We report an experimental and numerical study of the acceleration of electrons in a plasma interacting with a subterawatt laser pulse (intensity of ≈3 × 1018 W cm-2 at a pulse duration of 50 fs). A preplasma layer on the surface of a molybdenum target is formed by an additional laser pulse with a duration of 8 ns and an intensity of ≈2 × 1012 W cm-2. It is shown that an increase in the laser pulse duration to 1700 fs at a constant energy (and a proportional decrease in intensity) leads to an increase in the yield of bremsstrahlung γ-radiation by more than an order of magnitude when the nanosecond pulse is ahead of the femtosecond one by 15–25 ns. Interferometry data and results of diagnostics of optical and γ-radiation of a plasma demonstrate that the collisional ionisation of atoms by electrons oscillating in the field of such a laser pulse plays an essential role in the formation of electron density profile. The sensitivity of the described effect to the level of amplified spontaneous emission is determined, despite the nanosecond pulse impact. Numerical simulations show that at a large pulse duration, the acceleration of electrons is stipulated by the beaking of plasma waves excited during stimulated Raman scattering of laser radiation.

中文翻译：

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亚太瓦超短激光脉冲产生伽马辐射：优化前等离子体和脉冲持续时间

我们报告了与亚瓦级激光脉冲（在 50 fs 的脉冲持续时间下的强度 ≈3 × 1018 W cm-2）相互作用的等离子体中电子加速度的实验和数值研究。钼靶表面上的预等离子体层是通过额外的激光脉冲形成的，持续时间为 8 ns，强度为 ≈2 × 1012 W cm-2。结果表明，当纳秒脉冲为恒定能量时，激光脉冲持续时间增加到 1700 fs（强度成比例降低）导致轫致辐射 γ 辐射的产量增加一个数量级以上。比飞秒快 15-25 ns。干涉测量数据和等离子体的光学和 γ 辐射诊断结果表明，在这种激光脉冲场中振荡的电子对原子的碰撞电离在电子密度分布的形成中起着至关重要的作用。尽管有纳秒脉冲影响，但仍确定了所描述的效应对放大的自发辐射水平的敏感性。数值模拟表明，在大脉冲持续时间下，电子的加速是由激光辐射受激拉曼散射期间激发的等离子体波的喙形决定的。