Characterizations and Diagnostics of Compton Light Source

Characterizations and Diagnostics of Compton Light Source Advance Light Source (ALS) (LBNL) Ying K. Wu Duke Free Electron Laser Laboratory (DFELL) Acknowledgments: DFELL: B. Jia, G. Swift, H. Hao, J. Li, J. Zhang, M. Emamian, M. Busch, M. Pentico, P. Wallace, P. Wang, S. Huang, S. Mikhailov, V. Popov, V. Rathbone and W. Wu HIGS collaboration: A. P. Tonchev, B. Perdue, G. Rusev, H. Weller, M. Gai, M. W. Ahmed, and S. Stave

Outline Motivation of the dissertation Characterizations of Compton gamma ray beam An end to end spectrum reconstruction method A CCD based gamma ray imaging system Accurate measurements of electron beam energy and energy spread using Compton scattering technique Radiative polarization study of an electron beam

Motivation X ray beam discovery: X ray was first discovered by Wilhelm Roentgen in 1895. X ray beam has been used as a powerful tool for unraveling the structure of materials, crystal and molecules. X ray beam productions: Bremsstrahlung radiation X ray tube Hand with Rings taken in 1895 Synchrotron radiation Synchrotron light source

High Energy X ray or Gamma ray To study the structure of nuclear, a high energy x ray beam above MeV (gamma ray) is needed. 1. Synchrotron radiation is limited to 100keV, above which the brightness of radiation rapidly falls off. 2. Bremsstrahlung radiation has a broad energy spectrum, a relative low spectral flux and typically unpolarized. 3. Compton scattering of a laser beam and an electron beam can be used to generate high energy x ray beam. (Phy. Rev. Special Topics 8, 100702 (2005)) The Compton gamma ray beam is highly polarized, nearly mono energetic, and operated in a wide energy range.

Hist. and App. of Compton Scattering In 1920, Compton scattering was first observed by Arthur H. Compton. In 1963, Compton scattering of a laser beam and an electron beam was recognized as a useful mechanism to generate high energy x ray or gamma ray source. Several Compton scattering x ray or gamma ray source facilities have been brought to operations, such as High Intensity Gamma ray Source (HIGS) at Duke University. Compton scattering has been successfully used as a non destructive tool to diagnose electron beam, such as measuring energy and energy spread, polarization and transverse beam profile.

Scattered Photon Energy Momentum and energy conservation: Head on collision, Backscattering,

Coupled Spatial and Energy Distribution 800 nm photon scattering with 500 MeV electron, and the observation plane is 60 meters downstream from the collision point. High energy gamma photon are peaked around the center. Low energy gamma photon are distributed away from the center.

Scattering Cross Section For head on collision in lab frame, the angular differential cross section without the polarization effects of the finial particles is given by Azimuth angle of scattered photon Linear polarization of laser beam Azimuth angle of polarization vector

Circularly Polarized/Unpolarized Incoming Photons 800 nm laser beam scattering with 500 MeV electron beam, and the observation plane is at 60 m downstream of the collision point. Lawrence Berkeley National Laboratory

Linearly Polarized Incoming Photons 800 nm laser beam scattering with 500 MeV electron beam, and the observation plane is at 60 m downstream of the collision point.

Energy Spectrum of Scattered Photon Beam For unpolarized/circularly polarized photons scattering with unpolarized electrons, the gamma beam energy spectrum is given by 800 nm unpolarized laser photons scattering 500 MeV unpolarized electrons.

Beam Beam Scattering Assuming Gaussian electron and laser beam, neglecting the vertical emittance of the electron beam and the energy spread of the laser energy beam, we can obtain:

Effect of Electron Beam Energy Spread 500 MeV electron beam scattering with 800 nm laser beam, and the radius of the collimation aperture is 16mm Both low and high energy edges are smeared due to the electron beam energy spread

Effect of Collimation Aperture 500 MeV electron beam scattering with 800 nm laser beam, and energy spread of the electron beam is 0.2% 1. Collimation cuts down the lower energy gamma beam intensity, and determines the low energy edge of the spectrum. 2. For a large collimation aperture, the low and high energy edges of the spectrum are well separated. 3. For a tight collimation, the low and high energy edges begin to join together, and the peak of the spectrum shifts toward the higher energy end as the collimation aperture is decreased.

Monte Carlo Simulation Assumption for the analytical calculation: 1. Head on collisions; 2. Negligible angular divergence of laser beam 3. Far field collimation. Monte Carlo simulation: 1. A completely different approach to calculate the spatial and energy distribution of a Compton gamma ray beam 2. Allow us to study effects that cannot be accessed analytically.

Algorithm of Monte Carlo Simulation The electron bunch is divided into a number of macro particles. The coordinate and momentum of the macro particle are sampled according to the phase space function of the electron beam. The collision time is divided into a number of time steps. In each time step, the scattering probability is calculated for each macro particle according to the local intensity and wave vector of the laser beam. If the scattering event happens, the sampling of scattered photon is carried out in the electron rest frame, and then transferred back to the lab frame. After obtaining the scattered photon in the lab frame, the momentum of the scattered electron is modified. Such an electron can still interact with the laser beam in following time steps.

Spectra Calculated using Two Methods The gamma ray beam is produced by a 800 nm laser beam scattering with a 500 MeV electron beam, and collimated by a collimation aperture with radius of 12 mm.

Simulated Spatial Distribution of Gamma ray Beam linearly circularly The gamma ray beam is produced by Compton scattering of a 680 MeV electron beam and a 378 nm laser beam, and the observation plane is about 27 meters downstream of the collision point.

HIGS Facility at Duke University HIGS: High Intensity Gamma ray Source

Measured HIGS Beam Spectrum 5 MeV Detector response function True energy distribution 789 nm laser beam scattering with 466 MeV electron beam, and the radius of the collimation aperture is 12.7 mm.

Novel End to End Spectrum Reconstruction Method Accurately unfold the measured HIGS spectrum Simulate the measured gamma ray beam spectrum

5 MeV HIGS Beam

CCD Based Gamma ray Imaging System 1. Radiation transport toolkit Geant4 is used to optimize the scintillator thickness and surface finish type. 2. Optical software OSLO is used for lens system design.

Spatial Resolution Test Bar phantom Four groups of bars with different spacings. The maximum spacing is 2.5 mm and minimum is 1 mm.

Spatial Resolution Test Bar phantom Image Four groups of bars with different spacings. The maximum spacing is 2.5 mm and minimum is 1 mm. The spatial resolution is better than 1 mm

Measured Spatial Distribution of Gamma ray Beam circularly linearly 680 MeV electron beam scattering with a 378 nm laser beam, and the observation plane is about 27 meters downstream of the collision point. measured simulated

Collimator Alignment

Accurate Energy Measurement

Beam Beam Scattering Spectrum high energy edge is given by integrating all the single particle scatterings

Spectrum High Energy Edge Simple model [1] Gamma beam collimation and electron beam emittance effects are neglected. Comprehensive model [1] R. Klein, T. Mayer, P. Kuske, R. Thornagel, and G. Ulm. Nucl. Instr. and Meth.,A384:293 298, 1997.

Fitting of Spectrum High Energy Edge

Energy and Energy Spread Measurements The set energy of the electron beam is adjusted with an increment of 0.02 MeV. The relative increase is about 4*10^ 5. MeV

Measured Energy vs Set Energy

Uncertainty of the Energy Measurement The overall uncertainty of the measurement is about few 10^ 5. This accuracy is comparable to another technique, Resonant Spin Depolarization.

Sokolov Ternov effect Exponential build up process At Duke storage ring ( = 2.10 m, R = 17.1 m) operated at 1.15 GeV, Tp = 62 min The electron beam polarization can be measured using Compton polarimeter, however the setup could be complicated and challenging. We are going to use a simple technique, Touschek lifetime, to study the polarization buildup process of the electron beam.

Polarization Related Touschek Lifetime The Touschek lifetime depends on the electron beam polarization through intra beam scattering. For a flat beam with a non relativistic transverse momentum, the Touschek lifetime is given by Averaging over the storage ring Touschek lifetime difference between polarized and unpolarized beam

Experimental procedure 1. The electron beam is injected to 120 ma step by step with increments of 10 ma. 2. Then, the injection is stopped, and the electron beam current is monitored for about 300 minutes as it decay to 30 ma. 3. Finally, the electron beam is dumped, repeat the first run.

Polarization Measurements The polarized electron beam will be used to determine electron beam energy by the Resonant Spin Depolarization technique.

Conclusions Characterizations of Compton scattering gamma ray beam Develop an end to end spectrum reconstruction method Develop a CCD based gamma ray imaging system The electron beam energy and energy spread measurements using Compton scattering technique The electron polarization measurements using Touschek lifetime technique

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