HIGH-FREQUENCY VIBRATIONS OF COUPLED TEARING MODES IN A TOKAMAK PLASMA

The only coherent structures normally seen on the magnetic perturbatio n (B) spectra of ohmically heated tokamak plasmas are at the rotation frequencies, OMEGA(mn), of low-helicity tearing modes (Mirnov oscillat ions), i.e. B = B0 sin(mtheta + nphi + OMEGA(mn)t) where m, n are smal l integers. We describe a new coherent structure, at much higher frequ encies, which is shown to arise from small amplitude (DELTAtheta, DELT Aphi << 2pi) motion of the edge 3/1 tearing mode. A model which includ es 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 ro tation of both magnetic mode structures at the same (Mirnov) frequency but also high-frequency difference oscillations of small amplitude. I n addition, bouncing of the combined 2/1 and 3/1 mode structure (or th e 3/1 alone) in the potential well of the external field can occur aft er locking, The 'vibration' or 'bounce' frequencies are the only discr ete features on the magnetic, B, spectrum after suppression of the Mir nov frequency by forced mode locking and therefore provide a potential diagnostic for studying precursor growth in locked-mode major disrupt ions.