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July 2008

Edge fluctuation measurement using LiBP

In order to investigate the density pedestal physics, the edge density and density fluctuation profile were measured using newly developed lithium beam probe in Ip scan (0.8-1.7 MA), Ip ramp (0.8-1.0 MA), rotation scan (co and counter) and gas puffing scan experiments. Dataset of density pedestal profiles and dynamics of type I and grassy ELMs was obtained for the first time. It was found that type I and grassy ELMs had similar density pedestal, however, density loss fraction by grassy ELMs was very small and localized near the separatrix. Co-rotating plasmas had larger ELM amplitude than counter-rotating plasmas; it was caused by the difference of the pedestal pressure and its gradient between two cases. In gas puff and gas-jet experiments, the pedestal density was increased and the pedestal width became narrow by 1 to 2 cm when the gas puffing rate were increased. In some experiments, edge fluctuation with the frequency around 10 to 20 kHz was observed. The pedestal width will be investigated in terms of the edge fluctuation, gas puffing and Ip dependence.

JT-60U/JET ITB similarity experiment

Objective of this experiment is to understand the differences and similarities between JT 60U and JET ITB characteristics. Dr. X. Litaudon of CEA Cadarache participated in the experiment. The experiment was performed by using the plasma shape similar to the JET shape. Similar normalized quantities were obtained in reversed shear plasmas with qmin~3, qmin~2 and weak shear plasmas. The comparative experiments performed in JET show that the Mach number profile was clearly different between two devices, while other characteristics were very similar.

Tungsten transport study:

Detailed plasma rotation velocity scan experiments were performed to further investigate the previously observed trend that the tungsten accumulation tends to be significant with increasing toroidal rotation velocity in the direction opposite to the plasma current. The tungsten accumulation, indicated by W44+ intensity, was low at a rotation velocity lower than 50 km/s and became significantly high at a velocity higher than 100 km/s in the direction opposite to the plasma current. These results suggest a threshold may exist. The accumulated tungsten was expelled by EC wave injection at r/a~ 0.1 and 0.4, but it was not for EC wave injection at r/a~0.7. The accumulation level of Ar, Kr and W at the negative rotation velocity became significant in the order of the atomic number, indicating Z-dependence of impurity accumulation in the plasma core.

QH-mode study

Objective of this experiment is to obtain QH-mode with toroidal rotation in the same direction to the plasma current ('co-rotation') at the edge. The experiment was performed in high and low triangularity (δ~0.4/0.28) with predominantly co-NB injection. While QH-mode was not obtained in the high triangularity case, QH-mode with co-rotation at the edge was obtained in the low triangularity case. Dr. P. Gohil of General Atomics participated in the experiment.

Development of weak magnetic-shear plasma and development of integrated real-time control system

Development of steady-state full CD plasma was performed without using LHCD. Almost full-CD (fCD=0.97) was obtained at high βN=2.4 and fBS=0.52 in Ip=0.8 MA, Bt=2.1 T (q95=4.3). However, the current profile gradually peaked due to the near on-axis NBCD and m/n=2/1 NTM appeared due to the shrink of the q=2 rational surface into the ITB region. This discharge clearly shows an importance of the achievement of steady state current profile in high β plasmas as realized in the experiment performed in January 2008.
  Using LHCD scenario (Ip=0.8 MA, Bt=2.2 T, q95=5.2) as the target plasma, the integrated real-time control system was developed. The system simultaneously controlled the minimum of safety factor (qmin) to 1.5 by adjusting LHCD power, and difference of Ti at two spatial positions (which is equivalent to ∇Ti) to 1-1.8 keV by adjusting NB heating power.

Measurement of electron temperature during current quench

A measurement of electron temperature during the current quench phase of disruption is important to estimate the plasma conductivity and to compare with a theoretical model on current decay time (so called the L/R model). The electron temperature in the current quench phase was estimated from He I intensity ratio measurement. In experiments, a disruption was induced by intense neon gas puffing. Even with electron temperature obtained by this method, current decay time estimated by the L/R model did not agree with experimental observations. Instead, these values agreed well by taking into account the temporal change in the plasma internal inductance. This work was done in collaboration with Nagoya University, National Institute for Fusion Science and Ishikawa National College of Technology.