Speaker
Description
The operation of spherical tokamak ST-40 (a =0.27 m; R =0.5 m; b/a =1.4 – 2.0) with high toroidal magnetic fields of up to 2.2 T, plasma currents up to 750 kA, and plasma heating by neutral beam injection (NBI) has exhibited a variety of Alfvén eigenmodes (AEs) excited by the NBI-produced energetic ions. In these experiments, two NBI sources were employed, with the beam energy up to 55 keV and power up to 1 MW in the first NBI, and the beam energy of 25 keV and power 0.8 MW in the second NBI. Both the plasma and the beams were deuterium.
In this talk we will overview results of studies of Alfvén Eigenmodes in high-field high-temperature Spherical Tokamak ST-40, emphasising three main specifics of this device: low aspect ratio R/a <2.0, high temperatures – Ti/Te ~ 10keV/4keV and high, for spherical tokamaks, toroidal field, Bt > 2.0T. Comparison will be given for AEs observed in tokamaks with similar aspect ratio, but much lower Bt: NSTX and NSTX-U, MAST and MAST-U; tokamaks with similar plasma parameters but high aspect ratio: DIIID, AUG, KSTAR; and stellarators LHD and W7X.
Several distinct classes of beam-driven AEs have been identified, with different modes being most unstable in different ST-40 scenarios, observed over a broad frequency range, from TAEs down to BAAEs. Various n RSAEs have also been observed.
For the first time in tokamaks, Alfvén cascade eigenmodes have been observed after magnetic reconnection during merging-compression plasma formation. The observation of RSAEs was allowing an additional assessment of -profile evolution in time and were associated with reversed magnetic shear. Some cases exhibited not only up-sweeping but also down-sweeping ACs suggesting a weak reversed-shear configuration. Such q-profiles have not been predicted by the reconnection theory.
| Presentation type | Oral |
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