High-frequency vibrations of coupled tearing modes in a tokamak plasma

The only coherent structures normally seen on the magnetic perturbation (B.) spectra of ohmically heated tokamak plasmas are at the rotation frequencies, Omega mn, of low-helicity tearing modes (Mirnov oscillations), i.e. B=B0 sin(m theta +n phi + Omega mnt) where m,n are small integers. We describe a new coherent structure, at much higher frequencies, which is shown to arise from small amplitude ( Delta theta , Delta phi <<2 pi ) motion of the edge 3/1 tearing mode. A model which includes toroidal coupling between the 3/1 mode and an interior 2/1 mode, as well as coupling to an externally applied (or stray) resonant field, is able to explain the observations. The inter-mode coupling causes rotation of both magnetic mode structures at the same (Mirnov) frequency but also high-frequency difference oscillations of small amplitude. In addition, bouncing of the combined 2/1 and 3/1 mode structure (or the 3/1 alone) in the potential well of the external field can occur after locking. The 'vibration' or 'bounce' frequencies are the only discrete features on the magnetic, B., spectrum after suppression of the Mirnov frequency by forced mode locking and therefore provide a potential diagnostic for studying precursor growth in locked-mode major disruptions.