Discrete Alfvén eigenmodes excited by energetic particles in China Fusion Engineering Test Reactor

Abstract

In burning plasmas, discrete Alfvén eigenmodes (αTAEs, where α denotes a parameter of plasma pressure gradient) can be readily destabilized by energetic particles under the wave-particle resonance condition. In this study, we theoretically analyze the physical features of αTAEs in the China Fusion Engineering Test Reactor (CFETR), which is viewed as the next generation device in the Chinese magnetic confinement fusion program. One of the scientific objectives of the device is to achieve steady-state operation. There are three reference schemes for baseline steady-state operation, namely Case 1: NB+EC, Case 2: NB+EC+LH, and Case 3: EC+LH, where NB, EC, and LH represent neutral beam, electron cyclotron wave, and lower hybrid wave, respectively. Within the ideal magnetohydrodynamic (MHD) description, we focus on the physical characteristics of αTAEs and correlation between the radial profile of total current density and αTAEs for the three scenarios of CFETR operation with the help of a MHD eigenvalue code. The numerical results indicated that multiple branches of αTAEs can be trapped in potential wells of the ballooning drive under the CFETR operation condition. In the inner region, the bootstrap current density is large, and the total current density profile, defined by the peak of the radial profile of total plasma current density is relatively flat. Therefore, there are abundant αTAEs in this region. However, in the outer region, the bootstrap current is relatively small, and the total current density decreases gradually. So, αTAEs are hardly seen in this region. The αTAEs in the inner region are quasi-marginally stable within the ideal MHD description. In the CFETR operations, the energetic particles generated can destablize the MHD αTAEs under the wave-particle resonance conditions. We study the stability features of αTAEs interacting with energetic particles by employing linear gyrokinetic-MHD hybrid eigenvalue codes. The multiple branches of αTAEs are excited under the various resonance conditions. Moreover, the multiple branches of αTAEs can also be excited by energetic particles with virial energy. Furthermore, higher-frequency αTAEs may be destabilized by energetic particles with higher energy. These αTAEs could change the distribution of energetic particles in phase space and cause the loss of a large number of particles, which may affect the plasma confinement.