Speaker
Description
The behavior and confinement of fusion-born alpha particles are critical to the performance of magnetized deuterium-tritium (DT) plasmas. These alpha particles, produced in DT fusion reactions, emerge with energies primarily around 3.5 MeV, forming a narrow energy distribution. Initially, this results in an inverted velocity distribution, which can lead to the development of instabilities that affect their confinement[1]. One significant type of instability, occurring in the ion cyclotron frequency range, involves compressional Alfvén eigenmodes (CAEs)[2]and global Alfvén eigenmodes (GAEs)[3]. These high-frequency modes are of particular concern because they could potentially transfer energy directly from the fusion-born alpha particles to the bulk plasma fuel ions.
High frequency Alfvén eigenmodes in the ion cyclotron frequency range excited by ICRF heated ions are actively researched on the EAST tokamak[4]. As the ratio of hydrogen and deuterium (H/D) influence the ICRF power deposition ratio on minority hydrogen and the dominant deuterium, the mode excited by hydrogen and deuterium had been observed with different ratio of H/D[5]. With higher H/D (> $\sim$ 5%), the mode generally exited by hydrogen had been observed; with middle value of H/D (>$\sim$ 3%-5%), the mode normally exited by hydrogen and deuterium had been observed simultaneously; with lower H/D (< $\sim$ 3%), the mode usually exited by deuterium had been observed. We also found that the confinement of the plasma does influence the intensity of the high frequency mode. The general properties of this particular modes types are: (a) the mode is sub-cyclotron with the frequency $\omega \sim $0.65-0.8 $\omega_{ci}$ , where $\omega_{ci}$ corresponds to the on-axis cyclotron frequency of the fast ions; (b) small fluctuation of plasma density has a large influence of mode frequency which satisfied $f\sim n_e^{-1/2}$; (c) the high frequency Alfvén eigenmodes are always accompanied by sub-cyclotron modes in several experiments at EAST, consistent with the mode detected on other conventional tokamaks, namely ASDEX Upgrade[6] and DIII-D[7]. We also found that the excited location of high frequency Alfvén eigenmode driven by RF-accelerated minority hydrogen ions remains unchanged even though the cyclotron resonance location of hydrogen ions varies with $B_t$.
1. Kolesnichenko, Y.I., The Role of Alpha-Particles in Tokamak Reactors. Nuclear Fusion, 1980. 20(6)
2. Mahajan, S.M., D.W. Ross, and G.L. Chen, Discrete Alfven Eigenmode Spectrum in Magnetohydrodynamics. Physics of Fluids, 1983. 26(8)
3. Ochoukov, R., et al., Analysis of high frequency Alfvén eigenmodes observed in ASDEX Upgrade plasmas in the presence of RF-accelerated NBI ions. Nuclear Fusion, 2023. 63(4).
4. Liu, L.N., et al., Observation of high frequency Alfvén eigenmodes excited by RF-accelerated minority hydrogen ions in the EAST tokamak. Nuclear Fusion, 2024. 64(12).
5. Zhang, W., et al., Influence of ICRF-NBI synergy on fast ion distribution and plasma performance in second harmonic heating experiments with deuterium NBI at EAST. Nuclear Fusion, 2023. 63(5).
6. Ochoukov, R., et al., High frequency Alfven eigenmodes detected with ion-cyclotron-emission diagnostics during NBI and ICRF heated plasmas on the ASDEX Upgrade tokamak. Nuclear Fusion, 2020. 60(12).
7. DeGrandchamp, G.H., et al., Mode structure measurements of ion cyclotron emission and sub-cyclotron modes on DIII-D. Nuclear Fusion, 2022. 62(10).
| Presentation type | Oral |
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