Speaker
Description
Our group has recently demonstrated that relativistic electron acceleration can be achieved directly in ambient air by tightly focusing few-cycle infrared pulses with high-numerical-aperture optics, using mJ-class femtosecond laser systems. Owing to minimal B-integral accumulation which prevents intensity clamping, relativistic peak intensities approaching 1e19 W/cm² have been achieved, resulting in electron beams with energies up to 1.4 MeV and dose rates of 0.15 Gy/s.
Building on these experimental results, we present a combined theoretical and numerical investigation aimed at identifying the physical mechanism responsible for the acceleration and optimizing its performance. An analytical model for linearly polarized tightly focused ultrashort laser fields reflected by high-NA mirrors is derived and coupled to fully three-dimensional Particle-In-Cell simulations. By varying the laser wavelength (0.8–7 µm) and normalized vector potential (a₀ = 3.6–7.0), we confirm that acceleration is governed by the relativistic ponderomotive force, leading to preferential forward emission. A maximum electron kinetic energy of ≈1.4 MeV is predicted near a central wavelength of 1.8 µm, consistent with experimental results.
We further investigate the influence of polarization and plasma density on acceleration efficiency. In the generated near-critical plasmas, linearly and circularly polarized pulses outperform radially polarized pulses in terms of both maximum energy and total accelerated charge, while linear polarization yields lower divergence beams. Scaling analyses indicate that multi-MeV electrons can be generated with charges above 1 nanocoulomb.
These results establish tightly focused mJ-class lasers as a promising platform for compact, high-repetition-rate multi-MeV electron sources with potential applications in ultrafast imaging and FLASH radiotherapy.
| Keyword-1 | Electron acceleration |
|---|---|
| Keyword-2 | Laser-matter interaction |
| Keyword-3 | Numerical simulations |