TaCo Tuner-adjustable waveguide Coupler

1. TaCoTuner-adjustablewaveguideCoupler A new generation of waveguidecouplersfor standing wave cavities Publication: CERN-ATS-Note-2011-085 Frank Gerigk, Jean-Michel Giguet, RolfWegner, Patricia Ugena

2. Outline • why adjustable couplers for standing wave cavities • principles of couplers and their adjustment‘nozzle’ coupler, T-type coupler, TaCo • TaCos for Linac4/SPL • TaCo prototype • Advantages and potential applications of TaCo TaCo // Tuner-adjustable waveguide Coupler

3. Unpredictable Q-valuevariations EB welding shrinkage 0.2 to 0.3 mmdQ/Q ~ 1% tuningelements dQ/Q ~ 1..2% surface roughness dQ/Q ~ 3..5% TaCo // Tuner-adjustable waveguide Coupler

4. Whyadjustablecouplers 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) TaCo // Tuner-adjustable waveguide Coupler

5. “nozzle” coupler ‘nozzle’ coupler adaptation Δβ / β0  courtesy of Jim Stovall & Craig Deibele TaCo // Tuner-adjustable waveguide Coupler

6. T-typecoupler adjustment of coupling factor: LS principle: compensation of reactance by stub line TaCo // Tuner-adjustable waveguide Coupler

7. T-typecoupler, adaptation 1st Step: movable short circuit LS TaCo // Tuner-adjustable waveguide Coupler

8. T-coupler, adaptation 2. machining of appropriate short circuit (vacuum) (+ copper plating) delay 3. installation (helicoflex joint, 30 bolts M12) TaCo // Tuner-adjustable waveguide Coupler

9. TaCo–Tuneradjustablew.g. coupler plunger tuner 1. Cavity (slightly) detuned to compensate stub line reactance 2. Coupling factor can be decreased as well as increased TaCo // Tuner-adjustable waveguide Coupler

10. TaCofor 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 TaCo // Tuner-adjustable waveguide Coupler

11. TaCo, increase of coupling factor increase of coupling factor TaCo // Tuner-adjustable waveguide Coupler

12. TaCoprototype xt = 292 mm zt = -345 mm LS = 420 mm øt = 43 mm RRt = 2.0 mm Lt = 113 mm Pin=1 MW TaCo // Tuner-adjustable waveguide Coupler

13. TaCo prototype no breakdowns no multipactor high power test: Pin= 1.1 MW, Es,m= 8 MV/m TP= 0.8 ms, frep= 2 Hz, ttest= 72 h TaCo // Tuner-adjustable waveguide Coupler

14. 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 • TaCo applications: Linac4 / SPL26 TaCos (20 x decrease β, 5 x increase β, 1 x variable β) TaCo // Tuner-adjustable waveguide Coupler