Laser-plasma accelerators can produce high-quality electron beams, up to giga electronvoltsin energy, from a centimetre scale device. The properties of the electron beams and theaccelerator stability are largely determined by the injection stage of electrons into theaccelerator. The simplest mechanism of injection is self-injection, in which the wakefield isstrong enough to trap cold plasma electrons into the laser wake. The main drawback of thismethod is its lack of shot-to-shot stability. Here we present experimental and numericalresults that demonstrate the existence of two different self-injection mechanisms. Transverseself-injection is shown to lead to low stability and poor-quality electron beams, because of astrong dependence on the intensity profile of the laser pulse. In contrast, longitudinalinjection, which is unambiguously observed for the first time, is shown to lead to much morestable acceleration and higher-quality electron beams.
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