Dynamics of pedestal perturbations by ELMs and edge harmonic oscillations in DIII-D

Abstract

Edge density profile modifications associated with edge localized modes (ELMs) and edge harmonic oscillations (EHOs) have been measured using fast profile reflectometry in DIII-D H-mode plasmas. Specifically, high time (down to 10 µs) and spatial (∼4 mm) resolution density profile measurements from 0 × 10 19 to 3.1 × 10 19 m −3 provide us with new tools to study the dynamics of pedestal perturbations. During ELMs, it has been observed that the SOL density profile expands outwards to the vessel wall while the pedestal density reduces. These measurements provide direct evidence for enhanced particle radial transport to the vessel wall at the onset of ELMs. A large SOL density profile radial expansion velocity is observed during ELMs. A comparison of the velocities for different pedestal densities is made. The density scale length, density at the wall, and density fluctuations during ELMs are determined as a function of pedestal density, as well as particle transport during ELMs. Measurements in quiescent H-mode plasmas indicate that the pedestal density profile is modulated at the fundamental frequency of the EHO. A comparison between reflectometer and beam emission spectroscopy measurements regarding the EHO density fluctuation profile is presented.

SOL and pedestal density profile evolution during DIII-D ELMy and ELM-suppressed H-mode operation

“…As can be seen, the particle flux substantially reduces with increasing density. In previous work [9], the normalized density at the vessel wall, and normalized density fluctu- ation level in the pedestal were also shown to similarly decrease with increasing pedestal density.…”

“…doi:10.1016/j.jnucmat.2004.09.037 successfully used to measure the SOL and pedestal density profile modifications associated with ELMs in DIII-D [8][9][10]. The reflectometer system was recently upgraded to expand the measured density range to 0-6.4 · 10 19 m À3 , with high time resolution, such that the system now has the capability to resolve the whole pedestal profile evolution associated with ELMs.…”

Pedestal conditions for small ELM regimes in tokamaks

“…In these plasmas, counter-NBI (NB injection opposite to the plasma current) and a large clearance between the plasma separatrix and the first wall are required to obtain the QH phase. Edge Harmonic Oscillations (EHOs), which are considered to enhance the particle fluxes, were usually observed near the separatrix at the onset of the QH phase in all devices [15,16,19,13]. Edge pressure in QH-mode plasma on JT-60U seems to be lower than that in ELMing plasma as shown in Fig.…”

Experimentally inferred thermal diffusivities in the edge pedestal between edge-localized modes in DIII-D

“…13 These studies show that the relative drop of T e and n e at an ELM varies with plasma parameters and that there are conditions under which the relative changes in T e are small compared to the changes in n e . In addition, various measurements of time histories of edge parameters during a single ELM cycle show that the electron temperature, 14 the electron density, 15,16 and the ion temperature 17,18 in the pedestal all decrease rapidly at an ELM and recover on a slower time scale over the time prior to the next ELM.…”