TaCoTuner-adjustable waveguide Coupler

A new generation of waveguide couplers for standing wave cavities

Publication: CERN-ATS-Note-2011-085

Frank Gerigk, Jean-Michel Giguet, Rolf Wegner, Patricia Ugena

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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

09/12/11 TaCo // Tuner-adjustable waveguide Coupler

TaCo // Tuner-adjustable waveguide Coupler

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Unpredictable Q-value variations

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EB weldingshrinkage 0.2 to 0.3 mmdQ/Q ~ 1%

tuning elementsdQ/Q ~ 1..2%

surface roughnessdQ/Q ~ 3..5%

TaCo // Tuner-adjustable waveguide Coupler

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Why adjustable couplersCritical coupling of cavities (with beam) to minimise reflected power

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dd,unpred.bd,exp.

ext

PPPP

dunpred .0

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

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“nozzle” coupler

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‘nozzle’ coupler

adaptationΔβ / β0

 courtesy of Jim Stovall & Craig Deibele

TaCo // Tuner-adjustable waveguide Coupler

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T-type coupler

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adjustment of coupling factor: LS

principle: compensation of reactance by stub line

TaCo // Tuner-adjustable waveguide Coupler

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T-type coupler, adaptation

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1st Step: movable

short circuitLS

TaCo // Tuner-adjustable waveguide Coupler

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T-coupler, adaptation

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2. machining of appropriate short circuit (vacuum)

(+ copper plating)

delay

3. installation(helicoflex joint, 30 bolts M12)

TaCo // Tuner-adjustable waveguide Coupler

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TaCo – Tuner adjustable w.g. coupler

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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

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TaCo for LINAC4/SPL

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0 20 40 60 80 100 120 1400

20

40

60

80

100

L_t [mm]

b_re

l [%

]

0 20 40 60 80 100 120 1400

0.5

1

1.5

2

2.5

3

L_t [mm]

Em

ax [M

V/m

]

0 20 40 60 80 100 120 1400

2

4

6

8

10

L_t [mm]

P_1

0% [W

]

xt = 350 mmzt = 0 mmLS = 311 mmøt = 83 mmRRt = 10 mm

vacuum section

Pin=1 MW, cavity: ES~33 MV/m

TaCo // Tuner-adjustable waveguide Coupler

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TaCo, increase of coupling factor

0 20 40 60 80 100 120 140 1600

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

b_rel [%]

Em

ax [M

V/m

]

xt=100,zt= 0xt=290,zt= 0xt=350,zt= 0xt=470,zt= 0xt=290,zt= 200xt=290,zt=-250

increase of coupling factor

TaCo // Tuner-adjustable waveguide Coupler

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TaCo prototype

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70 80 90 100 110 120 130 1400

5

10

15

20

25

L_t [mm]

Em

ax [M

V/m

]

xt = 292 mmzt = -345 mmLS = 420 mmøt = 43 mmRRt = 2.0 mmLt = 113 mm

70 80 90 100 110 120 130 1400

0.5

1

1.5

2

2.5

3

L_t [mm]

simulationmeasurement Pin=1 MW

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TP= 0.8 ms,frep= 2 Hz,ttest= 72 h

high power test:

Pin= 1.1 MW,Es,m= 8 MV/m

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TaCo prototype

no breakdownsno multipactor

TaCo // Tuner-adjustable waveguide Coupler

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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 / SPL

26 TaCos (20 x decrease β, 5 x increase β, 1 x variable β)

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