The Effect Of Focal Geometry On Radiation From Atomic Ionization In An Ultrastrong/Ultrafast Laser Field

We use a tunneling-Monte-Carlo model to calculate the dynamics and emitted Larmor radiation from electrons ionized in an ultrashort/ultraintense pulsed laser focus over the intensity range from 10$^{17}$ to 10$^{20}$ W/cm$^{2}$. We find the spatial variation of a laser field can no longer be neglected at laser intensities leading to relativistic effects. Adopting a one-dimensional or plane wave approximation overestimates the total radiated energy by a factor as high as two orders of magnitude. Despite this, the spectral amplitude of the radiated high-energy photons from ionization in a laser focus is as high as that in the plane wave case since the laser focus imparts an extra boost of speed for electrons exiting the focus. Moreover, ionization in a laser focus limits the effective radiation volume to a few fraction of $\mu $m$^{3}$ leading to more coherent radiation. For the ionization of Na$^{10+}$ in a laser focus of intensity 1.22 10$^{20}$ W/cm$^{2}$, we find the peak radiation yield extending to photon energies of 580eV. In the plane wave case, we find radiation extending to photon energies of 560eV.