Radiation-induced increase of the dielectric loss tangent ( tanδ ) is a major problem for the realization of mm-wave windows in Electron Cyclotron wave systems in next generation fusion devices. In oxyde materials with dominant ionic bonding character, such as Al2O3 or MgAl2O4, increased loss levels are observed after neutron irradiation which can be related to displacement damage. Few experiments have been published so far on the transient effects effective only under irradiation, but at least for purely ionizing radiation, such as X-rays, none of such in-beam effects have been observed. A different situation can be expected for alternative window materials with a homopolar lattice, where contributions from free charge carriers to the dielectric loss dominate those from the ionic lattice polarization.
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