Current density measurement in Tokamak-type devices by longitudinal Thompson scattering: a proposal

A method is proposed for measuring the local current density in Tokamak-type devices by means of Thompson scattering of ruby laser light with the scattering k-vector parallel to the toroidal direction. The typically rather small shift of the profile ( approximately 10% of the width) may be determined by suitable summation of the signal observed in six to eight spectral channels. Questions of accuracy involving line-shape, photon statistics and channel calibration are discussed, and a null method is proposed for the last of these. A direct readout proportional to the current density may be obtained with accuracy better than about 10%.