Diagnostic developments for the study of internal transport barriers

The discovery, in the mid-1990s, of a variety of internal transport barriers (ITBs), capable of lowering transport down to neoclassical values, has been one of the major steps forward in the progress of plasma confinement. These phenomena, which usually show reduced transport together with the suppression of some instabilities, also offer a means to improve our understanding of the underlying processes which control the anomalous transport in toroidal plasma confinement devices. An understanding of ITBs requires extensive and detailed experiments, which imply new challenges for the diagnostic community. The present paper attempts to outline the main advances achieved as well as the prospects for improvement on selected diagnostics oriented towards ITB physics. We will concentrate on the latest achievements in three systems, each of them concerning, simultaneously, several parameters in the growth rate against stabilization/decorrelation processes (electric fields, sharp gradients and turbulence): heavy-ion beam probe (mainly a monitor for electric fields, but also able to provide fluctuation measurements of density and plasma potential), microwave reflectometry (used for turbulence and density profile studies) and a `classical' core diagnostic with capabilities to resolve sharp gradients and detailed structures in the density and temperature profiles, high-resolution Thomson scattering.