May 2008

Study of ITB formation and its structures
Objective of this experiment is to measure detailed ITB structures during ITB formation phase and steady-state phase in reversed shear plasmas using the newly installed modulation CXRS system that can measure ion temperature and toroidal rotation at 300 radial points every 50 ms. By changing the torque input, different evolution of ion temperature profile was obtained. The location of ITB shoulder moved inward with fixed ITB foot position in balanced injection case, while the location of ITB foot moved outward in counter injection case. Furthermore, it was found that the temperature at qmin increased when the qmin approached integer, and detailed plasma profiles at this transition were obtained.

Measurement of NBCD current profile
As the ITPA joint experiment SSO-6, NBCD current profile was measured in detail using motional Stark effect (MSE) diagnostics. Dr. J. Hobirk of Max-Planck-Institute für Plasmaphysik participated in the experiment on site and Drs. M. Murakami and J.M. Park of Oak Ridge National Laboratory participated remotely. Systematic scans of NBCD parameters, e.g. NBCD location (on-axis/off-axis), beam energy (85 keV, 330 keV), plasma triangularity (0.25, 0.4), and total heating power (6 MW, 13 MW), were performed. The database to compare the measurement with simulation was completed. Spatially localized change in magnetic pitch angle suggesting localized NBCD was observed in some discharges. Experimental validation of NBCD simulation code is ongoing using the database.

Multiple feedback control in reversed shear plasmas
The final goal of this experiment is to demonstrate simultaneous feedback control of pressure, current density and toroidal rotation in reversed shear plasmas using perp. NB, LHCD and tang. NB, respectively, with high bootstrap current fraction. Pressure gradient was evaluated by using ion temperature gradient and line averaged electron density in real time, which was close to actual pressure gradient evaluated from detailed measurements of temperature and density profiles. After the optimization of feedback gain for the pressure gradient control, time trace of the result value agreed well with that of the pre-programmed value. On the other hand, simultaneous control of current density and pressure profile was not successful due to arcing of LH launcher.

ITB control using real-time feedback control systems in positive shear plasmas
In a closed-loop feedback control system for ∇Ti, constant values of feedback gains over a wide range of Ti would not be suitable because non-linear dependence of ∇Ti on NB heating power becomes stronger as ITB grows up. To control ∇Ti in such a non-linear regime with ITB, the feedback control system was improved so that the proportional gain (GP : number of NB unit/(keV/m)) is a function of core Ti. Feedback control of ∇Ti in a plasma with strong ITB was demonstrated by using the upgraded system. The value of ∇Ti followed the reference value within the error of 1/GP. In the discharge, GP varied from ~1.8 to ~1.1 according to the change in Ti in the core region.

Supersonic molecular beam injection
Fuelling characteristics of supersonic molecular beam injection (SMBI), installed in collaboration with CEA-Cadarache, and effects of SMBI on confinement were investigated. The light from the SMBI measured using fast TV camera mainly emitted outside the separatrix even with background pressure (PBK) at 6 bar. However, the edge Ti quickly decreased at r/a~0.8, indicating that SMBI could directly affect the plasma parameters at r/a~0.8. The SMBI speed estimated from the fast TV camera was lower than expected. Ionization front could move slowly towards plasma boundary. Height of the density jump decreased with decreasing PBK. High confinement tended to be obtained with relatively smaller perturbation with lower frequency (~5 Hz) and lower PBK (~2-4 bar), although it also depended on target plasma confinement. H89PL~2 was successfully achieved at high density (/nGW~0.66) with SMBI.

Wall conditioning by ECH
It is important to establish a method for wall conditioning other than Taylor discharge cleaning and glow discharge cleaning toward operations in superconducting devices, where the toroidal magnetic field is always applied. The effectiveness of wall conditioning discharge using He plasmas generated by ECH was investigated. Uniform plasmas expanding into the whole vacuum vessel was obtained by applying horizontal magnetic field as well as toroidal magnetic field, and by applying fundamental ECH (both X- and O-mode). On the other hand, it was difficult to obtain such a good He plasma for second harmonic ECH. The effectiveness of the cleaning was investigated by comparing discharges with and without the wall conditioning after disruption. It was confirmed that only the plasmas with ECH wall conditioning could build up normally.