Zhusong Mei

Experimental validation of laser intensity is particularly important for the study of fundamental physics at extremely high intensities. However, reliable diagnosis of the focal spot and peak intensity faces huge challenges. In this work, we demonstrate for the first time that the coherent radiation farfield patterns from laser–foil interactions can serve as an in situ, real-time, and easy-to-implement diagnostic for an ultraintense laser focus. The laser-driven electron sheets, curved by the spatially varying laser field and leaving the targets at nearly the speed of light, produce doughnut-shaped patterns depending on the shapes of the focal spot and the absolute laser intensities. Assisted by particle-in-cell simulations, we can achieve measurements of the intensity and the focal spot, and provide immediate feedback to optimize the focal spots for extremely high intensity.

We demonstrate an all-optical method that online produces “microstructured” targets by manipulating a Laguerre-Gaussian (LG) prepulse. Besides the enhancement of the proton number, the cutoff energy is nearly doubled. Such transient “microstructured” targets are characterized using both offline and online approaches. Based on probe experiments and hydrodynamic simulations, we develop a simple model to describe the electron density distribution upon the arrival of the main pulse. Two-dimensional particle-in-cell simulations reveal that the annular plasma acts as an oscillating cavity for hot electrons and results in a cascaded acceleration effect. The combination of low-intensity LG prepulses and ultraintense Gaussian pulses opens another avenue for fine-tuning laser-plasma interactions spatiotemporally in interdisciplinary applications. Published by the American Physical Society 2024

In proton acceleration from laser-irradiated thin foil targets, adding foams on the front surface or connecting a helical coil on the rear surface of the foil has proven to be an effective scheme to enhance proton energy. In this paper, we make the first attempt to incorporate the above two enhancement schemes for laser-proton acceleration by simultaneously adding foams and connecting a helical coil to a thin foil target. By utilizing such integrated targets in the experiment, focused beams were generated. The maximum proton energy and the number of energetic protons are apparently enhanced. Moreover, quasi-monoenergetic peaks were formed at the high-energy end of the spectra. Particle-in-cell plasma simulations and electromagnetic beam dynamics simulations show that the double-layer target not only enhances the energy of protons but also leads to a multiple-fold increase in the number of escaped electrons, which results in an enhanced post-acceleration in helical coil subsequently.

Direct laser acceleration of electrons in near-critical-density (NCD) carbon nanotube foams (CNFs) has its advantages in the high-efficiency generation of relativistic electrons and broadband x-rays. Here, we report the first simultaneous measurement on the spectra of laser-driven electrons and x-rays from CNFs at moderately relativistic intensities of around 5×1019 W/cm2. The density and thickness of the CNFs were scanned in the experiments, indicating the optimized electron temperature of 5.5 MeV and x-ray critical energy of 5 keV. Two-dimensional particle-in-cell simulations confirm that the electrons, with a temperature significantly higher than the pondermotive scale, are directly accelerated by the laser along the NCD plasma channel, while the bright x-rays are emitted by these electrons through betatron radiation or Thomson backscattering inside the channel. The simultaneously generated electrons and x-rays, automatically synchronized with the femtosecond laser driver, are suitable for applications such as bi-modal radiography.

We introduce a LabVIEW-based control program that significantly improves the efficiency and flexibility in positioning and shooting solid targets in laser–plasma experiments. The hardware driven by this program incorporates a target positioning subsystem and an imaging subsystem, which enables us to install up to 400 targets for one experimental campaign and precisely adjust them in six freedom degrees. The overall architecture and the working modes of the control program are demonstrated in detail. In addition, we characterized the distributions of target positions of every target holder and simultaneously saved the target images, resulting in a large dataset that can be used to train machine learning models and develop image recognition algorithms. This versatile control system has become an indispensable platform when preparing and conducting laser–plasma experiments.

Post-acceleration of protons in helical coil targets driven by intense, ultrashort laser pulses can enhance ion energy by utilizing the transient current from the targets’ self-discharge. The acceleration length of protons can exceed a few millimeters, and the acceleration gradient is of the order of GeV/m. How to ensure the synchronization between the accelerating electric field and the protons is a crucial problem for efficient post-acceleration. In this paper, we study how the electric field mismatch induced by current dispersion affects the synchronous acceleration of protons. We propose a scheme using a two-stage helical coil to control the current dispersion. With optimized parameters, the energy gain of protons is increased by four times. Proton energy is expected to reach 45 MeV using a hundreds-of-terawatts laser, or more than 100 MeV using a petawatt laser, by controlling the current dispersion.

Laser plasma acceleration has made remarkable progress in the last few decades, but it also faces many challenges. Although the high gradient is a great potential advantage, the beam quality of the laser accelerator has a certain gap, or it is different from that of traditional accelerators. Therefore, it is important to explore and utilize its own features. In this article, some recent research progress on laser proton acceleration and its irradiation application, which was carried out on the compact laser plasma accelerator (CLAPA) platform at Peking University, have been introduced. By combining a TW laser accelerator and a monoenergetic beamline, proton beams with energies of less than 10 MeV, an energy spread of less than 1%, and with several to tens of pC charge, have been stably produced and transported in CLAPA. The beamline is an object–image point analyzing system, which ensures the transmission efficiency and the energy selection accuracy for proton beams with large initial divergence angle and energy spread. A spread-out Bragg peak (SOBP) is produced with high precision beam control, which preliminarily proved the feasibility of the laser accelerator for radiotherapy. Some application experiments based on laser-accelerated proton beams have also been carried out, such as proton radiograph, preparation of graphene on SiC, ultra-high dose FLASH radiation of cancer cells, and ion-beam trace probes for plasma diagnosis. The above applications take advantage of the unique characteristics of laser-driven protons, such as a micron scale point source, an ultra-short pulse duration, a wide energy spectrum, etc. A new laser-driven proton therapy facility (CLAPA II) is being designed and is under construction at Peking University. The 100 MeV proton beams will be produced via laser–plasma interaction by using a 2-PW laser, which may promote the real-world applications of laser accelerators in malignant tumor treatment soon.

In this work, the high-energy-density plasmas (HEDP) evolved from joule-class-femtosecond-laser-irradiated nanowire-array (NWA) targets were numerically and experimentally studied. The results of particle-in-cell simulations indicate that ions accelerated in the sheath field around the surfaces of the nanowires are eventually confined in a plasma, contributing most to the high energy densities. The protons emitted from the front surfaces of the NWA targets provide rich information about the interactions that occur. We give the electron and ion energy densities for broad target parameter ranges. The ion energy densities from NWA targets were found to be an order of magnitude higher than those from planar targets, and the volume of the HEDP was several-fold greater. At optimal target parameters, 8% of the laser energy can be converted to confined protons, and this results in ion energy densities at the GJ/cm³ level. In the experiments, the measured energy of the emitted protons reached 4 MeV, and the changes in energy with the NWA’s parameters were found to fit the simulation results well. Experimental measurements of neutrons from ²H(d,n)³He fusion with a yield of (24 ± 18) × 10⁶/J from deuterated polyethylene NWA targets also confirmed these results.