Controlling the parameters of a laser plasma accelerated electron beam is a topic of intense research with a particular focus placed on controlling the injection phase of electrons into the accelerating structure from the background plasma. An essential prerequisite for high-quality beams is dark-current free acceleration (i.e., no electrons accelerated beyond those deliberately injected). We show that small-scale density ripples in the background plasma are sufficient to cause the uncontrolled (self-)injection of electrons. Such ripples can be as short as $ensuremath{sim}50ext{ }ext{ }ensuremath{mu}mathrm{m}$ and can therefore not be resolved by standard interferometry. Background free injection with substantially improved beam characteristics (divergence and pointing) is demonstrated in a gas cell designed for a controlled gas flow. The results are supported by an analytical theory as well as 3D particle in cell simulations.
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机译：控制激光等离子体加速的电子束的参数是深入研究的主题，特别着重于控制电子从背景等离子体注入加速结构的注入相位。高质量光束的基本前提是暗电流自由加速（即，没有电子会加速到故意注入的电子之外）。我们表明，背景等离子体中的小规模密度纹波足以引起电子的不受控制的（自）注入。这样的波动可能短到$ ensuremath { sim} 50 text {} text {} ensuremath { mu} mathrm {m} $，因此无法通过标准干涉仪解决。在专为控制气流设计的气室中，证明了无束缚射流的背景特性得到了大幅改善（发散和指向）。分析理论以及细胞模拟中的3D粒子为结果提供了支持。
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