MHz 突发激光烧蚀过程中轻水和重水中的热电子积累
Thermo-electron accumulation in light and heavy water during MHz-burst laser ablation
AI总结：
本文首次研究重水中 MHz 突发激光烧蚀，发现 1 ps 突发模式产生最强等离子体辉光，并将其归因于热电子积累效应，为水下激光烧蚀和聚变相关研究提供新认识。
摘要：
激光诱导水烧蚀会引发多种物理效应，包括原子电离、液体的光学击穿、相爆炸、空化和冲击波传播。在重水中，这些效应还可以通过氘-氘聚变反应进一步放大，而聚变反应需要极高的能量水平。激光脉冲可以分组为突发脉冲，以在烧蚀等离子体羽流中达到所需能量。本研究旨在比较轻水和重水在单脉冲和突发模式超短激光辐照下的烧蚀等离子体辉光与热效应。值得注意的是，本研究首次引入了突发激光烧蚀在重水中的新应用。烧蚀在水面下方沿圆形激光扫描轨迹进行，采用两种不同的烧蚀机制：突发模式和单脉冲模式，并使用不同焦距的透镜和不同的脉冲持续时间。利用可见光和红外探测器、快速硅探测器以及热电偶监测吸收过程和等离子体辉光。研究表明，当使用 1 ps 激光脉冲时，重水中的突发机制产生了最强的等离子体辉光，较短脉冲产生的辉光较弱，最长脉冲产生的辉光最弱。令人惊讶的是，在较低的初始功率密度 2.6e13 W/cm2 下，等离子体辉光比在较高功率密度 8e13 W/cm2 下高四倍。这些发现与现有的超短激光脉冲在水中形成等离子体的理论进行了比较。突发模式中观察到的脉冲间等离子体辉光增强被归因于热电子积累效应。利用强场电离和雪崩电离模型计算了激发电子和水合电子的密度。此外，还讨论了脉冲奇偶性对重水中突发烧蚀辉光的影响。
英文摘要：
Laser-induced water ablation triggers various physical effects, including atom ionization, optical breakdown of the liquid, phase explosion, cavitation, and shockwave propagation. These effects can be further amplified in heavy water by deuterium-deuterium fusion reactions, which require extremely high energy levels. Laser pulses can be grouped in bursts to achieve the necessary energy within the ablation plasma plume. This study aims to compare the ablation plasma glow and thermal effects in light and heavy water under both single-pulse and burst-mode ultrashort laser irradiation. Notably, this research introduces the novel application of burst laser ablation in heavy water for the first time. The ablation was conducted beneath the water surface along a circular, laser-scanned trajectory, with two distinct ablation regimes: burst mode and single-pulse mode, utilizing lenses with varying focal lengths and different pulse durations. Absorption processes and plasma glow were monitored using visible and infrared detectors, a fast silicon detector, and a thermocouple. The study revealed that the burst regime in heavy water produced the most intense plasma glow when 1 ps laser pulses were used, with shorter pulses yielding less intense glow and the longest pulses yielding the least. Surprisingly, plasma glow at a lower initial power density of 2.6e13 W/cm2 was four times higher than at a higher power density of 8e13 W/cm2. These findings were compared with existing theories on plasma formation in water by ultrashort laser pulses. The observed increase in pulse-to-pulse plasma glow in burst mode was attributed to thermo-electron accumulation effects. The density of excited and hydrated electrons was calculated using both strong-field ionization and avalanche ionization models. Additionally, the influence of pulse parity on burst ablation glow in heavy water was discussed.
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