Speaker
Description
The relationship between the excitation of reversed shear Alfv\'en eigenmodes (RSAEs) and the formation of internal transport barrier (ITB) have been investigated in the EAST tokamak. A series of discharge conditions with the minimum safety factor, $q_{min} \approx 2$, have been established through the synergy effect of lower hybrid current drive (LHCD) and electron cyclotron resonance heating (ECRH). Under varying power conditions, the excitation of RSAEs instability is characterized by the presence of modes with lower toroidal mode numbers, typically $n \geq$ 1. The radial extent of RSAEs instability attains its peaks within the range of 1.98 $\leq R \leq$ 2.07 $m$ on the lower field side (LFS), corresponding to a normalized minor radius interval of 0.2 $\leq \rho \leq$ 0.4. The excitation of RSAEs instability is followed by an observed redistribution of fast ions, with the density predominantly increasing in the central region enclosed by the $q_{min}$.
The formation of the ITB is accompanied by the excitation of RSAEs instability, particularly under conditions where the minimum injection power of neutral beam injection (NBI) is approximately 4 $MW$. Following the ITB formation, the normalized ion temperature gradient scale length, $R/L_{T_i}$, is observed to be greater than or equal to 10, provided that the minimum safety factor, $q_{min}$, is approximately 2. Three effects of magnetic shear, flow shear and fast ions are analyzed. (1) One type of micro-instability is observed before the formation of ITB, characterized by the transport of thermal particles, is excited concurrently with the upward sweeping frequency of the RSAEs instability. The modification of the $q$-profile can be detected by the slight displacement of the inner $q$ = 2 surface ($\Delta \rho \approx$ 5%) of double tearing modes (DTM), and the suppression of micro-instability is noted upon altering the magnetic shear. (2) Negative radial Electric fields ($E_r$) are observed at the position of $q_{min} \approx$ 2, and the $E \times B$ shear rate significantly increases after the formation of ITB. (3) The phenomenon of coupled thermal particles on fast ions is observed following the suppression of the micro-instability, and it is noteworthy that the confinement of thermal particles is enhanced as the density of fast ions increases. Furthermore, the electron-ion-temperature ratio $\tau=T_e/T_i$ decreases dramatically, and larger Shafranov shift is also observed after the formation of ITB.
| Presentation type | Oral |
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