Distinct stages of radio frequency emission at the onset of pedestal collapse in KSTAR H-mode plasmas
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 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.
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 (fLH) 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 fLH ∼ 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.
Resonant Auger decay study of C1s -> pi* core-excited OCS
The present work aims at characterizing short-lived C1s((-1)) pi*((1)) core-excited states of the OCS molecule based on the analysis of the vibrational fine structure and lineshape profiles of the high-resolution resonant Auger decay spectra recorded at the excitation energies along the C1s -> pi* resonance in the binding energy region 15-19eV. Very different behavior in terms of lineshape and resonant enhancement is observed for the (A) over tilde, (B) over tilde and (C) over tilde final states. This is explained by (1) the variation in the C-O bond lengths for the states involved in the electronic relaxation and (2) different contributions in terms of Mulliken population to the molecular orbitals determining the electronic character of the corresponding states. Since the final-state geometries are known from a number of previous experiments and ab initio calculations, the geometry of the C1s((-1)) pi*((1)) intermediate states can be predicted in analogy with e.g. the N-2, CO2 and N2O molecules. (C) 2009 Elsevier B.V. All rights reserved. (Less)