to suppress the beam spread which may caused by beamlet-beamlet interaction in the multi-aperture grid due to the space charge force. Indeed, the investigation of the beam deflection, which was measured by the infrared camera on the target plate set 3.5 m away from the grid, indicates the beam spread is in proportion to the current density. Thin plates were attached on the extraction grid to modify the local electric field. The plate thickness was optimized to steer the beamlet deflection. The other is to reduce the stripping loss, where the electron of the negative ion beam is stripped and accelerated in the ion source and then collides with the grids. The ion source was modified to reduce the pressure in the accelerator column to suppress the beam-ion stripping loss. Up to now, long pulse injection of 17 sec for 1.6 MW and 25 sec for ~1 MW has been obtained by one ion source with these modifications.

No abstract

No abstract

No abstract

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...