investigating the diagnostics options and limitations of
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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
Jim Ögren | Halvtidsseminarium /392016-02-25 3
• 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?
Jim Ögren | Halvtidsseminarium /392016-02-25 4
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
540 klystrons20 MW, 148 µs
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 accelerator2.4 GeV, 1.0 GHz
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
Jim Ögren | Halvtidsseminarium /392016-02-25 5
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
Jim Ögren | Halvtidsseminarium /392016-02-25 6
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
Jim Ögren | Halvtidsseminarium /392016-02-25 7
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
Jim Ögren | Halvtidsseminarium /392016-02-25 8
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
Jim Ögren | Halvtidsseminarium /392016-02-25 9
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
Jim Ögren | Halvtidsseminarium /392016-02-25 10
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
Vertical position [mm]
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
Jim Ögren | Halvtidsseminarium /392016-02-25 12
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
Vertical position [mm]
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
Vertical position [mm]
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
Vertical position [mm]
Verti
cal w
idth
[mm
2 ]
No RFRF
We observed changes in beam size.
Jim Ögren | Halvtidsseminarium /392016-02-25 13
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
Jim Ögren | Halvtidsseminarium /392016-02-25 13
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
Jim Ögren | Halvtidsseminarium /392016-02-25 14
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
Jim Ögren | Halvtidsseminarium /392016-02-25 15
Change in beam size due to octupole
The full 4x4 transverse beam matrix
where
Symmetric:
Transfer matrix with coupling:
Transport
where
Jim Ögren | Halvtidsseminarium /392016-02-25 16
Full analytical expressionSingle particle on screen Beam size on screen
Average over distribution
Beam size on screen due to octupole
Jim Ögren | Halvtidsseminarium /392016-02-25 17
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
Vertical position [mm]
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
Vertical position [mm]
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
Vertical position [mm]
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
Vertical position [mm]
Verti
cal b
eam
siz
e [m
m2 ]
DataFit − linearFit − full
−2 −1 0 1 20
0.1
0.2
Vertical position [mm]
Cor
rela
tion
term
[mm
2 ]
DataFit − linearFit − full
• Parametrization of beam matrix• Start with random seed• Full analytical expressions,
nonlinear:Fit directly to elements of beam-matrix
Jim Ögren | Halvtidsseminarium /392016-02-25 19
Fit results
For more information:
• Retrieved the beam matrix elements
• Correlations small
Jim Ögren | Halvtidsseminarium /392016-02-25 20
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
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
Vertical position [mm]
Shift
in v
ertic
al p
ositi
on [m
m]
DataFit
−2 −1 0 1 20
0.1
0.2
0.3
0.4
Vertical position [mm]
Hor
izon
tal b
eam
siz
e [m
m2 ]
DataFit − linearFit − full
−2 −1 0 1 20
0.1
0.2
0.3
0.4
Vertical position [mm]
Verti
cal b
eam
siz
e [m
m2 ]
DataFit − linearFit − full
−2 −1 0 1 20
0.1
0.2
Vertical position [mm]
Cor
rela
tion
term
[mm
2 ]
DataFit − linearFit − full
FitC3l
Transverse position scan
Position shifts
Beam size
Jim Ögren | Halvtidsseminarium /392016-02-25 21
• 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
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
Vertical position [mm]
Shift
in v
ertic
al p
ositi
on [m
m]
DataFit
• Fit to determine offsets a, b:
Jim Ögren | Halvtidsseminarium /392016-02-25 22
Simulation
Jim Ögren | Halvtidsseminarium /392016-02-25 22
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
Jim Ögren | Halvtidsseminarium /392016-02-25 23
Simulation 2
Jim Ögren | Halvtidsseminarium /392016-02-25 23
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
1No OctupoleOctupole
Jim Ögren | Halvtidsseminarium /392016-02-25 24
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
1No OctupoleOctupole
Jim Ögren | Halvtidsseminarium /392016-02-25 25
• 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.
Centroid x position [mm]-2 -1 0 1 2
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
Centroid x position [mm]-2 -1 0 1 2
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%
Jim Ögren | Halvtidsseminarium /392016-02-25 26
Part II: Vacuum breakdowns
Jim Ögren | Halvtidsseminarium /392016-02-25 27
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
540 klystrons20 MW, 148 µs
CR2CR1
circumferencesdelay loop 73 mCR1 293 mCR2 439 m
BDS2.75 km
IPTA
BC2
delay loop2.5 km
540 klystrons20 MW, 148 µs
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 accelerator2.4 GeV, 1.0 GHz
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
Jim Ögren | Halvtidsseminarium /392016-02-25 28
• 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
Jim Ögren | Halvtidsseminarium /392016-02-25 29
Conditioning
• ConditioningStructures perform better over time Depends on the number of pulses not breakdowns
W. Wuensch, CLIC Workshop 2016
Jim Ögren | Halvtidsseminarium /392016-02-25 29
Conditioning
• ConditioningStructures perform better over time Depends on the number of pulses not breakdowns
W. Wuensch, CLIC Workshop 2016
E. Rodríguez Castro, CLIC Workshop
Jim Ögren | Halvtidsseminarium /392016-02-25 30
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.
Jim Ögren | Halvtidsseminarium /392016-02-25 31
• 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
Jim Ögren | Halvtidsseminarium /392016-02-25 32
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
Jim Ögren | Halvtidsseminarium /392016-02-25 32
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…
Jim Ögren | Halvtidsseminarium /392016-02-25 33
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
Jim Ögren | Halvtidsseminarium /392016-02-25 34
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
Jim Ögren | Halvtidsseminarium /392016-02-25 35
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?
Jim Ögren | Halvtidsseminarium /392016-02-25 36
• 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
Jim Ögren | Halvtidsseminarium /392016-02-25 37
• 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
Jim Ögren | Halvtidsseminarium /392016-02-25 38
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
Jim Ögren | Halvtidsseminarium /392016-02-25 39
Thank you for your attention!
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