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

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)) over bar = (0.3-1.5) x 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 50 kHz < f(AE) < 380 kHz. 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 steadyfrequency AEs, and back. The range of similar to intervals within which the AEs studied are chirping or steady-state is determined.