The sensitivity of a diamagnetic signal to several operational and geometrical factors is analysed. Among them are the flux conservation in the plasma, eddy currents induced in the outer structures in fast processes, toroidal shift and deformation of the plasma boundary due to its energy change and inhomogeneity of the confining magnetic field. It is shown that in each case, under proper experimental circumstances, the contribution, unaccounted in the traditional theory of diamagnetic measurements, can reach a level compared with β (ratio of the volume-averaged plasma pressure to the magnetic field pressure). The approach is fully analytical with all relevant dependences shown explicitly, allowing easy estimates and suggesting a resolution of the problem in order to restore the accuracy of finding β from diamagnetic measurements. This essentially extends the analysis by Yamaguchi et al (2006 Plasma Phys. Control. Fusion48 L73) of possible measures to improve the separation of the useful fraction of the measured diamagnetic signal. The approach is aimed at explaining the discrepancies between model estimates and experimental results, unification of a knowledge obtained in separate numerical studies, extending a theoretical basis of magnetic diagnostics and uncovering potential dangers in interpretations. This is also an essential step from traditional cylindrical theory to analytical derivations in the toroidal geometry. The results are equally applicable to tokamaks and stellarators.
Magnetic diagnostics: General principles and the problem of reconstruction of plasma current and pressure profiles in toroidal systems