  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.
