Shear-flow trapped-ion-mode interaction revisited. II. Intermittent transport associated with low-frequency zonal flow dynamics

Abstract

Équipe 107 : Physique des plasmas chaudsInternational audienceWe address the mechanisms underlying low-frequency zonal flow generation in turbulent system and the associated intermittent regime of ion-temperature-gradient (ITG) turbulence. This model is in connection with the recent observation of quasi periodic zonal flow oscillation at a frequency close to 2 kHz, at the low-high transition, observed in the ASDEX Upgrade [Conway et al., Phys. Rev. Lett. 106, 065001 (2011)] and EAST tokamak [Xu et al., Phys. Rev. Lett 107, 125001 (2011)]. Turbulent bursts caused by the coupling of Kelvin-Helmholtz (KH) driven shear flows with trapped ion modes (TIMs) were investigated by means of reduced gyrokinetic simulations. It was found that ITG turbulence can be regulated by low-frequency meso-scale zonal flows driven by resonant collisionless trapped ion modes (CTIMs), through parametric-type scattering, a process in competition with the usual KH instability

Shear-flow trapped-ion-mode interaction revisited. I. Influence of low-frequency zonal flow on ion-temperature-gradient driven turbulence

“…The electric potential is expressed in x d0 ᭝w units and the constants C e and C i introduced in the quasineutrality equation (4) 38 Without dissipation, three energetic subsystems interact to produce the complexity observed in the interchange-type turbulence: the kinetic energy of plasma E c , the energy of the zonal flow noted here E ZF , and the potential fluctuation of turbulence E turb contained in the initial pressure gradient. These quantities are defined by Eqs.…”

“…ZF is generated by the Reynolds stress and back reacts upon turbulence via vortex shearing. However, it is possible to connect the time variation of h/i a of ZF to dP fluctuations, driven by the interchange-type turbulence, or as we will see later in Paper II, 38 by resonant wave-particle interactions. Thus, the nature of ZF can be modified and its evolution is now described by A. Ghizzo and F. Palermo Phys.…”

“…28). In fact in Paper II, 38 we will show that both interchange and resonant counterpart CTIM coexist and can beat in a three-wave interaction leading to the (non zero) lowfrequency ZF growth. The analysis of the top panel in Fig.…”

“…(38) and the standard turbulence-induced Reynolds stress already mentioned by Diamond and Kim. 37 The nonlinear drive of zonal flows is governed by the Reynolds tensor in Eq.…”

“…(37). It is balanced by the (non-adiabatic) interchange-type transfer given by (38). It is the polarization drift nonlinearly occurring in the right-hand side of Eq.…”