Radiation from free electrons in a laser focus at 10 W/cm: modeling of photon yields and required focal conditions

In support of an experiment designed tomeasure the strength of radiation scattered from low-density free electrons in an intense laser focus, we model a variety of physical parameters that impact the rate of scattered photons. We employ a classical model to characterize duration of electron exposure 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 in the pulse or during a prepulse. When exposed to relativistic intensities, free electrons experience a Lorentz drift that causes redshifting of the scattered 800 nm light. This redshift can be used as a signature to discern light scattered from themore intense regions of the focus.We characterize the focal volume of initial positions leading to significant redshifting, given a peak intensity of 2 × 1018 W∕cm2. Under this scenario, the beam waist needs to be larger than several wavelengths for a pulse duration of 35 fs. 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. © 2015 Optical Society of America OCIS codes: (000.1600) Classical and quantum physics; (270.5580) Quantum electrodynamics; (290.5820) Scattering measurements; (350.5720) Relativity. http://dx.doi.org/10.1364/JOSAB.32.000743