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.

The ion heat pulse after a sawtooth crash is studied by a time-of-flight ͑TOF͒ neutral measurement. A rapid change of the bulk ion energy distribution near the edge is observed after a sawtooth crash. The delay time is measured, and the effective measuring position is estimated by a neutral transport code. Then a transient ion thermal conductivity, i HP , of about ͑7-18͒ m 2 /s is evaluated for the low confinement mode ͑L-mode͒ plasma. The simple diffusive model with constant i HP , however, does not explain the amplitude of the pulse in the ion energy distribution.

A timeofflight spectrometer for lowenergy neutral and ionized alkalis Rev. Sci. Instrum. 64, 672 (1993); 10.1063/1.1144195 Lowenergy neutral particle analysis by a timeofflight method on JFT2M (abstract) Rev. Sci. Instrum. 61, 3146 (1990); 10.1063/1.1141662 Timeofflight measurements of lowenergy electron energy distributions from ion-atom collisions Rev. Sci. Instrum. 46, 851 (1975); A disk chopper for timeofflight analysis of neutral particles Rev. Sci. Instrum. 45, 959 (1974);A time-or-flight method is used to measure the energy spectrum of charge exchange atoms emitted from the JFf-2M plasma in the energy range below 1000 eV /amu. The neutral flux is chopped in bunches of about 0.8 Jis by a slotted cylinder. The rotating cylinder is driven by a turbomolecular pump. After the flight of 1.5 m the particles are detected by a secondary particle detector. This is an electron multiplier modified from a photomultiplier which can be gated for an intense photon. The signal from the electron multiplier is digitized at a sampling time of 0.2 fis. To get good statistics of the arrival time distribution (ATD), eight or nine ATDs are averaged. Then the energy spectrum of charge exchange neutrals is measured by a 2 ms interval. The fast change «2-6 ms) of energy spectra at the L/ II and H / L transition is also reported. The average energy defined by JE(dr/dE)dE/S(dr/dE)dE increases by about 100 eV at the H transition Hke the increase of the ECE electron temperature at the edge. The neutral fiux defined by Hdr IdE) dE decreases iike the diverior Ha/Du signal. In the energy distribution of the flux, the number of particles whose energy less than about 400 eV decrease by the H transition but those greater than 500 eV do not change or increase. The flux at 200 eV in the H phase is two times lower than that in the L phase.

Rapid changes of the main ion energy distribution at transitions from L to //, H to L, and during edge localized modes (ELMs) are studied with the time-of-flight neutral measurement on the JFT-2M tokamak. At the L to H transition, 200-400 /us prior to the start of the H a drop, an increase of the high-energy outflux above an energy of 200 eV is observed. An energy of more than 200 eV for hydrogen corresponds to the collisionless condition v*, < 1 just inside the separatrix. A change of the energy distribution preceding that of the H c signal is also found for ELMs.

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