Self-generated zonal flows in the plasma turbulence driven by trapped-ion and trapped-electron instabilities

Abstract

Équipe 107 : Physique des plasmas chaudsInternational audienceThis paper presents a study of zonal flows generated by trapped-electron mode and trapped-ion mode micro turbulence as a function of two plasma parameters-banana width and electron temperature. For this purpose, a gyrokinetic code considering only trapped particles is used. First, an analytical equation giving the predicted level of zonal flows is derived from the quasi-neutrality equation of our model, as a function of the density fluctuation levels and the banana widths. Then, the influence of the banana width on the number of zonal flows occurring in the system is studied using the gyrokinetic code. Finally, the impact of the temperature ratio Te/Ti on the reduction of zonal flows is shown and a close link is highlighted between reduction and different gyro-and-bounce-average ion and electron density fluctuation levels. This reduction is found to be due to the amplitudes of gyro-and-bounce-average density perturbations ne and ni gradually becoming closer, which is in agreement with the analytical results given by the quasi-neutrality equation

Stimulated zonal flow generation in the case of TEM and TIM microturbulence

“…As expected this result is in agreement with equation (7) and the amplitude of the zonal flows observed previously in Fig.3 (b). We also noticed that the large density difference is in agreement with the results obtained in a previous study in which a strong steady zonal-flow regime is correlated with a large density difference in the case T e = T i (See Fig.5a in [18]). The species that is involved in the instability which drives turbulence acts the most in response to the zonal potential and separates from (adjusts to) the fluctuation level of the other species which then leads to the zonal flow amplification (reduction).…”

“…This is important because zonal flows have been shown to play an important role in suppressing nonlinear transport. The impact of the temperature ratio T i /T e on the reduction of zonal flows has been studied [18]. This reduction was found to be linked to the amplitudes of gyro-and-bounce-average density perturbations n e and n i gradually becoming closer.…”

“…Indeed Ref. [18] shows a natural transition from zonal flow-dominated regime to streamer-like regime as T i /T e decreases. Therefore it can be expected that stimulation is only efficient at relatively large temperature ratio.…”

“…Recently, a semi-Lagrangian nonlinear simulation code, TERESA-4D (Trapped Element REduction in Semi lagrangian Approach), has been developed based on this model for trapped ions, making efficient use of parallel computing [14,15]. We recently expanded TERESA to simultaneously describe both trapped-ion (TIM) and trapped-electron (TEM) driven modes [16,17,18]. The code's most interesting property is that it enables the processing of the full f problem for trapped ions and trapped electrons at very low numerical cost [17].…”

Impurity density gradient influence on trapped particle modes

“…More precisely, we look at the influence of the impurity profile on trapped particle driven modes, and study the transport of these impurities. This is done using the code TERESA, which is a semi-Lagrangian code based on a collisionless reduced bounce-averaged gyrokinetic model [17][18][19][20][21][22] . This code treats the passing particles adiabatically, and allows the study of Trapped Electron Modes (TEM) and Trapped Ion Modes (TIM).…”