Real time measurement of femtosecond dynamics of relativistic intense laser driven ultra-hot electron beams

In the interaction between ultra short and ultra strong lasers and matter, short pulse width and high energy electrons are generated, commonly referred to as "hot electrons". The generation and transport of hot electrons is one of the important fundamental issues in high-energy density physics of lasers. Superhot electrons can excite ultrafast electromagnetic radiation in a wide range of wavelengths (microwave gamma rays), drive ion acceleration, and rapidly heat substances, serving as energy carriers in the "fast fire" process of inertial confinement fusion. The properties of various secondary radiation and particle sources, plasma heating and energy deposition processes are closely related to the temporal, spatial, and energy characteristics, as well as the evolution dynamics of hot electrons. After years of research, people have gained a clear understanding of the energy and spatial characteristics of superheat electrons. However, due to the lack of suitable high-resolution measurement methods, the diagnosis of the time structure and dynamic processes of superheat electron beams still faces challenges.

A research team composed of Liao Guoqian, a specially appointed researcher, Li Yutong, and Zhang Jie from the Institute of Physics of the Chinese Academy of Sciences/Key Laboratory of Photophysics of the National Research Center for Condensed Matter Physics in Beijing, and other academicians, has explored for many years a new way for ultraintense lasers to interact with solid targets to generate high-power terahertz radiation, proposed a terahertz generation model based on coherent transition radiation of ultrahot electron beams, and developed a single shot ultra wideband terahertz detection technology based on non collinear autocorrelation.

On this basis, the team recently proposed a new method for diagnosing superheat electron beams using terahertz radiation. Using a self-developed high time resolution single shot terahertz autocorrelator, in-situ and real-time measurements of the time-domain structure and dynamics of superheat electron beams during the interaction between ultra strong lasers and thin film targets were achieved. In theory, a mapping relationship between terahertz radiation properties and the spatiotemporal characteristics of superheat electron beams was constructed, and the quantitative relationship between terahertz pulse width and electron beam pulse width, beam spot size, emission angle and other parameters was provided.

On the one hand, the pulse width of the ultra hot electron beam in the laser solid target interaction was accurately characterized at the order of tens of femtoseconds. It was found that the electron beam accelerated by the ultra strong laser had a pulse width similar to that of the driving laser during generation. Subsequently, during transmission, the longitudinal time width and transverse spatial size gradually widened due to velocity dispersion and angular divergence; On the other hand, for the first time, the dynamics of superheat electron backflow caused by secondary acceleration of laser pulses and target surface sheath field were directly observed. It was found that when a high contrast laser interacts with a thin film target, the electron beam bounces back and forth between the front and back surface sheath fields of the target, with a duration of up to 100 femtoseconds. These results demonstrate a single shot, non-destructive, in situ, and high temporal resolution method for characterizing hot electrons, which is of great significance for understanding and optimizing the spatiotemporal characteristics of ultrafast radiation and particle sources based on hot electrons, and developing related applications.

The relevant results were recently published in the Physical Review Letters under the title "Femtosecond dynamics of fast electron pulses in correlated laser foil interactions". This research work has received support from the National Natural Science Foundation of China, the Ministry of Science and Technology, and the Chinese Academy of Sciences.

Figure 1. Diagnosis of the pulse width of a superheat electron beam using terahertz coherent transition radiation.

Figure 2. Diagnosis of the dynamics of hot electron reflux based on multi cycle terahertz pulses.

Source: Institute of Physics, Chinese Academy of Sciences