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

Thatipamula, Shekar G,Yun, G S,Leem, J,Park, H K,Kim, K W,Akiyama, T,Lee, S G IOP 2016 Plasma physics and controlled fusion Vol.58 No.6

<P>Electromagnetic emissions in the radio frequency (RF) range are detected in the high-confinement-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 narrow-band 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 <I>μ</I>s) 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.</P>

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

Kim, M.H.,Thatipamula, S.G.,Lee, J.E.,Choi, M.J.,Park, H.K.,Akiyama, T.,Yun, G.S. International Atomic Energy Agency 2018 Nuclear fusion Vol.58 No.9

<P>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 2D 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 non-modal 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 <I>μ</I>s) 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.</P>

Atomic processes leading to asymmetric divertor detachment in KSTAR L-mode plasmas

Park, Jae Sun,Groth, Mathias,Pitts, Richard,Bak, Jun-Gyo,Thatipamula, S.G.,Juhn, June-Woo,Hong, Suk-Ho,Choe, Wonho International Atomic Energy Agency 2018 Nuclear fusion Vol.58 No.12

<P>The 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 a short poloidal connection from the <I>X</I>-point to the target and a much longer outer divertor leg on an inclined target lead to neutral accumulation towards the outer target, driving the 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 (Stangeby 2018 <I>Plasma Phys. Control. Fusion</I> <B>60</B> 4; Kotov and Reiter 2009 <I>Plasma Phys. Control. Fusion</I> <B>51</B> 115002; Stangeby and Sang 2017 <I>Nucl. Fusion</I> <B>57</B> 056007; Moulton <I>et al</I> 2017 <I>Plasma Phys. Control. Fusion</I> <B>59</B> 6) 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 the 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 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 roll-over threshold of the upstream density. The relative strengths of the various mechanisms contributing to momentum and power loss depend 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 D<SUB>2</SUB> 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 predominantly attributed 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 D<SUB>2</SUB>, which causes momentum and power loss.</P>

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

Chapman, B.,Dendy, R.O.,McClements, K.G.,Chapman, S.C.,Yun, G.S.,Thatipamula, S.G.,Kim, M.H. International Atomic Energy Agency 2017 Nuclear fusion Vol.57 No.12

<P>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 <img ALIGN='MIDDLE' ALT='$\Omega_{\rm cp}$ ' SRC='http://ej.iop.org/images/0029-5515/57/12/124004/nfaa8e09ieqn001.gif'/> at the outer plasma edge, we show that the driving population of energetic ions is likely to be a subset of the 3 MeV 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.</P>

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

Chapman, B.,Dendy, R.O.,Chapman, S.C.,McClements, K.G.,Yun, G.S.,Thatipamula, S.G.,Kim, M.H. International Atomic Energy Agency 2018 Nuclear fusion Vol.58 No.9

<P>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 <I>l</I> of the proton cyclotron frequency <I>f</I> <SUB>cp</SUB> at the outer midplane edge, where <I>l</I>  =  20–36. This frequency range exceeds estimates of the local lower hybrid frequency <I>f</I> <SUB>LH</SUB> 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 (8<I>f</I> <SUB>cp</SUB>) to 500 MHz (20<I>f</I> <SUB>cp</SUB>), which is probably driven (Chapman <I>et al</I> 2017 <I>Nucl. Fusion</I> <B>57</B> 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 fusion-born 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.</P>