Masaharu Nishikino

We experimentally investigated the control of the longitudinal and transverse transport of laser-driven ion beams by combining an energy-compressing cavity (ECC) with a rotating apertured target holder, which is interpreted as forming a transient electrostatic lens through self-charging. In this proof-of-principle experiment, when both the ECC and the target-holder electrostatic lens were employed, a localized beam spot was observed 4.9 m downstream, with full widths at half maximum of 4.6 and 5.0 mm in the horizontal and vertical directions, respectively. Time-of-flight (TOF) peaks corresponding to 0.35 and 0.11 MeV/u were also observed. To infer the effective parameters of the transient electrostatic lens, particle distributions generated by two-dimensional particle-in-cell simulations were coupled to ray-tracing calculations that accounted for the transient lens field, particle-dependent transit-time factors, arrival-time-dependent radio frequency phases in the ECC, and geometrical losses in the vacuum duct. Within the present source and transport model, comparison with the measured TOF peak positions yielded an effective lens potential of 25 kV and a discharge time constant of 12 ps. The simulations indicated selective transport from specific regions of the energy–divergence-angle distribution, with H+ contributing to the 0.35 MeV/u peak and both H+ and C4+ contributing to the 0.11 MeV/u peak. The analysis also suggests that, for laser-driven ion acceleration to serve as an injector for a heavy-ion radiotherapy system, transporting 4 MeV/u C6+ ions with initial divergence angles of several tens of degrees would require a holder potential exceeding 100 kV and a discharge time constant of several tens of picoseconds.

In relativistic plasma expansion, a relative motion of plasma components causes absorption of spectral lines to be Doppler-shifted with respect to the emission line centers. The shift of spectral line can be of an order or exceeding the natural linewidth, leading to a significant reduction in self-absorption and optical thickness of the plasma. In this paper we experimentally examine the effect of femtosecond relativistic laser plasma expanding from both sides of the solid foil target. In an experiment, a hot plasma x-ray source in a 1–2 μm thin 4-chlorostyrene foil was created by high-contrast ∼ 1 × 1021 W/cm2 pulses delivered by the J-KAREN-P facility. In addition, the rear side of the target was heated by 1/100 of the main pulse power with a controlled few-nanosecond time delay. It is demonstrated that the high velocity of the downstream and upstream (from the rear side) plasma expansion essentially induces plasma transparency for K-shell emission from He-like chlorine ions dominating the ion charge distribution.

To realize a compact and high-intensity ion beam facility based on laser-driven ion acceleration by Target Normal Sheath Acceleration, we constructed a dedicated beamline capable of transporting and controlling proton beams with kinetic energy of 1.5 MeV. The system consists of a quadrupole triplet electromagnet for spatial focusing and an energy-compressing cavity (ECC) for longitudinal phase rotation, enabling momentum compression. A wall current monitor (WCM), installed 4.2 m downstream of the source, enables real-time, non-destructive bunch diagnostics. Using this setup, a single-bunch proton with a kinetic energy of 1.5 MeV was generated from a 5.0 μm-thick nickel tape target and compressed by phase rotation in the ECC. The time-domain standard deviation of the bunch length, as measured by the WCM, was found to be less than σBunch≃ 0.14 ns (limited by the measurement resolution), and the bunch was transversely focused to a root-mean-square diameter of ∼10 mm at the WCM position, as determined from beam transport simulations. The single-shot irradiation fluence exceeded 5.5 × 107 (protons/cm2)/bunch with an energy spread of 4.6%, corresponding to a peak flux of ∼1017 (protons/cm2)/s. Such a low-energy, sub-nanosecond, high-flux single-bunch proton beam is extremely difficult to achieve with conventional ion accelerator systems. It enables experimental investigation of fundamental material damage processes with high temporal resolution, including early stage defect formation and atomic displacements. This laser-driven ions beam study is expected to significantly advance time-resolved applications requiring sub-nanosecond temporal resolution, particularly in the fields of advanced materials research.

High-repetition-rate laser systems require continuous protection of optical components from debris generated during laser–matter interactions. Conventional debris shields offer high optical quality at the cost of inflexible replacement mechanisms or allow easy roll-based replacement but suffer from degraded wavefront performance. In this study, we evaluated the feasibility of ultrathin glass, originally designed for flexible electronic applications, as a continuously replaceable debris shield combining excellent optical and mechanical properties. The optical transmission, wavefront error, and polarization behavior of 50-μm-thick ultrathin glass were characterized and benchmarked against fused silica and nitrocellulose film shields. The ultrathin glass showed high UV–NIR transmittance, minimal wavefront distortion (λ/5 PV, λ/20 RMS), and no observable birefringence. A compact roll-to-roll transport system was developed to support continuous shield renewal in vacuum. Optical stability was tested under active motion using a Ti:sapphire laser delivering 1-J and 40-fs pulses at 10 Hz. The beam pointing deviation remained below 40 µrad during shield translation. Furthermore, high-intensity ion acceleration experiments verified that the shield maintained stable focusing and target interaction over 4000 shots. Degradation was limited to the transmittance loss from debris accumulation with no laser-induced damage. The proposed shield system supports over 300 000 shots with a 20-m-long roll, which can be scaled to hundreds of meters for extended operation. These results demonstrate the viability of ultrathin glass as a practical and scalable debris protection solution for future laser facilities.

A novel three-dimensional surface analysis system for iron-based solid materials has been developed. This system is composed of a synchrotron Mössbauer source, an X-ray focusing device, a precision stage, a gas flow proportional counter for conversion electron measurements, and multiple multichannel scalers. The performance was evidenced by determining the spatial distribution of the iron oxide abundance on a laser-ablated iron foil surface. This system can be widely utilized as a powerful analytical tool in steel science for corrosion, welding, and laser surface modification.

The Modulation Transfer Function (MTF) characteristics of radiochromic film (RCF) dosimetry with optical systems were evaluated with an RCF, HD-V2, which is transferred with the line patterns of the resolution test chart. In the evaluation using a GT-X980 flatbed scanner, a high contrast spatial resolution with an MTF of ≥0.8 is limited to about 83.3 μm due to artifacts such as scattering of the HD-V2 transmitted light. To achieve high contrast at higher spatial frequencies, a test bench microdensitometer (TBMD) was developed, and its optical performance, dose equivalent response, and spatial resolution characteristics with MTF were evaluated. The TBMD had a minimum readout focal diameter of 13 μm, and its high contrast spatial resolution with an MTF of ≥0.8 is about 11.9 μm. The result that the optical density error increases with decreasing focal diameter in the TBMD supports our hypothesis that these errors are based on the non-uniformity of the shading and surface roughness of the HD-V2 active layer monomers.

This study reports the first experimental demonstration of surface contamination cleaning from a high-repetition supply of thin-tape targets for laser-driven carbon-ion acceleration. The adsorption of contaminants containing protons, mainly water vapor and hydrocarbons, on the surface of materials exposed to low vacuum (>10−3 Pa) suppresses carbon-ion acceleration. The newly developed contamination cleaner heats a 5-μm-thick nickel tape to over 400 °C in 100 ms by induction heating. In the future, this heating method could be scaled to laser-driven carbon-ion acceleration at rates beyond 10 Hz. The contaminant hydrogen is eliminated from the heated nickel surface, and a carbon source layer—derived from the contaminant carbon—is spontaneously formed by the catalytic effect of nickel. The species of ions accelerated from the nickel film heated to various temperatures have been observed experimentally. When the nickel film is heated beyond ∼150 °C, the proton signal considerably decreases, with a remarkable increase in the number and energy of carbon ions. The Langmuir adsorption model adequately explains the temperature dependence of desorption and re-adsorption of the adsorbed molecules on a heated target surface, and the temperature required for proton-free carbon-ion acceleration can be estimated.

Irradiation effects of poly(methyl methacrylate) (PMMA) induced by femtosecond-pulsed extreme ultraviolet (EUV) were investigated using Soft X-ray free electron laser (SXFEL) for realization of next generation extreme ultraviolet free electron laser (EUV-FEL) lithography. The sensitivity of PMMA upon exposure to femtosecond-pulsed SXFEL was much higher than that measured for conventional nanosecond-pulsed EUV source. The sensitivity enhancement upon exposure to femtosecond-pulsed SXFEL is similar to the result obtained using laser-induced-plasma based Soft X-ray laser (SXRL) (picosecond-pulsed EUV). This result speculates the reactions induced by femtosecond-pulsed SXFEL and picosecond-pulsed XRL were almost same, but it was different from those induced by nanosecond-pulsed EUV.

The ablation threshold fluence and crater morphology of amorphous and crystalline SiO ₂ glass were analyzed in the regime of an extreme ultraviolet femtosecond pulse. Despite the difference between the densities (or optical penetration depths) of amorphous and crystalline SiO ₂ glass, the ablation threshold fluences and crater morphologies were found to be comparable. In addition, we compared our experimental results at a 10.3 nm wavelength with those in a previous work at a 13.5 nm wavelength. We conclude that the impact of the difference in density or optical penetration depth of several tens of percent on the ablation process is limited.

We present first principles calculations of parametric amplification of attosec-ond coherent pulses around 100 eV at the single-atom level in high order harmonic generation from excited states of He⁺ and compare them with experimental measurements. Our results indicate the basic physics to achieve the desired attosecond XUV gain in the spectral regions far from the ionization threshold of the considered interacting media.