www.lightsource.ca main bullet #1 main bullet #2 main bullet #3 advances in coherent synchrotron...
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Main Bullet #1 Main Bullet #2 Main Bullet #3
Advances in Coherent Synchrotron Radiation at
the Canadian Light Source
Jack BergstromCLS 13th Annual Users Meeting
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Jack Bergstrom
Brant Billinghurst
TimMay
LesDallin
WardWurtz
All of the CLS staff who make
this work possible
Markde Jong
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f (GHz) 1/λ (cm-1) DevicesMicrowave 1-102 0.03-3 Oscillators
THz 102-104 3-300 Photoconductors
Infrared 104-106 300-30000 Thermal sources
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•Most Sources limited in intensity and brightness
P ≈ nW – μW
•Detector and imaging technology
•Many physical and chemical processes fall within the THz domain
•A “Gap” existed between the requirements and the availability of sources within the THz region
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Since 2004 accelerator-based technologies are producing intense Coherent Synchrotron Radiation (CSR) in the terahertz region
Electron Accelerator criteria: Electron Beam packaged in short bunches
• σ < few mm High Energy
• E > 500 MeV Radiating apparatus
• Dipole Magnet, Wiggler, etc. Extraction Beamline
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Normal Synchrotron Radiation
Coherent Synchrotron Radiation
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Bunch with N electrons undergoes acceleration a
Random radiation phases (incoherent)
2a2 Ne2
3c2
(Ne)2
Coherent Radiation Phases
P[coherent]
P[incoherent]= N ≈ 106 - 1010
Power =
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1. Bunch σ < λ (typically < 1 ps)This requires specialized electron machines
– Free electron Lasers (FEL)– Energy Recovering Linacs (ERL)
Power ~ 1W/cm-1
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I. Bursting Mode• Beam Instability• Micro-Bunching
Fill PatternFew Bunches - 1 to 10 mA /bunch
2. Bunch σ > λ (typically ≈ 1-10 ps)Can be done using Storage Rings
II. Continuous mode• Static Bunch-Shape Distortion • Shark fin charge profile
Fill PatternHundreds of Bunches 10 to 100 μA/Bunch
III. Laser ModulationIV. Femto-slicing
Power ~ 1mW/cm-1
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The CLS uses both Bursting and Continuous Modes
Bursting Mode at 2.9 GeV:1-3 bunches; Ib~ 7 mA
Continuous Mode at 1.5 GeV:70-210 bunches; Ib~ 30 μA
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E
Ez
Radiation from the bunch “tail”can effect the bunch head
This provides a longitudinal force on 2
The energy loss by 1 and the gain by 2 causes them to move closer together
This is called the longitudinal wakefield W(z)
This in turn causes Micro-Bunching
Transverse E field from 1 causes a longitudinal Ez field in the frame 2
121
2
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12
1
2
Energies Eo
1 loses energy ΔE1
2 gains energy ΔE2
Magnetic field with dispersion D
R
R1
R2
1 : Eν- ΔE1
2 : Eν+ ΔE2
ΔX=D*ΔE/Eo
R→R+ΔX
Since v≈cboth
particles travel the
same distance
Thus the distance between particles is reduced causing Micro-Bunching
Comment: (D/R) is called the
Momentum Compaction
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Time Scale Burst duration: 50-200 μs Burst Period: 1-10 ms
Threshold Current:Micro-bunch instability threshold Ibunch depends on the bunch length σ:
Ibunch ≈ 1-10 mA
σ ≈ few mm
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An important parameter in CSR isthe so-called Radiation Impedance Z(ω):
Fourier transform of the wakefield:Z(ω) = 1/c ∫W(z) e-iωz/c dz
The spectrum of the radiation becomes dP/dω = e2Z(ω)/π
This is Ohm’s law for CSR:Power α I2Z
Big Impedance → lots of CSR
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Ib<< Bursting threshold Bunch shape is static
ρ(z)
z
Standard Bunch Shape is a Gaussian
Frequency distribution: f(ω)=∫ρ(z)eiωz/c dz
Frequency components with ω ≈ 2πc/λ will radiate CSR at λ
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Gaussian bunch:
2
)( 2
)(
efwhere ω=2πc/λ
σ≈ few mmλ≈ 1 mm f(ω) = VERY SMALL
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Deform the bunch to produce high ω components
HOW ??Nature does it for free, using
Radiation Impedance
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3/1
3/1 2
1
2
3
3
)3/2()(
oo iZZ
Revolution frequency
Z(ω)Real part (Resistive)
Imaginary part (Reactive)
Re Z(ω) creates a static asymmetry within the bunch
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ρ(z)
z
Front Back
n electrons
•Shark fin profile•CSR power α n2
•Continuous emission
High FrequencyComponent
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Shark Fin CSR power α n2
Efficiency is much higher for short bunches
Storage ring is re-configured for σ ≈ few mm (versus ≈ 10 mm)
σ ≈ √α so reduce α
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0 10 20 30 40
0
20
40
60
80
100
120
Inte
nsi
ty (
arb
itra
ry u
nits
)
Frequency (cm-1)
0 10 20 30 40-0.05
0.00
0.05
0.10
0.15
0.20
Inte
nsity
(ar
bitr
ary
units
)
Frequency (cm-1)
CSR
SR
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0 10 20 300
20
40
60
80
100
Frequency (cm-1)
7.76mA 7.45mA 7.26mA 6.82mA
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Three Layers of Structure are observed in CSR Coarse Structure ≈ 1 cm-1
Fine Structure ≈ 0.073 cm-1
Very Fine Structure ≈ 0.016 cm-1 (Only Multi-bunch) Coarse and Fine Structures are independent of
storage ring operation Energy Current Fill pattern Time Structure (Bursting or Continuous)
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100
20
40
60
80
100
120
140
160
180
200
Frequency (cm-1)
100
20
40
60
80
100
120
140
160
180
200
Frequency (cm-1)
100
20
40
60
80
100
120
140
160
180
200
Frequency (cm-1)
9.0 9.2 9.4 9.6 9.8 10.00
20
40
60
80
100
Frequency (cm-1)
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Instrumentation ? Reflections ? Vacuum Chamber ?
Vacuum Chamber geometry determines the Radiative Impedance Z(ω)
• P(ω) ≈ I2Z(ω)
Structure in Z(ω)→Structure in P(ω) Modify Chamber→Modify Z(ω) →Modify P(ω) Experiment using a plunger to modify the chamber
caused no major changes to P(ω)
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Attributed to Bunch to Bunch Interference
9.0 9.1 9.20
20
40
60
80
Frequency (cm-1)
0.0167 cm-1
1/Bunch spacing
This is a Multi-bunch
effect observed only in the
continuous CSR mode
In this case the ring was filled with
210 bunches
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1↔12↔2...
1↔22↔3...
1↔32↔4...
1↔42↔5...
1↔52↔6...
1↔62↔7...
1↔72↔8...
1↔82↔9...
3 4 5 6 7 8 9 10 11 12 13
0
50
100
150
200
Inte
nsi
ty (
Arb
itra
ry u
nits
)
Frequency (cm-1) 6.00 6.02 6.04 6.06 6.08 6.10 6.12 6.14 6.16 6.18 6.20
Inte
nsity
(A
rbitr
ary
units
)
Frequency (cm-1)
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Pmb(ω)= Psb(ω) xsin (NbωT/2)
sin (ωT/2)
2
9.0 9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8 9.9 10.00
20
40
60
80
Frequency (cm-1)
Determined by bunch shapeand radiation impedance
6.00 6.02 6.04 6.06 6.08 6.10 6.12 6.14 6.16 6.18 6.20
Inte
nsi
ty (
Arb
itra
ry u
nits
)
Frequency (cm-1)
Correct Positions and Widths
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Psb(ω) α Ne2 (CSR)
Interference term α Nb2
Peak Power α Nb2 Psb(ω)→ (NbNe)2
Average Power α Nb Psb(ω)→ Nb(Ne)2
But ... This appears to be a solution in search of a problem.
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Please visit the poster entitled :
Photoacoustic Spectroscopy Photoacoustic Spectroscopy Using Coherent Synchrotron Using Coherent Synchrotron
RadiationRadiation
Which is being presented by Dr. Kirk Michaelian