Large changes in visible light and X-ray emission are observed when an injected pellet of solid D2 crosses the q = 1 surface of the JET tokamak. An explanation of this observation is offered and it is shown that this resonance effect can be used to follow the position of the q = 1 surface during a sawtooth cycle. A new method of determining the magnetic shear on the q = 1 surface is presented. Very low values of shear are found and the implications for theories of the sawtooth instability are discussed.
Sawtooth modulation of the poloidal field in TEXTOR under ohmic heating conditions