Impact of rotation and ion diamagnetic drift on MHD stability in QH-mode plasmas

We report the results of MHD stability analysis with plasma rotation and ion diamagnetic drift effects in QH-mode plasmas in JT-60U. It was found the plasma rotation evaluated by averaging both bulk ion rotation and impurity ion one is effectively stabilize the MHD modes. This trend can be reproduced by predicting plasma poloidal rotation based on the neoclassical theory.15th Japan-Korea Workshop on "Modeling and Simulation of Magnetic Fusion Plasmas

The multichannel motional Stark effect diagnostic in the JFT-2M tokamak

　　　The q-profile plays a key role in determining the equilibrium, stability, and transport of plasma in toroidal magnetic confinement configurations, where q denotes the inverse of the rotational transform of the magnetic field line. Accurate determination of the q-profile is thus essential for a quantitative analysis. Experimental techniques to improve the confinement and sustain it for a long pulse by active control of the q-profile, using radiofrequency (RF) wave, neutral beams, or bootstrap current, have been tested in tokamaks. Progress in these studies has largely been due to development of diagnostic techniques such as motional Stark effect (MSE) to measure the q-profile reliably.　　　A multichannel motional Stark effect polarimeter system, which is capable of simultaneous measurement of a radial electric field, has recently been installed on JFT- 2M. The diagnostic can measure the polarization angle at 18 radial location, which cover a region between just inside from the magnetic axis and the outboard edge of the plasma. After careful calibration of the instrument, the magnetic field pitch angle [γp=tan-1(BP/BT)] is obtained with a statistical uncertainty of about 0.1° with a time resolution of 10 ms. In order to determine the equilibrium selfconsistently, the free boundary equilibrium code (CUPID; CUrrent Profile IDentification code) has also been introduced on JFT-2M, which solves the Grad-Shafranov equation using internal magnetic field data measured by MSE in conjunction with external magnetic data measured by magnetic probes, flux loops, and Rogowski coils at several locations outside the plasma.　　　In L-mode discharges with different plasma current IP and its ramp up rates dIp/dt, it is found that sawtooth crashes emerges reproductively just after the q(0) pass through ~ unity, and it occurs at earlier time-slice as the discharges of larger IP (and smaller surface q values) at the flat top, irrespective of the dIp/dt. After sawtooth crash, the q(0) keeps constant value of below ~ unity throughout the sawtooth cycle as observed in many tokamaks. As the result of the equilibrium reconstruction by CUPID code, it is confirmed that plasma currents are ramped up with a broad profile (with a monotonic q-profile), and it is also observed a significant peaking of the current profile with the peaking of the soft X-ray emission in the plasma core, but sawtooth crash appears nothing yet showing q(0) > 1.　　　Outstanding feature of the MSE system on JFT-2M is that it makes possible to separate the effect of the radial electric field (Er) from the polarization angle measurements. By viewing two neural beam lines (one is co-parallel to the plasma current and the other is counter-parallel) simultaneously and near tangentially to the toroidal magnetic field from only one spectroscopic instruments, it provides the best sensitivity in Er measurements with good spatial resolution. The magnetic field pitch angle is also measured with a smallest uncertainty. Such a method of Er measurement with MSE using two beam lines, which is the first attempt in the world, can provide the best accuracy among other techniques. Preliminary data of δEr (the change in the radial electric field) profile in L-mode plasma has been obtained. Comparison with calculated δEr profile using CXRS measurements of the toroidal rotation velocity will also discussed

Structure generation of the edge radial current during the L-H transition on JT-60U

In this study, we analyzed the structure generation of the edge radial current (jr) by means of Poisson’s equation with a measured Er data (CXRS diagnostic) in JT-60U NBI heating plasmas. About 200 ms after the start of NBI heating, a slow L-H transition takes place, which evolves into a fully-developed H-mode spending a few 100ms. During this slow transition process, a smooth decrease in Da emission, increase in the edge line-averaged electron density and steepening of ion temperature take place. The Er-well bottom value at ~3 cm inside the LCFS becomes large up to -40 kV/m as a similar time-scale of the change in the density, while the jr shows a local Max. value of 0.01-0.02 A/m2 just after a slow L-H transition and its broader radial structure propagates toward plasma core region in the time-scale of ~100ms as seen in the pedestal development. On the other hand, we found that a localized jr structure with positive or negative polarities of its absolute peak value of 0.4-0.5 A/m2 occurred spontaneously during the later ELM-free H-phase at which a complex multi-stage Er-transition was seen with a fast time-scale. This observation suggests a co-existence of the non-linear physical mechanism for the jr generation at the plasma edge region in terms of its variation in the time-scale and radial structure. Comparison with a theoretical model, including fast-ion loss current due to the ripple loss effect, is also discussed

Structure generation of the edge radial current during the L-H transition on JT-60U

In this study, we analyzed the structure generation of the edge radial current (jr) by means of Poisson’s equation with a measured Er data (CXRS diagnostic) in JT-60U NBI heating plasmas [1-4]. About 200 ms after the start of NBI heating, a slow L-H transition takes place, which evolves into a fully-developed H-mode spending a few 100ms. During this slow transition process, a smooth decrease in Dα emission, increase in the edge line-averaged electron density and steepening of ion temperature take place. The Er-well bottom value at ~3 cm inside the LCFS becomes large up to -40 kV/m as a similar time-scale of the change in the density [5], while the jr shows a local Max. value of 0.01-0.02 A/m2 just after a slow L-H transition and its broader radial structure propagates toward plasma core region in the time-scale of ~100ms as seen in the pedestal development. On the other hand, we found that a localized jr structure with positive or negative polarities of its absolute peak value of 0.4-0.5 A/m2 occurred spontaneously during the later ELM-free H-phase at which a complex multi-stage Er-transition was seen with a fast time-scale. This observation suggests a co-existence of the non-linear physical mechanism for the jr generation at the plasma edge region in terms of its variation in the time-scale and radial structure. Comparison with a theoretical model, including fast-ion loss current due to the ripple loss effect, is also discussed.46th EPS Conference on Plasma Physic

On the possibility of limit-cycle-state of peeling mode near stability boundary in the quiescent H-mode

A model is proposed for the edge harmonic oscillation, in which the stationary coherent mode is sustained in the almost linear phase as has been observed in JT-60U. We study the coupled dynamics of the peeling mode amplitude and edge pressure gradient. The limit cycle oscillation is predicted. The peeling mode (which is almost in the linear phase) is in a dynamical stationary state with amplitude modulation. In this model, the time scales for the change of parameters that specify magnetic structures (such as magnetic shear and edge plasma current) are assumed to be much slower, so that are decoupled from the limit cycle dynamics. The condition that the limit cycle state appears is shown. The oscillation frequency of the modulation is given by the hybrid mean of the typical growth rate of the peeling mode and the additional loss rate of pressure gradient by the peeling mode