Youichi Sakawa

Generating 100-MeV protons is pivotal to developing laser-driven ion sources for many applications that range from medicine to laboratory astrophysics. An ultra-thin target is a way to increase the ion energy; however, such a target is easily destroyed by the interaction with laser prepulses. We have developed a large-area suspended graphene (LSG) target and demonstrated its durability by generating energetic protons and carbons by irradiating the thinnest target ever (2 nm) with an ultra-intense laser without a plasma mirror. Here, we show the experimental results of proton energies reaching 132 MeV by optimizing the laser conditions for the ultra-thin LSG targets. Ion diagnostics were primarily performed using a CR-39 stack detector combined with machine-learning-assisted analysis. Numerical simulations support the experimental results, where the radiation pressure of a relatively long-duration, low-intensity laser pulse slowly pushes the plasma, leading to the formation of a propagating electrostatic wave that efficiently accelerates protons, as in surfing acceleration.

This paper presents an experimental investigation of magnetic reconnection between two laser-produced expanding plasmas, focusing on the quantitative evaluation of the reconnection rate and energy conversion under varying initial conditions. By changing the separation distance between the drive laser focal spots (1 mm and 2 mm), we systematically controlled the inflow parameters. The reconnection region was probed using a two-directional laser Thomson scattering (LTS) system, which simultaneously measured the local plasma parameters parallel to the outflows and along the current sheet. Based on our established method incorporating macroscopic energy and mass conservation laws to derive the upstream magnetic field directly from LTS spectra, we characterized the temporal evolution of the current sheet and the reconnection rate. The larger spot separation allows the plasma bubbles to expand for a substantially longer time before the formation of a reconnection current sheet, resulting in different upstream conditions. Nevertheless, both configurations yielded comparable upstream magnetic fields and reconnection rates. These results suggest that the reconnection rate is relatively insensitive to the global inflow conditions and is primarily controlled by the local physics of the reconnection layer once a current sheet is formed.

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.

An experimental investigation of collisionless shock ion acceleration is presented using a multicomponent plasma and a high-intensity picosecond duration laser pulse. Protons are the only accelerated ions when a near-critical-density plasma is driven by a laser with a modest normalized vector potential. The results of particle-in-cell simulations imply that collisionless shock may accelerate protons alone selectively, which can be an important tool for understanding the physics of inaccessible collisionless shocks in space and astrophysical plasma.

The mechanism of generating collisionless shock in magnetized gas plasma driven by laser-ablated target plasma is investigated by using one-dimensional full particle-in-cell simulation. The effect of finite injection time of target plasma, mimicking the finite width of laser pulse, is taken into account. It was found that the formation of a seed-shock requires a precursor. The precursor is driven by gyrating ions, and its origin varies depending on the injection time of the target plasma. When the injection time is short, the target plasma entering the gas plasma creates a precursor; otherwise, gas ions reflected by the strong piston effect of the target plasma create a precursor. The precursor compresses the background gas plasma, and subsequently, a compressed seed-shock forms in the gas plasma. The parameter dependence on the formation process and propagation characteristics of the seed-shock was discussed. It was confirmed that the seed-shock propagates through the gas plasma exhibiting behavior similar to the shock front of supercritical shocks.

An experimental investigation of collisionless shock ion acceleration is presented using a multicomponent plasma and a high-intensity picosecond duration laser pulse. Protons are the only accelerated ions when a near-critical-density plasma is driven by a laser with a modest normalized vector potential. The results of particle-in-cell simulations imply that collisionless shock may accelerate protons alone selectively, which can be an important tool for understanding the physics of inaccessible collisionless shocks in space and astrophysical plasma.

The generation of highly charged ions in laser plasmas is usually associated with collisional ionization processes that occur in electron–ion collisions. An alternative ionization channel caused by tunnel ionization in an optical field is also capable of effectively producing highly charged ions with ionization potentials of several kiloelectronvolts when the laser intensity q > 10²⁰ W/cm². It is challenging to clearly distinguish the impacts of the optical field and collisional ionizations on the evolution of the charge state of a nonequilibrium plasma produced by the interaction of high-intensity, ultrashort PW-class laser pulses with dense matter. In the present work, it is shown that the answer to this question can be obtained in some cases by observing the X-ray spectral lines caused by the transition of an electron into the K-shell of highly charged ions. The time-dependent calculations of plasma kinetics show that this is possible, for example, if sufficiently small clusters targets with low-density background gas are irradiated. In the case of Ar plasma, the limit of the cluster radius was estimated to be R₀ = 0.1 μm. The calculation results for argon ions were compared with the results of the experiment at the J-KAREN-P laser facility at QST-KPSI.

High-power, short-pulse laser-driven fast electrons can rapidly heat and ionize a high-density target before it hydrodynamically expands. The transport of such electrons within a solid target has been studied using two-dimensional (2D) imaging of electron-induced Kα radiation. However, it is currently limited to no or picosecond scale temporal resolutions. Here, we demonstrate femtosecond time-resolved 2D imaging of fast electron transport in a solid copper foil using the SACLA x-ray free electron laser (XFEL). An unfocused collimated x-ray beam produced transmission images with sub-micron and ∼10 fs resolutions. The XFEL beam, tuned to its photon energy slightly above the Cu K-edge, enabled 2D imaging of transmission changes induced by electron isochoric heating. Time-resolved measurements obtained by varying the time delay between the x-ray probe and the optical laser show that the signature of the electron-heated region expands at ∼25% of the speed of light in a picosecond duration. Time-integrated Cu Kα images support the electron energy and propagation distance observed with the transmission imaging. The x-ray near-edge transmission imaging with a tunable XFEL beam could be broadly applicable for imaging isochorically heated targets by laser-driven relativistic electrons, energetic protons, or an intense x-ray beam.