18th Technical Meeting on Energetic Particles in Magnetic Confinement Systems

MUTLISCALE GYROKINETIC SIMULATIONS OF THE INTERACTION BETWEEN TURBULENCE AND FISHBONE

Not scheduled

20m

Oral Multiscale Physics and Instabilities involving Energetic Particles in Burning Plasmas

Speaker

Prof. Huishan Cai (Unviersity of Science and Technology of China)

Description

Internal Transport Barriers (ITBs) are regions in tokamak plasmas where reduced core transport leads to steep pressure gradients, indicating improved confinement. Moreover, the associated steep pressure gradients generate a high bootstrap current fraction, aiding plasma current drive and enabling potential steady-state operation. These advantages make ITBs important for advancing fusion energy research, motivating extensive studies on advanced scenarios incorporating ITBs.
Experiments have shown that fishbone instabilities frequently occur alongside ITBs in tokamak plasmas in ASDEX-U, EAST, HL-2A and so on. It suggests a strong correlation between the two phenomena. In EAST, the fishbone and ITB formation nearly occur simultaneously, and their sequence is hard to be distinguished by diagnose. Whether fishbone causes the ITB formation or not is still unclear.
In this work, based on the EAST discharge where ITBs form in presence of the weakly negative magnetic shear, for the first time, we conducted the cross-scale coupling simulations between microturbulence and fishbone instability. It is found that fishbone-induced zonal flow effectively reduces the ITG turbulence saturation amplitude, potentially facilitating the formation or maintenance of ITBs.
The electromagnetic multi-n nonlinear simulations (toroidal numbers n=10-34) reveal that the ITG mode predominantly locates between q_min and 𝑞=1. The presence of energetic particles (EPs) results in slightly reduced linear growth rates and lower saturation amplitudes of ITG modes, as well as lower zonal flow levels, due to the dilution and finite beta effect. Moreover, the ion thermal conductivity exhibits a slight reduction in the presence of EPs.
Electromagnetic gyrokinetic simulations show that a much larger zonal flow is generated by fishbone than that by ITG turbulence. The mode structure predominantly resides between the 𝑞=1 surfaces. As the instability evolves into the nonlinear phase, the behavior becomes irregular, and the radial scale of the mode diminishes.
Cross-scale coupling simulation between ITG turbulence and fishbone is carried out well by overcoming some numerical difficulties. It is found that the linear growth rate of ITG is slightly decreased, while the saturation amplitude is dramatically reduced, at least one half. This is caused by the strong zonal flow generated by fishbone.