Status of Wakefield Monitor Experimentsat the CLIC Test Facility

Reidar L. Lillestøl, E. Adli, J. PfingstnerUniversity of Oslo, Oslo, Norway

W. Farabolini, S. Doebert, R. Corsini, W. Wuensch, A. GrudievCERN, Geneva, Switzerland

N. Aftab, S. JaveedPakistan Institute of Nuclear Science and Technology, Islamabad, Pakistan

IPAC 201611th May 2016

IntroductionReidar L. LillestølIPAC 2016Busan, South Korea

I The proposed e+e− collider CLICis designed with a luminosity ofaround L = 1034/cm2/s.

I This requires emittances ofI εx = 660 nm,I εy = 20 nm.

I An important contributor toemittance growth is transversewakefields in the acceleratingstructures.

I Therefore, the structures areequipped with WakefieldMonitors (WFMs) for measuringthe transverse beam position inthe structures.

I Simulations show that theemittance growth has anacceptable level if the WFMresolution is 3.5 µm.

I WFM performance and operatingconditions are tested at CERN.

The CLIC Test Facility 3 (CTF3)Reidar L. LillestølIPAC 2016Busan, South Korea

Califes

Two-beam module

Linear acceleratorThermionic gun

10 m

2x

4x

Bunchcombination

A large part is the drive beam generationI Fully loaded acceleration.I Bunch combination in a delay loop and a

combiner ring, up to a factor 8.I Many experiments including phase

feed-forward, beam energy extraction,instrumentation etc.

I Will shutdown Dec. 2016.

The Califes linac emulates the CLICmain beam

I Two-beam accelerationI Instrumentation tests, external

users like ESA, etc.I May continue past 2016

The Califes Beamline in the CTF3Reidar L. LillestølIPAC 2016Busan, South Korea

A prototype CLICModule is installed,which uses the CTF3drive beam toaccelerate the probebeam in Califes

Beam parameters ValuesBeam energy 80–220 MeVRelative energy spread 1 %Bunch charge 0.01–1 nCAcceleration frequency 3 GHzNormalized emittances 2–70 µmRepetition rate 0.83–5 HzBunches per train 1–150Bunch frequency 1.5 GHzWFM signals 16

Wake Field Monitors (WFMs)Reidar L. LillestølIPAC 2016Busan, South Korea

I WFMs: Precise determination of the beamposition in accelerating structures.

I Four HOM damping waveguides withantennas are used for measuring dipolemodes (which depend on the beam position).

I Four TD26 structures are installed in themodule prototype in CTF3.

I A TE-like mode at 27.3 GHz and a TM-likemode at 16.9 GHz are expected fromsimulations (picked up with antennas ondifferent sides of the waveguides).

Simulated transverse impedance fornon-tapered TD26 structure (GdfidL)

Current WFM Readout System

AD

CA

DC

AD

C

Localoscillator

180 hybrids Waveguides and transitions BP Filters, limiters, log detectors

LP filters

TM

TE

Reidar L. LillestølIPAC 2016Busan, South Korea

Commissioning and Early ResultsReidar L. LillestølIPAC 2016Busan, South Korea

I The module including WFMs wascommissioned in early 2015, andbehaves as expected.

I Below: A position scan in bothtransverse planes, where signals in bothplanes are multiplied to find thestructure center.

0.0 0.2 0.4 0.6 0.8 1.00.0

0.2

0.4

0.6

0.8

1.0

Hor. beam pos. [mm] -1.0-0.5

0.00.5

1.0

Ver.beam

pos.[m

m]-1.0-0.5

0.00.5

1.0

Lef

t×U

p[a

rb.u

.]

100

300

500

700

900

450 460 470 480 490Time [ns]

0.0

0.5

1.0

1.5

2.0

WFM

am

plitu

de [u

W]

TE Left SS1, 17.11.15

1 bunch2 bunches5 bunches10 bunches20 bunches30 bunches50 bunches75 bunches100 bunches120 bunches140 bunches

450 460 470 480 490 500Time [ns]

0

1

2

3

4

5

6

7

WFM

am

plitu

de [u

W]

TM delta Horizontal SS1, 17.11.15

1 bunch2 bunches5 bunches10 bunches20 bunches30 bunches50 bunches75 bunches100 bunches120 bunches140 bunches

Signal buildup until steady-statedepending on the number of bunches(0.17 nC per bunch)

TE

TM

Downmix Scans – MethodReidar L. LillestølIPAC 2016Busan, South Korea

For measuring high frequency spectra, we have used a method we call downmix scans.

I When mixing a signal with frequency f0 with a LO at f0 + δf ,we are left with a signal peaked at δf .

I Changing the LO frequency will move the downmixed peak.

I We can downmix with a large span of frequencies, Fourier transforma mixed signal and obtain a frequency ’image’.

t

t

f

f

Frequency

fro

m F

FT

Local oscillator frequency

Downmix Scans – ResultsReidar L. LillestølIPAC 2016Busan, South Korea

24 25 26 27 28Frequency [GHz]

0

100

200

300

400

500

Freq

uenc

y [M

Hz]

24 25 26 27 28Frequency [GHz]

0.05

0.10

0.15

Nor

m. p

ower

spe

ctra

l den

sity

[arb

.u.]

40 bunches, SS140 bunches, SS22 bunches, SS12 bunches, SS2

I Upper right: A frequency image of aWFM signal for the TE-like mode,scanned from 23.7 to 28.8 GHz.

I For each frequency we canI Apply an image mask with 2 lines

that make up a ’V’ with the correctangle (fixed).

I Multiply and average with thefrequency image.

I Lower right: Spectra obtained withthis method.

I Spectra for 2 bunches (0.8 nC perbunch) are similar to the simulatedsingle-bunch spectrum.

I Spectra for 40 bunches (0.35 nC perbunch) have peaks at integers of thebunch frequency at 1.5 GHz.

I For CLIC, the number of bunchesduring machine tuning should takeinto account the WFM spectra.

WFM Resolution Estimates – Simple AnalysisReidar L. LillestølIPAC 2016Busan, South Korea

I A simple estimation of resolutioncan be done by comparing measuredto expected position, or comparingtwo signals.

I Right: 200 shots of TM-like mode inthe horizontal plane, comparing twoaccelerating structures.

I Obtained RMS of 6.6 µm, howeverthe TE-like mode had a spread of15 µm and all vertical signals had aspread of around 20 µm.

0.24 0.27 0.30 0.33 0.36 0.39 0.42Integrated TM-mode Hor. SS2 [mm]

0.10

0.15

0.20

0.25

0.30

Int.

TM H

or. S

S1 [m

m]

RMS = 6.6 um

Corr. coefficient = 0.97

50 bunches, 0.42 nC per bunch

Drive Beam Noise in WFMs (2015)Reidar L. LillestølIPAC 2016Busan, South Korea

300 400 500 600 700Time [ns]

0.0

0.5

1.0

1.5

2.0

Ampl

itude

[uW

]

TE-like

Up, SS1

w/probe beamw/o probe beam

0 1 2 3 4Bunch combination factor [-]

0.0

0.5

1.0

1.5

2.0

2.5

Mea

n D

B n

oise

in W

FMs

[uW

] Up SS1Up SS2 × 0.5Left SS1Left SS2

0 1 2 3 4Bunch combination factor [-]

0.00

0.05

0.10

0.15

0.20

0.25M

ean

DB

noi

se in

WFM

s [u

W]

∆ H, SS1∆ H, SS2∆ V, SS1∆ V, SS2

TE TM

I In the early commissioning, a large amount ofRF noise was seen coming from the drivebeam.

I The noise increased with the drive beamcurrent, and the TE-like mode had much morenoise than the TM-like mode.

Steps taken:I Careful grounding of

acceleratingstructures

I Recabling with highperformance cables

This significantlyreduced the noise level.

Current Noise Level in WFM SignalsReidar L. LillestølIPAC 2016Busan, South Korea

I After the steps to improve noise, we cannot see any significanteffect from the drive beam on the WFM signals.

I There is still some noise that likely comes from the Califesklystron, and this should be removed as well by shielding etc.

150 200 250 300 350Time [ns]

0.00.20.40.60.81.01.2

WFM

Am

plitu

de [u

W]

TE Left, SS1, drive beamTE Left, SS1, no drive beam

0 100 200 300 400 500Time [ns]

0.0

0.2

0.4

0.6

0.8

1.0

1.2

WFM

Am

plitu

de [u

W]

TE mode Up, SS2TE mode Up, SS1TE mode Left, SS2TE mode Left, SS1

WFM Resolution Estimates – SVDReidar L. LillestølIPAC 2016Busan, South Korea

1 2 3 4 5Position number

0

5

10

15

20

25

30

35

40

Res

olut

ion

[um

]

2015

1 2 3 4 5Position number

2016Preliminary

Inductive BPMWFM, TE mode, DownmixedWFM, TE mode, log detectorWFM, TM mode, log detector

1 2 3 4 5Position number

0

5

10

15

20

25

30

35

40

Res

olut

ion

[um

]

2015

1 2 3 4 5Position number

2016Preliminary

Inductive BPMWFM, TE mode, DownmixedWFM, TE mode, log detectorWFM, TM mode, log detector

1 2 3 4 5Position number

0

5

10

15

20

25

30

35

40

Res

olut

ion

[um

]

2015

1 2 3 4 5Position number

2016Preliminary

Inductive BPMWFM, TE mode, DownmixedWFM, TE mode, log detectorWFM, TM mode, log detector

No drive beam

With drive beam

I We can do a more careful resolution analysis usingmodel independent analysis with singular valuedecomposition (SVD).

I The first 2 singular values in our analysis representdispersion/drifts of the machine, and are removed.

I Right: Using the same data as before, we obtainresolutions of 2.3 and 4.6 µm for the TE-like mode.

I However, the TM-like mode is here worse, andseems to drive an eigenmode of their own.

I Left: Recently, there were afew hours with drive beam inthe module.

I A similar SVD analysis gives20–30 µm resolutions for theWFMs.

I However, the same is thecase for all recent data (alsowithout drive beam), so theremust be another cause thatmust be discovered!

I Also, here we have lowstatistics with 200 shots.

50 bunches0.12 nC/bunch

50 bunches0.42 nC/bunch

For CLIC, a cost-effectivereadout system should befound due to the 140,000WFM devices. We consider

I Log detectorsI Downmixing systemI DiodesI IQ demodulatorsI Electro-optical system

Conclusion

I CLIC Wakefield Monitors are tested in the Califes beamline in the CTF3.I Antennas in four damping waveguides measure one TE-like and one TM-like

dipole mode for finding the transverse beam position.I ’Downmix frequency scans’ have been used for finding high frequency spectra.

I The simulated single-bunch structure impedance was seen with 2 bunches.I However, for many bunches there are peaks at harmonics of the bunch frequency.

I At first there was a large amount of noise when the drive beam was operated,however this has now been heavily reduced. Some noise from the Califes klystronremains, but should be reduced as well.

I The best resolution found without drive beam in the module was 2.7 µm.I With drive beam the recently measured WFM resolution is 20–30 µm, however,

this is preliminary and further work is required. Also more statistics should betaken with the SVD analysis.

Thank you for your attention!

Reidar L. LillestølIPAC 2016Busan, South Korea