DEMONSTRATION OF COAXIAL COUPLING SCHEME AT 26 MV/m FOR 1.3 GHz TESLA-TYPE SRF CAVITIES * Yi Xie † , A. Kanareykin, Euclid TechLabs, Solon, Ohio T. Khabiboulline, A. Lunin, V. Poloubotko, A. Rowe, N. Solyak, V. Yakovlev, Fermilab, Batavia, Illinois J. Rathke, AES, Medford, New York Abstract 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 proto- type 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 fun- damental power coupler and two asymmetric HOM couplers both upstream and downstream. The couplers break the cav- ity 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 Figure 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; • It is compact. * This Work is supported by the US DOE SBIR Program DE-SC0002479. † yixie@fnal.gov, now at Fermilab. The magnetic field distribution for the operating mode is shown in Figure 2. One can see that the field in the corners is much smaller than in the main cavity. For the maximal acceleration gradient of 35 MeV/m the surface magnetic field is 147 mT. In this case in the corner it is about 0.15 mT, lower enough to definitely permit superconducting joints. Figure 1: Schematic of Euclid proposed quarter-wave coax- ial coupler. Figure 2: The magnetic field pattern in the coupler for the operating mode. ELECTROMAGNETIC DESIGN The first step in the coaxial coupler development was an adjustment of the shape to get the zero of the magnetic field at an accessible point for split flanges. The coaxial coupler which behaves like the coaxial resonator has an rf magnetic field and surface current minimum at the point λ rf /4 from the corner ( λ rf is the wavelength in vacuum). This position can’t be changed by appropriate choice of radii of the coaxial unit. Figure 3 shows the position of the magnetic field zero in the final design of the coaxial coupler. It satisfies the detachable design requirements. The coupling to the fundamental mode has been modeled by the HFSS eigenmode solver for the shape depicted above.