Compact X-ray & Emittance Measurement by Laser Compton Scattering Zhi Zhao Jan. 31, 2014
Jan 18, 2016
Compact X-ray & Emittance
Measurement
by Laser Compton Scattering
Zhi Zhao
Jan. 31, 2014
Outline
• Overview of Compton scattering
• ERL-based compact x-ray
• Emittance measurement
Overview of Compton scattering
Compton Scattering
Scattered photon: longer wavelength !
(h/mc)cos(); h/mc = 0.024 A
Inverse Compton Scattering
Scattered photon: high-energy, tunable, and
compact !
E = 0.511 * (MeV);
(1^2)^(-1/2)
ħ = ħ (1cos())/(1cos())
If , and =0; then h = 4 ^2*h
Drive laser: 1.2 eV (1.04 m), electron: = 60 (~30 MeV)
Photon energy: 17 keV (0.73 A)
Compton & Thomson Scattering
Compton scattering: if
Electron recoil is included !
Thomson scattering: if
Electron recoil is negligible !
ħ = ħ(1cos())/[(1cos()+ħ(1+cos()cos()/E0)
Including the electron recoil:
ħ ~ E0
ħ << E0
Nonlinear Laser-Electron Scattering
Electron’s dynamic:
• Electron oscillating approaching light speed
• Force of magnetic and electric comparable
• Nonlinear dynamics, i.e. multi-photon event, figure-8
movement
• Laser accelerator…
Nonlinear parameter, ~ 1 @ I=10^18
W/cm^2
Linear Laser-Electron Scattering
• Linear scattering regime
• Photon flux & brilliance
• Beam size & emittance
• 1 uJ, 50 MHz/1.3 GHz & 5 MeV, 80 pC,
• 2 mJ, 50 MHz/1.3 GHz & 30 Mev, 80 pC,
Our focus:
Working regimes
ERL-based Compact X-Ray
Photon Energy vs Scattering Angle
ħ = ħ(1cos())/(1cos())
Drive laser: 1.2 eV (1.04 m)
Electron: ~ 60 (30 MeV)
high-energy photon is concentrated around 1/ !
Cross-section vs Scattering Angle
d/dcos() =
3/8*th*(1/^2/(1cos())*(1+((cos()-)/(1cos())^2)
th 0.665
barns
• Small angle: bigger diff. cross-section
• Total cross-section around 1/ is ~ 0.165
barns
Key for Photon Flux
Flux is the product of electron current and photon flux
Flux per bunch, assuming Gaussian profile in electron and laser
Key factors:
•High electron bunch charge
•High laser pulse energy & High repetition
rate
•Small beam sizes at the interaction point
Photon Flux@80 pC, 1 uJ, 50 MHz, beam size of 1 mm X 2 mm:
1 MHz
Keys for Brightness
Keys:
Both photon flux and small
emittance
F is the photon flux per 0.1 % energy bandpass
Photon Brightness
Technical Approaches I: Small Storage Ring
Lyncean Technologies, Inc.,
• High repetition rate, small emittance, Cavity-enhanced laser
power
Technical Approaches I: Small Storage Ring
Thales/CEA, France
• High repetition rate, small emittance, Cavity-enhanced laser
power• Beam emittance and energy spread may grow; long pulse
duration
Technical Approaches II: Linac & SRF Linac
• High brightness, short pulse duration,
• High repetition rate, small emittance, Cavity-enhanced laser
power
• Compatible with ERL
MIT ICS Source: Planned
Technical Approaches II: ERL-based
• Cavity-enhanced laser power
• High-power laser generated by ERL (Jlab & Japan)
• We can easily generate X-ray and -ray if we reach 5 GeV!
Emittance Measurement
Emittance Measurement (I): Intensity Profile
• Beam size and divergence: can not be directly measured.
• Measuring beam sizes at three different locations
• Laser wires: Induced current from secondary emission or flux
Emittance Measurement (I): Intensity Profile
Scanning the beam transversely
Monitoring the X-ray yield
Fitting to find out the beam size
Three locations for determining
emittance
Copy from exp. of ILC
Emittance Measurement (II): X-ray Spectrum
Two factors:
•Intensity profile: determining beam size
•X-ray spectrum: deriving beam divergence
• Spectrum width and shaping: the function of
Spatial and temporal profiles of the electron and laser beams
as well as electron angular and energy spread
• Divergent angle decreasing the x-ray
energy
Signature, “low energy trail”
Emittance Measurement (II): X-ray Spectrum
Scattering photons:
Energy spectrum:
Model:
The spatial and temporal profiles of the electron and laser beams as well
as the electron angular and energy spread.
Intensity profile determine the beam size.
Scattered x-ray energy flux:
Deriving beam divergence by fitting X-ray spectrum
Intensity profile and Energy spectrum determine the
emittance.
Divergent angle: Signature, “low energy trail”
• Photon energy:
100 kV – 1 MeV
• low energy trail
Summary
• Potential X-ray & -Ray sources by ERL
• LCS for nondestructive beam diagnostic
• More effort is underway…