Euclid Techlabs LLC, in collaboration with JLab and Fermilab, has developed a new ceramic material with a finite DC electrical conductivity combined with a low RF loss tangent for use in high power coupler windows. The goal of the project was to develop windows with a loss tangent not exceeding that of alumina but with significantly increased DC conductivity for effective electrical discharge. Several SRF coupler windows operating in the 650 MHz and 1.5 GHz frequency ranges were fabricated and successfully tested at high power.
Euclid developed magnesium titanate ceramic elements with relative dielectric constant ε=15.2, a figure of merit, Q×f, in the range of 60,000–125,000 GHz, providing tan δ ~5.2×10-6 - 2.1×10-5 at 650 MHz, and increased conductivity from 10-12 S/m to 10-9 S/m correspondingly. This ability to tune the conductivity will allow the selection of the ideal combination of loss tangent and conductivity required to allow a window to effectively discharge any deposited charge.
Two 1.5 GHz waveguide window assemblies were fabricated using a tin-silver-titanium-magnesium active solder produced by S-Bond. Both were successfully tested at high power in vacuum up to 12 kW CW power, which was the limit of the klystron in travelling wave mode. The maximum temperature recorded on Window 1 was approximately 92°C, and on Window 2 was approximately 78°C. There was no evidence of multipacting or sparking during the high power test of the waveguide windows, or inspection afterward.
A 650 Mhz coaxial window assembly was fabricated using the same active solder as the waveguide window assemblies. The conductive ceramic coupler assembly was tested at Fermilab in conjunction with a spare alumina window coupler assembly. A 4.6 kV bias was applied to both uncoated windows during testing to suppress multipacting. Four field configurations were tested; a CW power of 30 kW was achieved with a stable window temperature for each. For three of the configurations, 50 kW CW was achieved, and 80 kW CW was reached for two configurations. The temperature of the conductive ceramic window as measured with an IR camera did not exceed 61°C for any configuration. For the configuration in which 30 kW CW was the limit, the alumina window flange temperature reached 39°C while the conductive ceramic window flange temperature was only 26°C. Residual gas analysis scans collected at the maximum conductive ceramic window operating temperature revealed no presence of any solder material components.