arXiv subjects

G. S. Yun

Publications and source records attributed to G. S. Yun.

6 recordsLinked to original sources

arXiv2019

Experimental evidence of the non-diffusive avalanche-like electron heat transport events and their dynamical interaction with the shear flow structure

We present experimental observations suggesting that the non-diffusive avalanche-like events are a prevalent and universal process of the electron turbulent heat transport in tokamak core plasmas. They are observed in the low confinement mode and the weak internal transport barrier tokamak plasmas in the absence of magnetohydrodynamic instabilities. In addition, the electron temperature profile corrugation, which indicates the existence of the $E \times B$ shear flow layers, is clearly demonstrated as well as their dynamical interaction with the avalanche-like events. The measured width of the profile corrugation is around $45ρ_i$, which implies the mesoscale nature of the structure.

physics.plasm-ph↗

arXiv2018

Semi-analytic Shape Function of High-harmonic Electron Cyclotron Emission in Tenuous Plasma

Semi-analytic expressions for the electron cyclotron emission (ECE) shape function are developed for arbitrary high harmonics. The integrand of the ${m}^{\mathrm{th}}$ harmonic ECE shape function is fitted with the readily-integrable test function which is parameterized by plasma temperature $T_e$, harmonic number $m$ and emission angle $θ$. Semi-analytic formulae for high harmonic ECE emissivity are obtained by integrating the test integrand with the fitting parameters gained from the regression analysis as well as the inductive studies for arbitrary cases. The developed expression matches the numerically-integrated ECE shape function very well and the overall differences between original shape functions and semi-analytic shape functions are evaluated. The expression can be used for rapid analysis of high-harmonic ECE spectra.

physics.plasm-ph↗

arXiv2017

Multiscale interaction between a large scale magnetic island and small scale turbulence

Multiscale interaction between the magnetic island and turbulence has been demonstrated through simultaneous two-dimensional measurements of turbulence and temperature and flow profiles. The magnetic island and turbulence mutually interact via the coupling between the electron temperature ($T_e$) gradient, the $T_e$ turbulence, and the poloidal flow. The $T_e$ gradient altered by the magnetic island is peaked outside and flattened inside the island. The $T_e$ turbulence can appear in the increased $T_e$ gradient regions. The combined effects of the $T_e$ gradient and the the poloidal flow shear determine two-dimensional distribution of the $T_e$ turbulence. When the reversed poloidal flow forms, it can maintain the steepest $T_e$ gradient and the magnetic island acts more like a electron heat transport barrier. Interestingly, when the $T_e$ gradient, the $T_e$ turbulence, and the flow shear increase beyond critical levels, the magnetic island turns into a fast electron heat transport channel, which directly leads to the minor disruption.

physics.plasm-ph↗

arXiv2017

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

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 diagnostic of the collapsing edge pedestal density.

physics.plasm-ph↗

arXiv2016

ExB flow-induced shearing-merging of filaments: a Ginzburg-Landau model of Edge-Localized Mode cycles

We derive and study a simple 1D nonlinear model for Edge Localized Mode (ELM) cycles. The nonlinear dynamics of a resistive ballooning mode is modeled via a single nonlinear equation of the Ginzburg-Landau type with a radial frequency gradient due to a prescribed ExB shear layer of finite extent. The nonlinearity is due to the feedback of the mode on the profile. We identify a novel mechanism, whereby the ELM only crosses the linear stability boundary once, and subsequently stays in the nonlinear regime for the full duration of the cycles. This is made possible by the shearing and merging of filaments by the ExB flow, which forces the system to oscillate between a radially-uniform solution and a non-uniform solitary - wave like solution. The model predicts a 'phase-jump' correlated with the ELM bursts.

physics.plasm-ph↗

arXiv2007

Observation of Kinetic Plasma Jets

Under certain conditions an intense kinetic plasma jet is observed to emerge from the apex of laboratory simulations of coronal plasma loops. Analytic and numerical models show that these jets result from a particle orbit instability in a helical magnetic field whereby magnetic forces radially eject rather than confine ions with sufficiently large counter-current axial velocity.

physics.plasm-ph↗