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Shielding for ionizing radiation is a critical safety measure for experiments performed with Joule-class lasers and this is becoming increasingly important for mJ-class lasers, especially at high average power. In-air experimental configurations of laser-generated radiation require further radiation safety considerations as the simpler implementation can lead to an even higher exposure risk. We present a straightforward method to generate MeV-ranged electron beams in ambient air through the tight focusing (NA $\cong$ 1) of a 12 fs, 3 mJ, infrared laser (λ$_0$ = 1.8 μm) operating at 100 Hz [1]. The measured dose rates range from 15 μGy/min at 6 m and up to 9 Gy/min at 0.1 m from the interaction zone. For reference, conventional radiation therapy for cancer treatments is typically performed at dose rates < 6 Gy/min. For an exposed researcher positioned at 1 m from the source, the delivered dose exceeds the public dose limit of 1 mGy/year (for electrons and x-rays), set by the Canadian Nuclear Safety Commission (CNSC) [2], in less than one second and warrants the implementation of radiation protection. We therefore advise to use high caution for researchers performing tight-focusing in air with mJ-class femtosecond lasers.
We show that relativistic peak intensities in ambient air are enabled by a very low B-integral from the use of a 1.8 μm central wavelength, a few-cycle pulse duration and a tight focusing geometry which altogether push further the intensity clamping limit. This generates a near-critical plasma in ambient air leading to a strong laser-to-electron energy conversion efficiency that explains the measured high dose rates. Three-dimensional Particle-In-Cell simulations confirm that the acceleration mechanism is based on the relativistic ponderomotive force and show theoretical agreement with the measured electron energies and divergence. Finally, we discuss the scalability of this method with the continuing development of high average power mJ-class lasers. This technique provides a promising approach for FLASH radiation therapy.
This work is supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), Compute Canada and Fonds de Recherche du Québec - Nature et Technologies (FRQNT).
[1] S. Vallières et al., "High Dose-Rate Ionizing Radiation Source from Tight Focusing in Air of a mJ-class Femtosecond Laser", arXiv, DOI:10.48550/arXiv.2207.05773 (2022).
[2] Nuclear Safety and Control Act, ”Radiation Protection Regulations”, Canadian Nuclear Safety Commission, URL: https://laws-lois.justice.gc.ca/eng/regulations/SOR-2000-203/FullText.html
