measurement principle and tuning of the clic crab cavity

Measurement Principle andTuning of the CLIC Crab Cavity

12. Feb. 2014

2

Outline

12.02.2014 Tuning of CLIC Crab Cavity

1. General information on the Crab Cavity and motivation for this talk

2. Electromagnetic field pattern close to the axis [Ex, Ey, Ez, Hx, Hy, Hz]

3. Strategy for Bead-pull measurements: basing them on Ey only

4. Results of 1st Bead-pull measurements

5. Tuning and measurement results of the Crab Cavity

6. Summary

Tuning of CLIC Crab Cavity 3

CLIC Crab Cavity

12.02.2014

• designed by Cockcroft Institute, Lancaster University, EDMS 1159170

• very good support from Graeme Burt and Praveen-Kumar Ambattu

• extracted fields on axis and off-axis (CST) from Praveen-Kumar Ambattu

• .sat Model from Praveen-Kumar Ambattu,(here simulated with HFSS)

• symmetric structure (constant impedance)

• directions: z: beamy: deflection

4

General information, Goal, Suggestions


General information:• Crab Cavity designed by Cockcroft Institute, Lancaster University and CERN• production coordinated by CERN (BE/RF/PM)• tuning by CERN (BE/RF/LRF)• preparation (bake out, etc.), installation and high power testing by CERN (BE/RF/PM,

BE/RF/MK, BE/RF/LRF)

Goal: develop a reliable tuning method and tune the CLIC Crab in short time=> no need to re-study RF design, data provided by Cockcroft Institute, Lancaster University=> no need to measure all electromagnetic field components=> find a reliable method and apply it !

Suggestions:• using a double bead-pull with a dielectric bead and a metallic bead + data processing to determine

the electric and the magnetic field (as Ben Hall used for his PhD)• using a double bead-pull with 2 different bead shapes (e.g. a small cylinder = needle and a

disc = washer) to couple to different field components complicated !

5

Is there a simple method for Bead-pulling?


Background:• the CLIC Crab Cavity is a multi-cell cavity• most important is the correct phase advance per cell for synchronism with the beam

=> automatically for good RF designs, the correct amplitude patterns settles• all cells need to be tuned = adjusted in volume to reach desired frequency / phase

advance=> several bead-pull measurements need to be performed to verify effect of tuning=> ideally a bead-pull measurement after each tuning operation=> about 30 (+ 10 at different frequencies) bead-pull measurements in total (best case)

• Reliability: Performing several bead-pull measurements in the same state shall give the same information (mainly phase advance per cell and amplitude profile)

• => we are looking for a way to get the necessary information for tuning (= phase advance per cell, amplitude profile) with a single, reliable bead-pull measurement


Electromagnetic field pattern

12.02.2014

• beam is deflected in y-direction by Ey and vz*μ0*Hx=ZF0*Hx(! fields taken at different moments in time)

observations for x~0, y~0:

• max(|Ey(z)|) @ location of irises

• max(|Hx(z)|) in middle of cells

• max(|Ez(z)|) in middle of cells

• max(|Ez(z)|)<max(|Ey(z)|)


z- dependency: backward travelling wave

12.02.2014

• negative group velocity at operation point

• wide pass band (11.9 to 12.8 GHz)

0 20 40 60 80 100 120 140 160 18011.9

12

12.1

12.2

12.3

12.4

12.5

12.6

12.7

phase advance per cell [°]

frequ

ency

[G

Hz]

Brillouin Diagram


EM-fields, deflecting Ey

12.02.2014

Ey(x,y) nearly homogeneous close to symmetry axis (here R0.5 mm)0 20 40 60 80 100 120 140

0

1000

2000

3000

4000

z [mm]

|E(x

,y,z

)| [V

/m]

Ey(x,y,z)

0 20 40 60 80 100 120 140-2000

-1000

0

1000

z [mm]

arg(

E(x

,y,z

)) [°

]

x=0.5mm, y=0x=0. y=0.5mmx=0, y=0

-1 0 1

x 107

-1

-0.5

0

0.5

1

x 107

Re(E^2)

Im(E

^2)

! Ey^2 in preparation for bead-pull pattern


EM-fields, deflecting Hx

12.02.2014

Hx(x,y) nearly homogeneous close to symmetry axis (here R0.5 mm)

peaks of ZF0*Hx are ~20% lower than peaks of Ey

0 20 40 60 80 100 120 1400

1000

2000

3000

4000

z [mm]

|ZF0

*Hx(

x,y,

z)|

[V/m

]

ZF0*Hx(x,y,z)

0 20 40 60 80 100 120 140-1500

-1000

-500

0

z [mm]

arg(

c0*H

x(x,

y,z)

) [°

]

x=0.5mm, y=0x=0. y=0.5mmx=0, y=0

-5 0 5

x 106

-5

0

5

x 106

Re(E 2̂)

Im(E

^2)


EM-fields, accelerating Ez

12.02.2014

Ez(x,y): ~lin. dependent on y, close to beam axis

smaller than Ey and Hx(here factor 3 for R0.5 mm)

Ez(0,0) not 0 in first and last cells => single feed effect

0 20 40 60 80 100 120 1400

500

1000

1500

z [mm]

|E(x

,y,z

)| [V

/m]

Ez(x,y,z)

0 20 40 60 80 100 120 140-6000

-4000

-2000

0

2000

z [mm]

arg(

E(x

,y,z

)) [°

]

x=0.5mm, y=0x=0. y=0.5mmx=0, y=0

-1 0 1

x 106

-1

-0.5

0

0.5

1x 10

6

Re(E^2)

Im(E

^2)


EM-fields, small Ex

12.02.2014

Ex negligible close to symmetry axis(factor >70 in respect to Ey for R0.5 mm)0 20 40 60 80 100 120 140

0

20

40

60

z [mm]

|E(x

,y,z

)| [V

/m]

Ex(x,y,z)

0 20 40 60 80 100 120 140-15000

-10000

-5000

0

5000

z [mm]

arg(

E(x

,y,z

)) [°

]

x=0.5mm, y=0x=0. y=0.5mmx=0, y=0

-2000 0 2000

-2000

-1000

0

1000

2000

Re(E 2̂)

Im(E

^2)


EM-fields, small Hy

12.02.2014

ZF0*Hy negligible close to symmetry axis(factor >100 in respect to Ey for R0.5 mm)

0 20 40 60 80 100 120 1400

10

20

30

40

z [mm]

|ZF0

*Hy(

x,y,

z)|

[V/m

]

ZF0*Hy(x,y,z)

0 20 40 60 80 100 120 140-15000

-10000

-5000

0

5000

z [mm]

arg(

c0*H

y(x,

y,z)

) [°

]

x=0.5mm, y=0x=0. y=0.5mmx=0, y=0

-1000 0 1000

-1000

-500

0

500

1000

Re(E^2)

Im(E

^2)


EM-fields, Hz

12.02.2014

different simulation (HFSS of .sat file)=> field worse adapted for 120°/cell compared to simulations from Praveen-Kumar Ambattu

Hz depends lin. on x close to beam axis

ZF0*Hz has similar magnitude as Ez

0 20 40 60 80 100 120 1400

500

1000

1500

z [mm]

|ZF0

*Hz(

x,y,

z)|

[V/m

]

ZF0*Hz(x,y,z)

0 20 40 60 80 100 120 140-1

0

1

2x 10

4

z [mm]

arg(

c0*H

z(x,

y,z)

) [°

]

x=0.5mm, y=0x=0. y=0.5mmx=0, y=0

-1 0 1

x 106

-1

-0.5

0

0.5

1

x 106

Re(E 2̂)

Im(E

^2)


Field summary and Bead-pull measurements

12.02.2014

• Summary of fields:Ex and Hy can be neglected in vicinity of beam axis (~100 smaller)=> only Ey, Hx, Ez, Hz need to be consideredEy(x,y), Hx(x,y) ~ y0 x0 (deflecting fields)Ez(x,y)~ y1 x0, Hz(x,y) ~ y0 x1

Ex(x,y), Hy(x,y) ~ 0

• Bead-pull measurement principle: monitoring change of input reflectionCharles W. Steele, IEEE Trans. on microwave theory and techniques, Vol. MTT-14, No.2 (February, 1966), p.70:

S11= S11,perturbed – S11,unperturbed = _{x,y,z} {(e.*E.)^2 – (ZF0*h.*H.)^2}E2, H2: complex fields squared (phase !) at position of beade, h: complex factors describing polarisation & magnetisation effect ofthe bead's material in the local EM field

• Study of different factors e., h. to investigate if bead-pull measurements can be bases on a single field component => making tuning procedure simple


Simulation of bead-pull measurement

12.02.2014

1 2 3 4 5 6 7 8 9 101112-0.1

-0.05

0

0.05

0.1

z [cells]

Re(

indi

v. c

ontr.

to d

S11

)

position: x=0.00 mm, y=0.00 mm

ey=1.00

1 2 3 4 5 6 7 8 9 101112

-0.1

-0.05

0

0.05

z [cells]

Im(in

div.

con

tr. to

dS

11)

1 2 3 4 5 6 7 8 9 1011120

0.05

0.1

z [cells]

abs(

indi

v. c

ontr.

to d

S11

)

1 2 3 4 5 6 7 8 9 1011120

0.05

0.1

z [cells]

|dS

11|

dS11=sum_{x,y,z} (e.*E.) 2̂ - (ZF0*h.*H.) 2̂

1 2 3 4 5 6 7 8 9 101112

-200

-100

0

z [cells]

sub-

rang

e ar

g(dS

11)

[°]

-0.1 0 0.1

-0.1

-0.05

0

0.05

0.1

Re(dS11)

Im(d

S11

)

Ey only =

reference



12.02.2014

small off-set y=0.5mmEy & Ez

=> changes seen for dS11 at locations min(|Ey|)

=max(|Ez|)

1 2 3 4 5 6 7 8 9 101112-0.1

-0.05

0

0.05

0.1

z [cells]

Re(

indi

v. c

ontr.

to d

S11

)


ex=1.00ey=1.00ez=1.00

1 2 3 4 5 6 7 8 9 101112

-0.1

-0.05

0

0.05

z [cells]

Im(in

div.

con

tr. to

dS

11)

1 2 3 4 5 6 7 8 9 1011120

0.05

0.1

z [cells]

abs(

indi

v. c

ontr.

to d

S11

)

1 2 3 4 5 6 7 8 9 1011120

0.05

0.1

z [cells]

|dS

11|

dS11=sum_{x,y,z} (e.*E.) 2̂ - (ZF0*h.*H.) 2̂

1 2 3 4 5 6 7 8 9 101112

-200

-100

0

z [cells]

sub-

rang

e ar

g(dS

11)

[°]

-0.1 0 0.1

-0.1

-0.05

0

0.05

0.1

Re(dS11)

Im(d

S11

)



12.02.2014

off-set y=0.5mmEy & Ez

but ez^2=4

=> changes seen for dS11 at locations min(|Ey|)

=> peaks of dS11

dominated by Ey stay in

the same location

(amplitude & phase)

1 2 3 4 5 6 7 8 9 101112-0.1

-0.05

0

0.05

0.1

z [cells]

Re(

indi

v. c

ontr.

to d

S11

)


ey=1.00ez=2.00

1 2 3 4 5 6 7 8 9 101112

-0.1

-0.05

0

0.05

z [cells]

Im(in

div.

con

tr. to

dS

11)

1 2 3 4 5 6 7 8 9 1011120

0.05

0.1

z [cells]

abs(

indi

v. c

ontr.

to d

S11

)

1 2 3 4 5 6 7 8 9 1011120

0.05

0.1

0.15

z [cells]

|dS

11|

dS11=sum_{x,y,z} (e.*E.) 2̂ - (ZF0*h.*H.) 2̂

1 2 3 4 5 6 7 8 9 101112

-200

-100

0

z [cells]

sub-

rang

e ar

g(dS

11)

[°]

-0.1 0 0.1

-0.1

-0.05

0

0.05

0.1

Re(dS11)

Im(d

S11

)


1 2 3 4 5 6 7 8 9 101112-0.1

-0.05

0

0.05

0.1

z [cells]

Re(

indi

v. c

ontr.

to d

S11

)


ey=1.00ez=3.00

1 2 3 4 5 6 7 8 9 101112

-0.1

-0.05

0

0.05

z [cells]

Im(in

div.

con

tr. to

dS

11)

1 2 3 4 5 6 7 8 9 1011120

0.05

0.1

z [cells]

abs(

indi

v. c

ontr.

to d

S11

)

1 2 3 4 5 6 7 8 9 1011120

0.05

0.1

0.15

z [cells]

|dS

11|

dS11=sum_{x,y,z} (e.*E.) 2̂ - (ZF0*h.*H.) 2̂

1 2 3 4 5 6 7 8 9 101112-50

0

50

100

z [cells]

sub-

rang

e ar

g(dS

11)

[°]

-0.1 0 0.1

-0.1

0

0.1

Re(dS11)

Im(d

S11

)


12.02.2014

off-set y=0.5mmEy & Ez

but ez^2=9

=> changes seen for dS11 at

locations min(|Ey|)

=> weird phase behaviour due to

"minimum passage"

=> peaks of dS11: phase locations

identic, small changes of amplitude


1 2 3 4 5 6 7 8 9 101112-0.1

-0.05

0

0.05

0.1

z [cells]

Re(

indi

v. c

ontr.

to d

S11

)


ey=1.00hx=0.50

1 2 3 4 5 6 7 8 9 101112

-0.1

-0.05

0

0.05

z [cells]

Im(in

div.

con

tr. to

dS

11)

1 2 3 4 5 6 7 8 9 1011120

0.05

0.1

z [cells]

abs(

indi

v. c

ontr.

to d

S11

)

1 2 3 4 5 6 7 8 9 1011120

0.05

0.1

z [cells]

|dS

11|

dS11=sum_{x,y,z} (e.*E.) 2̂ - (ZF0*h.*H.) 2̂

1 2 3 4 5 6 7 8 9 101112-300

-200

-100

0

100

z [cells]

sub-

rang

e ar

g(dS

11)

[°]

-0.1 0 0.1

-0.1

-0.05

0

0.05

0.1

Re(dS11)

Im(d

S11

)


12.02.2014

Ey & Hx

=> changes seen for dS11 at

locations min(|Ey|)

=> peaks of dS11: amplitude and

phase of peaks as for reference

=> difference seen for cell 1 &

12


1 2 3 4 5 6 7 8 9 101112-0.1

-0.05

0

0.05

0.1

z [cells]

Re(

indi

v. c

ontr.

to d

S11

)


ex=1.00ey=1.00ez=3.00hx=0.70

1 2 3 4 5 6 7 8 9 101112

-0.1

-0.05

0

0.05

z [cells]

Im(in

div.

con

tr. to

dS

11)

1 2 3 4 5 6 7 8 9 1011120

0.05

0.1

z [cells]

abs(

indi

v. c

ontr.

to d

S11

)

1 2 3 4 5 6 7 8 9 1011120

0.05

0.1

0.15

z [cells]

|dS

11|

dS11=sum_{x,y,z} (e.*E.) 2̂ - (ZF0*h.*H.) 2̂

1 2 3 4 5 6 7 8 9 101112

-200

-100

0

100

z [cells]

sub-

rang

e ar

g(dS

11)

[°]

-0.1 0 0.1

-0.1

0

0.1

Re(dS11)

Im(d

S11

)


12.02.2014

Ey, Ez & Hx

=> phases of peaks at

locations as reference,

=> amplitudes similar, only

different for first and last irises


Strategy for bead-pull measurements:

12.02.2014

• dielectric bead => bead does not perturb magnetic field => no (or negligible) S11

• close to beam axis (x=0,y=0):• Ex negligible• peaks(|Ey|^2) >> peaks(|Ez|^2)• moreover, Ez small where |Ey| reaches maxima in regular cells

==> Strategy for measuring and evaluating fields of operating mode in regular cells: • bead-pull with dielectric bead on axis (x=0,y=0) => S11

• search for peaks(S11) => Ey(zn)^2 + very small error==> amplitude and phase advance profile of regular cells can be evaluated accurately at the same time a sensitive method to validate the accuracy is provided by

comparing max(|Ez|) to min(|Ey|) (in the middle of cells) by looking at thecomplex bead-pull pattern S11

Coupling cells:• output coupling cell is adjusted to minimise the standing wave pattern• input coupling cell is adjusted to minimise the overall input reflection S11


Results of 1st RF measurements

12.02.2014

• nearly no frequency shift due to wire (for accelerating structures typically -0.50 MHz, here |df|0.04 MHz)

• perturbation by bead is quite small due to relatively low field strength (typical S11 usually ~ 0.1, here ~ 0.01)=> noise is was seen on the first measurements (VNA setting IF 200 Hz)=> simple solution: decrease VNA IF bandwidth to 100 Hz for measurements during tuning – signal was clean enough, no need for making a bigger bead

• due to an (un)fortunate setup-error the effect of going off-axis could be analysed



12.02.2014

flange was skewed (soft after brazing) => bead not on axis for the upper (first) cells



12.02.2014

dS11

iris number (iris 1 between cell 1 and 2)

arg(

dS11

) [°

]

off-axis

on-axisd_E~-120°


Tuning of the Crab Cavity

12.02.2014

centring V guiding the wire for bead-pull measurements

nitrogen supply

input (chosen and marked)

tuning pins (4 per cell)

temperature sensor

cooling block

output (marked)


Before tuning

12.02.2014

11.8 11.9 12 12.1 12.2 12.3 12.4-60

-50

-40

-30

-20

-10

0

f / GHz

S /

dB

-34.8

-26.3

11.9922

simulated by Graeme BurtS11

11.96 11.98 12 12.02 12.04-60

-50

-40

-30

-20

-10

0

f / GHz

S /

dB

-34.8

-26.3

11.9922


input reflection bead-pull @ 11.9922 GHz

1 5 100

0.02

0.04

0.06

0.08

sqrt(

abs(

S

11))

Bead-pulling at 11992.2 MHz,

1 5 10-122-120-118-116-114

/ cel

l (D

EG

)

phase advance between cells

cell#

-0.032 -0.03 -0.028 -0.026 -0.024 -0.022 -0.02 -0.018

-0.045

-0.044

-0.043

-0.042

-0.041

-0.04

-0.039

-0.038

-0.037

Real(S11)

Imag

(S11

)

combined S11 in complex plane

12

3

2

4

6

8

10

12


Tuning - E. Daskalaki, A. Degiovanni, C. Marrelli, M. Navarro Tapia, R. Wegner, B. Woolley

12.02.2014

cell

tuning record of |ds11|*sign(df) (mU)

1 2 3 4 5 6 7 8 9 10 11 12 13 141 in 2 3 4 5 6 7 +4.88 +9.6 +14.0 9 -8.9 -3.4

10 +7.0 +4.0 11 +7.4 +12.0 12 out +8.1 +10.2 +12.3 +8.7 -7.5 -3.2

celltuning record of |ds11|*sign(df) (mU)

15 16 17 18 19 20 21 22 23 24 25 26 sum1 in +9.8 +9.82 +10.5 +9.9 +20.43 +6.2 +21.7 +27.94 +5.3 +3.8 +9.15 +6.4 +3.1 +9.56 +5.0 +5.7 +10.77 +7.0 -3.4 +8.48 +23.69 -12.3

10 +11.011 +19.412out +28.6


Tuning of the Crab Cavity

12.02.2014

tuning pins (4 per cell)


After tuning

12.02.2014

input reflection bead-pull @ 11.9922 GHz

11.8 12 12.2 12.4 12.6 12.8-60

-50

-40

-30

-20

-10

0

f / GHz

S /

dB

-34.8-37.9

11.9922


11.96 11.98 12 12.02 12.04-60

-50

-40

-30

-20

-10

0

f / GHz

S /

dB

-34.8-37.9

11.9922


1 5 100

0.02

0.04

0.06

0.08

sqrt(

abs(

S

11))

Bead-pulling at 11992.2 MHz,

1 5 10

-120

-119

-118

/ cel

l (D

EG

)

phase advance between cells

cell#

-4 -2 0 2 4 6 8

x 10-3

-0.018

-0.016

-0.014

-0.012

-0.01

Real(S11)

Imag

(S11

)

combined S11 in complex plane

12

3

2

4

6

8

10

12


Summary

12.02.2014

• a simple and reliable bead-pull method has been identified to determine the phase advance and amplitude profile of the CLIC Crab Cavity with a single bead-pull measurement

• the CLIC Crab Cavity could easily be tuned. A few remarks: • the cells were initially very well in shape• the tuning range per cell is about 4 to 5 times smaller than for other accelerating

structures (T(D)24, T(D)26, DDS, etc.)* for the Crab Cavity the group velocity is higher (~3.3%)* the tuning pins placed ~45° off the max. magnetic field regionsbut due to the good initial shape, the tuning range was largely sufficient for tuning

• we had to hammer slightly harder for tuning compared to other structures


Thank you for your attention

12.02.2014

measurement principle and tuning of the clic crab cavity

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