Transition from chirping to steady NBI-driven Alfven modes caused by magnetic configuration variations in the TJ-Ⅱ stellarator

摘要

Beam-driven Alfven eigenmodes (AEs) have been studied in the TJ-Ⅱ low-magnetic-shear flexible heliac (B_0 = 0.95 T, 〈R〉 = 1.5 m, 〈a〉 = 0.22 m), in L-mode hydrogen plasmas with hydrogen NBI and ECR heating (P_(NBI ≤ 1.0 MW, E_(NBI) = 32 keV, P_(ECRH) ≤ 0.6 MW). In low-density plasmas in the range n_e = (0.3-1.5) × 10~(19) m~(-3), a large variety of AEs have been observed in the plasma core with the heavy ion beam probe diagnostic and Mirnov coils, in the frequency range 50kHz ＜f_(AE) ＜ 380kHz. In experiments in which the vacuum rotational transform t is varied during the shot (dynamic configuration scan), some AEs exhibit changes in their nonlinear evolution from bursting-amplitude AEs with chirping frequency to steady-frequency AEs, and back. The range of t intervals within which the AEs studied are chirping or steady-state is determined.