Abstract
This paper reports on our efforts to develop a flangeable coaxial coupler for both higher order mode and fundamental coupling for nine-cell ILC-type cavities, which were designed in the early 1990's for pulsed operation with a duty factor less than 1%. The design of the coupler has been done in such a way that the rf magnetic flux $B$ at the flange connection was minimized and only a field of $<5\text{ }\text{ }\mathrm{mT}$ would be present for an operation at an accelerating field ${E}_{\mathrm{acc}}\ensuremath{\sim}36\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ ($B\ensuremath{\sim}150\text{ }\text{ }\mathrm{mT}$) in the cavity. Even though we achieved reasonably high $Q$ values at low field, the cavity/coupler combination was limited in the cw mode to only $\ensuremath{\sim}7\text{ }\text{ }\mathrm{MV}/\mathrm{m}$, where a thermally initiated degradation occurred. We believed that this limitation was caused by poor cooling of the shorting plate and inner tube in the coaxial coupler; therefore, we have improved the cooling conditions by initially drilling radial cooling channels every 30 degrees, then every 15 degrees into the shorting plate and eventually removing the ``bridges'' between the channels. This paper reports on our experiences with the modified coaxial coupler under cw and pulsed conditions.
Highlights
In the coaxial coupling (CC) scheme, shown in Fig. 1, both fundamental power coupler (FPC) and higher order mode (HOM) couplers are shielded by the inner tube, which is supported by the Nb disk welded to it and to the beam tube
We subsequently learned from the material supplier that the mechanical strength of the material strongly depends on the annealing conditions during material casting and rolling and can be improved. (ii) The original design of the coaxial coupler components did not provide sufficient cooling to the shorting plate between inner and outer conductors
The way forward to better performance of the assembly will be a replacement of the present Nb1%Zr flanges with material of higher hardness, which will better withstand the pressures applied during the assembly with the gasket
Summary
In the coaxial coupling (CC) scheme, shown in Fig. 1, both fundamental power coupler (FPC) and higher order mode (HOM) couplers are shielded by the inner tube, which is supported by the Nb disk welded to it and to the beam tube. In the coaxial coupling (CC) scheme, shown, both fundamental power coupler (FPC) and higher order mode (HOM) couplers are shielded by the inner tube, which is supported by the Nb disk welded to it and to the beam tube. The disk is an electric short in the coaxial line, which is formed by the inner and outer tubes, and separates electrically two mirrored coupling devices and neighboring cavities. The pair of mirrored coupling devices can be flanged between two cavities. The flanges are located $35 mm apart from the end irises, at the positions where the standing wave of the magnetic flux has its notch. The TESLA HOM damping scheme fulfills the specification for the ILC project, which is the TESLA successor, and can be used for the superconducting cavities in the main accelerator
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