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Description
The configuration of reversed shear q(r)-profile is existence for the burst of sawteeth-like events, and the collapse is triggered periodically by the magnetic reconnection of double tearing modes (DTM) [1], with abbreviation of DTRC (double tearing reconnection crash). The excitation conditions of DTRC has strong relationship with the impurity ions, the power threshold of ECRH, the influence of resonant magnetic perturbation (RMP), and the beam direction of NBI, et al. One example is given in Fig. 1, and the energetic ions (neutron yield) is collapse swiftly when the density of copper impurity ions achieves the threshold condition. The internal transport barrier of electron temperature (e-ITB) [2-4] is formed before the DTRC, and the confinement of energetic ions (thermal particles) is improved dramatically, where one kind micro-instability is excited accordingly before the formation of e-ITB (the thermal particles are transport in the ion $B\times\nabla B$ drift direction that is similar to turbulence [5]).
Series of Alfven Eigenmodes are aroused during the oscillation of DTRC after the injection of energetic ions, i.e. Beta-induced Alfven Eigenmodes (BAEs), Reversed Shear Alfven Eigenmodes (RSAEs) and Beta-induced Alfven-Acoustic Eigenmode (BAAE) as shown in Fig. 2. The BAAE coexists and possesses similar characteristics as the pair of BAEs-RSAEs, e.g. the radial structures of BAAE and BAEs are deformed into the analogous triangle shapes along poloidal direction, and the frequencies of BAAE and RSAEs are sweeping upward synchronously for the decreasing of $q_{min}$ ($q_{min}\leq1$), and the mode structure of BAAE is achieved experimentally with m = 4/n $\approx$ 4. The excitation conditions of BAAE in EAST are also discussed in Fig. 3, and the BAAE is damped easily for $\tau_o \leq$ 150 ms, where the $\tau_o$ is the period of DTRC oscillation. It needs to be stressed that the gradient of electron temperature at vicinity of $q_{min}$ and the portion of energetic ions in central region are increasing with $\tau_o$ [4], and the BAAE should be excited by the special distribution of energetic ions, where the gradient of electron temperature might reduce the excitation threshold of BAAE.
Reference
[1]. Chang, Z., et al. "Off-Axis Sawteeth and Double-Tearing Reconnection in Reversed Magnetic Shear Plasmas in TFTR." Physical Review Letters 77(17): 3553 (1996).
[2]. Ming Xu, et al. "Characteristics of off-axis sawteeth with an internal transport barrier in EAST." Nuclear Fusion 59(8): 084005 (2019).
[3]. Ming Xu, et al. “Excitation of Alfven Eigenmodes and Formation of ITB during off-axis Sawteeth in EAST”, Oral on 16th IAEA technical meeting on Energetic Partciles.
[4]. Ming Xu, et al. " Excitation of Beta-induced Alfven-Acoustic Eigenmode during Sawteeth-like Oscillation in EAST." Accepted in Nuclear Fusion.
[5]. Brower, D. L., et al. "Multichannel Scattering Studies of the Spectra and Spatial Distribution of Tokamak Microturbulence." Physical Review Letters 54(7): 689-692 (1985).
| Affiliation | Institute of Plasma Physics, Chinese Academy of Sciences |
|---|---|
| Country or International Organization | China |
