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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