SPL Seminar 2012 BE-RF-LRF HOM couplers for SPL cavities Kai Papke 1 Comparison of different design approaches

SPL Seminar 2012 BE-RF-LRF Outline 2 Introduction Design Requirements Design Approaches Transmission Line Model 3D-Simulations Optimizations of the Designs Summary Outlook

SPL Seminar 2012 BE-RF-LRF Introduction 3 HOM-Coupler used to extract or dissipate unwanted, higher order modes in the cavity induced by the beam Typically placed at the same position as the power coupler (immediately before/ behind the cavity) Due to the unknown polarization of the modes, two couplers are required with different orientations

SPL Seminar 2012 BE-RF-LRF Equivalent Circuit 4 HOM-Coupler Very high impedance for the notch frequency (704.4MHz) Low impedance for “all” other modes Extracted power Resonant circuit Antenna

SPL Seminar 2012 BE-RF-LRF Design Requirements 5 Monopole modes with the highest R/Q 1 1 M.Schuh, F. Gerigk. “Influence of higher order modes on the beam stability in the high power superconducting proton linac“, Phys. Rev. ST Accel. Beams, 2011 Dipole modes with the highest R/Q 1 For SPL only the monopole modes are interesting In case of recirculators or synchrotrons should be also considered Schematic layout of the linac (Conceptual design of the SPL II) H - source RFQchopperDTLCCDTLPIMS 3 MeV 50 MeV 102 MeV MHz β=0.65β= MeV 4/5 GeV MHz 160 MeV Cut-off frequencies of cut-off tubes for the different mode types

SPL Seminar 2012 BE-RF-LRF Design Requirements 6 2 frequency regions relatively closed to each other Narrowband coupler Monopole modes with the highest R/Q 1 1 M.Schuh, F. Gerigk. “Influence of higher order modes on the beam stability in the high power superconducting proton linac“, Phys. Rev. ST Accel. Beams, 2011 Schematic layout of the linac (Conceptual design of the SPL II) TM 020,1/5  TM 020,2/5  TM 011,3/5  TM 011,4/5  TM 011,  TM 011,4/5  TM 011,  TM 021 TM 022,  3 frequency regions which are widely spread Broadband coupler or application of multiple couplers H - source RFQchopperDTLCCDTLPIMS 3 MeV 50 MeV 102 MeV MHz β=0.65β= MeV 4/5 GeV MHz 160 MeV

SPL Seminar 2012 BE-RF-LRF Design Approaches 7 Number of factors such as operational frequency, power dissipation, multipacting, field emission, surface resistance & heating evaluation as well as personal preference  different approaches Beam pipe Cavity Narrowband coupler with hook 2-inductive stub coupler (rescaled TESLA-style) Broadband coupler with probe Narrowband coupler with probe

SPL Seminar 2012 BE-RF-LRF E and H-field coupling Notch inductance which together with the red labeled capacitance creates the notch filter Notch capacitor couples to the outer conductor Mutual inductance of the resonant circuits fixing of the hole inner conductor may support liquid helium for cooling Capacitive coupling for impedance matching Design Approaches 8 Every part of the coupler (also the non labeled) affects more or less its frequency dependent impedance characteristic Coaxial port (50 Ohm) 8

SPL Seminar 2012 BE-RF-LRF Transmission Line Model 9 Initial estimation for design by a transmission line models Calculation of the lumped circuit elements for a desired frequency spectrum Transformation in Transmission Lines gives the lengths of the coupler parts Approximation by lumped circuit model Initial design 9

SPL Seminar 2012 BE-RF-LRF Transmission Line Model 10 Transmission Line Models reduce the problem to a few parameters Fast and easy computation of the impedance characteristic (with e.g. Mathematica) Only a rough estimation of the 3D-Model but helpful tool to provide a first design Very useful to get an idea of the parametric dependencies Subsequently the optimization will be done with CST MW, HFSS, … 10

SPL Seminar 2012 BE-RF-LRF Transmission Line Model Example for a parametric dependency (3D-Simulation and TML-Model) Simplified 3D-Model Splitted modes of the two coupled resonant circuits Notch filter l1 l1 TM01 - TEM (Narrowband type v2) TML-Model 11

SPL Seminar 2012 BE-RF-LRF Transmission Line Model Circuit equivalents for the different design approaches Notch filter Narrowband coupler with hook Broadband coupler with probe 2-Stub coupler (narrow/ broad) 1-Stub coupler (narrow) 12

SPL Seminar 2012 BE-RF-LRF To reduce numerical effort consider first a coupler model without beam pipe Compared with model including tapered beam pipe the characteristic resonances only slightly shifted Computation time – Reduced model : < 2min 1 – Extended model: 15-30min 2 Parametric investigations and first optimization may be carried out with the reduced mode especially for S2(3)1(1) 3D-Simulations 13 Reduced model S2(3)1(1) (TM01 – TEM) S2(3)1(1) (TM01 – TEM) 1 CST Microwave Studio - Fast Resonant Solver, Intel® Core™ i GHz, 8 GB-RAM 2 CST Microwave Studio - Fast Resonant Solver, Intel® Xeon® CPU 3.47GHz, 48 GB-RAM (~10GB used) Extended model

SPL Seminar 2012 BE-RF-LRF 3D-Simulations Coupling to the beam pipe 65 mm Cut-off frequencies of the beam pipe (TE11, TM01, TE21) TM01 - TEM (Narrowband type v1) Cut-off frequency of TM01-Mode always appeared rbeam2 Parasitic notch appears when beam pipe is tapered behind the pick-up tube (also affected by pick-up position, HOM coupler type as well as by the orientation of the antenna) ?

SPL Seminar 2012 BE-RF-LRF Narrowband Coupler with Hook 15 Optimization of the narrowband coupler for beta = 0.65 h1 h2 r0 l1 l2 l3 C1 Ct M12 Design only suitable for small band width In case of beta = 1.00 coupler could only be optimized for one frequency Notch filter Reduced model Extended model

SPL Seminar 2012 BE-RF-LRF Narrowband Coupler with Hook 16 Influence of the orientation of the coupling antenna Rise the coupling for S2(3)1(1) and also for other transmissions that corresponds with TM monopole modes Possibility to tune the notch frequency Normal position alpha TE11 - TEM TM01 - TEM

SPL Seminar 2012 BE-RF-LRF High quality factor for the first resonance Good transmission behavior for the higher frequency range (>2 GHz) Parasitic Notch is not a problem Broadband Coupler with Probe 17 Optimization of the broadband coupler for beta = 1.00 l1 l2 l4 C1 d l3 l5 h1 M23 Notch filter M12 Reduced model Extended model

SPL Seminar 2012 BE-RF-LRF More difficult to adjust the notch filter Less damping of the fundamental mode Similarly behavior to broadband coupler for higher frequencies (>2 GHz) 2-Stub Coupler 18 Optimization of the 2-Stub coupler for beta = 1.00 (broadband) l1 l2 C1 d l3 hm M23 Notch filter M12 r  Reduced model Extended model

SPL Seminar 2012 BE-RF-LRF Summary 19 Narrowband coupler with hook Broadband coupler with probe 2-Stub coupler (narrow/ broad) 1-Stub coupler (narrow) Small bandwidth Preferred for medium beta cavities Higher damping of the Higher Order Modes More sensitive to Multipacting 1 Larger bandwidth (3 resonances) Preferred for high beta cavities Relatively high quality factor for 1 st HOM resonance necessary to achieve a good damping Larger bandwidth (2 resonances) More difficult to adjust notch filter Very appropriate for cooling Less sensitive to Multipacting 1 Will be investigated in future Probably more preferred for medium beta cavities 1 S. Molloy, R. Ainsworth,, R. Ruber,. “Multipacting analysis for the superconducting RF Cavity HOM couplers in ESS“, Proceedings of IPAC2011, 2011

SPL Seminar 2012 BE-RF-LRF Outlook Maybe other designs will be considered Extension of the models including the cavity (Q ext ) Multipacting analysis Investigation of the heating characteristic (heating losses) Mechanical design Building of the first prototypes and experimental analysis 20 SPL: only focused on monopole modes  2 couplers per cavity Recirculators: monopole & dipole modes have to be considered  4 couplers per cavity SPL: only focused on monopole modes  2 couplers per cavity Recirculators: monopole & dipole modes have to be considered  4 couplers per cavity