Relativistic electron beams driven by kHz single-cycle light pulses (1611.09543v2)

Abstract: Laser-plasma acceleration is an emerging technique for accelerating electrons to high energies over very short distances. The accelerated electron bunches have femtosecond duration, making them particularly relevant for applications such as ultrafast imaging or femtosecond X-ray generation. Current laser-plasma accelerators are typically driven by Joule-class laser systems that have two main drawbacks: their relatively large scale and their low repetition-rate, with a few shots per second at best. The accelerated electron beams have energies ranging from 100 MeV to multi-GeV, however a MeV electron source would be more suited to many societal and scientific applications. Here, we demonstrate a compact and reliable laser-plasma accelerator producing high-quality few-MeV electron beams at kilohertz repetition rate. This breakthrough was made possible by using near-single-cycle light pulses, which lowered the required laser energy for driving the accelerator by three orders of magnitude, thus enabling high repetition-rate operation and dramatic downsizing of the laser system. The measured electron bunches are collimated, with an energy distribution that peaks at 5 MeV and contains up to 1 pC of charge. Numerical simulations reproduce all experimental features and indicate that the electron bunches are only $\sim 1$ fs long. We anticipate that the advent of these kHz femtosecond relativistic electron sources will pave the way to wide-impact applications, such as ultrafast electron diffraction in materials with an unprecedented sub-10 fs resolution.

Relativistic Electron Beams Driven by Kilohertz Single-Cycle Light Pulses

The paper "Relativistic Electron Beams Driven by kHz Single-Cycle Light Pulses" presents an innovative approach to laser-plasma acceleration, a domain of considerable interest due to its potential to produce high-energy electron beams over very short distances. This research demonstrates a compact and efficient laser-plasma accelerator capable of generating high-quality electron beams with energies in the few-MeV range at kilohertz repetition rates. The paper notably employs near-single-cycle light pulses, drastically reducing the required energy input traditionally necessitated by Joule-class laser systems. This methodological refinement enables more frequent operation and miniaturization of the laser system.

Experimental Insights

The platform described showcases its ability to overcome limitations associated with conventional Joule-class systems, which include large size and low repetition rates. By utilizing multi-mJ laser systems capable of producing near-single-cycle pulses with a duration of 3.4 fs, the authors achieved a kilohertz repetition rate for electron beam acceleration. Experimental measurements indicated peak electron energies of 5 MeV, with collimated beam properties and sub-picocoulomb charges per shot. Numerical simulations aligned closely with these experimental results, suggesting these electron bunches have durations of approximately 1 fs.

A breakthrough in the reduction of laser energy requirements by three orders of magnitude without compromising beam quality is notable. This is achieved through a landscaping of the laser-plasma interaction that promotes the nonlinear bubble regime—a state conducive to producing high-quality beams with narrow energy spreads and minimal divergence.

Implications and Theoretical Considerations

The adoption of this methodology has substantial implications for the future of both scientific and practical applications, including ultrafast electron diffraction and femtosecond X-ray generation. These applications benefit immensely from the high repetition-rate and reliability of the electron source, which enhances temporal resolution and signal strength necessary for detailed imaging and diffraction at atomic scales.

The paper underscores the importance of precise control over the laser parameters, such as chirp and carrier-envelope phase (CEP), both of which play critical roles in optimizing electron injection and acceleration. Experimental observations indicated that introducing a small positive chirp compensates for plasma-induced dispersion effects, thereby maximizing beam charge and energy—an aspect confirmed through particle-in-cell simulations.

Prospects for Future Research

Looking forward, the research community is poised to explore further miniaturization of laser systems while enhancing their stability under these single-cycle pulse regimes. The ability to leverage compact, commercial-grade laser systems with mJ-level energies expands accessibility and reduces operational costs, enabling broader experimental deployments.

Continued investigations are likely to focus on refining control over chirp and CEP, as well as experiments aimed at verifying the scalability of this approach with denser plasmas or varied gas compositions. Additionally, the high sensitivity of system performance to laser parameters implies potential benefits from active stabilization technologies, which could mitigate the charge fluctuation observed due to the experimental setup's proximity to the threshold conditions for electron injection.

In summary, this paper advances the understanding and applicability of laser-plasma accelerators, paving the way for a new class of compact and rapid electron sources compatible with diverse research applications requiring ultrafast temporal resolution. The implications for material science, physics research, and medical imaging are particularly promising as this technology matures.