Alfvén eigenmode properties and dynamics in the TJ-II stellarator

Alfven eigenmodes (AEs) were studied in neutral beam injection (NBI) heated plasmas in the TJ-II stellarator using a heavy ion beam probe (HIBP) in the core, and by Langmuir (LP) and Mirnov probes (MP) at the edge. AEs were detected over the whole plasma radius by the HIBP with a spatial resolution of about 1 cm. AE-induced oscillations were detected in the plasma density ne, electric potential φ and poloidal magnetic field Bpol with frequencies 50 kHz < fAE < 300 kHz. The LP, MP and HIBP data showed a high level of coherency for specific branches of AEs. Poloidal mode wave-vectors kθ, mode numbers m(m < 8) and propagation velocities Vθ ∼ 30 km s−1 were detected for various branches of AEs, having different radial locations. When the density rose due to NBI fuelling, the AE frequency decreased as predicted by the Alfven law . During the AE frequency decay the following new AE features were observed: (i) the poloidal wave-vector kθ and mode number m remained constant, (ii) the cross-phases between the oscillations in Bpol, ne and electric potential remained constant, having an individual value for each AE branch, (iii) Vθ decreased proportional to the AE frequency. The interaction of the AEs with the bulk (thermal) plasma resulted in clearly pronounced quasi-coherent peaks in the electrostatic turbulent particle flux spectra. Various AE branches exhibited different contributions to the particle flux: outward, inward and also zero, depending on the phase relations between the oscillations in Epol and ne, which are specific for each branch. A comparison with MHD mode modelling indicated that some of the more prominent frequency branches can be identified as radially extended helical AEs.