Effects of alpha particles on the transport of helium ash driven by collisionless trapped electron mode turbulence

Abstract

The effects of alpha (α) particles on the transport of helium ash driven by collisionless trapped electron mode (CTEM) turbulence are analytically studied using quasi-linear theory in tokamak deuterium (D) and tritium (T) plasmas. Under the parameters used in this work, the transport of helium ash is mainly determined by the diffusion due to very weak convection. It is found that the ratio between helium ash diffusivity and effective electron thermal conductivity (D_He/χ_eff) driven by CTEM turbulence, which is a proper normalized parameter for quantifying the efficiency of helium ash removal, is smaller than unity. This indicates the less efficient removal of helium ash through CTEM turbulence as compared with ion temperature gradient (ITG) turbulence in [Angioni, et al, 2009 Nucl. Fusion, 49 055013]. However, the efficiency of helium ash removal is increased 55% by the presence of 3% α particles with their density gradient being twice that of electrons, and this enhancement can be further strengthened by steeper profile of α particles. This is mainly because the enhancement of helium ash diffusivity by α particles is stronger than that of the effective electron thermal conductivity. Moreover, the higher fraction of T ions, higher temperature ratio between electrons and thermal ions as well as flatter electron density profile, the stronger enhancement of D_He/χ_eff, and α particles further strengthen the favorable effects of these parameters on the removal of helium ash.

Summary of the 10th Conference on Magnetically Confined Fusion Theory and Simulation (CMCFTS)

“…Weixin Guo et al presented the helium ash transport driven by collisionless trapped electron mode (CTEM) turbulence in the presence of alpha (α) particles. They found that although D He /χ eff <1, which was less efficient for the removal of helium ash as compared with ITG mode [20], D He /χ eff could be increased by 50% with the presence of 3% α particles with their density gradient being twice that of electrons [21]. This is mainly because the destabilizing effect of α particles [22] on helium ash diffusivity is stronger than that on electron thermal conductivity.…”

“…Xiaolong Zhu presented the hybrid simulation of the avalanche transport of energetic particles observed in National Spherical Torus eXperiment (NSTX), which was believed to be a strong nonlinear process, leading to significant EP transport. It is found that the existence of multiple modes and the overlapping of their phase-space wave-particle resonance region are crucial for the onset of avalanche EP transport [35].…”