Main Bullet #1 Main Bullet #2 Main Bullet #3 Advances in Coherent Synchrotron Radiation at the Canadian Light Source Jack Bergstrom CLS 13 th Annual Users Meeting

Jack Bergstrom Brant Billinghurst Tim May Les Dallin Ward Wurtz All of the CLS staff who make this work possible Mark de Jong

f (GHz)1/λ (cm -1 )Devices Microwave Oscillators THz Photoconductors Infrared Thermal sources

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

 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

Normal Synchrotron Radiation Coherent Synchrotron Radiation

Bunch with N electrons undergoes acceleration a Random radiation phases (incoherent) 2a 2 Ne 2 3c 2 (Ne) 2 Coherent Radiation Phases P[coherent] P[incoherent] = N ≈ Power =

1. Bunch σ < λ (typically < 1 ps) This requires specialized electron machines – Free electron Lasers (FEL) – Energy Recovering Linacs (ERL) Power ~ 1W/cm -1

I. Bursting Mode Beam Instability Micro-Bunching Fill Pattern Few 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 Pattern Hundreds of Bunches 10 to 100 μA/Bunch III. Laser Modulation IV. Femto-slicing Power ~ 1mW/cm -1

The CLS uses both Bursting and Continuous Modes Bursting Mode at 2.9 GeV: 1-3 bunches; I b ~ 7 mA Continuous Mode at 1.5 GeV: bunches; I b ~ 30 μA

E EzEz 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 E z field in the frame

Energies E o 1 loses energy ΔE 1 2 gains energy ΔE 2 Magnetic field with dispersion D R R1R1 R2R2 1 : E ν - ΔE 1 2 : E ν + ΔE 2 ΔX=D*ΔE/E o R→R+ΔX Since v≈c both particles travel the same distance Thus the distance between particles is reduced causing Micro-Bunching Comment: (D/R) is called the Momentum Compaction

 Time Scale  Burst duration: μs  Burst Period: 1-10 ms  Threshold Current: Micro-bunch instability threshold I bunch depends on the bunch length σ: I bunch ≈ 1-10 mA σ ≈ few mm

An important parameter in CSR is the 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 = e 2 Z(ω)/π This is Ohm’s law for CSR: Power α I 2 Z Big Impedance → lots of CSR

 I b << Bursting threshold  Bunch shape is static ρ(z) z Standard Bunch Shape is a Gaussian  Frequency distribution:  f(ω)=∫ρ(z)e iωz/c dz  Frequency components with ω ≈ 2πc/λ will radiate CSR at λ ρ(z) z Standard Bunch Shape is a Gaussian

Gaussian bunch: where ω=2πc/λ σ≈ few mm λ≈ 1 mm f(ω) = VERY SMALL

Deform the bunch to produce high ω components HOW ?? Nature does it for free, using Radiation Impedance

Revolution frequency Z(ω) Real part (Resistive) Imaginary part (Reactive) Re Z(ω) creates a static asymmetry within the bunch

ρ(z) z FrontBack n electrons Shark fin profile CSR power α n 2 Continuous emission High Frequency Component

 Shark Fin  CSR power α n 2  Efficiency is much higher for short bunches  Storage ring is re-configured for  σ ≈ few mm (versus ≈ 10 mm)  σ ≈ √α so reduce α

CSR SR

 Three Layers of Structure are observed in CSR  Coarse Structure ≈ 1 cm -1  Fine Structure ≈ cm -1  Very Fine Structure ≈ cm -1 (Only Multi-bunch)  Coarse and Fine Structures are independent of storage ring operation  Energy  Current  Fill pattern  Time Structure (Bursting or Continuous)

 Instrumentation ?  Reflections ?  Vacuum Chamber ?  Vacuum Chamber geometry determines the Radiative Impedance Z(ω) P(ω) ≈ I 2 Z(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)

 Attributed to Bunch to Bunch Interference 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

1↔1 2↔2... 1↔2 2↔3... 1↔3 2↔4... 1↔4 2↔5... 1↔5 2↔6... 1↔6 2↔7... 1↔7 2↔8... 1↔8 2↔9...

P mb (ω)=P sb (ω) x sin (N b ωT/2) sin (ωT/2) 2 Determined by bunch shape and radiation impedance Correct Positions and Widths

P sb (ω) α N e 2 (CSR) Interference term α N b 2 Peak Power α N b 2 P sb (ω)→ (N b N e ) 2 Average Power α N b P sb (ω)→ N b (N e ) 2 But... This appears to be a solution in search of a problem.

Please visit the poster entitled : Photoacoustic Spectroscopy Using Coherent Synchrotron Radiation Which is being presented by Dr. Kirk Michaelian