Abstract
Ionization-induced injection mechanism was introduced in 2010 to reduce the laser intensity threshold for controllable electron trapping in laser wakefield accelerators (LWFA). However, usually it generates electron beams with continuous energy spectra. Subsequently, a dual-stage target separating the injection and acceleration processes was regarded as essential to achieve narrow energy-spread electron beams by ionization injection. Recently, we numerically proposed a self-truncation scenario of the ionization injection process based upon overshooting of the laser-focusing in plasma which can reduce the electron injection length down to a few hundred micrometers, leading to accelerated beams with extremely low energy-spread in a single-stage. Here, using 100 TW-class laser pulses we report experimental observations of this injection scenario in centimeter-long plasma leading to the generation of narrow energy-spread GeV electron beams, demonstrating its robustness and scalability. Compared with the self-injection and dual-stage schemes, the self-truncated ionization injection generates higher-quality electron beams at lower intensities and densities, and is therefore promising for practical applications.
Highlights
Laser wakefield electron acceleration (LWFA) was proposed in 1979 by Tajima and Dawson[1]
Based on particle-in-cell (PIC) simulations[25] we have shown a self-truncation of the ionization injection (STII) process, where the electron injection occurs only over a distance of few-hundred micrometers at the very front region of the He-N2 mixed gas target
The STII conditions differ from previous ionization injection experiments in the following two major points; the first is the use of unmatched laser spot size in order to allow for a significant evolution of the laser pulse
Summary
Laser wakefield electron acceleration (LWFA) was proposed in 1979 by Tajima and Dawson[1]. The “ionization-induced electron injection” mechanism[17,18] was proposed to achieve this goal It utilizes the high ionization-threshold for the K-shell electrons of a high-Z gas (such as nitrogen), which is mixed with the low-Z gas (He) in order to control the initial injection phase of the ionized K-shell electrons. A dual-stage target with a short injection stage of doped gas was proposed in order to improve the relative energy-spread[23,24]. The main advantage of the STII is the fact that it is a single-stage scheme which can generate as low as few percent energy-spread electron beams This is realized by using the so-called unmatched large laser spot size at the beginning, where the laser intensity is high-enough to ionize the K-shell electrons of N2, injecting them into the wake formed in the background He plasma. There has been no detailed investigation on this mechanism, including its flexibility toward changes in laser-plasma parameters, and in particular its applicability for long laser-plasma acceleration lengths which are needed for accelerating electron beams to GeV energies
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