Search Results (5 total)

Recent laser wakefield acceleration experiments have demonstrated the generation of femtosecond, nano-Coulomb, low emittance, nearly monokinetic relativistic electron bunches of sufficient quality to produce bright, tunable, ultrafast x-rays via Compton scattering. Design parameters for a proof-of-concept experiment are presented using a three-dimensional Compton scattering code and a laser-plasma interaction particle-in-cell code modeling the wakefield acceleration process; x-ray fluxes exceeding 10²¹   s^-1 are predicted, with a peak brightness >10¹⁹   photons/(mm² mrad² s   0.1%   bandwidth)).

No monochromatic (Δω_x/ω_x<1%), high peak brightness [>10²⁰   photons/(mm²×mrad²×s×0.1%   bandwidth)], tunable light sources currently exist above 100 keV. Important applications that would benefit from such new hard x-ray and γ-ray sources include the following: nuclear resonance fluorescence spectroscopy and isotopic imaging, time-resolved positron annihilation spectroscopy, and MeV flash radiography. In this paper, the peak brightness of Compton scattering light sources is derived for head-on collisions and found to scale quadratically with the normalized energy, γ; inversely with the electron beam duration, Δτ, and the square of its normalized emittance, ε; and linearly with the bunch charge, eN_e, and the number of photons in the laser pulse, N_γ:   B-^ _x∝γ²N_eN_γ/ε²Δτ. This γ² scaling shows that for low normalized emittance electron beams (1 nC, 1  mm·mrad, <1  ps, >100  MeV), and tabletop laser systems (1–10   J, 5 ps) the x-ray peak brightness can exceed 10²³   photons/(mm²×mrad²×s×0.1%   bandwidth) near ℏω_x=1   MeV; this is confirmed by three-dimensional codes that have been benchmarked against Compton scattering experiments performed at Lawrence Livermore National Laboratory. The interaction geometry under consideration is head-on collisions, where the x-ray flash duration is shown to be equal to that of the electron bunch, and which produce the highest peak brightness for compressed electron beams. Important nonlinear effects, including spectral broadening, are also taken into account in our analysis; they show that there is an optimum laser pulse duration in this geometry, of the order of a few picoseconds, in sharp contrast with the initial approach to laser-driven Compton scattering sources where femtosecond laser systems were thought to be mandatory. The analytical expression for the peak on-axis brightness derived here is a powerful tool to efficiently explore the 12-dimensional parameter space corresponding to the phase spaces of both the electron and incident laser beams and to determine optimum conditions for producing high-brightness x rays.

The Dirac-Lorentz equation describes the dynamics of a classical point charge in an electromagnetic field, accounting for radiative effects in a manifestly covariant and gauge-invariant manner. The validity of this equation is assessed by direct comparison between the Dirac-Lorentz dynamics of an electron subjected to a plane wave in vacuum and the well-known recoil associated with Compton scattering. In the small recoil limit, the classical Dirac-Lorentz is shown to yield the correct momentum transfer. For larger values of the recoil, the quantum scale appears explicitly, and the classical Dirac-Lorentz equation does not properly model this situation, as shown by deriving an exact analytical solution for a monochromatic plane wave of wave number k₀ to any order in k₀r₀, where r₀ is the classical electron radius.

We present a detailed comparison of the measured characteristics of Thomson backscattered x rays produced at the Picosecond Laser-Electron Interaction for the Dynamic Evaluation of Structures facility at Lawrence Livermore National Laboratory to predicted results from a newly developed, fully three-dimensional time and frequency-domain code. Based on the relativistic differential cross section, this code has the capability to calculate time and space dependent spectra of the x-ray photons produced from linear Thomson scattering for both bandwidth-limited and chirped incident laser pulses. Spectral broadening of the scattered x-ray pulse resulting from the incident laser bandwidth, perpendicular wave vector components in the laser focus, and the transverse and longitudinal phase spaces of the electron beam are included. Electron beam energy, energy spread, and transverse phase space measurements of the electron beam at the interaction point are presented, and the corresponding predicted x-ray characteristics are determined. In addition, time-integrated measurements of the x rays produced from the interaction are presented and shown to agree well with the simulations.

Detailed experimental studies of the first operation of an X-band (8.547 GHz) rf photoinjector are reported. The rf characteristics of the device are first described, as well as the tuning technique used to ensure operation of the 11 / 2-cell rf gun in the balanced π-mode. The characterization of the photoelectron beam produced by the rf gun includes: measurements of the bunch charge as a function of the laser injection phase, yielding information about the quantum efficiency of the Cu photocathode ( 2×10^-5 for a surface field of 100 MV/m); measurements of the beam energy (1.5–2 MeV) and relative energy spread ( Δγ/γ₀  =  1.8±0.2%) using a magnetic spectrometer; measurements of the beam 90% normalized emittance, which is found to be ɛ_n  =  1.65π mm mrad for a charge of 25 pC; and measurements of the bunch duration ( <2 ps). Coherent synchrotron radiation experiments at Ku-band and Ka-band confirm the extremely short duration of the photoelectron bunch and a peak power scaling quadratically with the bunch charge.