Scientists Observe Complex Turbulence Behavior in Tokamak H-mode Pedestal Region
count: [2019-09-23]

Scientists from Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP) obtained a new discovery in H-mode pedestal region on EAST tokamak in their recent experiments.

High-confinement mode (H-mode) in tokamak plasma is characterized by the spontaneous formation of pedestal, i.e a localized region with steep density and temperature gradients at the plasma edge. The pedestal performance is strongly correlated with the global energy confinement. Therefore, great attentions have been paid on the pedestal region recent years.

Recently, scientists from ASIPP observed a low-frequency (~1 kHz) oscillation (LFO) in H-mode pedestal region on EAST tokamak. It had a magnetic component with a toroidal mode number (n=0), indicating an axisymmetric mode, while its poloidal magnetic structure demonstrated an m = 1 standing wave with in–out asymmetry.

The pedestal density fluctuation, pedestal density profile, and the particle flux onto the divertor were all modulated by this low frequency oscillation.

Before this low-frequency oscillation occured, the density fluctuation in the pedestal region was mainly dominated by low frequency (<400 kHz). During this low-frequency oscillation, the density fluctuation of the low frequency presented suppression and recovery alternatively.

At the same time, in the low-frequency density fluctuation suppression stage, the high-frequency density fluctuation (>500kHz) appeared, the temperature pedestal height (Te,ped) tended to be saturated and the density pedestal height (ne,ped) still kept growing.

This phenomenon implies that high frequency density fluctuations could drive plasma heat transport.

Theses results demonstrate complex turbulence behavior in pedestal region. The influence of pedestal turbulence on particle and heat transport still deserves further study. (XIANG Haoming reports)

 

Low frequency oscillation (LFO) emerges in ELM-free phase in H-mode pedestal region (Image by XIANG Haoming)

 

LFO modulates the density profile (Image by XIANG Haoming)

 

LFO in the inter-ELM phase and the relationship between the high frequency fluctuations and the pedestal evolution (Image by XIANG Haoming)