Generating MeV temperature electrons typically requires laser intensities of 10¹⁸ W cm⁻². However, achieving this at lower, non-relativistic intensities (~10¹⁶ W cm⁻²) using high-repetition-rate lasers is crucial for developing compact and ultrafast electron sources. Current methods, like laser wakefield acceleration (LWFA), become inefficient at moderate intensities (10¹⁷ W cm⁻²) due to reduced acceleration gradients. While LWFA has been demonstrated using millijoule-class lasers, achieving relativistic electron temperatures at these lower intensities remains a challenge. Target modification strategies have shown promise, but haven't efficiently produced relativistic electrons using moderate-intensity table-top lasers. Parametric processes like two-plasmon decay (TPD) have been explored, but typically require high-energy (100 mJ) laser pulses to reach sufficient intensities. This paper presents a novel technique using dynamic target structuring of micro-droplets to overcome these limitations and generate high-energy electron beams at significantly lower intensities.

Existing research on high-energy electron generation predominantly focuses on relativistic laser intensities (10¹⁸ W cm⁻²). Laser wakefield acceleration (LWFA) is a successful method, but its efficiency diminishes at lower intensities. Studies have demonstrated electron acceleration using millijoule-class lasers in gas targets, achieving energies up to 1.5 MeV. However, these typically require high intensities and are not suitable for creating compact, high-repetition-rate sources. Target modification has been investigated as a means to achieve relativistic electron generation at lower intensities, but previous experiments have not yielded significant success. The use of parametric processes such as two-plasmon decay (TPD) has also been explored, although it typically requires high-energy laser pulses and long pulse durations to create the necessary long scale-length pre-plasma. This work builds on these prior studies by introducing a novel approach that overcomes the limitations of existing methods.

This research employs a dynamic target structuring technique using micro-droplets. The method involves two collinear laser pulses: a pre-pulse and a main pulse. The pre-pulse, with 5% of the main pulse energy, arrives 4 ns before the main pulse and creates a concave surface on the liquid droplet. The main pulse then interacts with this dynamically shaped concave surface, generating plasma waves through the two-plasmon decay (TPD) instability. This process accelerates electrons to MeV energies. The experiments use a 1 kHz, 25 fs, 2 mJ, 800 nm Ti-Sapphire laser system, focusing the pulses onto 15 µm methanol droplets. The on-target intensity was 4 × 10¹⁶ W cm⁻². Electron energies were measured using a magnetic field bending electron spectrometer (ESM) with LANEX and image plate (IP) detectors. Bremsstrahlung X-ray measurements were also conducted for corroboration. Shadowgraphy was employed to visualize the pre-pulse induced droplet deformation. Two-dimensional hydrodynamic (h2d) and particle-in-cell (PIC) simulations (using SMILEI and EPOCH codes) were performed to model the laser-droplet interaction and understand the underlying acceleration mechanism. The simulations incorporated the modified target geometry and plasma parameters obtained from the hydrodynamic simulations. Optical emission spectra were also measured to further investigate the TPD process.

The experiments successfully generated two beams of electrons with hot electron temperature components of 200 keV and 1 MeV at a laser intensity of 4 × 10¹⁶ W cm⁻², significantly lower than what's typically required. The maximum electron energy exceeded 4 MeV. The electron emission was confined to the laser polarization plane, directed along two backward cones at ±50°. The 200 keV component scaled as I^(1/2) in this intensity regime. The presence of the pre-pulse was crucial, causing significant target modification. Shadowgraphy images revealed the formation of a concave cup-like structure on the droplet surface due to the pre-pulse. 2D PIC simulations, using the modified target geometry from hydrodynamic simulations, reproduced the experimental electron energy spectrum and angular distribution, confirming the TPD mechanism. The simulations showed that the optimal plasma scale length was 8λ, enhancing the relativistic temperature component. Stimulated Raman scattering (SRS) at 3ω₀/2 was observed, further supporting the TPD mechanism. High-resolution electron radiography (14 µm resolution) was demonstrated using the generated electron beams, achieving this at 100 times lower intensity than relativistic intensity sources.

The results demonstrate a novel and efficient method for generating relativistic electron beams at significantly lower laser intensities than previously possible. The dynamic target structuring combined with the two-plasmon decay instability enabled the generation of high-energy electrons with a compact, high-repetition-rate laser system. The observed correlation between experimental results and simulations strongly supports the identification of TPD as the dominant acceleration mechanism. The ability to generate high-flux, directional electron beams at these low intensities opens new avenues for applications such as high-resolution electron radiography and microscopy. The tunability of the electron emission angle through laser intensity control adds to the versatility of this technique.

This research presents a new scheme for generating MeV-class electron beams using a dynamically structured liquid droplet target and a millijoule-class laser. The use of a pre-pulse to create a concave surface optimizes the two-plasmon decay instability, leading to highly efficient electron acceleration. The results demonstrate the potential for developing compact, high-repetition-rate, ultra-fast electron sources for applications like high-resolution radiography and microscopy. Future work could explore scaling this technique to even higher repetition rates and investigating the generation of ultrashort electron pulses.

The simulations were performed in 2D, potentially overestimating the hot electron temperature and flux due to neglecting energy loss through processes like SRS in directions perpendicular to the polarization plane. While the study extensively investigates TPD, the potential interplay of other parametric instabilities like SRS could be further investigated. The study focused on methanol droplets, and further research could explore the applicability of this method to other materials and droplet sizes. The long-term drift in laser-droplet alignment, although corrected, might be further minimized with a more advanced feedback system.