Low Kick Coupler for Superconducting Cavities

Results from the high current, low emittance photo injector at Cornell revealed that even with two opposing input couplers, the beam emittance is affected by the coupler kick. As a result, a coupler with low transverse kick is proposed for use in superconducting accelerating cavities. In this coupler, a rectangular waveguide transforms into a coaxial line inside the beam pipe. The geometry of the coupler is tuned to minimize the transverse kick that is important for linear accelerators with low emittance. The coupler can be used in ERL injectors or other linacs for high brightness light sources. INTRODUCTION The proposed Cornell Energy Recovery Linac (ERL) [1] and other new ultra high brightness synchrotron light sources need high quality low emittance electron beams. That means that beam has to be formed by electron gun and accelerated with minimal destructive effects. The starting part of the accelerator, the injector, produces the strongest effect on the beam due to low beam energy. The input coupler of superconducting cavities is usually an antenna type coupler attached to a side port in the beam pipe near the cavity. Due to axial asymmetry of this design, electromagnetic fields in the vicinity of the coupler have transverse components and produce a transverse kick to the electron beam. That leads to the emittance dilution of the beam. To reduce this effect, the injector cavities of Cornell ERL are equipped with two identical input couplers symmetrically placed on two sides of the beam pipe [2]. That design eliminates the dipole transverse kick. However, there exist quadrupole field components and this quadrupole focusing distorts the beam [3]. Therefore we consider a possibility to build a new coupler which produces axially symmetric fields and generates very low transverse kick to the beam. That coupler has a tube inside the beam pipe which forms a coaxial line with the beam pipe. This coaxial line is coupled to a power transmission line. Couplers of that type were designed, built and used for S-DALINAC superconducting cavities in early 90s [4]. In that design the coaxial transmission line was coupled to the coaxial line in the beam pipe by movable antenna. Later similar couplers were considered at DESY [5] and FNAL [6]. Unfortunately, these designs were not perfect. For practical sizes of the beam pipe, two modes can propagate in the beam pipe coaxial line: a fully axially symmetric TEM mode and a dipole TE11 mode. The TE11 mode is excited in the coaxial line due to asymmetric coupling to the transmission line. This dipole mode produces a transverse kick to the beam. During the early study of ERL injector design, a symmetric coupler inside the beam pipe of small diameter was considered [7]. In such a case, the TE11 mode could not propagate along the coaxial line of that smaller size. Unfortunately, this was not compatible with the HOM and BBU requirements set by the linac design. A similar solution was also found at Darmstadt, operating at 3 GHz The idea was to use a rectangular waveguide instead of a coaxial transmission line to and to optimize the geometry of the field transformer between the waveguide and the beam pipe coaxial line for minimizing the excitation of the TE11 mode [8]. SYMMETRIC COUPLER FOR ERL INJECTOR CAVITY Figure 1 shows the model of the ERL injector cavity with the Darmstadt type of symmetric coupler. The Figure 1: 2-cell cavity with the symmetric coupler. Figure 2: Calculated S-parameters of the coupler waveguide-to-coax field transformer (in dB). ____________________________________________ * Work is supported by NSF Grants NSF DMR-0807731 and NSF PHY-1002467. † vgv1@cornell.edu S1(TE10),1(TE10) S2(TEM),1(TE10) S2(TE11),1(TE10) THPP019 Proceedings of LINAC2014, Geneva, Switzerland ISBN 978-3-95450-142-7 876 C op yr ig ht © 20 14 C C -B Y3. 0 an d by th e re sp ec tiv e au th or s