Progress and Future Plans of the BNL ATF Compton Source Oliver Williams University of California, Los Angeles

Outline Simulation and characterization of source by absorption in foils Attempts at phase contrast imaging (PCI) New endeavors A look at source polarimetry and electro-optic sampling

Electron Beam ParameterValue Energy64-72 (85) MeV Beam size (RMS) 30 μm Bunch length (FWHM) ps Emittance2 mm-mrad Energy spread % Charge300 pC Laser Beam ParameterValue Laser energy2 J Waist size60 μm Pulse length6 ps Bandwidth~0.6% Wavelength10.6 μm Laser potential (a L ) 0.38 X-rays ParameterValue Total Photons (N T ) 1x10 9 (on-axis: 2x10 7 ) Energy (on-axis) keV Bandwidth2-3% Pulse length ps Full opening angle ~8 mrad Peak Brightness (B peak ) ph/s/mm 2 /mrad 2 (0.1% BW) Simulated parameters

Simulated x-ray spectra ICS spectra for various acceptance angles (0.5 to 10 mrad) at 65 MeV and Δγ/γ=1.0%. ICS spectra on-axis (1 mrad accepted) and the effects of e-beam energy spread. Reducing to 0.1% spread results in the dominance of beam angles on the bandwidth. Tail from e-beam angles Note: Code does not include nonlinear effects

Energy characterization by K-edge foils (Low-pass filters)

Simulated intensity distribution before and after foil “Undulator equation” Off-axis red-shifting

Analyzing the photons 250 μm Be-window Insertable Ni, Fe, and Ag foils 1 mrad pinhole on remote 2-axis control Remotely insertable Si-diode detector 250 μm Be-window MCP image intensifier (CCD camera not pictured)

No foil Iron foil Nickel foilSilver foil MCP Low energy photons preferentially attenuated E x far above Fe K-edge Ni K-edge 1.2 keV higher than Fe

72 MeV70 MeV 68 MeV

66 MeV 65 MeV 64 MeV

Lobe observation angle -Max simulated lobe intensity shows peak at 6.9 keV -Fit simulation curve to data by adding energy offset (~290 eV) -Energy offset could be due to absolute e-beam energy calibration or nonlinear induced red-shifting (a L >0) ΔE e =1.3 MeV => ΔE x = 290 eV

Measured on-axis flux and BW Need ~65 MeV e-beam to create ≤1 mrad null (with <50% photons transmitting on-axis) ΔE= 1.3 MeV = 290 eV => BW = 4.0% Measured ~2x10 6 photons through 1 mrad pinhole placed on-axis B peak = in pulses from 4 ps to 300 fs

Circular polarization and sub-ps pulses 68 MeV, 4 ps FWHM e-beam (2Δ γ/γ=ΔE/E=1%) 68 MeV, 300 fs FWHM e-beam (2Δγ/γ=ΔE/E=2%)

Phase Contrast Imaging (PCI) Different index of refraction results in interference effects Produces edge enhancement Require small source sizes (10’s of microns)

1 st attempts at PCI Zoomed intensity lineout Simulated lineout Phase peaks No edge enhancement MCP taken image, 500 μm PET Full profile lineout

Upgrades for successful PCI Detector better suited to many keV x-rays – Hamamatsu CMOS detector – 50 micron/pixel – Direct detection and image processing – Carbon-fiber window = req. >10 keV x-rays Upgraded linac: 72 to now 85 MeV max – Up to 13 keV x-rays (fundamental) – Smaller divergence angle (6 mrad) Helium-filled transport line between object and detector increased signal by x3 for ~11.5 keV

Recent attempts at PCI Wires of various material and diameter characterized Preliminary analysis shows obvious edge enhancement and good agreement with simulations “Vespa” starlet shown; tomographic centerfold coming soon!

Possible new endeavors Two-color digital subtraction imaging – Medical apps, cultural heritage (e.g. paint on canvas) Verifying ICS polarization rate – Polarized positrons – Polarization-dependent materials (e.g. XMCD) – Polarization of harmonics? Electro-optic sampling – Non-destructive bunch length measurement – Time stamping of x-ray pulse arrival for pump-probe experiments (e.g. non-thermal melting requires sub- ps x-rays)

Polarization analyzer Index of refraction, n~1 for x-ray energies Yields Brewster’s (polarization) angle ~45 o for x-rays At this angle only s-pol x-rays are reflected Consider in-hand silicon crystal

Analyzer cont. Si (111) crystal, 333 symmetric reflection 8.39 keV photons, Bragg angle ~ 45 o, same as Brewster Rotate crystal about beam axis (χ-angles) Circular pol yields constant signal J. Samson, Rev. Sci. Instr., Vol. 47, pp

Electro-optic sampling Use nonlinear crystal (e.g. ZnTe ) E-field of electron bunch imprinted on crystal Acts as polarization gate resolution: probe laser pulse length and crystal thickness window: crystal width and laser spot size Provocation based on measurements done at UCLA Pegasus Lab

e- beam and laser parameters in comparison Pegasus – Laser Ti:Sapphire regen µm 40 fs FWHM 3 mJ (<5% needed) – e- beam 200 fs rms < 10 pC 3.5 MeV BNL ATF – Laser Nd:YLF µm 200 fs FWHM 100 µJ – e- beam 150 fs rms 300 pC 80 MeV

CCD readout from Pegasus EO signal

EO effect comparison The peak current (thus peak E fields) are larger for the ATF beam (Q/L) = ~ 2 BNL ATF, ~ 0.05 Pegasus The opening angle of the fields (~ 1/ γ ) is smaller by a factor of 80 MeV/3.5 MeV = 23 Conclusion: The EO-effect signal is much larger for the ATF beam

Summary ATF Compton source characterized: – ~2x10 6 photons over 1 mrad – 4-5% bandwidth – ps x-ray pulses (assuming equal bunch length) – Linear or circularly polarized x-rays – Easily tunable photon energy, 6 to 9 keV (13 keV since upgrade) – B peak = photons/s/mm 2 /mrad 2 (0.1% BW) Phase contrast imaging can be done with ICS (given good detector) Next up: Two-color subtraction, source polarization rate, and electro-optic sampling Thanks! Vitaly Yakimenko and ATF staff, Massimo Carpinelli and Co., UCLA, and of course, the audience