High field X-ray laser physics
Authors: Shen Bai-Fei , Ji Liang-Liang , Zhang Xiao-Mei , Bu Zhi-Gang , Xu Jian-Cai ,   Mathematics & Science College, Shanghai Normal University, Shanghai 200234, China,   State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China
Published: April 2021

<sec>Development of coherent X-ray source, especially X-ray free electron laser (XFEL), offers a new approach to reaching a strong X-ray field. High field laser physics will extend from optical to X-ray regime since the X-ray beam has high photon energy, high intensity and ultrashort pulse duration. Till now, nonlinear atomic physics and nonlinear molecular physics have been explored based on intense X-ray beam sources. They will extend to relativistic physics and quantum electrodynamics (QED) physics area with X-ray intensity increasing, and thus offering a new opportunity to innovatively investigate the particle acceleration and radiation, QED vacuum, dark matter generation and vacuum birefringence. This review provides an overview of the wake field acceleration, vacuum birefringence as well as axion generation and detection based on strong X-ray laser field. Intense X-ray pulse will show unique potential both in basic science and in practical applications. Finally, an outlook for the future development and perspectives of high-field X-ray physics is described. </sec><sec>The invention of chirped pulse amplification results in the generation of the light intensity in the relativistic regime (> 10<sup>18</sup> W/cm<sup>2</sup>). Laser-plasma interaction in this regime motivates multiple disciplines such as laser-driven particle acceleration, laser secondary radiation sources, strong-field physics, etc. While petawatt (PW) lasers have been established in various institutions, several projects of building 10 PW or even 100 PW lasers are proposed. However, pushing the laser power to the next level (EW) confronts significant challenges. Current technology is approaching to its limit in producing large aperture size optics due to the damage threshold of optical material. Alternatively, plasma is considered as a potential medium to amplify or compress laser pulses. This requires further validation in future studies. </sec><sec>In recent years, XFEL has made significant progress of producing high brightness light sources. Based on self-amplified spontaneous emission (SASE) or self-seeding in undulators, the XFEL provides a brightest light source up to the hard X-ray wavelength. The existing major XFEL facilities are LCLS-II in USA, EuXFEL in Europe, SACLA in Japan, Swiss FEL in Switzerland and PAL-XFEL in South Korea. In China, a new facility SHINE consisting of a high-repetition rate hard X-ray FEL and ultra-intense optical laser is under construction. After implementing the tapered undulator in XFEL, the peak power of X-ray pulses now reaches multi-terawatt. The pulses can also be compressed to an attosecond level. </sec><sec>Following this trend, it is expected that the coherent XFEL will be able to generate a super strong light field, thus pushing strong-field physics to the X-ray regime. The relativistic threshold for 1-nm X-ray is about 10<sup>24</sup> W/cm<sup>2</sup>, which we believe will be achievable in the near future. Such relativistic X-ray pulses can be used to stimulate relativistic dynamics in solid materials, realizing high-gradient low-emittance particle acceleration in solids. This may open a new path towards high-energy physics, advanced light sources, fast imaging, etc. In addition, the combination of strong X-rays and ultra-intense lasers offers a new opportunity to study the light-by-light scattering in vacuum and detecting the candidate particles for dark matter. The field of strong-field X-ray physics is largely unexplored realm. In this review, we show a few key science cases brought up by high power X-rays and shed light on this important direction.</sec><sec>The ultra-intense coherent X-ray laser with a wavelength in a range from 100 nm to less than 0.1nm can interact directly with the nanostructured materials with solid density. Benefiting from the ultra-intense field and ultra-high critical density, acceleration field with gradient of TeV/cm can be stimulated on a nanometer scale, and thus ultra-high energy particle beams can be obtained. The available nanometer material technique promotes such a development. For example, the recent research reported that high-repetition/few-attosecond high-quality electron beams can be generated from crystal driven by an intense X-ray laser. Beside electrons, ions including protons are expected to be accelerated to ultra-high energy via target normal sheath or light pressure acceleration mechanisms on a nanometer scale if the X-ray is intense enough. It should be noted that ultra-high acceleration gradient is not the unique advantage of the X-ray laser driven acceleration. A more important quality is the beam emittance that can be low enough because of the small size of the beam source. This is very significant for ultrafast microscopy to achieve a high resolution.</sec><sec>In classical physics, photon-photon interaction is prohibited in vacuum. However, according to the QED theory, vacuum is full of quantum fluctuation, in which virtual particle-antiparticle pairs emerge and annihilate in ultra-short instants. When excited by strong fields, the vacuum fluctuation appears as a weak nonlinear medium and allows photon-photon interaction therein, which is referred to as vacuum polarization. Based on the effective field theory, the vacuum polarization can be described by Euler-Heisenberg Lagrangian density, and then classical Maxwell equations are modified. Vacuum polarization can induce some novel physical effects, including vacuum birefringence, light-by-light scattering, vacuum diffraction, etc. Up to now, none of these effects has been verified experimentally under strong fields. The XFEL is regarded as a promising probe to explore these vacuum polarization effects. In this paper, the research progress of vacuum polarization driven by strong fields is summarized, the potential detection proposal using XFEL is discussed.</sec><sec>Dark matter is one of the puzzles in contemporary physics. Till now, we still have not known what particles constitute it. Axion is a spinless massive hypothetical boson that is proposed as the solution to strong CP problem. It is the particle beyond the standard model and has extremely weak interaction with the standard-model particle like photon, and hence there appears a significant obstacle to detecting it. Therefore, axion and axion-like-particles (ALPs) are a kind of promising candidate of dark matter. In this paper, we summarize the research progress of axions and ALP detection, including detecting the axions sources from universe, the production and detection of artificial axions and ALPs. It is shown that the XFEL is a potential tool for detecting the artificial axions and ALPs under strong electromagnetic fields.</sec><sec>The XFEL provides a coherent ultrafast X-ray beam for exploring particle acceleration and radiation, QED vacuum, dark matter generation, vacuum birefringence, etc. The probing of these dynamics requires different X-ray diagnoses, including the measurement of polarization purity, spectrum, pulse duration and focal condition. The X-ray polarization purity has been improved to a 10<sup>-10</sup> level by using 6 reflections based on channel-cut silicon crystal and it will efficiently probe the vacuum birefringence. The pulse duration of isolated X-ray pulse in FEL reaches as short as 200 as, which allows probing ultrafast electron dynamics. A new self-seeding scheme using the Bragg reflection in SACLA is developed to obtain a narrow spectrum of 3 eV, 10 times smaller than that in the current SASE scheme. Therefore, the fast development of X-ray diagnostics will finely characterize X-ray beam itself and offer a unique tool for understanding the underlying phenomena for different applications.</sec><sec>The peak intensity of coherent X-ray beam will reach to a relativistic level in future. A possible way is CPA technology, which is well developed in intense near-infrared laser system and may produce an ultrahigh intense attosecond X-ray pulse. High field X-ray laser physics will offer new opportunities both for basic science and for revolutionary application.</sec>