Numerical study of electron acceleration and betatron radiation based on interaction of petawatt femtosecond laser with near-critical-density plasma

Laser-driven betatron radiation is a wide-energy-spectrum X-ray source analogous to synchrotron radiation. Compared to the quasi-monochromatic X-ray spectra of synchrotron radiation or free-electron lasers, the broad energy spectrum of betatron radiation is more favorable for X-ray absorption spectroscopy. Additionally, laser-driven betatron radiation features a small source size, short pulse duration, low divergence, and high brightness, making it comparable to third-generation synchrotron sources.

The photon energy yield of betatron radiation is closely related to the quality of the electron beam, plasma density, and transverse oscillation amplitude. However, current technology faces two major challenges: first, there is a trade-off between electron beam charge and energy, with single-shot charges typically limited to the hundreds-of-pC range; second, the radiation conversion efficiency is significantly influenced by target parameters, necessitating breakthroughs through innovative target structures.

For typical petawatt-class femtosecond laser facility parameters, a capillary-type gas-cell structure target is proposed to generate a near-critical density plasma with a hundred-micrometer scale and a steep density gradient. This gas-cell structure target features low back pressure and minimal gas injection. Due to the confinement by the gas cell walls, a more stable platform-like gas density distribution can be produced within the cell.

Particle-in-cell simulation methods were employed to study the electron acceleration and betatron radiation processes resulting from the interaction of petawatt-class femtosecond lasers with this near-critical density plasma. By adjusting the gas density and laser pulse width, a high-charge and high-energy electron beam can be induced to undergo transverse oscillations within the plasma channel, thereby generating a high-brightness betatron radiation source with a peak photon energy of approximately 8 keV and a brightness of $ 1.75\times {10}^{20}\;\mathrm{p}\mathrm{h}\cdot {\mathrm{s}}^{-1}\cdot {\mathrm{m}\mathrm{m}}^{-2}\cdot {\mathrm{m}\mathrm{r}\mathrm{a}\mathrm{d}}^{-2}\cdot $$ {\left(0.1{\text{%}} \mathrm{b}\mathrm{w}\right)}^{-1} $.

The results indicate that appropriate gas density and laser pulse width are conducive to the stable formation of plasma channels. Within these channels, electrons undergo effective laser wakefield acceleration firstly. These accelerated high-energy electrons interact directly with the tail of the laser. Through betatron resonance and direct laser acceleration, their yield and cutoff energy can be further enhanced. Additionally, the study focuses on the impact of gas density and laser pulse width on the betatron radiation source and elucidates the underlying mechanisms.