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