Dynamic spectra of radio frequency bursts associated with edge-localized modes

Yun

et al. 2017

Sci Rep

Solitary perturbations (SPs) localized both poloidally and radially are detected within ~100 μs before the partial collapse of the high pressure gradient boundary region (called pedestal) of magnetized toroidal plasma in the KSTAR tokamak device. The SP develops with a low toroidal mode number (typically unity) in the pedestal ingrained with quasi-stable edge-localized mode (QSM) which commonly appears during the inter-collapse period. The SPs have smaller mode pitch and different (often opposite) rotation velocity compared to the QSMs. Similar solitary perturbations are also frequently observed before the onset of complete pedestal collapse, suggesting a strong connection between the SP generation and the pedestal collapse.

Section: Resultsmentioning

confidence: 59%

Section: Resultsmentioning

confidence: 99%

See 1 more Smart Citation

Solitary perturbations in the steep boundary of magnetized toroidal plasma

Yun

et al. 2017

Sci Rep

“…The duration of the proton ICE features during KSTAR ELM crashes is brief, typically a few microseconds; see for example, figure 6 of [1] and figure 1 below. Temporal correlation between an ELM crash and the ICE phenomena observed on KSTAR [1] may be due in part to the action of the ELM in 'flushing out' particles from the edge region.…”

Section: Introductionmentioning

confidence: 99%

Nonlinear wave interactions generate high-harmonic cyclotron emission from fusion-born protons during a KSTAR ELM crash

et al. 2018

The radio frequency detection system on the KSTAR tokamak has exceptionally high spectral and temporal resolution. This enables measurement of previously undetected fast plasma phenomena in the ion cyclotron range of frequencies. Here we report and analyse a novel spectrally structured ion cyclotron emission (ICE) feature in the range 500 MHz to 900 MHz, which exhibits chirping on sub-microsecond timescales. Its spectral peaks correspond to harmonics l of the proton cyclotron frequency f cp at the outer midplane edge, where l = 20-36. This frequency range exceeds estimates of the local lower hybrid frequency f LH in the KSTAR deuterium plasma. The new feature is time-shifted with respect to a brighter lower-frequency chirping ICE feature in the range 200 MHz (8f cp ) to 500 MHz (20f cp ), which is probably driven (Chapman et al 2017 Nucl. Fusion 57 124004) by 3 MeV fusion-born protons undergoing collective relaxation by the magnetoacoustic cyclotron instability (MCI). Here we show that the new, fainter, higher-frequency chirping ICE feature is driven by nonlinear wave coupling between different neighbouring spectral peaks in the lower-frequency ICE feature. This follows from bispectral analysis of the measured KSTAR fields, and of the field amplitudes output from particle-in-cell (PIC) simulations of the KSTAR edge plasma containing fusionborn protons. This reinforces the identification of the MCI as the plasma physics process underlying proton harmonic ICE from KSTAR, while providing a novel instance of nonlinear wave coupling on very fast timescales.

“…The last panel of Figure 1 From the filter-bank data, the trends of RF emission can be observed during multiple ELM cycles or the whole time range of discharge. KSTAR is equipped with another fast RF digitizer system (~5 GSa/s), which enables us to see the details of RF emission spectra with a high spectral resolution [11]. The high temporal (~1 μs) and spectral (~1 MHz) resolution are essential to resolve the fast ELM crash process, which proceeds within several hundreds of microseconds.…”

Section: Introductionmentioning

confidence: 99%

Distinct stages of radio frequency emission at the onset of pedestal collapse in KSTAR H-mode plasmas

et al. 2018

Using a high-speed and broadband radio frequency (RF) (0.1-1 GHz) spectrum analyzer developed on the KSTAR tokamak, it is found that several distinct stages of RF emission appear at the pedestal collapse in high confinement discharges. Comparison with 2-D electron cyclotron emission (ECE) images has revealed that each stage is related to the instantaneous condition at the outboard mid-plane edge. First, high-harmonic ion cyclotron emissions (ICE) are intensified with the appearance of a nonmodal filamentary perturbation in the edge within several tens of microseconds before the collapse. Then, the RF emission becomes broad toward high-frequency range (< 500 MHz) at the burst onset of the non-modal filament. During the pedestal collapse initiated by the filament burst, rapid chirping (1-3 μs) appear with additional filament bursts. The strong correlation between the RF spectra and the perturbation structure provides important clues on the stability of edge-localized modes and on the ion dynamics in the plasma boundary.