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…