Magnetic diagnostics: General principles and the problem of reconstructionof plasma current and pressure profiles in toroidal systems

Restrictions on magnetic diagnostics are discussed. Being related to the in tegral nature of the measurable quantities, these follow from the fundament al laws of electromagnetism. A series of examples demonstrating the strengt h of these restrictions is analysed. The general rule is emphasized that in formation obtained from external magnetic measurements is insufficient for reliable evaluation of plasma current and pressure profiles in tokamaks and in stellarators. The underlying reason is that outside the plasma the self -held of the equilibrium plasma currents is determined by the boundary cond itions on the plasma surface alone.