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内容記述 |
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.<br /> 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.<br /> 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.<br /> 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. |
要旨・審査要旨 / Abstract, Screening Result (272.4 kB)
