Thermal and electrical analysis of alumina and beryllia coax high-power windows under irradiation

Characteristics of dielectric insulators in vacuum windows of coaxial 10 to 100 MHz transmission lines in high-power steady-state use under irradiation are simulated with respect to electric, nuclear, mechanical, and thermal properties. Neutron fluence ~5×10¹⁸ n/cm² at the window is obtained to be sufficiently small to allow beryllia, but not alumina, to be used as dielectric. In beryllia (10^-3 displacements per atom (dpa) due to irradiation) or in un-irradiated alumina (97.5% purity), the temperature is found to rise by not more than 125°C with maximum stress <140 MPa for 50 kV peak voltage at 60 MHz, provided niobium, titanium or materials with similar thermal expansion coefficients are used in water cooled conductors. The tangential electric field is kept well below the surface discharge limit 2 MV/m by using potential rings together with a sufficiently large inclination angle of the conical ceramic with respect to the radial coaxial direction, but high normal fields exceeding the vacuum breakdown limit are obtained near the potential rings. Abandoning the potential rings and deforming the equipotential lines by shaping the ceramic-conductor joint can reduce both tangential and normal field components below the breakdown limit, which appears to be in agreement with recent voltage test experiments