Takashi Morita

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.

Wakefield acceleration has been considered a possible mechanism for producing cosmic rays, including extremely high-energy ions, especially in astrophysical environments. In the laboratory, however, laser-driven wakefield acceleration of ions remains challenging because ions are much heavier than electrons. A central issue is how ions enter the accelerating field. One possible route with present laser facilities is to use dense plasma, in which the wake moves more slowly and ions can be trapped by the bow wake formed at the laser front. Here we experimentally demonstrate bow-wake acceleration of protons using resorcinol-formaldehyde foam targets. We obtain protons with energies up to 60 MeV. Three-dimensional particle-in-cell simulations support the interpretation that the protons are accelerated by the bow wake. These results show that dense plasma can enable ion injection into laser-driven wakes and provide a laboratory route toward bow-wake acceleration of high-energy ions.

Magnetic reconnection, a fundamental process driving energy release across laboratory, space, and astrophysical plasmas, efficiently converts magnetic field energy into plasma kinetic and thermal energy. Here, we present the simultaneous, direct observation of plasma outflow acceleration across different spatial scales in laser-produced plasmas: the near-field (local acceleration) and the far-field (the resulting plasma jet). Local plasma flow near the X-point is measured using the Doppler shift of Thomson scattering spectra. The resultant high-speed plasma jet emanating from the reconnection region is subsequently detected by an ion collector with a Faraday cup. The near- and far-field velocity measurements exhibit a similar tendency across three different initial plasma conditions. This simultaneous diagnostic approach provides a powerful tool for the detailed experimental investigation and validation of magnetic reconnection physics.

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.

A laser fusion rocket has been proposed for interplanetary flight. This rocket generates a high-energy plasma via laser-fusion and expels it via a magnetic nozzle. The magnetic nozzle is a key component for the rocket performance, and we have investigated the impulse bit generated from the magnetic nozzle by using simulations and experiments in relatively low energy regime of a few joules. In addition, the energy dependence on the thrust performance is an essential factor to evaluate and design the laser fusion rocket. Here, we conducted numerical simulations in energy regimes from a few joules to mega joules to understand the thrust performance both for small-scale experiments and for full-scale fusion rockets. We find that the momentum efficiency does not depend on the propellant mass, material, and plasma energies, and the impulse bit is expressed as the power-law of the plasma energy and mass. These relations are important for designing missions and for estimating the thrust of fusion rockets.

Magnetic reconnection in laser-produced magnetized plasma is investigated by using optical diagnostics. The magnetic field is generated via the Biermann battery effect, and the inversely directed magnetic field lines interact with each other. It is shown by self-emission measurement that two colliding plasmas stagnate on a midplane, forming two planar dense regions, and that they interact later in time. Laser Thomson scattering spectra are distorted in the direction of the self-generated magnetic field, indicating asymmetric ion velocity distribution and plasma acceleration. In addition, the spectra perpendicular to the magnetic field show different peak intensity, suggesting an electron current formation. These results are interpreted as magnetic field dissipation, reconnection, and outflow acceleration. Two-directional laser Thomson scattering is, as discussed here, a powerful tool for the investigation of microphysics in the reconnection region.

A developing supercritical collisionless shock propagating in a homogeneously magnetized plasma of ambient gas origin having higher uniformity than the previous experiments is formed by using high-power laser experiment. The ambient plasma is not contaminated by the plasma produced in the early time after the laser shot. While the observed developing shock does not have stationary downstream structure, it possesses some characteristics of a magnetized supercritical shock, which are supported by a one-dimensional full particle-in-cell simulation taking the effect of finite time of laser-target interaction into account.

Laser experiments are becoming established as tools for astronomical research that complement observations and theoretical modeling. Localized strong magnetic fields have been observed at a shock front of supernova explosions. Experimental confirmation and identification of the physical mechanism for this observation are of great importance in understanding the evolution of the interstellar medium. However, it has been challenging to treat the interaction between hydrodynamic instabilities and an ambient magnetic field in the laboratory. Here, we developed an experimental platform to examine magnetized Richtmyer-Meshkov instability (RMI). The measured growth velocity was consistent with the linear theory, and the magnetic-field amplification was correlated with RMI growth. Our experiment validated the turbulent amplification of magnetic fields associated with the shock-induced interfacial instability in astrophysical conditions. Experimental elucidation of fundamental processes in magnetized plasmas is generally essential in various situations such as fusion plasmas and planetary sciences.

Anomalous transport of electrons has considerable impact on the overall performance of Hall thrusters. Numerous studies have shown that high-frequency (on the order of MHz) fluctuations in both the electric field and plasma density are one of the causes of the anomalous transport. This phenomenon has yet to be measured in detail, however, because it is difficult to evaluate the density distribution of neutral particles and the dispersion relation of plasma oscillation in the several MHz range. It was therefore necessary to develop a device to measure the fluctuations in order to elucidate the cause of the anomalous transport. Here we focus on electric field fluctuations in the azimuthal direction inside a Hall thruster, and measure floating potential difference between two Langmuir probes. When a discharge current increases under a strong magnetic field, the azimuthal fluctuation in the frequency of 1-5 MHz is enhanced and fluctuations at ~100 kHz appear. These results suggest that azimuthal fluctuations in the electric field play an important role in the anomalous transport of electrons in the Hall thruster.