Imaging electron angular distributions to assess a full-power petawatt-class laser focus
- Univ. of Maryland, College Park, MD (United States)
- Centro de Laseres Pulsados, Salamanca (Spain); Universidad de Salamanca (Spain)
- Centro de Laseres Pulsados, Salamanca (Spain)
- Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
- Univ. of Alberta, Edmonton, AB (Canada)
- Universidad de Salamanca (Spain)
Here we present a 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 ×1019 to a few ×1020W/cm2, an f/10 optic produced a focal spot in the paraxial regime. This allows a plane-wave parametrization of the peak intensity given by tanθc=2/a0 (a0 being the normalized vector potential and θc the minimum ejection angle) to be compared with our measurements. Qualitative agreement was found using an a0 inferred from the pulse energy, pulse duration, and focal spot distribution with a modified parametrization, tanθc=2η/a0 ($$η=2.02^{+0.26}_{–0.22}$$). 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→0° limits the usability of angular measurement. For 800 nm, this limit is at an intensity ~1021W/cm2.
- Research Organization:
- Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- USDOE National Nuclear Security Administration (NNSA); National Science Foundation (NSF); Natural Sciences and Engineering Research Council of Canada (NSERC); Junta de Castilla y León; Ministerio de Ciencia e Innovacion; LaserLab Europe V
- Grant/Contract Number:
- AC52-07NA27344
- OSTI ID:
- 2407212
- Report Number(s):
- LLNL--JRNL-858032; 1087488
- Journal Information:
- Physical Review A, Journal Name: Physical Review A Journal Issue: 5 Vol. 108; ISSN 2469-9926
- Publisher:
- American Physical Society (APS)Copyright Statement
- Country of Publication:
- United States
- Language:
- English
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