ABSTRACT
A tokamak relies on the axisymmetric magnetic fields to confine fusion plasmas and aims to deliver sustainable and clean energy. However, misalignments arise inevitably in the tokamak construction, leading to small asymmetries in the magnetic field known as error fields (EFs). The EFs have been a major concern in the tokamak approaches because small EFs, even less than 0.1%, can drive a plasma disruption. Meanwhile, the EFs in the tokamak can be favorably used for controlling plasma instabilities, such as edge-localized modes (ELMs). Here we show an optimization that tailors the EFs to maintain an edge 3D response for ELM control with a minimized core 3D response to avoid plasma disruption and unnecessary confinement degradation. We design and demonstrate such an edge-localized 3D response in the KSTAR facility, benefiting from its unique flexibility to change many degrees of freedom in the 3D coil space for the various fusion plasma regimes. This favorable control of the tokamak EF represents a notable advance for designing intrinsically 3D tokamaks to optimize stability and confinement for next-step fusion reactors.
ABSTRACT
The investigation of impurity behavior in fusion plasmas is a critical issue in fusion plasma research. The effective charge (Zeff) profile is a widely used measure of the impurity levels in fusion plasmas. In this study, the visible bremsstrahlung emissivity profile is reconstructed using toroidal visible bremsstrahlung (TVB) arrays at Korea Superconducting Tokamak Advanced Research (KSTAR). KSTAR TVB arrays have recently been developed and calibrated using a halogen light source and an integrating sphere. The reconstruction algorithm has been developed using the Phillips-Tikhonov method, and the reconstruction accuracy is assessed with test profiles. Electron density and temperature profiles from Thomson scattering diagnostics are fitted for Zeff calculations. Subsequently, the Zeff profiles in the edge localized mode suppression experiment are reconstructed. In addition, line-averaged Zeff values in the 2020 KSTAR campaign are presented, which are mostly distributed from two to four.
ABSTRACT
An ultrafast charge exchange spectroscopy (UFCES) system has been designed for measuring fluctuations in ion temperature and toroidal rotation velocity. The UFCES on the KSTAR tokamak is a powerful tool for investigating plasma instabilities and long-wavelength turbulence related to ion temperature gradient and flow. The UFCES system is designed to measure the C VI line (n = 8 â 7, λ0 = 529.05 nm) from the charge exchange reaction between a deuterium-heating neutral beam and the intrinsic carbon impurity in KSTAR. The ion temperature and toroidal rotation velocity at two radial positions will be observed simultaneously with UFCES. The key difference between the UFCES system and conventional charge exchange spectrometers is the application of high-throughput collection optics, a high-efficiency transmission grating combined with prisms, and a high-speed detector. We use a comprehensive spectrum simulation code with input parameters of KSTAR's plasmas and a neutral beam injection system to estimate the performance of the designed UFCES system. The results simulated with the code show that the diagnostic achieves a turbulence-relevant time resolution of 10 µs with a high enough signal-to-noise ratio. Furthermore, a preliminary test is performed using a complementary metal-oxide semiconductor camera and Ne spectral lamp to confirm the linear dispersion and curvature radius.
ABSTRACT
Measuring rotation profiles with a reliable spatial resolution is one of the critical diagnostics in understanding the plasma behavior especially for the edge transport. In the KSTAR experiments, it has been consistently observed from the charge exchange spectroscopy measurements that the magnetic perturbations not only suppresses edge localized modes (ELMs) but also reduces toroidal rotations. In this paper, toroidal velocities of the carbon impurity and their profile evolutions during ELMy and ELM-suppressed phases are presented. The rotation profiles are shown to collapse immediately after an ELM burst and continue to build up until the next burst that accompanies another collapse. Toroidal rotations following the resonant magnetic perturbations applications are observed to be reduced along with the ELMs suppressed.
ABSTRACT
A multi-purpose soft x-ray array diagnostic system was developed for measuring two-dimensional x-ray emissivity profile, electron temperature, Ar impurity transport, and total radiation power. A remotely controlled filter wheel was designed with three different choices of filters. The electron temperature profile can be determined from the ratio of two channels having different thickness of Be layer, and the Ar impurity concentration transport can be determined from a pair of Ar Ross filters (CaF(2) and NaCl thin films). Without any filters, this diagnostic system can also be used as a bolometer.
ABSTRACT
The charge exchange spectroscopy (CES) system on Korea Superconducting Tokamak Advanced Research (KSTAR) was installed last year and had been applied to measure the C VI ion temperature and rotation velocity profiles. The ion temperature and rotation velocity profiles had been estimated from the C VI 5290.5 Å (n = 8-7) charge-exchange spectrum signal measured by a Czerny-Turner type spectrometer and a thinned back-illuminated charge coupled device (CCD) camera. However, the Czerny-Turner type spectrometer used for the KSTAR CES system showed so low signal to noise ratio for KSTAR plasmas in the 2010 experimental campaign that the time resolution of the CES system had been limited to 100 ms due to the increased exposure time of the attached CCD camera. Then, new two-grating spectrometer had been developed in order to improve the time resolution of the CES system. The spectrometer consists of two gratings (1200 g/mm and 1800 g/mm each) with additive configuration, concave mirrors (f = 50 cm), and a cylindrical lens (f = 50 cm). The time resolution of the CES system increases by a factor of 2-4 with the two-grating spectrometer. The C VI ion temperature and rotation velocity profiles obtained by the two-grating spectrometer are compared to those by Czerny-Turner type spectrometer in this paper.
ABSTRACT
The charge exchange spectroscopy (CES) system including collection assemblies, lens design, and cassettes for the KSTAR experiment was installed to obtain profiles of the ion temperature and the toroidal rotation velocity from charge exchange emission between plasma ions and beam neutrals near the plasma axis by using a modulated neutral beam and a background system. We can measure the charge exchange spectra of an impurity line such as the 529 nm line of carbon VI to get ion temperature and rotation profiles in KSTAR. The CES and background systems will have absolute intensity and spectral calibrations using a calibrated source and various spectral lamps. The calibration was done inside the tokamak after all CES systems are installed and the optical systems are slid into the cassettes. This requires that the diagnostic systems are installed near the vacuum vessel inside the cryostat maintaining the superconducting state of the superconducting coils. Repeated spectral calibrations of the spectrometer and charge coupled device for CES will be made in the diagnostic room during the experimental campaign. We show a detailed description of the KSTAR CES system, how to calibrate, and the results of calibration.
