progress and future plans of the bnl atf compton source
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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 - PowerPoint PPT PresentationTRANSCRIPT
Progress and Future Plans of the BNL ATF Compton Source
Oliver WilliamsUniversity 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
Parameter Value
Energy 64-72 (85) MeV
Beam size (RMS)
30 μm
Bunch length (FWHM)
0.15-4 ps
Emittance 2 mm-mrad
Energy spread 0.5-1.0%
Charge 300 pC
Laser Beam
Parameter Value
Laser energy 2 J
Waist size 60 μm
Pulse length 6 ps
Bandwidth ~0.6%
Wavelength 10.6 μm
Laser potential (aL)
0.38
X-rays
Parameter Value
Total Photons (NT)
1x109
(on-axis: 2x107)
Energy (on-axis)
7-8.9 keV
Bandwidth 2-3%
Pulse length 0.15-4 ps
Full opening angle
~8 mrad
Peak Brightness(Bpeak)
1019-1020 ph/s/mm2/mrad2
(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
Ea
E L
L
x
22
2
2
21
4
“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 foil Silver foil
MCP
Low energy photons preferentially attenuated
Ex far above Fe K-edge
Ni K-edge 1.2 keV higher than Fe
72 MeV 70 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 (aL>0)
ΔEe=1.3 MeV => ΔEx= 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 ~2x106 photons through 1 mrad pinhole placed on-axis
• Bpeak= 1018-1019 in pulses from 4 ps to 300 fs
Circular polarization and sub-ps pulses
68 MeV, 4 ps FWHM e-beam (2Δγ/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)
1st 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 ~45o for x-rays
• At this angle only s-pol x-rays are reflected
• Consider in-hand silicon crystal
Brnr )(1tan
in 1~
Analyzer cont.
• Si (111) crystal, 333 symmetric reflection
• 8.39 keV photons, Bragg angle ~ 45o , same as Brewster
• Rotate crystal about beam axis (χ-angles)
• Circular pol yields constant signal
J. Samson, Rev. Sci. Instr., Vol. 47, pp 859-860
Electro-optic sampling• Use nonlinear crystal (e.g. ZnTe <110>)
• 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 parametersin comparison
• Pegasus– Laser• Ti:Sapphire regen.• 0.800 µm• 40 fs FWHM• 3 mJ (<5% needed)
– e- beam• 200 fs rms• < 10 pC• 3.5 MeV
• BNL ATF– Laser• Nd:YLF• 1.047 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 pC/fs @ BNL ATF, ~ 0.05 pC/fs @ Pegasus• The opening angle of the fields (~ 1/γ) is
smaller by a factor of 80 MeV/3.5 MeV = 23Conclusion:• The EO-effect signal is much larger for the ATF
beam
Summary• ATF Compton source characterized:
– ~2x106 photons over 1 mrad– 4-5% bandwidth– 0.3-4 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)– Bpeak=1018-1019 photons/s/mm2/mrad2 (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