Search Results (5 total)

Wang et al. [Phys. Rev. E 65, 028501 (2002)] claim that an electron interacting in vacuum with a unipolar plane electromagnetic wave will permanently gain energy, thus circumventing the Lawson-Woodward Theorem. We demonstrate that realistic, three-dimensional unipolar impulses cannot permanently impart energy to electrons in vacuum, leaving only the idealistic, one-dimensional, plane-wave impulse as a topic for academic discussion. We also note in passing that the version of the Lawson-Woodward theorem, which they are employing, is an erroneous version that has emerged in the laser acceleration literature over the last decade, and we direct the reader to the relevant papers containing the correct version of the theorem. Finally, we show that both our work and the proposal of Wang et al. are consistent with the correct version of this theory, and that the criticism by Wang et al. is thus without merit.

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.

The validity of the concept of laser-driven vacuum acceleration has been questioned, based on an extrapolation of the well-known Lawson-Woodward theorem, which stipulates that plane electromagnetic waves cannot accelerate charged particles in vacuum. To formally demonstrate that electrons can indeed be accelerated in vacuum by focusing or diffracting electromagnetic waves, the interaction between a point charge and coherent dipole radiation is studied in detail. The corresponding four-potential exactly satisfies both Maxwell’s equations and the Lorentz gauge condition everywhere, and is analytically tractable. It is found that in the far-field region, where the field distribution closely approximates that of a plane wave, we recover the Lawson-Woodward result, while net acceleration is obtained in the near-field region. The scaling of the energy gain with wave-front curvature and wave amplitude is studied systematically.

The relativistic dynamics of an electron submitted to the three-dimensional field of a focused, ultrahigh-intensity laser pulse are studied numerically. The diffracting field in vacuum is modeled by the paraxial propagator and exactly satisfies the Lorentz gauge condition everywhere. In rectangular coordinates, the electromagnetic field is Fourier transformed into transverse and longitudinal wave packets, and diffraction is described through the different phase shifts accumulated by the various Fourier components, as constrained by the dispersion relation. In cylindrical geometry, the radial dependence of the focusing wave is described as a continuous spectrum of Bessel functions and can be obtained by using Hankel’s integral theorem. To define the boundary conditions for this problem, the beam profile is matched to a Gaussian-Hermite distribution at focus, where the wave front is planar. Plane-wave dynamics are verified for large f numbers, including canonical momentum invariance, while high-energy scattering is predicted for smaller values of f at relativistic laser intensities.

The relativistic dynamics of an electron subjected to the classical electromagnetic field of an ultrashort laser pulse is studied theoretically at arbitrary intensities. Frequency modulation effects associated with the nonlinear relativistic Doppler shift induced on the backscattered radiation are analyzed in detail. For circular polarization, an exact analytical expression for the full nonlinear spectrum is derived. For linear polarization, it is found that the scattering of coherent light by a single electron describing a well-behaved trajectory can yield anharmonic spectra when the laser ponderomotive force strongly modulates the electron’s proper time. At ultrahigh intensities, these nonlinear relativistic spectra exhibit complex structures. In addition, the temporal laser pulse shapes best suited to generate narrow Compton backscattered spectral lines at ultrahigh intensities are discussed. © 1996 The American Physical Society.