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

Beam-driven Alfvén eigenmodes (AEs) have been studied in the TJ-II low-magnetic-shear flexible heliac (B0  =  0.95 T, 〈R〉  =  1.5 m, 〈a〉  =  0.22 m), in L-mode hydrogen plasmas with hydrogen NBI and ECR heating (PNBI  ⩽  1.0 MW, ENBI  =  32 keV, PECRH  ⩽  0.6 MW). In low-density plasmas in the range ne¯  =  (0.3–1.5)  ×  1019 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 50 kHz  <  fAE  <  380 kHz. In experiments in which the vacuum rotational transform ι 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 ι intervals within which the AEs studied are chirping or steady-state is determined.