The rapid development of ultra-intense and ultra-short lasers has provided unprecedented new experimental methods and extreme physical conditions, made it possible to reach new frontiers of ultra-fast and intense interactions between lasers and matter, and given birth to a large number of new principles, new phenomena, and revolutionary techniques. Plasma-based acceleration driven by an ultra-intense and ultra-short laser may contribute to the emergence of new particle-acceleration technologies and generation of novel ultra-fast radiation sources. These novel particle and radiation sources can provide new means and opportunities for frontier interdisciplinary studies in areas such as high-energy particle physics, nuclear photonics, materials science, and biomedicine, making it a hot spot and emerging field on the world scientific and technological frontiers.
The accelerating electric field of a laser-driven plasma wakefiled can reach 100 GV/m, which is more than three orders of magnitude higher than that of a traditional electron accelerator. A high-energy GeV electron beam can thus be produced over a centimeter-scale acceleration length, thus greatly reducing the scale and cost of the accelerator. The electron beams produced via laser wakefield acceleration also have the advantages of an ultrashort pulse duration and inherent high-precision synchronization with the driving laser. In addition, by designing an appropriate and effective scheme, the electron-injection and acceleration processes can be optimized to produce high-quality and high-energy ultrafast electron sources with ultrahigh brightness comparable to that from a traditional accelerator.
Laser-wakefield-driven electron beams can be used as low-cost and desktop femtosecond radiation sources such as for betatron X-ray radiation, inverse Compton scattering, bremsstrahlung radiation, and undulator radiation. These novel radiation sources usually have high brightness, good collimation, a femtosecond pulse duration, and energy tunability, covering a wide spectral range from extreme ultraviolet to gamma rays. Therefore, research in this area is occurring around the world, and this is an important research topic for high-field laser physics and new accelerators. Such laser wakefield acceleration and novel radiation sources are thus of great scientific significance for the development and application of synchrotron radiation, free electron lasers, and high-energy particle physics.
After nearly 20 years of development, great progress has been made in both experimental and theoretical studies on laser-driven plasma acceleration. It is now transitioning from laser acceleration to laser accelerators. On one hand, the energy gain of laser wakefield electron acceleration has been significantly extended to 7.8 GeV. On the other hand, the specific qualities of the accelerated electron beams produced via laser wakefield acceleration, such as the energy spread, divergence, emittance, and stability of the electron beam, are also being optimized to a great extent. However, the comprehensive performance has to meet higher requirements for practical application, and there are still many key scientific issues and technical difficulties that need to be further explored and solved in the future. In particular, the energy spread of the electron beam is usually on the order of several percent, and such a large energy spread has greatly hindered its practical application. In order to obtain more stable and brighter high-energy electron beams, the electron injection and acceleration in the plasma wakefield should be accurately controlled and optimized to minimize the energy spread and divergence, which can also improve the application performance of novel radiation sources. Therefore, the basic principles and parameter characteristics of a plasma wakefield driven by a femtosecond intense laser are first briefly introduced. The mechanisms and characteristics of different electron injection methods are then analyzed and compared (Table 1). Second, based on the research results and progress made by our group in recent years, the schemes and technologies for exploring energy chirp control in a plasma wakefield with a structured plasma profile are summarized and analyzed in relation to the generation of ultrahigh-brightness electron beams with an ultralow energy spread at a per-mille level (Fig.5). Third, we discuss how these high-quality electron beams are used to produce novel radiation sources and greatly improve their application performance, including enhanced betatron X-ray radiation (Fig.9), quasi-monoenergetic all-optical self-synchronized Compton scattering γ‑rays (Fig.18), and free-electron lasing in an undulator (Fig.22). Some of the progress in other related frontier research fields is also discussed in relation to laser wakefield electron acceleration and novel radiation source generation. Finally, the prospects for a laser wakefield electron accelerator and its further practical applications are outlined.
A high-quality electron beam source and novel radiation source based on laser wakefield acceleration have the advantages of a compact size, easy tuning, small source size, femtosecond pulse duration, high brightness, good collimation, and high-precision synchronization control, which can provide new methods and tools for frontier interdisciplinary research such as high-energy particle physics, nuclear photonics, materials science, and biomedicine. Although significant progress has been made in the past decade in improving the quality of an electron beam such as its energy spread and six-dimensional brightness, the wakefield accelerator is still in a very early phase in view of the energy spread and stability of the electron beam, especially for electron beams with energy levels below 100 MeV or above 1 GeV, when compared with traditional accelerators. This dilemma is mainly limited by the scalability and stability of the existing schemes. The key issue or challenge facing the wakefield acceleration community is to devise more effective schemes to generate electron beams with an ultralow energy spread (0.1%‒0.01%), ultralow emittance (~1 μm·mrad), high repetition rate, and stability. Benefiting from the rapid and continuous development of ultrashort pulse laser technology in terms of the repetition rate, waveform control, and stability of the high-power laser, it is believed that the qualities and brightness of these high-energy ultrafast electron beams will be further improved by advancing the existing schemes, which will further facilitate the development of novel radiation sources. All these advances will greatly promote the continuous development of high-quality laser wakefield electron accelerators and their practical applications in the years to come.
