Optical materials and coatings are exposed to the flux of energetic particles when used in either space applications or nuclear energy plants. The study of their behavior in such an environment is important to avoid failure of the optical components during their operation. The optical performance of several thin-film materials (${\mathrm{HfO}}_2$, ${\mathrm{Ta}}_2{\mathrm{O}}_5$, ${\mathrm{Nb}}_2{\mathrm{O}}_5$, ${\mathrm{TiO}}_2$, ${\mathrm{SiO}}_2$) and coatings, under irradiation with high-dose gamma rays (5.8 MGy) and exposure to low-energy (60 keV) protons, has been investigated. Some variations of optical properties have been detected in silicon oxide after irradiation, while the other materials are stable in such conditions.

We measure current-voltage characteristics as function of magnetic field and temperature in Nb strips of different thickness and width. The instability voltage of the flux-flow state related to the vortex critical velocity v$\ast${} is studied and compared with the Larkin-Ovchinnikov theory. Beside the usual power-law dependence v$\ast${}$\approx${}B$-${}1/2, in the low-field range a new crossover field, Bcr1, is observed below which v$\ast${} decreases by further lowering the external magnetic field B. We ascribe this unexpected crossover to vortex channeling due to a fanlike penetration of the applied magnetic field as confirmed by magneto-optic imaging. The observation of Bcr1 becomes a direct evidence of a general feature in type-II superconducting films at low fields that is a channel-like vortex motion induced by the inhomogeneous magnetic state caused by the relatively strong pinning.

Fundamental and higher harmonics of the ac magnetic susceptibility have been measured on ${\text{LaFeAsO}}_{0.92}{\text{F}}_{0.08}$ samples as a function of the temperature, at various amplitudes and frequencies of the ac magnetic field, with a small superimposed dc field parallel to the ac field. The granularity of the samples has been investigated and the intergrain and intragrain contributions have been clearly individuated looking at both the first and the third harmonics. The vortex dynamics has been also analyzed, and a comparison with the magnetic behavior of both the ${\text{MgB}}_2$ and the cuprate superconductors has been performed. Some vortex dissipative phenomena, i.e., the thermally activated flux flow and the flux creep, have been detected in the presented experimental data, similar to what have been obtained on YBCO. Nevertheless, although the general behavior is similar, several differences have been also evidenced between these different classes of superconductors, mainly in the third harmonics. We infer that different vortex dynamics has to be included into the analysis of the magnetic response in this iron-based material.

The magnetic relaxation has been investigated in type-II superconductors when the initial magnetic state is realized with entrapped and shielded flux contemporarily. This flux state is produced by an inversion in the magnetic field ramp rate due to, for example, a magnetic field overshoot or undershoot. The investigation has been faced both numerically and by measuring the magnetic relaxation in BSCCO tapes. Numerical computations have been performed in the case of an infinite thick strip and of an infinite slab, showing a quickly relaxing magnetization in the first seconds. As verified experimentally, the effects of the overshoot (or the undershoot) cannot be neglected simply by cutting the first $10–100\mathrm{s}$ in the magnetic relaxation. On the other hand, at very long times, the magnetic states relax toward those corresponding to field profiles with only shielded flux or only entrapped flux, depending on the amplitude of the field change with respect to the full penetration field of the considered superconducting samples. In addition, we have performed numerical simulations in order to reproduce the relaxation curves measured on the BSCCO(2223) tapes; this allowed us to interpret correctly also the first seconds of the $M\left(t\right)$ curves.

The flux lines dynamics have been investigated on MgB2 bulk superconductors obtained by reactive liquid infiltration by measuring the ac magnetic susceptibility. The fundamental and third harmonics have been studied as a function of temperature, dc magnetic field, ac field amplitude and frequency. In order to determine the dynamical regimes governing the vortex motion, the experimental results have been compared with susceptibility curves obtained by numerical calculations of the non-linear diffusion equation for the magnetic field. The frequency behaviour of the third harmonic response, that cannot be explained by frequency dependent critical state models, has been related to the current dependence of the flux creep activation energy U(J) in the diffusion coefficient. In this way we have shown that the measured curves are correctly interpreted within the framework of a vortex glass description.