April 1996


Beam conditioning of the negative-ion-based neutral beam (N-NB) has been continued to increase the beam power and acceleration voltage. No injection into the plasma was attempted during April. Using Cs in the two ion sources, the total neutral beam power was increased up to 2.1 MW at a beam acceleration voltage of 300 keV; the beam pulse duration in the extraction test was up to 0.5 s due to the maximum heat load limit for the beam dump.


The April run in JT-60U was devoted to high performance campaign. Experiment started from shear reversal experiments in which the plasma currents were increased up to 2.1 MA from the last year level of 1.2 MA for a high elongation configuration (k~1.8). High performance plasmas were successfully achieved with clear internal transport barriers as observed in the last year. A typical discharge was produced to have 8.2 MJ at 2.1 MA for only 15 MW injection. This discharge indicated a long energy confinement time around 0.7 sec with a dW/dt fraction less than 20% to the heating power, and a peaked electron density profile with ~ 8x1019 m-3 at center during the main beam injection. The fusion reactivity was dominated by thermal reactions with the central Ti value around 20 keV, where the equivalent QDT would be ~0.4. Further reduction of light impurities such as carbon and oxygen would improve the fusion performance in the shear reversal mode discharges. High-bp H-mode experiments have been conducted to commission high current target discharges and produce high performance plasmas. Utilizing a fast current ramp-up operation newly developed in April, the high current target discharges without sawteeth before main beam injection were successfully created up to 2.7 MA. The best performance was attained at 2.2 MA with the stored energy of 9 MJ and the fusion triple product of nD(0)tETi(0)~1.3x1021 m-3skeV, both of which were the new record values in JT-60U. Suppression of m=3 mode activities associated with the low qedge below 4 appeared to be an issue to optimize the high current high-bp H-mode discharges above 2.5 MA for further improvement in performance. Fusion-produced tritons were directly measured after the turn-off of the high power beam injection using the tangentialy viewing charge-exchange neutral particle analyzer introduced from the Ioffe Institute, Russia. The analysis of this result is underway.


Analyses of SOL plasma profiles and particle confinement for L-mode discharges are ongoing for presentation at the 12th PSI conference to be held in St.Raphael, France on May 20-25. The profile measurements of Te and ne with a fast reciprocating probe and Ti with a poloidal charge exchange spectroscopy system in the SOL plasmas for PNB=4 MW show that the ion pressure was significant larger than electron pressure due to Ti~3 x Te in the midplane SOL, and the e-folding decay length of Ti,mid was 2.5-3 times larger than Te,mid. The total pressure in the midplane SOL was found to be balanced with the divertor total pressure ( Ti~Te ). The effect of the particle source distribution on the global particle confinement is analyzed. The results show that the particle confinement time tends to be increased with the ratio of NB fueling rate to wall recycling and gas-puffing fueling rate. Based on the recent L-mode database, the confinement time of central fueled particle was found to be three times longer than that of edge fueled particle.