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Superconducting ILC-type cavities have an rf input coupler that is welded on. A detachable input coupler will reduce conditioning time (can be conditioned separately), reduce cost and improve reliability. The problem with placing an extra flange in the superconducting cavity is about creating a possible quench spot at the seal place. Euclid Techlabs LLC has developed a coaxial coupler which has an on the surface with zero magnetic field (hence zero surface current). By placing a flange in that area we are able to avoid disturbing surface currents that typically lead to a quench. The coupler is optimized to preserve the axial symmetry of the cavity and rf field. The surface treatments and rf test of the prototype coupler with a 1.3 GHz ILC-type single-cell cavity at Fermilab will be reported and discussed. INTRODUCTION The standard 1.3 GHz TESLA type SRF cavity has a fundamental power coupler and two asymmetric HOM couplers both upstream and downstream. The couplers break the cavity axial symmetry that in turn causes a rf field distortion and transverse wake field which may cause beam emittance dilution [1]. In order to preserve the axial symmetry of the acceleration channel, different schemes of coaxial coupler were proposed and developed [2], [3]. We suggested another design for the coupling unit, as shown in Fig. 1. This coupler has the following features: • The coupler unit preserves the axial symmetry of the acceleration channel. There are no RF kicks or wakes that lead to emittance dilution; • It is a quarter-wave resonant coupler for the operating mode; • The coupler is detachable, because in the operating mode the currents are small in the coupler corners, and non-welded superconducting joints may be used. Thus, the coupler unit can be a separate device that can be treated independently of the structure; • It may be manufactured of low RRR niobium (reduced cost) compared with the main cavity;

Demonstration of Coaxial Coupling Scheme at 26 MV/m for 1.3 GHz Tesla-Type SRF Cavities