We study numerically the mechanisms of proton acceleration in gas–foil targets driven by an ultraintense femtosecond laser pulse. The target consists of a near-critical-density hydrogen gas layer of a few tens of microns attached to a $2\ \mathrm {\mu }$ m-thick solid carbon foil with a contaminant thin proton layer at its back side. Two-dimensional particle-in-cell simulations show that, at optimal gas density, the maximum energy of the contaminant protons is increased by a factor of $\sim$ 4 compared with a single foil target. This improvement originates from the near-complete laser absorption into relativistic electrons in the gas. Several energetic electron populations are identified, and their respective effect on the proton acceleration is quantified by computing the electrostatic fields that they generate at the protons’ positions. While each of those electron groups is found to contribute substantially to the overall accelerating field, the dominant one is the relativistic thermal bulk that results from the nonlinear wakefield excited in the gas, as analysed recently by Debayle et al. (New J. Phys., vol. 19, 2017, 123013). Our analysis also reveals the important role of the neighbouring ions in the acceleration of the fastest protons, and the onset of multidimensional effects caused by the time-increasing curvature of the proton layer.

中文翻译：

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使用气箔靶增强激光驱动的质子加速

我们在数值上研究了由超强飞秒激光脉冲驱动的气箔靶中的质子加速机制。目标由一个几十微米的近临界密度氢气层组成，附着在一个 $2\ \mathrm {\mu }$ m 厚的固体碳箔，其背面有一个污染物薄质子层。二维粒子细胞模拟表明，在最佳气体密度下，污染物质子的最大能量增加了 1 倍 $\sim$ 4 与单箔目标相比。这种改进源于激光几乎完全吸收到气体中的相对论电子中。确定了几个高能电子群体，并通过计算它们在质子位置产生的静电场来量化它们对质子加速的各自影响。虽然发现这些电子基团中的每一个都对整个加速场有很大贡献，但主要的是相对论热体积，它是由气体中激发的非线性尾流场产生的，正如 Debayle 最近分析的那样等。(新 J. 物理学。, 卷。19, 2017, 123013)。我们的分析还揭示了相邻离子在加速最快质子中的重要作用，以及质子层随时间增加的曲率引起的多维效应的开始。