Wendell T. Hill III | Searcharxiv

arXiv2025

Ponderomotive-expulsion: toward creating an electron-free volume

We describe a demonstration of a prototype approach to clear the laser focal volume of free electrons and disable their atomic and molecular sources. Employing two temporally separated, copropagating pulses, we exploited a pump-probe setup in our experiment. The pump ionized a low-density gas and expelled free and nascent electrons from its focal volume. The probe, traversing the same focal volume, expelled any remaining free and probe-induced nascent electrons. We gauged the effectiveness of the approach by capturing the spatial distribution of ejected electrons with image plates while we varied the relative intensity and time delay between the pump and probe. When we injected the pump 300 fs before the probe, we found the electron spatial distribution significantly altered and the yield suppressed, proving ponderomotive expulsion works. However, the yield was enhanced when we set the temporal spacing between the pump and probe to 150 fs. Simulations show the enhancement is due to Airy rings of the focused pump expelling electrons inward toward the propagation axis. Our results show that the complete removal of focal-volume electrons was inhibited by spatial overlap fluctuations and the stronger probe generating ionization outside the cleared-volume of the pump. We discuss ways to mitigate these impediments and propose alternate two-beam arrangements to achieve more efficient focal-volume clearing.

physics.ins-det↗

arXiv2023

Imaging electron angular distributions to assess a full-power petawatt-class laser focus

We present a novel technique to assess the focal volume of petawatt-class lasers at full power. Our approach exploits quantitative measurement of the angular distribution of electrons born in the focus via ionization of rarefied gas, which are accelerated forward and ejected ponderomotively by the field. We show that a bivariate ($θ, ϕ$) angular distribution, which was obtained with image plates, not only enables the peak intensity to be extracted, but also reflects nonideality of the focal-spot intensity distribution. In our prototype demonstration at intensities of a few $\times 10^{19}$ to a few $\times 10^{20}$ $\mathrm{W/cm^2}$, an f/10 optic produced a focal spot in the paraxial regime. This allows a plane-wave parameterization of the peak intensity given by $\tan{θ_c} = 2/a_0$ ($a_0$ being the normalized vector potential and $θ_c$ the minimum ejection angle) to be compared with our measurements. Qualitative agreement was found using an $a_0$ inferred from the pulse energy, pulse duration and the focal spot distribution with a modified parameterization, $\tan{θ_c} = 2η/a_0$ ($η= 2.02^{+0.26}_{-0.22}$). This highlights the need for (i) better understanding of intensity degradation due to focal-spot distortions and (ii) more robust modeling of the ejection dynamics. Using single-shot detection of electrons, we showed that while there is significant shot-to-shot variation in the number of electrons ejected at a given angular position, the average distribution scales with the pulse energy in a way that is consistent with that seen with the image plates. Finally, we note that the asymptotic behavior as $θ\to 0^{\circ}$ limits the usability of angular measurement. For 800 nm, this limit is at an intensity $\sim 10^{21}\ \mathrm{W/cm^2}$.

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