Speaker
Description
Recurrent bursting Alfvénic instabilities, accompanied by a rapid and violent release of stored free energy, are deleterious to the plasma confinement and threaten vacuum vessel integrity. Unlike beam-driven Alfvén eigenmodes (AEs), which often exhibit bursting behavior [1], AEs during ion cyclotron resonance frequency (ICRF) heating typically maintain steady amplitudes [2,3], despite theoretical predictions suggesting the possibility of a bursting state [4,5]. This work reports the first comprehensive simulations of ICRF-induced AEs with fully relaxed minority ion distributions, where shear Alfvén wave (SAW) induced minority ion transport is self-consistently included during the tail formation. Bursting ICRF-induced AEs are obtained in multi-$n$ and single-$n$ simulations for a plasma with a low magnetic field $B_0=1.5\,\rm{T}$ and an ICRF resonance layer located at the magnetic axis, where $n$ is the toroidal mode number. The maximum minority ion beta in the non-bursting case reaches almost double that of the busting case. The results also show that resonance layer position and magnetic field strength play a decisive role in the occurrence of bursting ICRF-induced AEs, outweighing factors like RF power and multi-n interactions [6]. An increase in magnetic field strength can avoid the bursting of ICRF-induced AEs with a reduced minority ion transport, which benefits burning plasmas operating in strong magnetic fields.
Fig. 1 shows the bursting [top] and non-bursting [bottom] ICRF-induced AEs in a plasma with a low toroidal field $B_0=1.5\,\rm{T}$, with ICRF resonance layers located at the magnetic axis and located at the ρ⁄a=0.4 of the outer equatorial plane, respectively. The absorbed RF power is $6\,\rm{MW}$. A bursting mode is observed during the continuous hybrid simulation phase [$E_{kin}$ evolutions in blue] in the on-axis heating case. In these simulations, dominant harmonics with $𝑛 \le 8$ are retained. It should be noted that the ICRF-induced bursting mode can even be reproduced by considering only a single toroidal harmonic, which is very different from the results of beam-induced bursting mode [6]. The significant redistribution of minority ions can be noticed in both cases, indicating the necessity of including AE-induced transport in evaluating the ICRF heating. A much higher minority ion beta is achieved in the non-bursting case, but the stored minority ion energy is comparable between these two cases. The triggering mechanism of the bursting event will be presented at the conference. With a higher magnetic field $B_0=3.0\,\rm{T}$, the bursting AEs during on-axis heating will turn into a non-bursting state. The modes remain non-bursting even at a high RF power of $18\,\rm{MW}$.
References
[1] Wong K L, Fonck R J, Paul S F, et al. Phys. Rev. Lett., 66, 1874 (1991).
[2] García-Muñoz M, Hicks N, Van Voornveld R, et al. Phys. Rev. Lett., 104, 185002 (2010).
[3] Kazakov Y O, Ongena J, Wright J C, et al. Nat. Phys., 13, 973 (2017).
[4] Berk H L, Breizman B N, Pekker M. Phys. Rev. Lett., 76,1256 (1996).
[5] Lilley M K, Breizman B N, Sharapov S E. Phys. Rev. Lett., 102, 195003 (2009).
[6] Bierwage A, Shinohara K, Todo Y, et al. Nat. Commun., 9, 3282 (2018).
| Presentation type | Oral |
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