Sub-microsecond temporal evolution of edge density during edge localized modes in KSTAR tokamak plasmas inferred from ion cyclotron emission

During edge localised mode (ELM) crashes in KSTAR deuterium plasmas, bursts of spectrally structured ion cyclotron emission (ICE) are detected. Usually the ICE spectrum chirps downwards during an ELM crash, on sub-microsecond timescales. For KSTAR ICE where the separation of spectral peak frequencies is close to the proton cyclotron frequency Ω<sub>cp</sub> at the outer plasma edge, we show that the driving population of energetic ions is likely to be a subset of the 3MeV fusion protons, born centrally on deeply passing orbits which drift from the core to the edge plasma. We report first principles modelling of this scenario using a particle-in-cell code, which evolves the full orbit dynamics of large numbers of energetic protons, thermal deuterons, and electrons self-consistently with the electric and magnetic fields. The Fourier transform of the excited fields in the nonlinear saturated regime of the simulations is the theoretical counterpart to the measured ICE spectra. Multiple simulation runs for different, adjacent, values of the plasma density under KSTAR edge conditions enable us to infer the theoretical dependence of ICE spectral structure on the local electron number density. By matching this density dependence to the observed time-dependence of chirping ICE spectra in KSTAR, we obtain sub-microsecond time resolution of the evolving local electron number density during the ELM crash

Thin foil bolometer development for shattered pellet injection studies on KSTAR tokamak

The shattered-pellet injection (SPI) is widely believed as an effective tool for mitigating plasma disruption in magnetic confinement fusion devices, as the frozen pellets can penetrate deep into the core. The investigation of SPI and consequent radiation profile on KSTAR tokamak was initiated since 2019 experiments. In conjunction with the existing radiation diagnostics such as electron cyclotron emission imaging (ECEI) and radio frequency (RF) spectrometers [1-2], two types of thin foil bolometers are developed along with an extreme ultra-violet bolometer for measurement of high radiative flux during SPI in KSTAR. The first type is a thin foil infrared (IR) bolometer consisting of a graphite coated platinum foil and an IR detector. The foil is sandwiched between two copper rings for good thermal conduction, from the foil to the metal support structure. The calibration and time response of the foil are obtained by the IR detector facing the foil, which is illuminated with a laser beam on the other side. The decay time of the IR detector signal from saturation after turning-off the laser is found comparable (~100 ms) for varying foil thicknesses (0.5 m and 2.5 m). Multiple layers of EMI shields were proven critical for preventing high amplitude EMI noises both in the laboratory tests using a Tesla coil and in the commissioning test on the KSTAR. The second type is a resistive foil pulsed current bolometer. Each bolometer unit has a pair of resistive foils for enhanced signal contrast and reduction of system noises. A prototype is underway which shall be compared with the IR bolometer. Work supported by the Korea Institute of Fusion Energy under the KSTAR collaboration program.1

Radio frequency emissions driven by energetic ions from neutral beam in KSTAR low confinement mode plasma

The tangential neutral beam injection in KSTAR low confinement mode plasma is rapidly accompanied by the electromagnetic emissions in radio frequency (RF) range (0.1–1 GHz). The RF emission is initially onset within 1 ms from the beam injection, at discrete frequencies with steadily increasing intensity. The frequency spacing for these discrete emission lines corresponds to the deuteron cyclotron frequency, at a location midway between the magnetic axis and the edge. Further, the observed discrete frequencies lie in the lower hybrid frequency (f LH) range in a broad region on the low field side (LFS). As the initial RF emission becomes saturated, there is another onset of intense RF bursts occurring at discrete frequencies, broadening the emission frequency range further, either at higher or lower frequencies. In some cases, the time interval of the intense RF bursts at the dominant frequency is comparable with the toroidal rotation period at the radial location where f LH ~ dominant frequency. The rapid rise and saturation of RF emission intensity in a broad frequency range indicate that a small population of fast ions is sufficient for the growth of energetic particle driven instabilities on the LFS. The multiple onsets in RF emission and the intense RF burst repetition frequency comparable with toroidal rotation frequency indicate the possibility that reorganization in the anisotropic fast ion population results in localized growth of the above instabilities. A gradual decay of RF emission intensity over few tens of milliseconds indicate that enhanced population of fast ions has damping effect on these instabilities.11Nsciescopu

Intense whistler-frequency emissions at the pedestal collapse in KSTAR H-mode plasmas

The edge confinement barrier of high-confinement mode (H-mode) plasma involves a variety of plasma waves alongside with fluid instabilities and collective particle transport during the barrier collapse. We demonstrate a new method of resolving the plasma waves by measuring the modulations embedded in the second harmonic electron cyclotron emission (ECE). Utilizing mm-wave heterodyne detection and fast digitization technologies on the KSTAR tokamak, we resolve not only the frequency spectrum but the wavenumber as well. At the plasma boundary during the barrier collapse, we observe multiple bursts of broadband whistler-frequency (<8 GHz) waves, together with intense narrowband emissions. The narrowband emissions exhibit rapid rising and falling tones within a few microseconds. Bispectral analyses reveal the existence of nonlinear interactions between the broadband and the narrowband waves, confirming their concurrence in the barrier zone. We estimate that the vertical wavenumber of the narrowband waves is comparable to that of a whistler wave, utilizing multiple mm-wave mixer channels. Our work opens a new experimental way to study wave-wave and wave-particle interactions in magnetically confined plasmas.11Nsciescopu

Radio frequency (RF) waves at the onset of pedestal collapse in KSTAR H-mode plasmas: from ion cyclotron harmonics to whistler waves

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Intense and short bursts of whistler-frequency waves during the pedestal collapse in KSTAR H-mode plasmas

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Whistler-frequency bursts during pedestal collapse in KSTAR H-mode plasmas

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Analysis of intense bursts of whistler-frequency waves during the edge transport barrier collapse in KSTAR H-mode plasmas

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Radiation measurement in plasma disruption by thin-foil infrared bolometer

A thin-foil infrared bolometer has been developed to measure the plasma radiation quantitatively during plasma disruptions in the KSTAR tokamak. We present analytic solutions of a 0D heat transfer model, which enable the estimation of the plasma radiation from the bolometer signal. The analytical solutions for the linear response regime give practical ways by which the radiation power and energy can be estimated from the cooling time scale of the bolometer signal. A useful way of evaluating the linear response of the system is also introduced. The analysis is complemented by 2D heat transfer simulations. The bolometer signals from the shattered pellet injection experiments in the 2020 KSTAR campaign are analyzed and interpreted according to the heat transfer models.11Nsciescopu