TaCo Tuner-adjustable waveguide Coupler A new generation of waveguide couplers for standing wave cavities Publication: CERN-ATS-Note-2011-085 Frank Gerigk, Jean-Michel Giguet, Patricia Ugena Tirado, Rolf Wegner

TaCo - Tuner adjustable waveguide Copler Rolf Wegner outline why adjustable couplers for standing wave cavities principles of couplers and their adjustment ‘nozzle’ coupler, RLWG coupler, T-type coupler, TaCo TaCos for Linac4/SPL TaCo prototype Advantages and potential applications of TaCo 19-Oct-2011 TaCo - Tuner adjustable waveguide Copler Rolf Wegner

Unpredictable Q-value variations tuning elements dQ/Q ~ 1..2% surface roughness dQ/Q ~ 3..5% EB welding shrinkage 0.2 to 0.3 mm dQ/Q ~ 1% 19-Oct-2011 TaCo - Tuner adjustable waveguide Copler Rolf Wegner

Example: surface roughness Example 350 MHz cavities: Ra demanded 0.8 µm δs ~ 3.5 µm ρ/δs ~ Ra/δs = 0.227 add. Losses ~ 7.5% machining: Ra ~ 0.7 µm Ra/δs = 0.199 add. Losses ~ 5% => unpredictable dQ ~ 2.5% Ralph Lending: "New criteria for microwave component surface". 19-Oct-2011 TaCo - Tuner adjustable waveguide Copler Rolf Wegner

Why adjustable couplers Critical coupling of cavities (with beam) to minimise reflected power coupling factor (critical coupling: βd = 1) small adaptation of coupling factor desired to compensate for unpredictable variation => safety margin (often adjustment in one direction only; proton Linacs: cavities often have different dimensions) 19-Oct-2011 TaCo - Tuner adjustable waveguide Copler Rolf Wegner

TaCo - Tuner adjustable waveguide Copler Rolf Wegner ‘nozzle’ coupler ‘nozzle’ coupler adaptation Δβ / β0  courtesy of Jim Stovall & Craig Deibele 19-Oct-2011 TaCo - Tuner adjustable waveguide Copler Rolf Wegner

‘nozzle’ coupler, equiv. circuit f0: resonant frequency of cavity f: frequency of drive Q0: cavity Q-value, B: cavity bandwidth ZL: waveguide impedance (f) β0: coupling factor adjustment of coupling factor: mechanical adaptation 19-Oct-2011 TaCo - Tuner adjustable waveguide Copler Rolf Wegner

‘nozzle’ coupler, remarks 1. Cavity operated at resonance 2. Coupling factor can (only) be increased 19-Oct-2011 TaCo - Tuner adjustable waveguide Copler Rolf Wegner

‘nozzle’ coupler, types Iris shape: circular, elliptical, racetrack or rectangular Waveguide shape: straight (not tapered) LAMPF SCL, Fermilab SCS, J-PARC ACS, SLAC LCLS RF-gun tapered (one or both directions) SNS SCS, LANL OCS 19-Oct-2011 TaCo - Tuner adjustable waveguide Copler Rolf Wegner

Ridged-Loaded Wave-Guide (RLWG) coupler ‘dog-bone’ iris + small opening, small perturbation, small frequency shift - sensitivity, difficulties seen: multipacting, arcing, overheating used for nearly all RFQs and for cavities requiring small freq. shift 19-Oct-2011 TaCo - Tuner adjustable waveguide Copler Rolf Wegner

TaCo - Tuner adjustable waveguide Copler Rolf Wegner T-type coupler used for RF guns, proton accelerators adjustment of coupling factor: LS principle: compensation of reactance by stub line 2 knobs ? amplitude and phase? 19-Oct-2011 TaCo - Tuner adjustable waveguide Copler Rolf Wegner

T-type coupler, equiv. circuit coupling factor: 1. Cavity (slightly) detuned 2. Coupling factor can (only) be decreased 19-Oct-2011 TaCo - Tuner adjustable waveguide Copler Rolf Wegner

T-type coupler, adaptation 1. movable short circuit => LS 19-Oct-2011 TaCo - Tuner adjustable waveguide Copler Rolf Wegner

T-type coupler, adaptation 2. machining of appropriate short circuit (vacuum) (+ copper plating) ==> delay 4. installation (helicoflex joint, 30 bolts M12) 19-Oct-2011 TaCo - Tuner adjustable waveguide Copler Rolf Wegner

TaCo - Tuner-adjustable w.g. Coupler plunger tuner LS fixed 19-Oct-2011 TaCo - Tuner adjustable waveguide Copler Rolf Wegner

TaCo - Tuner adjustable waveguide Copler Rolf Wegner TaCo, remarks 1. Cavity (slightly) detuned to compensate stub line reactance 2. Coupling factor can be decreased as well as increased 19-Oct-2011 TaCo - Tuner adjustable waveguide Copler Rolf Wegner

TaCo - Tuner adjustable waveguide Copler Rolf Wegner TaCo for Linac4/SPL xt = 350 mm zt = 0 mm LS = 311 mm øt = 83 mm RRt = 10 mm vacuum section Pin=1 MW, cavity: ES~33 MV/m 19-Oct-2011 TaCo - Tuner adjustable waveguide Copler Rolf Wegner

TaCo, increase of coupling factor 19-Oct-2011 TaCo - Tuner adjustable waveguide Copler Rolf Wegner

TaCo - Tuner adjustable waveguide Copler Rolf Wegner TaCo prototype xt = 292 mm zt = -345 mm LS = 420 mm øt = 43 mm RRt = 2.0 mm Lt = 113 mm Pin=1 MW 19-Oct-2011 TaCo - Tuner adjustable waveguide Copler Rolf Wegner

TaCo - Tuner adjustable waveguide Copler Rolf Wegner TaCo prototype high power test: Pin= 1.1 MW, Es,m= 8 MV/m TP= 0.8 ms, frep= 2 Hz, ttest= 72 h no breakdowns no multipactor 19-Oct-2011 TaCo - Tuner adjustable waveguide Copler Rolf Wegner

TaCo - Tuner adjustable waveguide Copler Rolf Wegner Advantages of TaCo Coupling factor can be increased as well as decreased => cavity design for desired coupling => safety margins included in coupler design TaCo machined in one process (T-type: short circuit machined after RF measurements -> delay) RF matching simple and quick (final RF contact between coupler and cavity) Change of beam current => exchange of plunger tuner Movable tuner => variable coupling during operation 19-Oct-2011 TaCo - Tuner adjustable waveguide Copler Rolf Wegner

TaCo’s (potential) applications Linac4 / SPL 26 TaCos (20 x decrease β, 5 x increase β, 1 x variable β) Replacement of nearly all T-type couplers (! Emax) (TERA / ADAM side coupled structures, RF guns, ESS Bilbao) Replacement of ‘nozzle’ couplers 1. no waveguide taper => direct replacement 2. waveguide taper => no taper + bigger coupling slot (e.g. bridge couplers of π/2 mode structures [=many structures]) 3. waveguide taper => plunger tuner placed in taper TaCo principle could also be applied to RLWG couplers 19-Oct-2011 TaCo - Tuner adjustable waveguide Copler Rolf Wegner