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January 1998


In spite of the extensive in-vessel work in November and December last year, the mass pattern of the residual gas and the wall conditions were remarkably favorable. Accordingly, after the Taylor type discharge of 30 minutes and one overnight glow discharge cleaning enabled the stable 15 second discharge in the 2nd day of the campaign. As to the equilibrium control capabilities, it was found that the plasma center was satisfactorily controlled with the resolution of 2 to 3 cm. However, the X-point height detection error was larger than expected. To improve the controllability, additional number of numerical equilibrium data were produced to revise the coefficients toward experiments planned in February.
For high values of the DT equivalent fusion gain QDT in the next campaign from middle March to June, an working group meeting has been started for careful analyses of impurity influx, the lower limit of safety factors (qmin and q95), location of the internal transport barrier, H-mode transition etc.
As one of the international collaborations for MHD stability analyses, a joint analysis started between Princeton Plasma Physics Laboratory and JAERI treating the characteristics of the instabilities locally destabilized around the internal and the edge transport barriers.


Together with the restart of JT-60U operation in January 1998, the conditioning of NNB was started.
Recent results of RF experiments on JT-60U were presented by S. Ide in the 2nd European Topical Conference on Heating and Current Drive in Fusion Devices held in Brussels. Topics presented are; RF coupling to new divertor configuration, LHCD experiments on R/S plasmas, interaction of LHW and energetic ions and ICRF heating in R/S plasmas.


Tests, adjustments and calibrations of divertor diagnostics were carried out in the first experimental week after the vacuum vent for maintenance. Effective surface areas and positions of divertor target Langmuir probes with eroded tips were estimated by using separatrix sweep operation and comparing the measured result with calculated separatrix positions by the FBI code. Inserted positions of sensor heads of a reciprocating X-point Langmuir probe and a reciprocating mid-plane Langmuir probe were also checked. ASDEX gauges were calibrated in the range of 0.025-0.25 Pa in the toroidal magnetic field of 3 T. All channels of a repaired divertor bolometer viewing the X-point vicinity and bolometers viewing from the top were confirmed to work well.
Wall conditioning proceeded well. The vacuum pressure decreased to order of 10-5 Pa in a week after the restart of experiment. Although NBI port aging was conducted in this week, effective charge of OH plasma decreased to 2.5, and carbon and oxygen concentrations were 2.5% and 1%, respectively.