Radial Localization of Alfven Eigenmodes and Zonal Field Generation

GTC gyrokinetic particle simulations of DIII-D discharge #142111 near 525ms find a radial localization of toroidal Alfven eigenmode (TAE) due to the modification of the MHD mode structure, i.e., nonperturbative contribution by energetic particles (EP). The EP-driven TAE has a radial mode width much smaller than that predicted by the MHD theory. The TAE radial position peaks at and moves with the location of strongest EP pressure gradients. Experimental data confirms that the eigenfunction drifts quickly outward in the radial direction. The EP contribution also breaks the radial symmetry of the ballooning mode structure and induces a dependence of the TAE frequency on the toroidal mode number, in excellent agreement with the experimental measurements. GTC nonlinear simulation finds that zonal fields (zonal flows and zonal currents) are driven by TAE mode coupling (passive generation). The growth rate of the zonal fields is almost twice of the TAE growth rate. Zonal flows induce TAE saturation without EP profile relaxation, while zonal currents have little effects on the TAE saturation. Furthermore, GTC nonlinear simulations of beta-induced Alfven eigenmode (BAE) show that the mode frequency and intensity have a fast chirping due to the radial variations of mode amplitude and guiding center dynamics. Zonal fields localized near mode rational surface are generated by BAE. The effects of zonal fields on BAE depend on the growth rate of BAE instability.