Winthrop J. Brown

PLEIADES: A picosecond Compton scattering x-ray source for advanced backlighting and time-resolved material studies

The PLEIADES (Picosecond Laser-Electron Inter-Action for the Dynamical Evaluation of Structures) facility has produced first light at 70 keV. This milestone offers a new opportunity to develop laser-driven, compact, tunable x-ray sources for critical applications such as diagnostics for the National Ignition Facility and time-resolved material studies. The electron beam was focused to 50 μm rms, at 57 MeV, with 260 pC of charge, a relative energy spread of 0.2%, and a normalized emittance of 5 mm mrad horizontally and 13 mm mrad vertically. The scattered 820 nm laser pulse had an energy of 180 mJ and a duration of 54 fs. Initial x rays were captured with a cooled charge-coupled device using a cesium iodide scintillator; the peak photon energy was approximately 78 keV, with a total x-ray flux of 1.3×106 photons/shot, and the observed angular distribution found to agree very well with three-dimensional codes. Simple K-edge radiography of a tantalum foil showed good agreement with the theoretical divergence-...

Characterization of a bright, tunable, ultrafast Compton scattering X-ray source

The Compton scattering of a terawatt-class, femtosecond laser pulse by a high-brightness, relativistic electron beam has been demonstrated as a viable approach toward compact, tunable sources of bright, femtosecond, hard X-ray flashes. The main focus of this article is a detailed description of such a novel X-ray source, namely the PLEIADES (Picosecond Laser–Electron Inter-Action for the Dynamical Evaluation of Structures) facility at Lawrence Livermore National Laboratory. PLEIADES has produced first light at 70 keV, thus enabling critical applications, such as advanced backlighting for the National Ignition Facility and in situ time-resolved studies of high- Z materials. To date, the electron beam has been focused down to σ x = σ y = 27 μm rms, at 57 MeV, with 266 pC of charge, a relative energy spread of 0.2%, a normalized horizontal emittance of 3.5 mm·mrad, a normalized vertical emittance of 11 mm·mrad, and a duration of 3 ps rms. The compressed laser pulse energy at focus is 480 mJ, the pulse duration 54 fs Intensity Full Width at Half-Maximum (IFWHM), and the 1/ e 2 radius 36 μm. Initial X rays produced by head-on collisions between the laser and electron beams at a repetition rate of 10 Hz were captured with a cooled CCD using a CsI scintillator; the peak photon energy was approximately 78 keV, and the observed angular distribution was found to agree very well with three-dimensional codes. The current X-ray dose is 3 × 10 6 photons per pulse, and the inferred peak brightness exceeds 10 15 photons/(mm 2 × mrad 2 × s × 0.1% bandwidth). Spectral measurements using calibrated foils of variable thickness are consistent with theory. Measurements of the X-ray dose as a function of the delay between the laser and electron beams show a 24-ps full width at half maximum (FWHM) window, as predicted by theory, in contrast with a measured timing jitter of 1.2 ps, which contributes to the stability of the source. In addition, K -edge radiographs of a Ta foil obtained at different electron beam energies clearly demonstrate the γ 2 -tunability of the source and show very good agreement with the theoretical divergence-angle dependence of the X-ray spectrum. Finally, electron bunch shortening experiments using velocity compression have also been performed and durations as short as 300 fs rms have been observed using coherent transition radiation; the corresponding inferred peak X-ray flux approaches 10 19 photons/s.

Brightness Optimization of Ultra-Fast Thomson Scattering X-ray Sources

We present simple scaling relations of the brightness a Thomson x‐ray source on the electron and laser beam parameters in the case of a head (180 degree) interaction geometry. In particular, it is shown that a direct relation exists between the x‐ray brightness and the electron beam brightness. These relations are discussed in the context of the PLEIADES Thomson x‐ray source, where 107 photons per pulse, with photon energies as high as 140 keV, have been produced by colliding a 0.25 nC, picosecond electron bunch with a 500 mJ, 50 fs, 800 nm laser pulse. The estimated peak brightness of the source is about 1016 photons/s/mm2/mrad2/0.1 %b.w. A comparison of the current performance of the source and the predicted performance using optimized parameters is presented.

An adjustable permanent magnet quadrupole (PMQ) final focus system for low energy experiments

The final focus system for the Thomson X-ray scattering experiment termed PLEIADES (Picosecond Laser-Electron InterAction for Dynamic Evaluation of Structures) at LLNL demands ultrahigh field gradient quadrupoles in order to focus initially small beams to 10-20 /spl mu/m spot sizes. The scheme present here circumvents limitations due to chromatic aberrations and space-charge effects in this relatively low energy (<100 MeV) system. The final focus scheme is based on an ultrahigh gradient (/spl les/300 T/m) quadrupole which employs the Halbach 16-piece, permanent magnet design. Use of this optimized geometry, NdFeB material, and a small (5 mm) bore allows the desired field gradient, and a few cm focal length, to be achieved. The adjustability of the focusing system is obtained by changing the relative longitudinal positions of subcomponent focusing and defocusing magnets on precision movers. We present the results of RADIA 3D design simulations, and also discuss the results of beam dynamics simulations of the PLEIADES system using the tracking code.

PLEIADES: High Peak Brightness, Subpicosecond Thomson Hard-X-ray source

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. First light was observed at 70 keV early in 2003, and the experimental data agrees with 3D codes developed at LLNL. The x-rays are generated by the interaction of a 50 fs Fourier-transform-limited 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 brightness is expected to exceed 1020 ph/mm2/s/mrad2/0.1% BW. The beam is well-collimated (10 mrad divergence over the full spectrum, 1 mrad for a single color), and the source is a unique tool for time-resolved dynamic measurements in matter, including high-Z materials.

Pulse compression via velocity bunching with the LLNL Thomson X-ray source photoinjector

We report the compression of a high brightness, relativistic electron beam to rms lengths below 300 femtoseconds using the velocity compression technique in the LLNL Thomson X-ray source photoinjector. The results are consistent with analytical and computational models of this process. The emittance evolution of the beam during compression is investigated in simulation and found to be controllable with solenoid focusing.

Generation of high brightness x-rays with the PLEIADES Thomson x-ray source

The use of short laser pulses to generate high peak intensity, ultra-short 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 (Picosecond Laser Electron InterAction for the Dynamic Evaluation of Structures) is a next generation Thomson scattering x-ray source being developed at Lawrence Livermore National Laboratory (LLNL). Ultra-fast picosecond x-rays (10-200 keV) are generated by colliding an energetic electron beam (20-100 MeV) with a high intensity, sub-ps, 800 nm laser pulse. The peak brightness of the source is expected to exceed 10/sup 20/ photons/s/0.1% bandwidth/nm/sup 2//mrad/sup 2/. Simulations of the electron beam production, transport, and final focus are presented. Electron beam measurements, including emittance and final focus spot size are also presented and compared to simulation results. Measurements of x-ray production are also reported and compared to theoretical calculations.

PLEIADES: a subpicosecond Thomson x-ray source for ultrafast materials probing

The use of ultrafast laser pulses to generate very high brightness, ultrashort (10-14 to 10-12 s) pulses of x-rays is a topic of great interest to the x-ray user community. In principle, femtosecond-scale pump-probe experiments can be used to temporally resolve structural dynamics of materials on the time scale of atomic motion. However, further development of this field is severely hindered by the absence of a suitably intense x-ray source that would drive the development of improved experimental techniques and establish a broader range of applicability. We report on a project at the Lawrence Livermore National Laboratory to produce a novel x-ray source and essential experimental techniques that will enable unprecedented dynamic measurements in matter. Based on scattering of a sub-50-fs, multi-terawatt, multi-beam laser from a co-synchronous and highly focused relativistic electron bunch, PLEIADES (Picosecond Laser Electron Interaction for Dynamic Evaluation of Structures) will produce tunable, ultrafast, hard x-ray (10- 200 keV) probes that greatly exceed existing 3rd generation synchrotron sources in speed (100 fs - 1 ps), peak brightness (1020 ph/mm2s mrad2 0.1% BW, and >109 ph/pulse), and simplicity (100-fold smaller). Such bright, ultrafast high energy x-rays will enable pump-probe experiments using radiography, dynamic diffraction, and spectroscopy to address the equation of state and dynamics of phase transitions and structure in laser heated and compressed heavy dense metals of interest for materials science.

PLEIADES: Generation of ultrashort X-ray pulses by inverse Compton scattering

We report on a project to produce a novel x-ray source for high-density material science. By scattering a sub-50-fs, multi-terawatt, laser from a co-synchronous and highly focused relativistic electron bunch, PLEIADES (Picosecond Laser Electron Interaction for Dynamic Evaluation of Structures) produces tunable, ultrafast, hard x-ray (10-200 keV) probes.

RF phase stability and electron beam characterization for the PLEIADES Thomson X-ray source

We report on the performance of an S-band RF photocathode electron gun and accelerator for operation with the PLEIADES Thomson scattering X-ray source at LLNL. To produce picosecond, high brightness X-ray pulses, picosecond timing, Terahertz bandwidth diagnostics, and RF phase control are required. Planned optical, RF, X-ray and electron beam measurements to characterize the dependence of electron beam parameters and synchronization on RF phase stability are presented.