Observations on turbulence and beam-ion driven modes in TEXTOR

Abstract

At a sufficiently weak toroidal magnetic field, neutral beam injection heated, limited plasmas in the TEXTOR tokamak exhibit bursts of beam-ion driven 'fishbone' and Alfvén modes, which are characterized for the first time using the multi-antenna reflectometer and Mirnov coils. In H-mode the fishbone triggers edge-localized modes (ELMs) and in L-mode it triggers previously unobserved bursts of particle recycling, resembling the ELMs. The reflectometer phase shows statistically significant bispectral coherence between the fishbone and the Alfvén modes, indicative of non-linear coupling between them. Additionally, using conditional averaging techniques, two ELM precursor modes are found that are not related to the beam ions. The first is a coherent mode with toroidal mode number n = −2, which is also seen with the Mirnov coils. Bispectral analysis of the reflectometer signals shows that this mode modulates the amplitude of broadband turbulence in the pedestal. The second ELM precursor is a semi-coherent, down-chirping mode with a poloidal wavelength of 6 cm.