Speaker
Description
Ion cyclotron resonance heating (ICRH) is a key auxiliary heating method for fusion reactors[1], supporting multiple heating schemes including minority, harmonic, and three-ion heating. These schemes generate fast ions that can efficiently drive Alfvén eigenmodes (AEs)[2], which can induce fast-ion redistribution or loss, potentially impacting plasma-wall interactions and first-wall components.
EAST has studied high-$\beta_p$ discharges ($B_t$ = 2.5 T, $I_p$ = 400 kA), which are characterized by a high fraction of bootstrap current, excellent confinement properties, and the potential for non-inductive steady-state operation, and are considered promising candidates for future fusion reactors[3], using pure radio-frequency heating ($P_{\rm ECRH}$ = 2 MW, $P_{\rm LHW}$ = 2 MW, $P_{\rm ICRH}$ = 2–4 MW) with hydrogen minority heating (H/(H+D) $\sim 4\%$) to drive core-localized TAE (n = 3)[4] and BAE (n = 2). Investigating fast-ion-driven AEs and their impact on confinement in this regime is highly relevant for the development of reactor-relevant plasmas.
The core fast-ion beta, $\beta_f = 2\mu_0 p_f/B^2$, is identified experimentally as a key parameter for the excitation of AEs. The experimental results indicate the existence of a finite $\beta_f$ threshold for AE destabilization, with the TAE threshold higher than the BAE threshold, i.e. $\beta_{f,\mathrm{TAE}}^{\mathrm{thre}} > \beta_{f,\mathrm{BAE}}^{\mathrm{thre}}$. The AE amplitude decreases as $\beta_f$ is reduced and eventually vanishes when $\beta_f$ falls below the corresponding threshold.
Experimentally, when the fast ions have sufficient energy to resonate with TAE, they tend to preferentially destabilize TAE rather than BAE. BAE is destabilized only when the fast-ion drive is insufficient to excite TAE, suggesting a competition between TAE and BAE for the available fast-ion free energy.
The injection of NBI beams is observed to have a stabilizing effect on both TAE and BAE. Since the beam energy is relatively low, $E_{\rm NBI} \sim 55~\mathrm{keV}$, the injected beam ions may contribute more to damping than to resonant drive for these modes. In addition, second-harmonic deuterium heating can compete with hydrogen minority heating for the absorbed ICRH power. Numerical results show that the power fraction absorbed by deuterium is only about 4%, but this can still reduce the fast-hydrogen-ion drive. From this perspective, injecting relatively low-energy NBI beams to enhance damping relative to fast-ion drive may provide an effective approach for mitigating AE activity.
No clear degradation of fast-ion or bulk plasma confinement is observed for these core-localized AEs. Under the current long-pulse high-$\beta_p$ operating scenarios, these AEs appear to have little impact on steady-state plasma operation.
[1] M. Salewski et al., Nucl. Fusion 65, 043002 (2025).
[2] F. Zonca and L. Chen, Phys. Plasmas 21, 072120 (2014).
[3] S. Ding and A. M. Garofalo, Rev. Mod. Plasma Phys. 7, 4 (2023).
[4] C.Y. Pan et al., Nucl. Fusion 66, 016026 (2026).