Measurement Principle and - PowerPoint Presentation

Slide1

Measurement Principle andTuning of the CLIC Crab Cavity

12. Feb. 2014

Slide2

Outline

12.02.2014

Tuning of CLIC Crab Cavity

2

General information

on the Crab Cavity and

motivation

for this talk

Electromagnetic

field pattern

close to the axis [Ex,

Ey

,

Ez

,

Hx

,

Hy

, Hz]

Strategy for

Bead-pull

measurements: basing them

on

Ey

only

Results of

1st Bead-pull measurements

Tuning

and

measurement results

of the Crab Cavity

Summary

Slide3

CLIC Crab Cavity

3

12.02.2014

Tuning of CLIC Crab Cavity

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: beam

y: deflection

Slide4

General information, Goal, Suggestions

12.02.2014

Tuning of CLIC Crab Cavity

4

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 !

Slide5

Is there a simple method for Bead-pulling?

12.02.2014

Tuning of CLIC Crab Cavity

5

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

Slide6

Electromagnetic field pattern

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Tuning of CLIC Crab Cavity

beam is deflected in y-direction by

Ey

and

vz

*

μ

0*

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 cellsmax(|Ez(z)|) in

middle of cellsmax

(|

Ez

(z)|)<

max(|

Ey

(z)|)

Slide7

z- dependency: backward travelling wave

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12.02.2014

Tuning of CLIC Crab Cavity

negative group velocity at operation point

wide pass band (11.9 to 12.8 GHz)

Slide8

EM-fields, deflecting Ey

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12.02.2014

Tuning of CLIC Crab Cavity

Ey

(

x,y

) nearly homogeneous close to symmetry axis

(here R

0.5 mm)

! Ey^2 in preparation for bead-pull pattern

Slide9

EM-fields, deflecting Hx

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12.02.2014

Tuning of CLIC Crab Cavity

Hx

(

x,y

) nearly homogeneous close to symmetry axis

(here R

0.5 mm)

peaks of ZF0*

Hx

are ~20% lower than peaks of Ey

Slide10

EM-fields, accelerating Ez

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12.02.2014

Tuning of CLIC Crab Cavity

Ez

(

x,y

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

smaller than

Ey

and

Hx

(here factor 3 for R0.5 mm)Ez(0,0) not 0 in first and last cells => single feed effect

Slide11

EM-fields, small Ex

11

12.02.2014

Tuning of CLIC Crab Cavity

Ex negligible close to symmetry

axis

(factor >70 in respect to

Ey

for R

0.5 mm)

Slide12

EM-fields, small Hy

12

12.02.2014

Tuning of CLIC Crab Cavity

ZF0*

Hy

negligible close to symmetry

axis

(factor

>100

in respect to

Ey

for R0.5 mm)

Slide13

EM-fields, Hz

13

12.02.2014

Tuning of CLIC Crab Cavity

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

Slide14

Field summary and Bead-pull measurements

14

12.02.2014

Tuning of CLIC Crab Cavity

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

)~ y

1 x0, Hz(x,y) ~ y0 x1 Ex(

x,y), Hy(x,y)

~ 0

Bead-pull measurement principle: monitoring change of input reflection

Charles

W. Steele, IEEE Trans. on microwave theory

and techniques

,

Vol

. MTT-14, No.2 (February,

1966), p.70:

S

11

= S

11,perturbed

– S

11,unperturbed

= _{

x,y,z

} {(e.*E.)^

2 –

(ZF0*h.*H.)^2}

E

2

, H

2

: complex fields squared (phase !) at position of bead

e, h: complex factors describing polarisation & magnetisation effect of

the 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

Slide15

Simulation of bead-pull measurement

15

12.02.2014

Tuning of CLIC Crab Cavity

Ey

only

=

reference

Slide16

Simulation of bead-pull measurement

16

12.02.2014

Tuning of CLIC Crab Cavity

small off-set y=0.5mm

Ey

&

Ez

=> changes seen for dS11 at locations min(|

Ey

|) =max(|

Ez

|)

Slide17

Simulation of bead-pull measurement

17

12.02.2014

Tuning of CLIC Crab Cavity

off-set y=0.5mm

Ey

&

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)

Slide18

Simulation of bead-pull measurement

18

12.02.2014

Tuning of CLIC Crab Cavity

off-set y=0.5mm

Ey

&

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

Slide19

Simulation of bead-pull measurement

19

12.02.2014

Tuning of CLIC Crab Cavity

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

Slide20

Simulation of bead-pull measurement

20

12.02.2014

Tuning of CLIC Crab Cavity

Ey

,

Ez

&

Hx

=> phases of peaks at locations as reference,

=> amplitudes similar, only different for first and last irises

Slide21

Strategy for bead-pull measurements:

21

12.02.2014

Tuning of CLIC Crab Cavity

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

S

11

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)

=> S11search for peaks(S

11) => 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 the

complex bead-pull pattern S

11

Coupling cells:

output coupling

cell is adjusted to minimise the standing wave pattern

input

coupling

cell

is

adjusted to

minimise the overall input reflection S

11

Slide22

Results of 1st RF measurements

22

12.02.2014

Tuning of CLIC Crab Cavity

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

Slide23

Results of 1st RF measurements

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Tuning of CLIC Crab Cavity

flange was skewed (soft after brazing)

=> bead not on axis for the upper (first) cells

Slide24

Results of 1st RF measurements

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12.02.2014

Tuning of CLIC Crab Cavity

dS11

iris number (iris 1 between cell 1 and 2)

arg

(dS11)

[°]

off-axis

on-axis

d

_E

~-120°

Slide25

Tuning of the Crab Cavity

25

12.02.2014

Tuning of CLIC Crab Cavity

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)

Slide26

Before tuning

26

12.02.2014

Tuning of CLIC Crab Cavity

i

nput reflection

bead-pull @ 11.9922 GHz

Slide27

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

27

12.02.2014

Tuning of CLIC Crab Cavity

cell

tuning record of |ds11|*sign(

df

) (

mU

)

1

2

3

4

5

6

7

8

9

10

11

12

13

14

1

in

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

3

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

4

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

5

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

6

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

7

 

 

 

 

 

 

 

 

 

 

 

 

 

 

+4.8

8

 

 

 

 

 

 

 

 

 

 

 

 

+9.6

+14.0

 

9

 

 

 

 

 

 

 

 

 

 

 

-8.9

 

 

-3.4

10

 

 

 

 

 

 

 

 

 

+7.0

+4.0

 

 

 

 

11

 

 

   +7.4+12.0        12out+8.1+10.2+12.3+8.7  -7.5-3.2      

cell

tuning record of |ds11|*sign(

df

) (

mU

)

15

16

17

18

19

20

21

22

23

24

25

26

sum

1

in

 

 

 

 

 

 

 

 

 

 

+9.8

 

+9.8

2

 

 

 

 

 

 

 

 

 

 

+10.5

 

+9.9

+20.4

3

 

 

 

 

 

 

 

 

+6.2

+21.7

 

 

 

+27.9

4

 

 

 

 

 

+5.3

 

+3.8

 

 

 

 

 

+9.1

5

 

 

 

 

+6.4

 

+3.1

 

 

 

 

 

 

+9.5

6

 

+5.0

 

+5.7

 

 

 

 

 

 

 

 

 

+10.7

7

 

+7.0

-3.4

 

 

 

 

 

 

 

 

 

 

+8.4

8

 

 

 

 

 

 

 

 

 

 

 

 

 

+23.6

9

 

 

 

 

 

 

 

 

 

 

 

 

 

-12.3

10

 

 

 

 

 

 

 

 

 

 

 

 

 

+11.0

11

 

 

 

 

 

 

 

 

 

 

 

 

 

+19.4

12

out

 

 

 

 

 

 

 

 

 

 

 

 

+28.6

Slide28

Tuning of the Crab Cavity

28

12.02.2014

Tuning of CLIC Crab Cavity

tuning

pins (4 per cell

)

Slide29

After tuning

29

12.02.2014

Tuning of CLIC Crab Cavity

i

nput reflection

bead-pull @ 11.9922 GHz

Slide30

Summary

30

12.02.2014

Tuning of CLIC Crab Cavity

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

C

avity

could easily be tuned

. A few remarks: the cells were initially very well in shapethe 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 tuningwe had to hammer slightly harder for tuning compared to other structures

Slide31

Thank you for your attention

31

12.02.2014

Tuning of CLIC Crab Cavity