Ultra-intense laser pulse characterization using ponderomotive electron scattering

Abstract

We present a new analytical solution for the equation of motion of relativistic electrons in the focus of a high-intensity laser pulse. We approximate the electron's transverse dynamics in the averaged field of a long laser pulse focused to a Gaussian transverse profile. The resultant ponderomotive scattering is found to feature an upper boundary of the electrons' scattering angles, depending on the laser parameters and the electrons' initial state of motion. In particular, we demonstrate the angles into which the electrons are scattered by the laser scale as a simple relation of their initial energy to the laser's amplitude. We find two regimes to be distinguished in which either the laser's focusing or peak power are the main drivers of ponderomotive scattering. Based on this result, we demonstrate how the intensity of a laser pulse can be determined from a ring-shaped pattern in the spatial distribution of a high-energy electron beam scattered from the laser. We confirm our analysis by means of detailed relativistic test particle simulations of the electrons' averaged ponderomotive dynamics in the full electromagnetic fields of the focused laser pulse.for providing this key laser parameter is the combination of three different and distinct measurements: (a) pulse energy of the fully amplified, collimated beam; (b) pulse duration of a fraction of the collimated beam, typically not at full amplification; and (c) focus imaging of typically a not fully amplified and with attenuating elements transported beam. There is a number of shortcomings of this approach: (a) it does not necessarily yield the energy concentrated within the focus; (b) it may differ across the beam profile due to radial dispersion and further nonlinear effects [41,42]. (c) It is a time-and spectrum-integrated measurement. Due to the previous effects as well as radial group delay and chromatic aberrations [43,44], the pulse duration in the focus can be much longer than measured with (b) and exhibit time structures varying with the focal position. Hence, the typical procedure yields rather an upper limit of peak intensity.Atomic effects have been proposed and employed to yield a measure of peak intensity [45][46][47], but were restricted to non-relativistic intensities. At higher intensity, atomic ionization is followed by significant electron acceleration which can also provide information on peak intensity and focus size [48,49]. Experimental realizations of this concept were successfully implemented at mildly relativistic intensities [50,51]. Relativistic, intensity-dependent plasma effects like ion acceleration [52][53][54][55], on the other hand, are not feasible since they are quite sensitive to the temporal contrast of the laser pulse [9, 56], as the plasma formation prior to the pulse peak strongly affects the energy conversion during the peak. Hence, measurements of laser-scattered electron distributions could be a reasonable alternative, especially when relying on beam profile measurements which are much simpler than spectral measuremen...

Optimal laser focusing for positron production in laser-electron scattering

“…Besides optimising the positron yield considered in this manuscript, the equivalent intensity distributions are going to be useful to calculate the asymptotic energy spread [32][33][34]52,53] and divergence of the interacting electron beams [36,[54][55][56], which are also imprinted on the emitted photon beams in the hard x-ray and gamma-ray range. The analytical description can be generalized to tight-focusing regime beyond the paraxial approximation considering interaction at an angle for the regions with curved wavefronts.…”

From local to nonlocal: higher fidelity simulations of photon emission in intense laser pulses

“…1, angular structure in the photons emerges differently in the LMA and LCFA simulations. In the former, it is the emission rate and the Analytical predictions for the scattering angle are also given in [76], but these are derived under the assumptions that the laser transverse intensity profile is flat up to a radius equal to the waist, and that the pulse duration is infinitely long. Neither condition applies here.…”

Kilo-Tesla axial magnetic field generation with high intensity spin and orbital angular momentum beams

“…The ponderomotive force acting on an electron associated with a high intensity focal spot can be given by, F p = −m e c 2 ∇ 1 + |a| 2 where a = eE/m e cω 0 is the normalized vector potential, e and m e are the electron charge and mass respectively, and c is the speed of light. In the standard case of a high-intensity Gaussian beam a finite energy maybe transferred from the laser to plasma electrons through ponderomotive scattering [22]. The same is true for higher order LG modes [23], with some possibility for electron trapping and additional energy transfer within the donut mode leading to a possible increased absorption rate [24,25].…”