Electron Cyclotron Resonance (ECR) plasmas with high density and high temperature are required by the injectors for the Accelerators and by interdisciplinary studies in Astrophysics and Nuclear Astrophysics. Understanding and further developing magnetic traps requires accurate measurements of the physical properties of the confined plasma, such as density, temperature and ionization state, not allowed by the commonly used diagnostic methods (imaging, low resolution spectroscopy). Here, we show the potentiality of a few diagnostic applications of high resolution spectroscopy in the visible range, routinely used in Astrophysics to measure stellar physical parameters such as effective temperature, surface gravity, chemical abundances. In addition, we show that polarimetry is a powerful technique to derive the morphology of stellar magnetic fields, whose knowledge is necessary for a correct interpretation of spectra from magnetized plasmas. An application of these non-invasive methods to B-min ECR plasma concerning optical emission is discussed in view of a better understanding of the plasma structure, magnetic confinement properties and heating processes.