High-flux bright x-ray source from femtosecond laser-irradiated microtapes

communications physics Article https://doi.org/10.1038/s42005-024-01575-z High-flux bright x-ray source from femtosecond laser-irradiated microtapes Check for updates 1234567890():,; 1234567890():,; Xiaofei Shen 1 , Alexander Pukhov 1 & Bin Qiao 2 Betatron x-ray sources from laser-plasma interaction are characterized by compactness, ultrashort duration, broadband spectrum and micron source size. However, high-quality measurements with good statistics, especially in a single shot, require fluxes and energies beyond the current capabilities. Here, we propose a method to enhance the flux and brightness of the betatron sources without increasing the laser energy. By irradiating an edge of a microtape target with a femtosecond laser, a strong surface plasma wave (SPW) is excited at the edge and travels along the lateral plasma-vacuum interfaces. Tens of nC of electrons are peeled off and accelerated to superponderomotive energies by the longitudinal field of the SPW, whilst undergoing transverse betatron oscillations, leading to emission of hard x-rays. Via three-dimensional particle-in-cell simulations, we demonstrate that a tabletop 100 TW class femtosecond laser can produce an ultrabright hard x-ray pulse with flux up to 107 photons eV−1 and brilliance about 1023 photons s−1 mm−2 mrad−2 0.1%BW−1, paving the way for single-shot x-ray measurements in ultrafast science and high-energy-density physics. Ever since its discovery in 1895 by Röntgen, x-ray radiation has always been pushing the frontiers of our knowledge as one of the most powerful tools for exploring the properties of matter1–3. State-of-the-art x-ray sources can produce ultrabright x-rays at keV photon energies4–8. They have become indispensable for many applications in fundamental research, industry and medicine. However, despite the extensive demand, only a few dedicated synchrotrons and x-ray free-electron lasers exist in the world, because limited by the weak acceleration field of radio-frequency accelerators, these facilities are usually very large and expensive. Furthermore, the natural narrowband spectrum from undulators imposes restrictions on their applications in several important diagnostics for material science and highenergy-density (HED) physics9. In relativistic laser plasma-based accelerators, the excited accelerating field strength can be several orders of magnitude higher, and therefore, laserplasma based radiation sources have been proposed to be an attractive alternative for modern x-ray applications. In the last two decades, x-ray sources based on laser wakefield accelerators (LWFA) have been widely investigated10–25. However, in LWFA, the accelerated electron beam is characterized by moderate charge (about hundreds of picocoulombs to nanocoulombs)26,27, which determines that the conversion efficiency of laser energy to photons and the photon number are relatively low. The total number of photons in full spectrum is usually on the order of 108, and the peak flux is about 104–105 photons eV−13,28. This is not sufficient to make high-quality measurements, especially in a single-shot. For instance, to achieve good statistics, x-ray absorption spectroscopy techniques, as essential tools for probing both electronic and atomic structural properties of matter, require the flux satisfying Nph > 106 eV−13,29–32, where Nph is the number of photons in the energy band of interest. This is simply ffiffiffiffiffiffiffiffi pdetermined by the requirement that the random statistic noise SN ¼ 1= N ph should be smaller than 1/1000 of the signal, while recent experiments indicate that due to background noise, an even higher flux about 107 photons eV−1 is necessary to fulfill the required signal-to-noise ratio33. On the other hand, for many applications, especially in HED experiments3,33–38, single-shot measurements are crucial because of complex target designs and low repetition rate of powerful lasers. Though it is possible to make such measurements by utilizing implosion-based x-ray sources39–41, they are based on large, expansive and unique facilities and their brilliance is relatively low3. Therefore, it is critical to develop tabletop ultrabright high-flux x-ray sources. Recently, there has been a growing focus on enhancing the photon flux of x-rays at keV range. Most of the works are based on delicate manipulation of the LWFA by making a trade-off between high brilliance and high flux33,42–47 due to the limitation of the beam loading effect48. Another approach to achieving higher flux involves using electrons from direct laser acceleration (DLA) mechanism with near-critical-density plasmas49–53 or microwires54,55, where compared to the LWFA, much larger number of electrons but with lower energy and larger angular spread are produced. The divergence of the emitted x-rays is usually around hundreds of mrad, 1 Institut für Theoretische Physik I, Heinrich-Heine-Universität Düsseldorf, 40225 Düsseldorf, Germany. 2Center for Applied Physics and Technology, HEDPS, e-mail: ; SKLNP, and School of Physics, Peking University, Beijing 100871, China. Communications Physics | (2024)7:84 1 Article https://doi.org/10.1038/s42005-024-01575-z leading to rather low brilliance56–61. Moreover, electron acceleration and coherent harmonics generation in laser-microtape target interaction at oblique incidence62–64 and MeV electron acceleration driven by THz surface waves in laser-microwire interaction65 have been investigated. Until now, in spite of the significant demand in diverse applications, how to achieve ultrabright femtosecond x-rays with a flux of up to or even beyond 107 photons eV−1 at keV range still remains an open question. In this paper, we demonstrate a scheme for generation of high-flux high-brilliance x-ray radiations, where a linearly-polarized (LP) femtosecond laser pulse is incident on an edge of a submillimeter wide microtape, as illustrated in Fig. 1a. When the laser pulse impinges on the edge, a strong surface plasma wave (SPW) is easily excited and significant amounts of electrons are peeled off from the tape into vacuum. These electrons will be accelerated forward to superponderomotive energies, while undergoing betatron oscillations near the lateral plasma-vacuum interfaces, leading to intense x-ray bursts. Our three-dimensional (3D) particle-in-cell (PIC) simulations demonstrate that by utilizing a readily available tabletop 150 TW femtosecond laser, hard x-ray sources characterized by flux up to 107 photons eV−1 around 5 keV and peak brilliance about 1023 photons s−1 mm−2 mrad−2 0.1%BW−1 can be generated. The energy conversion efficiency is about 2.5 × 10−4. We have identified the existence of the SPW and demonstrated that the radiating electrons are primarily accelerated by the longitudinal electric field of the SPW and undergo betatron oscillations in the self-generated quasistatic transverse electric and magnetic fields at the vicinity of the interfaces. Our theoretical model fits well with the numerical results. R (...truncated)