Optical ionization effects in kHz laser wakefield acceleration with few-cycle pulses

TL;DR

This paper demonstrates the advantages of using hydrogen in kHz laser wakefield acceleration with few-cycle pulses, showing improved beam quality and continuous operation enabled by a novel differential pumping scheme.

Contribution

It provides the first comprehensive study of gas effects in kHz LWFA, highlighting hydrogen's superior performance and introducing a differential pumping system for continuous high-repetition-rate operation.

Findings

Hydrogen yields higher quality electron beams with up to 10 MeV energy.

Differential pumping enables continuous kHz operation, unlike burst mode systems.

Ionization effects in nitrogen and helium degrade performance, mitigated in hydrogen.

Abstract

We present significant advances in Laser Wakefield Acceleration (LWFA) operating at a 1 kHz repetition rate, employing a sub-TW, few-femtosecond laser and a continuously flowing hydrogen gas target. We conducted the first comprehensive study assessing how the nature of the gas within the target influences accelerator performance. This work confirms and elucidates the superior performance of hydrogen in kHz LWFA. Our system generates quasi-monoenergetic electron bunches with energies up to 10 MeV, bunch charges of 2 pC, and angular divergences of 15 mrad. Notably, our novel scheme relying on differential pumping enables continuous operation at kHz repetition rates, contrasting with previous systems that operated in burst mode to achieve similar beam properties. Particle-in-cell simulations explain hydrogen's superior performances: the ionization effects in nitrogen and helium distort the laser pulse, negatively impacting accelerator performance. These effects are strongly mitigated in hydrogen plasma, thereby enhancing beam quality. This analysis represents a significant step forward in optimizing and understanding kHz LWFA. It underscores the critical role of hydrogen and the imperative need to develop hydrogen-compatible target systems capable of managing high repetition rates, as exemplified by our differential pumping system. These advances lay the groundwork for further developments in high-repetition-rate LWFA technology.