investigating the diagnostics options and limitations of

Investigating the diagnostics options and limitations of the accelerating structures of CLIC

Halvtidsseminarium

25/2/2016

Jim Ögren

Jim Ögren | Halvtidsseminarium /392016-02-25 2

• Introduction to CLIC• Accelerating structures• Part I: nonlinear fields

Octupole field Beam characterization Alignment

• Part II: vacuum breakdownsField emission Experiment at UU

• Summary• Outlook

Outline


• Hadron colliders good discovery machines

• Need for lepton collider for precision measurements

• Guaranteed program:Higgs and top physics

• Potential program:SUSY models Particle at 750 GeV? To be seen from LHC data

High-energy physics post-LHC?

Why build a lepton collider in the 500 GeV - 3 TeV range?


Compact Linear Collider (CLIC)

• √s = 380 GeV - 3 TeV• High-gradient: 100 MV/m• Two-beam acceleration• Drive beam:

high intensity, low energy • Main beam:

low intensity, high energy • PETS:

Power extraction and transfer structure

power-extraction and transfer structure (PETS)

accelerating structures

quadrupolequadrupole

RF

beam-position monitor

12 GHz, 68 MW

main beam 1.2 A, 156 ns 9 GeV – 1.5 TeV

drive beam 100 A, 239 ns 2.38 GeV – 240 MeV

Source: CLIC CDR

Sou

rce:

CLI

C C

DR

(c)FT

TA

BC2

delay loop2.5 km

decelerator, 25 sectors of 878 m

540 klystrons20 MW, 148 µs

CR2CR1

circumferencesdelay loop 73 mCR1 293 mCR2 439 m

BDS2.75 km

IPTA

BC2

delay loop2.5 km


drive beam accelerator2.4 GeV, 1.0 GHz

CR2CR1

BDS2.75 km

50 kmCR combiner ringTA turnaroundDR damping ringPDR predamping ringBC bunch compressorBDS beam delivery systemIP interaction pointd ump


Drive Beam

Main Beambooster linac2.86 to 9 GeV

e+main linace– main linac, 12 GHz, 72/100 MV/m, 21 km

e+injector2.86 GeV

e+PDR389 m

e+DR

427 me– injector

2.86 GeV

e–DR

427 m

BC1


CLIC test facility 3 (CTF3)

30 GHz test stand 150 MeV e– linac

magnetic chicane pulse compression frequency multiplication

photo injector tests and laser CLIC experimental area (CLEX) with two-beam test stand, probe beam and test beam line

28 A, 140 ns

total length about 140 m

10 m

delay loop

combiner ring

3.5 A, 1.4 μs

• Drive beam studies • Two-beam acceleration and 100 MV/m

achieved • Breakdown studies • Two-beam test-stand (TBTS) • CALIFES 190 MeV e- injector

• Drive beam: 125 MeV, 20 A, 0.83 Hz

• Probe beam:200 MeV

• Rep. rate: 1.66 Hz

Source: CLIC CDR

Source: CLIC CDR


CLIC Acceleration structure

• Transverse wakefield damping slots • Four radial waveguides connected to each cell • Four-fold symmetry => Octupole component of

fundamental frequency mode

• Prototype: TD24_vg1p8 • 24 accelerating cells

+ input/output couplers • 12 GHz traveling wave

disc-loaded structure

W. Wuensch, CLIC Workshop 2016


Observation of octupole component

0 1 2 3 4 5 6 7

−1

−0.8

−0.6

−0.4

−0.2

0

0.2

0.4

0.6

0.8

1

RF accOctupole component

Courtesy of Wilfrid Farabolini, CERN

Courtesy of Alexej Grudiev, CERN

Simulation: Observation in CTF3:

• RF kick can be expressed in magnetic units

• At Vz = 22.8 MV => integrated octupole strength 73.4 kTm/m3

• Kick strongest at RF phase zero-crossing

Screen

ACS

Correctors

Quadrupoles

MT

Q


Octupole kicks

Position shift of center-of-mass on downstream screen due to octupole field:

where

Simulation of beam:

Single particle kick due to octupole:

Effect on beam:

−4 −3 −2 −1 0 1 2 3 4−4

−3

−2

−1

0

1

2

3

4Octupolar field

x

y


Measurement at CTF3

• Scan incoming transversely • Acquiring images with and without

radio-frequency (RF) power in structure • Compare center-of-mass position for

beam with and without RF • Beam at zero-crossing RF phase • E = 194 MeV, ~single bunch Source: TBTS webpage

Screen

ACS

Correctors

QuadrupolesMTV0790

QF0360


Position scan

x

yNo RF With RF

Screen

ACS

Correctors

Quadrupoles

Jim Ögren | Halvtidsseminarium /392016-02-25

−1.5 −1 −0.5 0 0.5 1 1.5−0.2

−0.1

0

0.1

0.2Horizontal position shift

Vertical position [mm]

Shift

in h

oriz

onta

l pos

ition

[mm

]

DataFit

−1.5 −1 −0.5 0 0.5 1 1.5−0.4

−0.2

0

0.2

0.4Vertical position shifts


Shift

in v

ertic

al p

ositi

on [m

m]

DataFit

11

Position shifts

Simultaneous least square fit, ansatz:

Result: C3l = 14±2 kTm/m3

At Vz = 5.1 MV, simulation C3l = 16.4 kTm/m3


Moving position in ACS changes beam size

−1.5 −1 −0.5 0 0.5 1 1.5 20

0.1

0.2

0.3

0.4


Hor

izon

tal w

idth

[mm

2 ]

No RFRF

−1.5 −1 −0.5 0 0.5 1 1.5 20

0.1

0.2


Cor

rela

tion

[mm

2 ]

No RFRF

−1.5 −1 −0.5 0 0.5 1 1.5 20

0.1

0.2

0.3

0.4


Verti

cal w

idth

[mm

2 ]

No RFRF

We observed changes in beam size.


By changing the strength of a quadrupole and observing the change in beam size we can determine the incoming beam distribution.

Single particle description

Quadrupole scan

Screen Quadrupole

L

Single particle transport

Beam transport

For quadrupole + drift, 2D

By Andre.holzner - python/matplotlib, CC BY-SA 3.0, https://en.wikipedia.org/w/index.php?curid=37948467

https://en.wikipedia.org/w/index.php?curid=37948467


Measuring the beam matrix

−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.5−10

−8

−6

−4

−2

0

2

4

6

8

10

x [arb. length]

B y [arb

. mag

netic

uni

ts]

By = kx3

If y=0

Position dependent gradient:

• Locally, beam with transverse spread in octupolar field experience a linear gradient

• Effect (locally) similar to a quadrupole • Position scan in octupole yield same information as a quadrupole scan


Change in beam size due to octupole

The full 4x4 transverse beam matrix

where

Symmetric:

Transfer matrix with coupling:

Transport

where


Full analytical expressionSingle particle on screen Beam size on screen

Average over distribution

Beam size on screen due to octupole


Simulation

• Simulated 105 particles • Parameters similar to CTF3 • Input beam distribution with correlations • Full analytical expression and linear

expression

−1.5 −1 −0.5 0 0.5 1 1.50.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8


Hor

izon

tal w

idth

[mm

2 ]

Multi−particleAnalytical − linearAnalytical − full

Horizontal width

−1.5 −1 −0.5 0 0.5 1 1.50

0.05

0.1

0.15

0.2

0.25

0.3

0.35


Verti

cal w

idth

[mm

2 ]

Multi−particleAnalytical − linearAnalytical − full

Vertical width

Jim Ögren | Halvtidsseminarium /392016-02-25

−2 −1 0 1 20

0.1

0.2

0.3

0.4


Hor

izon

tal b

eam

siz

e [m

m2 ]

DataFit − linearFit − full

18

Fit data from scan

−2 −1 0 1 20

0.1

0.2

0.3

0.4


Verti

cal b

eam

siz

e [m

m2 ]


−2 −1 0 1 20

0.1

0.2


Cor

rela

tion

term

[mm

2 ]


• Parametrization of beam matrix• Start with random seed• Full analytical expressions,

nonlinear:Fit directly to elements of beam-matrix


Fit results

For more information:

• Retrieved the beam matrix elements

• Correlations small


Summary

Screen

ACS

Correctors

Quadrupoles

−4 −3 −2 −1 0 1 2 3 4−4

−3

−2

−1

0

1

2

3

4Octupolar field

x

y

−1.5 −1 −0.5 0 0.5 1 1.5−0.2

−0.1

0

0.1



Shift

in h

oriz

onta

l pos

ition

[mm

]

DataFit

−1.5 −1 −0.5 0 0.5 1 1.5−0.4

−0.2

0

0.2



Shift

in v

ertic

al p

ositi

on [m

m]

DataFit

−2 −1 0 1 20

0.1

0.2

0.3

0.4


Hor

izon

tal b

eam

siz

e [m

m2 ]


−2 −1 0 1 20

0.1

0.2

0.3

0.4


Verti

cal b

eam

siz

e [m

m2 ]


−2 −1 0 1 20

0.1

0.2


Cor

rela

tion

term

[mm

2 ]


FitC3l

Transverse position scan

Position shifts

Beam size


• From position shifts we can determine the electromagnetic center of the structure

• Scan position and monitor beam position with a beam position monitor or screen

Accelerating structure alignment

−1.5 −1 −0.5 0 0.5 1 1.5−0.2

−0.1

0

0.1



Shift

in h

oriz

onta

l pos

ition

[mm

]

DataFit

−1.5 −1 −0.5 0 0.5 1 1.5−0.4

−0.2

0

0.2



Shift

in v

ertic

al p

ositi

on [m

m]

DataFit

• Fit to determine offsets a, b:


Simulation


Simulation

Centroid x position [mm]-2 -1 0 1 2

Cent

roid

y p

ositio

n [m

m]

-1.5

-1

-0.5

0

0.5

1

1.5

2No OctupoleOctupole


Simulation 2


Simulation 2

Centroid x position [mm]-1 -0.5 0 0.5 1

Cent

roid

y p

ositio

n [m

m]

-1

-0.5

0

0.5



Finding the center

2

x [mm]

Horizontal Position Shift

0

-2-2-1

y [mm]

01

1

-1

-0.5

0

0.5

2

Posi

tion

shift

[mm

]

2

x [mm]

0

Vertical Position Shift

-2-2-1

y [mm]

01

1

-1

-0.5

0

0.5

2

Posi

tion

shift

[mm

]

Centroid x position [mm]-1 -0.5 0 0.5 1

Cent

roid

y p

ositio

n [m

m]

-1

-0.5

0

0.5



• In CLIC modules: power at least two structures at the same time

Aligning 2 structures

• If distance between octupole is small compared to distance to screen we can simplify expression.

• Only total effect, i.e. the sum of the offsets not the individual contribution from offsets in each octupole.


Cent

roid

y p

ositio

n [m

m]

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2Kicks 1 Octupole

No OctupoleOctupoleCenter 1stCenter 2nd


Cent

roid

y p

ositio

n [m

m]

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2Kicks 2 Octupoles

No OctupoleOctupoleCenter 1stCenter 2nd

Fitting gives sum of offsets within ~10%


Part II: Vacuum breakdowns


CLIC and RF Breakdowns• Compact Linear Collider

High-gradient, high power ~140,000 accelerating structures Breakdowns limit performance

• Vacuum dischargesComplex phenomenon DC-experiments Field emission studies

RF breakdown

(c)FT

TA

BC2

delay loop2.5 km

decelerator, 25 sectors of 878 m


CR2CR1

circumferencesdelay loop 73 mCR1 293 mCR2 439 m

BDS2.75 km

IPTA

BC2

delay loop2.5 km



CR2CR1

BDS2.75 km

50 kmCR combiner ringTA turnaroundDR damping ringPDR predamping ringBC bunch compressorBDS beam delivery systemIP interaction pointd ump


Drive Beam

Main Beambooster linac2.86 to 9 GeV

e+main linace– main linac, 12 GHz, 72/100 MV/m, 21 km

e+injector2.86 GeV

e+PDR389 m

e+DR

427 me– injector

2.86 GeV

e–DR

427 m

BC1

R. Behrisch, in Physics of Plasma-wall Interactions in Controlled fusion, NATO ASI series B 131 (1986) 495


• Electrons tunnel through barrier under presence of external field.

• Fowler-Nordheim eq:

Field Emission

Field enhancement β can be determined from the slope b:

• Microscopic protrusions and surface features enhances the local field: positive

negative


Conditioning

• ConditioningStructures perform better over time Depends on the number of pulses not breakdowns



Conditioning

• ConditioningStructures perform better over time Depends on the number of pulses not breakdowns


E. Rodríguez Castro, CLIC Workshop


Scanning electron microscope

Vacuum chamber

electron beam

x

y

z

T

R

Stage holder

x

yzW tip

Cu sample

Right: SEM Down right: vacuum chamber Below: 3 degrees of freedom tip and sample surface, 5 degrees of freedom on SEM stage holder.


• Continuation of T. Muranaka’s work• Cu sample• W tip, radius of curvature 5 μm.• Piezo-motors for 3D control with

position sensors with nm precision• Keithley 6517a Electrometer for

measuring FE currentsSourcing up to 1 kV Range from sub-pA to mA 50 Hz sample rate

• SEMEnvironmental SEM Field emitting gun, 10-30 kV Vacuum ~7×10-5 mBar

In-situ SEM setup


Knowing the gap distance• Find surface method:

Set low voltage 1 V Approach tip in steps of 2 nm Measure current Repeat until current exceeds threshold

• High reproducibility10 repeated times: σ ≈ 20 nm

• Small marks on surfaceUse surrounding positions

Tip at surface


Knowing the gap distance• Find surface method:

Set low voltage 1 V Approach tip in steps of 2 nm Measure current Repeat until current exceeds threshold

• High reproducibility10 repeated times: σ ≈ 20 nm

• Small marks on surfaceUse surrounding positions

Tip at surface

Accidents happen…


Preliminary results: Voltage scans

Voltage [V]120 130 140 150 160 170 180 190

Cur

rent

[A]

×10-7

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

Voltage [V]0 50 100 150 200 250

Cur

rent

[A]

×10-6

0

0.2

0.4

0.6

0.8

1

1.2

1/V [V-1] ×10-35 5.5 6 6.5 7 7.5 8

Ln(I/

V2 )

-36

-34

-32

-30

-28

-26

-24Fit: a =-3990.8738 and b = -3.9381. R2 = 0.99447.

DataROI

1/V [V-1]0 0.005 0.01 0.015 0.02

Ln(I/

V2 )

-36

-34

-32

-30

-28

-26

-24Fit: a =-3990.8738 and b = -3.9381. R2 = 0.99447.

DataROIy=ax+b Gap distance

500 nm

β = 31

Scan step0 20 40 60 80 100

Beta

0

10

20

30

40

50

60

70

80

90

100

100 Voltage scans. Gap distance = 500 nmMean value = 29, standard dev. = 7

Repeated 100 times


De-conditioning?

Voltage [V]0 500 1000 1500 2000 2500 3000

Cur

rent

[A]

×10-8

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Voltage [V]0 500 1000 1500 2000 2500 3000

Cur

rent

[A]

×10-8

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Voltage [V]0 500 1000 1500 2000 2500 3000

Cur

rent

[A]

×10-8

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Scan step0 5 10 15 20

Max

vol

tage

[V]

2500

2600

2700

2800

2900

3000

3100

3200

• Multiple scans at DC-spark setup at CERN.• Emission at lower voltages in later scans• Maximum voltage decrease over time,

seems like surface performs worse over time => de-conditioning?

Scan 1 Scan 10 Scan 20


Multiple F-N curves

Voltage [V]0 50 100 150 200 250 300 350 400 450

Cur

rent

[A]

×10-7

0

1

2

3

4

5

6

7

8

9

• In some cases we observed multiple F-N characteristics.• Many measurements also yielded no F-N characteristics at all

Voltage [V]0 100 200 300 400 500 600 700

Cur

rent

[A]

×10-6

0

0.2

0.4

0.6

0.8

1

1.2

Pulling/burning away a protrusion? Dynamic changes on surface?


• Ramp voltage to a certain current threshold and then reverse voltage

• Mostly symmetric behaviour but in some cases asymmetric.

Activation effect Removal of oxidation layer? Other changes on surface?

Activation effect

Data number0 200 400 600 800 1000 1200

Cur

rent

[A]

×10-7

0

0.5

1

1.5

2Gap distance = 700 nm

Volta

ge [V

]

0

100

200

300

400


• In some voltage scans we have seen crater formation

• Assign given set of I-V measurements to an observed surface change

• Crater size similar to tungsten tip ~5 μm.

Crater due to breakdownBefore

After


SummaryThe CLIC accelerating structures have an octupole component that can be utilised for

Measuring the full transverse beam matrix Find the EM center of the structures and aligning the beam

Vacuum discharges are a limiting factor for high-gradient acceleration Field emission is important for both evolution of a breakdown and for conditioning We have a setup for studying field emission inside a SEM here at UU, also a DC-spark setup at CERN. So far we have seen different I-V characteristics and there are many questions that need further investigation

OutlookFurther investigate the possibility of utilising the octupole component for beam alignment. Hopefully test method at CTF3. Continue DC field emission measurements:

Correlate with surface changes (UU) Long-term data (CERN) and conditioning

Summary and outlook


Thank you for your attention!

investigating the diagnostics options and limitations of

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