The plasma current position is measured based on the current multipole moment method in the Korea Superconducting Tokamak Advanced Research (KSTAR) device [G. S. Lee et al., Nucl. Fusion 41, 1515 (2001)]. Since a limited number of magnetic probes and flux loops were installed for the first campaign, an iteration method is used to interpolate the magnetic field in the area absent of probes. A numerical technique is developed which automatically compensates for the linear drift of the integrator. The measured plasma position is cross checked with the charge coupled device images. In KSTAR, the plasma current is displaced toward the lower outboard direction during the current initiation phase. This displacement is expected due to eddy currents but the exact amount and direction needs more study.
The development of an integrator for magnetic diagnostics becomes more important as the pulse length of fusion devices gets longer and longer, especially for present-day superconducting fusion devices. A small offset in the signal can cause a significant drift in the integrator output for long pulse experiments. A lock-in amplifying digital integrator has been developed for Wendelstein 7-X (W7-X). It succeeds in suppressing the drift to a low value but requires about 100 ms for data processing. To shorten the data processing time, a Field Programmable Gate Array (FPGA) built in the digitizer is utilized. Since there is no need to transfer the data to an external computer, the integration can be done in real time. The microprocessor built in the digitizer directly transfers the data integrated in the internal FPGA into the reflective memory installed in the same compact Peripheral Component Interconnect chassis. These features result in a very compact system design. The design and the preliminary results of the digital integrator will be presented.
Frequency modulation reflectometer has been developed to measure the plasma density profile of the Korea Superconducting Tokamak Advanced Research tokamak. Three reflectometers are operating in extraordinary polarization mode in the frequency range of Q band (33.6-54 GHz), V band (48-72 GHz), and W band (72-108 GHz) to measure the density up to 7 × 10(19) m(-3) when the toroidal magnetic field is 2 T on axis. The antenna is installed inside of the vacuum vessel. A new vacuum window is developed by using 50 μm thick mica film and 0.1 mm thick gold gasket. The filter bank of low pass filter, notch filter, and Faraday isolator is used to reject the electron cyclotron heating high power at attenuation of 60 dB. The full frequency band is swept in 20 μs. The mixer output is directly digitized with sampling rate of 100 MSamples/s. The phase is obtained by using wavelet transform. The whole hardware and software system is described in detail and the measured density profile is presented as a result.
Mirnov coils (MCs) are used to identify the toroidal and poloidal mode number of tearing mode instabilities in the KSTAR tokamak. The mode number is analyzed by accumulating the phase of MC signals from a Morlet wavelet transform. There is ambiguity in identification of the poloidal mode number due to the phase distortion in the MC measurements. The extreme case is so-called phase folding. Although the phase distortion caused by the eddy currents in the in-vessel components and by the coupling of multimodes has been studied intensively, little attention is given to the effect of the orientation of MCs. The poloidal MCs are not aligned to the plasma shape but to the vacuum vessel. The effect of this misalignment is quantitatively analyzed by simulating the MC measurements of magnetic field fluctuations in a tokamak with mathematical modeling of perturbed current filaments on rational surfaces. The maximum MC response to a perturbed current filament occurs when the current filament is located at an angle different from the geometric installation angle of the MC. This angle is defined as the effective angle. The characteristics of the effective angle are investigated depending on the plasma shape, mode number, and distribution of the perturbed currents. The ambiguity in the identification of the poloidal mode number is reduced when the MC measurements are analyzed in the effective angular coordinate instead of the geometric coordinate. The only requirement for the calculation of the effective angles is an equilibrium reconstruction to identify the rational q surfaces.
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