Fig. 1 Thomson Scattering Geometry
Fig. 2 Experiment Layout
Fig. 3 X-ray Image
Fig. 4 Thomson Interaction
Fig. 1 Thomson Scattering Geometry
Fig. 2 Experiment Layout
Fig. 3 X-ray Image
Fig. 4 Thomson Interaction
The Picosecond Laser-Electron Inter-Action for the Dynamic Evaluation of Structures (PLEIADES) facility, is a unique, novel, tunable (10-200 keV), ultrafast (ps-fs), hard x-ray source that greatly extends the parameter range reached by existing 3rd generation sources, both in terms of x-ray energy range, pulse duration, and peak brightness at high energies. Ultra-fast x-rays are generated by the interaction of a sub-ps, 800 nm laser pulse produced by the TW-class FALCON CPA laser and a highly focused, relativistic (20-100 MeV), high brightness (1 nC, 0.3-5 ps, 5 mm-mrad, 0.2% energy spread) photo-electron bunch. The resulting x-ray flux is expected to exceed 
photons for pulse durations of 100 fs to 5 ps using interaction geometries ranging from 90 degree (side-on collision) to 180 degree (head-on collision) and the peak brightness of 
. The use of short laser pulses to generate high peak intensity, ultra-fast x-ray pulses enables exciting new experimental capabilities, such as femtosecond pump-probe experiments used to temporally resolve material structural dynamics on atomic time scales.
PLEIADES (PicosecondLaser-Electron Inter-Action for the Dynamic Evaluation of Structures) x-ray source is based on relativistic Thomson scattering process (also known as inverse Compton scattering). In this type of source, the hard x-rays are generated by scattering a high-power, ultra-short laser from a relativistic electron bunch. The scattered photons are relativistically upshifted in energy into the hard x-ray range as
and 
represent the peak frequency of the scattered x-ray photons and incidence/initial laser photons frequency, respectively, 
is the angle between the electron and the laser beams, and 
is the relativistic factor defined as
where the numerator term is the total electron energy and the denominator term is the rest mass electron energy.
An alternative explanation of the mechanism of the x-ray production is as viewed in the frame of the moving electrons, the incident laser pulse train appears as an undulator of wavelength 
. The undulator wavelength and hence the laser wavelength as seen by the electrons is relativistically contracted. By transforming the wavelength of the emitted x-rays back into the laboratory coordinate frame, radiated photon energy picks up a second factor 
. Fig. 1 illustrates the geometrical relations of interacting beams.
In the laboratory frame, x-rays are generated and confined in a narrow cone of opening angle 
in the direction of the electron beam propagation, and the pulse duration is limited by the transit time of the laser pulse through the electron bunch. The x-ray output is defined by the Thomson scattering cross-section
where 
is the classical electron radius. The number of x-ray photons produced,
, is given by
where 
is the number of electrons. The laser beam and electron beam diameters are 
and 
, respectively.
The PLEIADES facility consists of a Ti-Sapphire laser system generating bandwidth limited 820 nm laser pulses of 54 fs with up to 540 mJ of energy, the RF photo-injector gun which is driven by a picosecond, 1.2 mJ, UV laser and four (2.5 meter each) accelerator sections used to accelerate the electron beam to beam energies of 20 - 100 MeV. The RF photo-injector gun is synchronized to the interaction drive laser for a maximum probability of photon-electron beam interactions.
A schematic of the interaction region is shown in Fig. 4. There are two interaction schemes: a 180 degree laser (the current experimental scheme) incidence angle with respect to electron beam direction for a maximum x-ray flux output with a time length of ~femtosecond. The focal length between the Permanent Magnet Quadrupole (PMQ) final focus triplet and the interaction region is ~1 to 2 cm depending on the initial electron beam energy to allow for maximum focus strength and minimum electron bunch spot size (~15
). A 30 degree dipole magnet is positioned right after the interaction is used to bend the electron beam out of the x-ray beam path. As for focusing the laser beam into the interaction chamber, an off-axis, 1.5 m focal length parabolic mirror (f/30) is used to reach a minimum spot size of about 15 FWHM at the interaction point. The focused laser beam is finally used to steer the focused laser beam into the interaction point to scatter off of the counter-propagating relativistic electron bunch. There are plans to replace this with a beryllium flat, which will be more transparent to the x-ray beam. Diagnostic instruments such as an optical CCD camera (spatial overlap) and an optical streak camera (temporal overlap) are set up in the vicinity of interaction chamber for alignment of two counter-propagating beams. A 16 bit CCD array fiber coupled to a cesium iodide scintillator which is placed in the down-stream side of the accelerator is used for imaging of x-rays. Fig. 3 is an actual x-ray image taken with the CCD camera. The measured integrated flux is
.
[1] D. J. Gibson, et. al., Physics of Plasma, 11, 2857 (2004)
[2] W.J. Brown et al., Phys. Rev. Special Topics - Accel. and Beams, 7, 060702 (2004)
[3] J. K. Lim et al., Phys. Rev. Special Topics - Accel. and Beams, 8, 072401 (2005)
[4] J. K. Lim et al., Particle Accelerator Conference Paper (2005)
[5] J. K. Lim et al., Particle Accelerator Conference Poster (2005)
[1] LLNL PLEIADES