Influence of plasma shaping on the parity of core-localized toroidal Alfvén eigenmode in an advanced tokamak configuration

TL;DR

This work tackles how plasma shaping and magnetic shear influence the parity of core-localized TAEs/EPMs driven by energetic particles in a CFETR-like tokamak. It employs a hybrid kinetic-MHD framework implemented in NIMROD, with EFIT and CHEASE equilibria, to perform parametric scans over $q_{\min}$, $\beta_h$, and elongation. The main findings show a robust dominance of odd-parity, anti-ballooning structures in weak/reversed shear with finite elongation, and a shaping-induced transition to even-parity ballooning TAEs as elongation approaches circularity, while $q_{\min}$ and $\beta_h$ have limited impact on parity. These results have practical implications for designing advanced tokamaks by leveraging plasma shaping to control AE parity and stability in burning-plasma regimes.

Abstract

Toroidal Alfvén eigenmodes (TAEs) and energetic particle modes (EPMs) can both be excited by energetic particles from auxiliary heating and fusion-born alpha particles in a tokamak. Using the hybrid kinetic-MHD model implemented in the NIMROD code, the excitation of these modes and their properties are investigated in an advanced tokamak configuration with reversed magnetic shear in the core region. The dominant TAE/EPM is found to exhibit odd parity with an anti-ballooning structure when the plasma has elongated, non-circular two-dimensional shaping. As the plasma shaping becomes more circular with reduced elongation, the mode parity undergoes a transition to even parity accompanied by a ballooning structure. These results may help explain the dominant parity of TAE/EPMs observed in advanced tokamak configurations with different plasma shaping.

Figures (12)

• Figure 1: (a) Contour plot of equilibrium poloidal flux in (R, Z) coordinate. The red curve represents the last closed flux surface (LCFS). (b) The mesh grid of flux coordinates used in the calculation. The blue lines represent the constant poloidal fluxes, and the red lines the poloidal angles. (c) and (d) are the 1D radial profiles of safety factor and pressure. The radial coordinate $\rho$ represents the square root of the normalized poloidal flux, and the minimum value of the $q$ profile is specified as $q_{\text{min}} = 2.37$. The equilibrium is based on the CFETR case eq5_2
• Figure 2: The dependences of the linear TAE/EPM (a) frequency and (b) growth rate on the toroidal mode number.
• Figure 3: Contours of perturbed normal velocity component from NIMROD simulations for toroidal mode number: (a) n=1, (b) n=2, (c) n=3, (d) n=4, (e) n=5 and (f) n=6.
• Figure 4: (a) Radial profiles of two most dominant poloidal Fourier components with toroidal mode number $n=3$ from NIMROD simulation, and (b) the corresponding Alfvén continuum calculated using GTAW.
• Figure 5: The dependences of (a) frequency and (b) growth rate on minimum safety factor $q_{\text{min}}$ for the toroidal mode number $n=3$, and contours of perturbed normal velocity component from NIMROD simulations for (c) $q_{\text{min}}=2.15$ and (d) $q_{\text{min}}=2.50$ modes.