An experiment currently underway at BYU is designed to test whether the size of a free electron wave packet affects the character of scattered radiation. Using a semi-classical argument wherein the wave packet is treated as a diffuse charge distribution, one would expect strong suppression of radiation in the direction perpendicular to the propagating field as the wave packet grows in size to be comparable to the wavelength of the driving field. If one disallows the interaction of the wave packet with itself, as is the case when calculating the rate of emission using QED, then regardless of size, the electron wave packet radiates with the strength of a point-like emitter. In support of this experiment, we explore a variety of physical parameters that impact the rate of scattered photons. We employ a classical model to characterize the exposure of electrons to high-intensity laser light in a situation where the electrons are driven by strong ponderomotive gradients. Free electrons are modeled as being donated by low-density helium, which undergoes strong-field ionization early on in the pulse or during a pre-pulse. When exposed to relativistic intensities (i.e. intensities sufficient to cause a Lorentz drift at a significant fraction of c), free electrons experience a Lorentz drift that causes redshifting of the scattered 800 nm laser light. This redshift can be used as a key signature to discern light scattered from the more intense regions of the focus. We characterize the focal volume of initial positions leading to significant redshifting, given a peak intensity of 2 x 10^18 W/cm 2 , which is sufficient to cause a redshift in scattered light of approximately 100 nm. Under this scenario, the beam waist needs to be larger than several wavelengths for a pulse duration of 35 fs in order to ensure free electrons remain in the focus sufficiently long to experience intensities near the peak pulse intensity despite strong ponderomotive gradients. We compute the rate of redshifted scattered photons from an ensemble of electrons distributed throughout the focus and relate the result to the scattered-photon rate of a single electron. We also estimate to what extent the ionization process may produce unwanted light in the redshifted spectral region that may confound the measurement of light scattered from electrons experiencing intensities greater than 1.5 x 10^18 W/cm^2.



College and Department

Physical and Mathematical Sciences; Physics and Astronomy



Date Submitted


Document Type





high-intensity laser, photon scattering, radiation, relativistic electron, classical and quantum physics