Determination of the shear on the q = 1 surface of the JET tokamak

Abstract
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.