Inductive lenses for improved magnetic field symmetry in a corrugated theta-pinch coil

The magnetic field distribution of corrugated theta-pinch coils is measured in an analogue experiment (experimental error 0.05%) in order to determine the perturbation of the rotational symmetry induced by the current feed slot.The maximum deviations from rotational symmetry are found near the coil surface around the current feed slot. They have a value of a few per cent and are proportional to the width of the current feed slot. The perturbations decrease towards the coil axis to a few tenths per cent only.The distribution of a locally varying inductance of the current transmission line is computed and applied by moulding inductive lenses into the transmission line plates. The influence of the inductive lenses is measured. Within a radial region of about 1/2 of the coil radius the perturbations are found to be reduced to the level of the experimental error. The residual perturbations outside this region are mainly attributed to the limited space available for the application of the inductive lenses in the transmission line system.