Pre- and post-irradiation studies on mm-wave losses in reference window materials for electron cyclotron wave systems

Abstract

Dielectric materials for transmission windows in electron cyclotron (EC) wave systems are studied for the radiation effects on their dielectric properties (permittivity, loss tangent). Based on comparative neutron irradiation experiments performed at cryogenic and reactor pool temperatures, it is shown here that fast neutron fluences up to at least 2–4 × 10 20 n/m 2 ( E > 0.1 MeV) do not critically affect the mm-wave losses in HEMEX grade sapphire, the reference material for the cryogenically cooled high power EC window. Fast neutron (10 20 n/m 2) and electron (5 × 10 −6 dpa) irradiations on specially developed CVD diamond grades for conventionally cooled windows demonstrate acceptable mm-wave loss levels, whereas loss at lower frequencies can be significantly affected. In-beam measurements under X-ray irradiation show no additional loss terms in sapphire and CVD diamond, at least up to 0.45 Gy/s. Dielectric data obtained for synthetic quartz indicate that the observed changes due to neutron damage are not severe for fast neutron fluences at least up to 10 22 n/m 2.