Speaker
Description
In magnetically confined fusion plasmas, shear Alfvén waves (SAWs) can be resonantly excited by energetic particles (EPs), and in turn, induce EP anomalous transport loss across magnetic surfaces, resulting in plasma performance degradation and possibly damage of plasma facing components. With the EP anomalous transport rate determined by the amplitude and spectrum of the SAW instabilities, it is necessary to investigate the nonlinear evolution and saturation process of SAW instabilities. Refs. [1,2] investigated the nonlinear saturation of toroidal Alfvén eigenmode (TAE) via ion induced scattering in uniform plasmas and obtained the saturation amplitude for TAE and the corresponding EP transport coefficient. In ref. [3], the three-wave parametric decay process of TAE in nonuniform plasmas is investigated and effects of plasma nonuniformity are analyzed, which show both quantitative and qualitative differences with uniform plasma results. Most importantly, the spontaneous decay condition is changed from frequency downward cascading to the normal cascading of toroidal mode number, which is quite similar with the cascading behavior of drift wave turbulence.
In this work, based on the parametric dispersion relation obtained in ref. [3], the wave-kinetic equation for multi-TAEs interaction in nonuniform plasmas is firstly derived. Following the approximation treatment in ref. [4], the nonlinear spectrum evolution equation in poloidal wavenumber space is then yielded. By fixed point method, the nonlinear saturation spectrum can be solved numerically, while analytically, the scaling of this saturation spectrum can be derived both in long-wavelength and short-wavelength limits. On the other hand, solving the spectrum evolution equation as an initial value problem, the evolution process of spectrum intensity in poloidal wavenumber space and the final saturation spectrum are recovered, as shown in figure 1. With the saturation spectrum solved above, the corresponding magnetic perturbation is estimated as \delta B_r/B_0~10^{-4}, which shows lower saturation amplitude than the uniform plasma result [2], as expected.
References:
[1] Hahm T S, Chen L., Physical review letters, 1995, 74(2): 266.
[2] Qiu Z, Chen L, Zonca F., Nuclear Fusion, 2019, 59(6): 066024.
[3] Cheng Z, Shen K, Qiu Z, Nuclear Fusion, 2024, 64(6): 066031.
[4] Hahm T S, Tang W M., Physics of Fluids B, 1991, 3(4): 989.
| Presentation type | Oral |
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