Chen Lin

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.

Discharge capillary plasma waveguides are key components for long-distance laser guiding, multi-GeV laser wakefield acceleration, and staged accelerator coupling. Their optical axis, matched spot size, and effective channel depth directly affect laser guiding quality and beam pointing stability, while in curved waveguides the geometric centerline is not necessarily the optical equilibrium trajectory. Here we extend the established laser centroid-oscillation diagnostic by adding a far-field angular measurement to the conventional near-field exit-position measurement. Combining the two measurements reduces the phase ambiguity of position-only diagnostics and gives a more constrained inversion of the centroid-oscillation phase. In a straight discharge capillary, the two measurements give consistent phases near 154 degrees, corresponding to a matched spot size of about 53.3 μm and an effective channel depth of about 1.25×10^{18} cm^{-3} under the selected guiding condition. The same diagnostic is applied to a curved capillary with a 1.25 m curvature radius, where the far-field response indicates a preferred nonzero relative injection offset. This combined measurement provides an in situ phase-space diagnostic for straight and curved plasma waveguides.

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.

The development of laser-plasma accelerators has brought new opportunities for tumor radiotherapy. After appropriate energy selection and beam transport, laser-accelerated proton beams can provide depth-dose characteristics relevant to proton radiotherapy. Owing to their ultrashort pulse duration and extremely high instantaneous dose rate, they also provide a promising platform for ultra-high-dose-rate and FLASH-related radiobiological studies. However, the low-frequency single-shot mode and dose fluctuations of laser proton beams, resulting from the transient nonlinear acceleration process, limit the accuracy of irradiation doses. Traditional irradiation control methods are inadequate to address these issues. Here, we propose a model-based adaptive dose-delivery method based on sequential decision optimization, which can maintain stable dose control with an accuracy of 5% under unstable laser-accelerated proton beams. This method involves adjusting the beam transport system and employing feedback control systems to monitor and adjust beam parameters in real time, adaptively compensating for upstream instabilities without altering the conditions at the driver end. Simulation results show that these schemes significantly outperform traditional methods in terms of dose delivery accuracy and irradiation efficiency, providing important references for the further development of laser proton radiotherapy technology.

Laser-driven proton acceleration has attracted considerable interest owing to its potential in versatile applications, including cancer therapy. Proton energies depend critically on the on-target intensities, but the detrimental impact of focal spot degradation induced by spatiotemporal couplings on the acceleration remains unclear. In this study, we demonstrate that residual angular chirp (AC), stemming from minor misalignments of the grating compressor in a petawatt (PW)-class laser system, acts as a critical bottleneck for proton acceleration. Experimental results reveal that even around 100 μrad of grating misalignment induces substantial focal-spot elongation and a pronounced reduction in peak intensity. By implementing an in situ spectral-blocking diagnostic, we are able to effectively eliminate the residual AC and restore a near-diffraction-limited focus. This optimization leads to a significant recovery of the on-target intensity, resulting in a twofold increase in the proton cutoff energy. Our work represents a successful demonstration of diagnosis and elimination of residual AC. This provides a practical reference for generating high-energy proton beams and supporting their diverse applications in a PW-class laser.

Achieving diffraction-limited focusing of high-power laser pulses to generate ultrahigh intensities is crucial for developing compact laser-driven particle accelerators and exploring strong-field quantum electrodynamics. However, accurately diagnosing and optimizing the focal spots of petawatt (PW) laser pulses remains a significant challenge. In this work, we present an experimental methodology utilizing a twin-focus scheme to precisely characterize the intensity distribution and wavefront of focused PW femtosecond laser pulses, and employ it to elucidate their power-dependent evolution. Furthermore, we optimize the focal spots at full power via our in situ wavefront correction method called “HotLoop,” achieving a Strehl ratio of 0.80 for 1 PW laser pulses. Consequently, the cutoff proton energies in laser proton acceleration experiments are significantly enhanced. The success of this approach underscores the necessity of in situ high-energy wavefront correction for ultrahigh-intensity laser–matter interactions.

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.

The laser proton accelerator achieves an exceptionally high gradient of TV/m, offering a novel approach for the development of compact proton irradiation devices. As the first laser-driven proton radiotherapy device worldwide, the Compact Laser Plasma Accelerator (CLAPA-II) at the Beijing Laser Acceleration Innovation Center faces significant challenges for its timing system. Beyond high-precision synchronization, the system must support complex radiotherapy logic, including multi-subsystem coordination, online operation mode switching, and strict safety interlocks. This paper presents the design and implementation of the CLAPA-II timing system, capable of generating hundreds of synchronized trigger pulses with sub-nanosecond precision and robust resistance to the transient EMP generated during laser-target interactions. Two schemes are introduced: a baseband distribution system based on the Stanford DG645, delivering an RMS jitter of 252.2 ps for early-stage experiments; and an event-driven timing system using the SINAP EVG-EVR architecture, achieving an RMS jitter of 29.6 ps and enabling flexible, event-code-based control of the entire proton therapy workflow. The event system natively synchronizes with the 80 MHz laser oscillator and supports millisecond-level switching between treatment modes, offering a robust and scalable solution for clinical applications. This study offers a comprehensive technical foundation for the integration of laser accelerators into proton therapy.

Laser-driven proton accelerators offer a compact and innovative alternative to conventional radio frequency accelerators, yet they are limited by the low energy efficiency and insufficient proton energy gain. To overcome these limitations, helical post-acceleration structures have been proposed as a promising solution. However, existing designs suffer from dispersion-induced field collapse and synchronization challenges, particularly in long-length helices exceeding centimeters. In this work, we present a self-consistent and self-adaptive numerical framework to design the pitch distribution of shielded helical structures for long-distance proton post-acceleration. Our method features a flexible synchronization condition based on suppressed dispersion, allowing phase-stable and transverse focusing at the same time. With the condition that corresponds to a petawatt laser, we achieve roughly 150% proton energy gain from 37 MeV, better Root Mean Square (RMS) emittance, and smaller beam spot variance at the same time within 40 mm of acceleration length. This study provides a general and practical design methodology for next-generation laser-driven ion accelerators.