R&D of RF coupleres for Crab cavity
The Graduate University for Advanced Studies(KEKB Crab Cavity Group)
NAKANISHI Kota

1. Design of Input coupler
2. Design of Coaxial coupler
3. Development of Nb film on Cu substrate2004 KEKB Machine advisory committee

0. Cryostat of Crab Cavity (Top View)

1. Design of Input coupler
RF power source

Design condition
Item

RF window

antenna

Requirement

Diameter ofouterconductor(window side)

151.9 mm

Diameter of
innerconductor(window side)

66 mm

ReasonTo use RFwindow
designed foraccelerationcavities

Diameter ofouterconductor(cavity side)

120 mm

To use crab
cavity forverticalmeasurement

RF frequency

509 MHzKEKB
specification

1. Design of Input coupler
Case 1 (Plain)#!"*!")!"(=>!'+=>!'&=>!(+=>!(&=>!)+9:;<!"'!"&!"%!"$!"#!"!
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L=171mm
VSWR(@509MHz)=1.34'++(++)++

1. Design of Input coupler
Case 2 (step)!"'!"&#40Ω
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Z1=32.8Ω

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When Z1=32.8Ω, reflection coefficient of T-stub and step are same.
The L1 must be decided to make these reflections are canceled.
 L1=141mm#!

1. Design of Input coupler#!"*!")!"(9:;<!"'!"&!"%!"$=>?.3!
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Low VSWR value and wide band width are obtained.
Passband (VSWR < 1.2) lies between 309 and 652 MHz.

1. Design of Input coupler
Case 3 (taper)!"#'!"#&89:;!"#%!"#$!"#!

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This model was proposed by a manufacturer.
Reflection caused by the taper is negligible.

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1. Design of Input coupler!"&!"%

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Passband (VSWR < 1.2) lies between 474 and 538MHz.
VSWR(@509MHz)=1.147
Model of case 3 is acceptable.

2. Design of Coaxial coupler

Coaxial coupler must behave as band stop filter.

passstoppass

2. Design of Coaxial coupler

cavityload&+,#$)2'(,"1-,./)-.00+1,!"#$%&'#%()*+'&,

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2. Design of Coaxial coupler-./0,-,,+!,)*-1,-233-2*)-21*

S11(@413)=0.228

ls1=190mm(to adjust magnitude of reflection)!"#$%!#&&'("The ls2 was adjusted to makethese reflections are canceled.
The ls2 must be chosen 85mm.

2. Design of Coaxial coupler
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'( )*+ ,- ./)0 1&$$The ln must be chosen 142mm, because of stopping 509MHz.
Then S11(@413MHz) = 0.586

2. Design of Coaxial coupler"#!

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When lj=100mm and rj=70mm, reflection coefficients are almost same.
And if the lns is chosen =340mm, these reflections are canceled.

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2. Design of Coaxial coupler

3. Development of Nb films on Cu substrate

The coaxial coupler is exposed high electric and magnetic field only the tip.
It is very luxury that this part is made of Nb.
If Nb/Cu technique can be applied, everyone may be happy.

3. Development of Nb films on Cu substrateRF
1.E+10ArNbmagnet

Q12'+30

1.E+09

1.E+081.E+07

1.E+0605

10Esp[MV/m]!"#$%&'()*'+,-./0

CuSuperconducting film was obtained.

15

3. Development of Nb films on Cu substrate

Simplified coaxial coupleres ware made.
Measurement of Nb film property can be simplified.

3. Development of Nb films on Cu substrate!"#$ArRF3. Development of Nb films on Cu substrate

Nb film was obtained.
The film was not peeled by thermal cycles. (10times 77K ⇔ RT)

3. Development of Nb films on Cu substrate/0"12#,-)"1-&11"'()*"+,-.

31$4-)"1-&11"!"#$%&The Nb film property may be easily measured using the coaxial cavity.

3. Development of Nb films on Cu substrate

Grinding

Barrel polishing

As welding
After barrel polishing3. Development of Nb films on Cu substrate

Electro polishing
The sample measurement setup was readied.
We must make Nb/Cu samples, as next step.