Speaker
Description
Fast-ion driven Alfvén Eigenmodes (AEs) are observed during ICRF heated high-βp plasmas on EAST since AEs quiescence was discussed in [1]. Multiple high frequency modes are observed for shot 112670 at fTAE1 ~ 145 kHz, n = 3, δB/B ~ 4× 10-4 and fTAE2 ~ 175 kHz, n = 4, δB/B ~ 1.2×10-5 measured by high-frequency mirnov coil, at B0 ~ 2.45T, Ip ~ 350kA , n ~ ramp-up from 3.5× 1019m-3 to ~ 4× 1019m-3 at PECRH ~ 2 MW, PLHW ~ 2 MW, and PICRH ~ 2.1 MW. Here, 37MHz ICRF minority hydrogen heating scheme, located at plasma core, creates fast ion population which drives TAEs unstable from TRANSP+TORIC. In the experiment, it is observed that the evolution of mode frequency depends on electron density, following the theoretical TAE frequency characteristic fth = VA/(4πqR)∝(ne)1/2 [2]. As the electron density increases, the amplitude of the AE mode progressively diminishes. At higher plasma densities, the slowing-down time of fast ions is decreased, reduce the ICRF absorption power, and weaken the high-energy ion tail, thereby it reduces fast-ion beta βf. According to reflectometer diagnostic, both TAE modes are located at a radial position at approximately ρ ∼ 0.2. The gyro-fluid code FAR3d[3] are applied to study the properties of these instabilities, including the effect of the acoustic modes, EP finite Larmor radius damping effects. The simulation indicates a narrow TAE gap in the inner plasma region around ρ ~ 0.2. The simulation indicates a narrow TAE gap in the inner plasma region around ρ ~ 0.2, showing a consistence between the measurements and the frequency range of the dominant modes. It is also found that the linear growth rate of the n = 4 TAE is higher than that of the n = 3 TAE. The result shows the characteristic of βf profile, as the main driver of AE instability, is peaked at ρ ∼ 0.2 which is the location of ICRF power deposited.
The threshold of ICRF power to excite AEs is studied in the experiments attributing to enough βf to drive TAE instability. A further study for the impact of ICRF resonance layer position shows that when Bt is increased from 2.51T to 2.56T, TAE activities gradually disappear, attributed to the reduced βf with the ICRF power deposited outwards. In addition, to investigate the impact of the synergetic effect between NBI and ICRF on ICRF driven TAEs, the experiment shows a stabilizing effect of beam ions that TAE activity are mitigated. The changes in kinetic profiles and fast-ion velocity distribution can impact ICRF tail and TAEs.
t-ions, due to different fast-ion velocity distribution.
| Presentation type | Oral |
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