Mingfeng Huang

Laser-accelerated proton beams are highly sensitive to laser and target parameters, and distinct beam behaviors can arise in different parameter regimes. Here, we demonstrate a duration-controlled pre-pulse scheme for the transverse modulation of proton beams in target-normal sheath acceleration. At a fixed pre-pulse energy (10 mJ) and main-pulse delay (300 ps), we vary the pre-pulse duration from 50 to 150 ps. In addition to the enhanced high-energy proton yield observed in all cases, the low-energy proton beam exhibits a distinct transverse response to pre-pulse duration. Within the pre-pulse scan, longer pre-pulses compress the beam profile and strengthen the annular signal distribution at the beam edge. Overall, the beam remains broader than the no-pre-pulse reference. Successive hydrodynamic and particle-in-cell simulations reveal that the expanding proton beam is modulated inward by a transient kilo-tesla azimuthal magnetic field. Electrons initially in the transverse periphery of the pre-plasma move toward the axis and reach the target rear, reorganizing the rear current. The resulting magnetic-field structure exerts an inward force on forward-moving protons and modifies their transverse distribution. This work clarifies the physical mechanism governing proton-beam divergence modification under ablation pre-pulses in the present experimental regime.

Laser-accelerated proton beams deliver ultrashort, high transient flow energy deposition that remains challenging to diagnose in real time. Here, we report a proof-of-principle observation of acoustic waves generated by laser-accelerated protons in solid media and investigate solid-state thermoacoustic conversion as a potential approach for sensing ultrahigh-intensity proton beams. Localized proton energy deposition induces transient heating and stress formation in an aluminum target, producing measurable MHz acoustic signals after the electromagnetic pulse interference is separated from the acoustic response. By combining experiments with multiphysics simulations, we examine the proton-driven origin of the signals, quantify their propagation-distance-dependent attenuation, and observe an approximately linear correlation between the reconstructed acoustic pressure and the estimated proton flow intensity. Measurements with a tungsten target further support the robustness and multi-material applicability of the solid-state acoustic response. These results extend proton-induced acoustics from liquid-based range-verification studies toward solid media, where thermoacoustic signals may provide a complementary observable for transient high-current proton beams in vacuum-compatible and electromagnetically harsh environments, with possible extensions to acoustic signaling through solid structures under extreme conditions.

Owing to their broad energy spectrum, short pulse width, and high particle numbers, laser-accelerated protons offer significant advantages for rapidly evaluating the durability of materials and understanding microscopic damage mechanisms. In this study, WO3 nanowires were synthesized to assess their resistance when exposed to irradiation from laser-accelerated proton beams. Unlike traditional bulk materials, the surface of WO3 nanowires showed no apparent damage after irradiation; however, advanced characterization techniques reveal a unique surface-to-core amorphization damage phenomenon in nanowires. Comparative experiments with a traditional accelerator (nine orders lower dose rate) confirm that ultrashort pulses are indispensable for triggering this amorphization. Our findings establish WO3 nanowires as promising radiation-resistant candidates and provide insight into how the microscopic damage process varies with the dosage of laser-driven protons.

Plasma can support extremely high-gradient strong electromagnetic fields, making it suitable not only for laser acceleration but also for short-distance manipulation of the charged particle beams, thereby enabling the miniaturization of particle accelerators. Straight capillary discharge plasma lenses have previously garnered widespread attention from researchers for their excellent focusing capabilities. We introduced the theory of using a curved capillary discharge plasma channel to deflect charged particle beams [ , 031301 (2021)] and, in this paper, present the first experimental demonstration of plasma-based deflection and focusing of laser-accelerated proton beams. Combined with a dipole magnet, the emittance after transmission was measured, revealing that magnetic field nonlinearity and multiple Coulomb scattering tend to increase the proton beam’s emittance. This work is of great significance for the development of compact beam transport systems.

Particle-in-cell simulations have been carried out to investigate the behavior of pulsed proton beams accelerated by laser within underdense plasma. The results reveal that wakefields generated by the proton beam effectively focus the proton beam, improving the beams' spatial distribution and energy spread. Additionally, electron rotations, inspired by an external longitudinal magnetic field, form a transverse focusing field near the proton beam, by which the proton beam can be focused more effectively. This implies the possibility of utilizing plasma as a viable medium for transport and focusing laser-accelerated broadband-energy proton beams, offering solutions to preserve and improve the proton beam properties, which is crucial for a variety of applications of laser-accelerated proton beams.

We demonstrate the laser-driven post-acceleration experiment, utilizing a miniature slow-wave structure. Experiments on a terawatt laser system showed a significant increase in proton cutoff energy, highlighting the technique's potential, especially for high-power laser systems. The slow-wave structure consists of a helix and a shielded metallic shell covered on the outside. The transmission properties of electromagnetic pulses (EMPs) generated by laser–foil interactions along the structure are studied. Through an electromagnetic field perspective, we derived dispersion relations for helices with and without metallic shield. Our findings, supported by theory, simulations, and experiments, demonstrate the structure's ability to transmit high-frequency EMPs with limited dispersion.

Active plasma lensing, a compact method for intensifying the focus of charged particle beams by providing a magnetic field gradient of kT/m, has emerged as a sought-after technology in laser plasma accelerator applications. However, the utilization of active plasma lenses faces significant hurdles when dealing with laser-driven proton pulses, characterized by their broad bandwidth and high divergence. To address this challenge, we developed a novel active plasma lens with a variable radius, specifically designed to optimize lens geometry in accordance with the beam envelope, and performed the first measurement of its focusing ability. The experimental findings reveal that, compared to conventional cylindrical active plasma lenses, our radius-varying lens exhibits a 2.0-fold improvement in single-energy transmission efficiency, while maintaining comparable achromatic ability. This breakthrough is anticipated to significantly contribute to the miniaturization of laser proton accelerators. Published by the American Physical Society 2024

A new high transmission efficiency, easily tunable, and cost-effective beam collection system is proposed for laser-accelerated proton beams with large divergence angles and wide energy spectra. In previous experiments conducted at the compact laser plasma accelerator platform of Peking University, a beamline was initially constructed with a collection system based on an electromagnetic quadrupoles (EMQs) triplet. However, due to the limited acceptance angle, the EMQs exhibited a monoenergetic transmission efficiency of only 10% to 20% for laser accelerated proton beams. Permanent magnet quadrupoles (PMQs), known for their high magnetic field gradients, compact size, and lower expenses, are well suited for integration with other readily adjustable transmission elements like EMQs and solenoids, effectively enhancing the beam collection capabilities of the system. In this paper, we show that by introducing a pair of centimeter-sized PMQs in front of the EMQs for beam prefocusing, the beam’s transverse size is quickly compressed, enabling transmission of highly divergent protons. Experimental results demonstrate that the prefocusing by PMQs increased the system’s transmission efficiency by a factor of 2.44 to 6.01 compared to the original setup, while also enhancing the energy selection based on stronger chromatic effect. This method can be extended to 100 MeV high-energy proton beamlines and is crucial for applications of laser plasma accelerators. Published by the American Physical Society 2024