Experimental study of pedestal turbulence on EAST tokamak

Abstract

Turbulence in the pedestal region of the EAST tokamak has been observed and studied using reflectometry. In lower hybrid wave (LHW) or neutral beam injection (NBI) dominated heating plasma, a coherent mode (CM) was usually observed in the ELM-free phase just after the L-H transition. The CM rotated in the electron diamagnetic drift (EDD) direction in the laboratory frame with a poloidal wave number (k θ ) of 0.5 cm −1 -0.7 cm −1 and its frequency usually chirped from 80 -100 kHz down to 40 -50 kHz as the pedestal evolved. The appearance of this mode reduced the increasing rate of pedestal pressure, implying that the CM may have an effect on outward pedestal transport. This mode can exist every ten milliseconds and is finally replaced by broadband (BB) fluctuation in the later ELM-free phase. It was found that the appearance and disappearance of the CM was correlated to the pedestal pressure. In the inter-ELM phase, the pedestal turbulence is generally dominated by BB fluctuation with poloidal wave numbers from 0 to 3 cm −1 rotating in the EDD direction in the laboratory frame. Analysis shows that the pedestal pressure increasing rate dp e, ped /dt decreases with the amplitude of the BB fluctuation, implying that the BB fluctuation may play an important role in pedestal evolution. The preliminary observation on the fluctuation just inside the pedestal top is also presented.

Effects of intermittency on turbulent transport in magnetized plasmas

“…Thus, V 0 = 1. These choices are in agreement with the gross features of turbulence spectra from incompresible plasma and fluids [29,30,54,55].…”

“…The present theoretical analysis requires apriori knowledge of the statistical properties of the potential φ: the turbulence spectrum S(k, ω) and the distribution P (φ). To acquire such information, one needs to use high-quality gyro-kinetic simulations [27,28] completed by diagnostic techniques [14,29,30]. In tokamak devices, the spectrum shows a fast decay in frequency and along the radial direction with a peaked profile (at some specific wave-number k 0 ) along the poloidal direction [30][31][32][33][34][35].…”

Effects of intermittency via non-Gaussianity on turbulent transport in magnetized plasmas

“…Thus, V 0 = 1. These choices are in agreement with the gross features of turbulence spectra from incompressible plasmas and fluids (Levinson et al 1984;Boldyrev 2005;Casati et al 2009;Gao et al 2015).…”

“…The present theoretical analysis requires a priori knowledge of the statistical properties of the potential φ: the turbulence spectrum S(k, ω) and the distribution P(φ). To acquire such information, one needs to use high-quality gyro-kinetic simulations (Jenko & Dorland 2001;Wang et al 2006) complemented by diagnostic techniques (Casati et al 2009;Gao et al 2015;Gonçalves et al 2018). In tokamak devices, the spectrum shows a fast decay in frequency and along the radial direction with a peaked profile (at some specific wavenumber k 0 ) along the poloidal direction (Fonck et al 1993;Jenko & Dorland 2002;Casati et al 2009;Holland et al 2009;Shafer et al 2012;Qi et al 2019).…”

Non-Oberbeck–Boussinesq zonal flow generation

“…However, the latter breaks down, if the background density varies over more than one order of magnitude or if the relative density fluctuations exceed roughly 10 percent. This for example prevails in the edge of tokamak fusion plasmas, where experimental measurements typically feature relative density fluctuation levels around the order 0.1 in the edge and up to unity at the last closed flux surface [25][26][27][28][29][30][31][32][33]. Moreover, typical edge background density gradient (e-folding) lengths reach from 50ρ s0 in lowconfinement to 10ρ s0 in high-confinement tokamak plasmas [34,35].…”