Ion cyclotron emission (ICE) is detected during edge localised modes (ELMs) in the KSTAR tokamak at harmonics of the proton cyclotron frequency in the outer plasma edge. The emission typically chirps downward (occasionally upward) during ELM crashes, and is driven by confined 3MeV fusion-born protons that have large drift excursions from the plasma core. We exploit fully kinetic simulations at multiple plasma densities to match the time-evolving features of the chirping ICE. This yields a unique, very high time resolution (< 1µs) diagnostic of the collapsing edge pedestal density. PACS numbers: 52.35.Hr, 52.35.Qz, 52.55.Fa, 52.55.Tn Understanding the physics of edge localised modes (ELMs) [1][2][3][4] in magnetically confined fusion (MCF) plasmas is crucial for the design of future fusion power plants. The same is true of the physics of the energetic ions born at MeV energies [5,6] from fusion reactions between fuel ions in the multi keV thermal plasma. The crash of an ELM involves impulsive relaxation of the edge magnetic field, releasing energy and particles from the plasma at levels which may not be compatible with sustained operation of the next step fusion experiment, ITER [7,8]. The confinement of fusion-born ions while they release energy, collisionally or otherwise, to the thermal plasma, was a key physics objective of the unique deuterium-tritium plasma experiments in TFTR [9] and JET [10], and will be central to ITERs research programme. Here we report an unexpected conjunction of ELM physics with fusion-born ion physic. We show how this can be exploited as a diagnostic of plasma edge density with unique, very high (< 1µs) time resolution. This is achieved through particle orbit studies combined with first principles kinetic plasma simulations that explain high-time-resolution measurements of ion cyclotron emission (ICE) from the medium-size tokamak KSTAR [11]. We show that ICE from KSTAR deuterium plasmas is driven by a small subset of the fusion-born proton population, originating in the core of the plasma and passing through the edge region where they radiate collectively through the magnetoacoustic cyclotron instability (MCI) [13][14][15][16][17][18][19][20][21][22]. The MCI can occur because of the spatially localised population inversion in velocity space that is caused by the large drift excursions of 3.0 MeV fusionborn protons on deep passing orbits. Our simulations of the MCI in its saturated nonlinear regime show that the frequency spectrum excited depends strongly on the plasma density. By comparing MCI spectra simulated at different densities with high-time-resolution measurements of ICE spectra during ELM crashes in KSTAR, we are able to infer the time evolution of the collapsing edge density at sub-microsecond resolution, which is unprecedented. Recently, ICE has been detected from the outer mid-plane of KSTAR [23-25], with spectral peak frequencies at local proton cyclotron harmonics; see e.g. Fig. 1. The only energetic protons in KSTAR plasmas are produced in deuteron-deuteron (D-D) ...

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.

Electromagnetic emissions in the radio frequency (RF) range are detected in the highconfinement-mode (H-mode) plasma using a fast RF spectrometer on the KSTAR tokamak. The emissions at the crash events of edge-localized modes (ELMs) are found to occur as strong RF bursts with dynamic features in intensity and spectrum. The RF burst spectra (obtained with frequency resolution better than 10 MHz) exhibit diverse spectral features and evolve in multiple steps before the onset and through the ELM crash: (1) a narrowband spectral line around 200 MHz persistent for extended duration in the pre-ELM crash times, (2) harmonic spectral lines with spacing comparable to deuterium or hydrogen ion cyclotron frequency at the pedestal, (3) rapid onset (faster than ~1 μs) of intense RF burst with wide-band continuum in frequency which coincides with the onset of ELM crash, and (4) a few additional intense RF bursts with chirping-down narrow-band spectrum during the crash. These observations indicate plasma waves are excited in the pedestal region and strongly correlated with the ELM dynamics such as the onset of the explosive crash. Thus the investigation of RF burst occurrence and their dynamic spectral features potentially offers the possibility of exploring H-mode physics in great detail.

Intense bursts of suprathermal radiation, with spectral peaks at frequencies corresponding to the deuteron cyclotron frequency in the outer midplane edge region, are often detected from deuterium plasmas in the KSTAR tokamak that are heated by tangential neutral beam injection (NBI) of deuterons at 100 keV. Identifying the physical process by which this deuterium ion cyclotron emission (ICE) is generated, typically during the crash of edge localised modes, assists the understanding of collective energetic ion behaviour in tokamak plasmas. In the context of KSTAR deuterium plasmas, it is also important to distinguish deuterium ICE from the ICE at cyclotron harmonics of fusion-born protons examined by Chapman et al (2017 Nucl. Fusion 57 124004; 2018 Nucl. Fusion 58 096027). We use particle orbit studies in KSTAR-relevant magnetic field geometry, combined with a linear analytical treatment of the magnetoacoustic cyclotron instability (MCI), to identify the sub-population of freshly ionised NBI deuterons that is likely to excite deuterium ICE. These deuterons are then represented as an energetic minority, together with the majority thermal deuteron population and electrons, in first principles kinetic particle-in-cell (PIC) computational studies. By solving the Maxwell–Lorentz equations directly for hundreds of millions of interacting particles with resolved gyro-orbits, together with the self-consistent electric and magnetic fields, the PIC approach enables us to study the collective relaxation of the energetic deuterons through the linear phase and deep into the saturated regime. The Fourier transform of the excited fields displays strong spectral peaks at multiple successive deuteron cyclotron harmonics, mapping well to the observed KSTAR deuterium ICE spectra. This outcome, combined with the time-evolution of the energy densities of the different particle populations and electric and magnetic field components seen in the PIC computations, supports our identification of the driving sub-population of NBI deuterons, and the hypothesis that its relaxation through the MCI generates the observed deuterium ICE signal. We conclude that the physical origin of this signal in KSTAR is indeed distinct from that of KSTAR proton ICE, and is in the same category as the NBI-driven ICE seen notably in the TFTR tokamak and LHD heliotron–stellarator plasmas. ICE has been proposed as a potential passive diagnostic of energetic particle populations in ITER plasmas; this is assisted by clarifying and extending the physics basis of ICE in contemporary magnetically confined plasmas.

Atomic processes leading to asymmetric divertor detachment in KSTAR L-mode plasmas. Nuclear Fusion, 58(12), [126033]. https://doi. AbstractThe experimentally observed in/out detachment asymmetry in KSTAR L-mode plasmas with deuterium (D) fueling and carbon walls has been investigated with the SOLPS-ITER code to understand its mechanism and identify important atomic processes in the divertor region. The simulations show that the geometrical combination of a vertical, inner target with short poloidal connection from X-point to target and a much longer outer divertor leg on an inclined target lead to neutral accumulation towards the outer target, driving outer target detachment at lower upstream density than is required for the inner target. This is consistent with available Langmuir probe measurements at both target plates, although the inner target profile is poorly resolved in these plasmas and further experiments with corroborating diagnostics are required to confirm this finding. The pressure and power loss factors defined in the two-point model [1][2][3][4] of the divertor scrape-off layer (SOL) and the sources contributing to the loss factors are calculated through post-processing of the SOLPS-ITER results. The momentum losses are mainly driven by plasma-neutral interaction and the power losses by plasma-neutral interaction and carbon radiation. The presence of carbon impurities in the simulation enhances pressure and power dissipation compared to the pure D case. Carbon radiation is a strong power loss channel which cools the plasma, but its effect on the pressure balance is indirect. Reduction of the electron temperature indirectly increases the momentum loss by decreasing the static pressure and increasing the volumetric reaction rates which are responsible for the loss of momentum. As a result, the addition of carbon saturates the momentum and power losses in the flux tube at lower upstream densities, reducing the rollover threshold of upstream density. The relative strengths of the various mechanisms contributing to momentum and power loss depends on the radial distance of the SOL flux tubes from the separatrix (near/far SOL) and the target (inner/outer target). This is related to the strong D2 molecule accumulation near the outer strike point, which makes the deuterium gas density at the outer target 2-10 times higher than that at the inner target. A large portion of the recycled neutral particles from both targets reach and accumulate in the outer SOL, which is attributed in strong part to the target inclination and gap structure between the central and outboard divertors and hence to the impact of geometry. The accumulated neutrals enhance the reactions involving D2 which cause momentum and power loss.