Wakefield and Kick measurements

at

Antonio Gilardi

Kyrre Ness Sjobak

Acknowledgment:

Pasquale Arpaia – Hikmet Bursali – Roberto Corsini – Wilfrid Farabolini – Davide Gamba

– Andrea Latina – Vivien Rude

CLIC project meeting, 23/7/2020

1

Alignment of CLIC structure

2

● To achieve high luminosity, CLIC main beam accelerating structures

must be aligned better than 3.5 µm relative to the beam○ This is achieved by pre-alignment (14 µm) followed by

active alignment using movers on the girders that hold the structures

● Wake fields are an important cause of emittance degradation○ Need to verify that they are correctly captured by simulation models

● Wake field monitors are used to monitor the offset of the beam

relative to the accelerating structure

2x TD26s with Wakefield Monitors

3

eBunch

Ideal trajectory

Real trajectory

Well known effect, the WAKEFIELD

4Sjobak, Kyrre, et al. "Status of the CLEAR electron beam user facility at CERN." 10th Int. Particle Accelerator Conf.(IPAC’19), Melbourne, Australia. 2019.

5Sjobak, Kyrre, et al. "Status of the CLEAR electron beam user facility at CERN." 10th Int. Particle Accelerator Conf.(IPAC’19), Melbourne, Australia. 2019.

CLIC BPM

CLIC

components

C) D)

GIRDER

VERTICAL MOVER

GIRDER

VERTICAL MOVER

GIRDER

CLIC ACCELLERATING

STRUCTURE

RADIATION

MONITORRADIATION

MONITOR

RADIATION

MONITOR

CLIC BMPsBEAM

SCREEN

WFM

6

CLIC BPM

CLIC

components

C) D)

GIRDER

VERTICAL MOVER

GIRDER

VERTICAL MOVER

GIRDER

CLIC ACCELLERATING

STRUCTURE

RADIATION

MONITORRADIATION

MONITOR

RADIATION

MONITOR

CLIC BMPsBEAM

SCREEN

WFM

7

•All the preliminary scans were done to

understand the limitation of the method

•The main limitation comes from a

mechanical constraint while limiting the

aperture.8

Scanning the

beam position

using a

magnetic bump

Calibration of bump accuracy about 1% (as long as energy is correctly measured)

Imposed V position [mm]Imposed V position [mm]

V p

ositio

n o

n s

cre

en [m

m]

V p

ositio

n o

n s

cre

en [m

m]

2020-0

5-28

_14-32-3

3_V7

30-3

0pC-

1.6p

s-1b

unch

-205

MeV

2020-0

5-28

_14-25-5

2_V6

20-3

0pC-

1.6p

s-1b

unch

-205

MeV

205 MeV, 1 bunch, 16

pC

205 MeV, 1 bunch, 16

pC

9

10

SLOPE

ENERGY

CHARGE

11

SLOPE

ENERGY

CHARGE

12

SLOPE

ENERGY

CHARGE

13

Medium Charge: 500pCLow Charge: 50pC

1 ― Evaluate the passive

kick received by the beam

2 ― Asses the electric

center of the structure

Measurements

14

Measurements1 ― Evaluate the passive

kick received by the beam

Kick independent of #bunches,

scaling with bunch charge

Dominated by short range wake 15

10 bunches

1 bunch

16 bunches

Arpaia, P., Corsini, R., Gilardi, A., & Sjobak, K. N. (2019, May). Beam–based alignment of the

CLIC high-gradient X-Band accelerating structure using beam-screen. In 2019 IEEE

International Instrumentation and Measurement Technology Conference (I2MTC) (pp. 1-6).

IEEE.

Measurements1 ― Evaluate the passive

kick received by the beam

2 ― Asses the electric

center of the structure

Kick independent of #bunches,

scaling with bunch charge

Dominated by short range wake 16

For comparison with WFM

(around 100um)

10 bunches

1 bunch

16 bunches

Arpaia, P., Corsini, R., Gilardi, A., & Sjobak, K. N. (2019, May). Beam–based alignment of the

CLIC high-gradient X-Band accelerating structure using beam-screen. In 2019 IEEE

International Instrumentation and Measurement Technology Conference (I2MTC) (pp. 1-6).

IEEE.

Average beam position

-3.1

-3

-2.9

-2.8

-2.7

-3.2

-3.3

-1.5 -1 -0.5 0 0.5

Structure position [mm]

X p

ositio

n [m

m]

-1.5 -1 -0.5 0 0.5

Structure position [mm]

-3.4

-3.3

-3.2

-3.1

-3

-2.9

-2.8

-2.7

-2.6

X p

ositio

n [m

m]

Average Beam Position X

BLUE = 1 Bunch SLOPE=0.37248

RED = 10 Bunches SLOPE=0.26134

BLACK = 16 Bunches SLOPE=0.41451

3 ― Bunch-length dependency

17

Bunch length measured with RF deflector,

beam optics for better resolution

Scanning charge and

bunch length,

measuring kick

18

3 ― Charge dependency

Single bunch

Single bunch ― Same bunch length

19

Early result from kick measurements in CLEAR

(Spring 2019)From the Wakefield simulations we expect: 115 V/(pC m mm).

From the data we got a value of: 1200 V /(pC m mm).

Thanks to a first comparison with simulation a wrong calibration

of the ICT in the CLEAR beam line was discovered.

This was corrected in spring 2019

After correction

ICT

BP

M

CST Simulation

20

● WFMs measure the

wakefields on their

way to the HOM load

● 2 pickups on each arm

● Hybrids make Δ and Σ

for each plane (H/V)

● No active electronics

in tunnel

● Filters (18 / 24 GHz),

power limiters,

log detectors in gallery

Wake Field Monitor (WFM)

signals and setup

Extended

wakefield

damping WG

RF

lo

ad

s

ab

so

rb H

OM

po

we

r

BEAM 21

● WFMs measure the

wakefields on their

way to the HOM load

● 2 pickups on each arm

● Hybrids make Δ and Σ

for each plane (H/V)

● No active electronics

in tunnel

● Filters (18 / 24 GHz),

power limiters,

log detectors in gallery

Wake Field Monitor (WFM)

signals and setup

Extended

wakefield

damping WG

RF

lo

ad

s

ab

so

rb H

OM

po

we

r

BEAM

4 HOM WGs

pr cell

22

● WFMs measure the

wakefields on their

way to the HOM load

● 2 pickups on each arm

● Hybrids make Δ and Σ

for each plane (H/V)

● No active electronics

in tunnel

● Filters (18 / 24 GHz),

power limiters,

log detectors in gallery

Wake Field Monitor (WFM)

signals and setupPickups

8 in total

Hybrid

for Σ and Δ

4 in total

Extended

wakefield

damping WG

RF

lo

ad

s

ab

so

rb H

OM

po

we

r

BEAM

4 HOM WGs

pr cell

23

WFM signal treatment

● WFM signal measured by log-

detectors behind filters○ Filtering on beam harmonics

🠞 Works best for trains,

needs about 30 CLEAR bunches

● Convert Log-detector output to

equivalent input volts

● Integrate linearized voltage signal

Single number proportional to

wakefield excitation

'2019-07-13_01-19-34_bumpscanH_730_50bunches' 195 pC/bunch, ver long bunch

24

Lin

eari

zed

data

Center measurement

● Least squares fit of V-function as a

function of beam position

● Fitted parameters:

x0 [mm], y0 [V], a [V/mm]

● Method is robust to “softness”

around minimum and automatic

'2019-07-13_01-19-34_bumpscanH_730_50bunches' 195 pC/bunch, ver long bunch

25

Separation

of

H and V

plane

is good

2019-07-13_03-36-21_bumpscanH_730_30bunches_defaultcharge

2019-06-27_17-55-09_bumpScanV-screen730-highCharge

(But some cross-talk

from H to TE Δ V)

26

WFM, calibration with charge

● See a clear linear response in

charge as expected

● Much stronger signal

for TM than TE

● Note that quality of charge

measurements were a problem

during these runs

30 bunches, ≅3.5 ps. 13/7/2019

27

WFM: Mystery of offset x0

between signals

● Often notice that minima are

separated by ≅200 µm

● One hypothesis was that this is

caused by beam angle○ Calibrated bump can control angle and

position simultaneously

○ Experiment makes hypothesis

seem unlikely

● Will compare to center

defined by kicks○ Installed 2nd upstream profile monitor

during winter shutdown so that initial

beam trajectory can be re-established

after changing charge setting

50 bunches, 195 pC/bunch, very long bunches. 13/7/2019

28

TM Σ hor. very

weak, canceled

by phase shifter

Conclusions and outlook

● Magnetic bump commissioned; fast and little impact to rest of machine○ Position offset systematic error≅ 1%

○ We plan to verify results of kick and WFM using module mover

● Have undertaken detailed studies on effect of charge on kick

● Data indicate strong domination of short range over long range kick

for CLEAR bunch lengths

● Currently investigating the effect of bunch length on kick

● Kick is compared to Karl Bane formula and CST simulation

● WFM characterization continues○ Will properly map charge-, trainlength- and bunch length dependence,

and compare zero positions to kick; more instrumentation installed in winter shutdown

● Data quality challenging; need a low emittance beam for many combinations

of charge, bunch length, and number of bunches

29

30

31

Early result from kick measurements in CLEAR

(Spring 2019)

WFM: Mystery of offset x0

between signals

32

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