Diagnostic of the current density profile in the vicinity of q = 1: The Hα emission of ablated pellets

The strong striation observed in large tokamaks when a pellet crosses the q=l surface has been shown to be due to an almost shearless annular region and to be extremely sensitive to the details of the magnetic structure. Therefore, if this q-profile exists before pellet injection, analysis of the shape of the striation can be a powerful diagnostic of the local current density distribution. The paper presents a method of precisely calculating the width, depth and shape of the striation for an arbitrary pellet trajectory throughout a discharge. Different mechanisms that are possibly responsible for the flattening of q in the vicinity of q=l (e.g. turbulence or magnetic reconnection) are discussed and it is shown that pellet induced perturbation is not likely to produce such a feature (which confirms its interest as a diagnostic). A simple parameterization of plasma rotation and of a magnetic island is introduced and the model is applied in a simulation of striations displaying important profile distortions even for shallow current perturbations, which could therefore be detected, demonstrating the strong sensitivity of the method. At present, there are no sufficiently documented experimental data, but the available diagnostics on TORE SUPRA, JET, TFTR and JT-60 could be used to study systematically the q=l structure with this method.