枝本 雅史

Laser fusion rocket can achieve large thrust and high specific impulse by utilizing huge amount of fusion energy to heat up a propellant. One of the issues of this system is heavy neutron shields which are heated by neutrons. The propulsion performance of a conical target that can reduce neutron radiation to the coil is calculated by numerical analysis. The impulse bits of the spherical and the conical targets are 12 Ns and 0.75 Ns, respectively, and the propellant in the conical target does not contribute to the thrust. This result suggests that the target shape should be optimized to establish both the neutron shielding and large thrust.

We have successfully developed a portable pulsed magnetic field generation system incorporating a number of techniques to avoid the effects of noise, including shielding, a self-power capability, and a high-capability semiconductor switch. The system fits into a cubical box less than 0.5 m in linear dimensions and can easily be installed in experimental facilities, including noisy environments such as high-power laser facilities. The system can generate a magnetic field of several tesla sustainable for several tens of microseconds over a spatial scale of several centimeters. In a high-power laser experiment with Gekko-XII, the system operated stably despite being subjected to a high level of electrical noise from laser shots of 600 J. Published by AIP Publishing.

A magnetic thrust chamber is an important system of a laser fusion rocket, in which the plasma kinetic energy is converted into vehicle thrust by a magnetic field. To investigate the plasma extraction from the system, the ions in a plasma are diagnosed outside the system by charge collectors. The results clearly show that the ion extraction does not strongly depend on the magnetic field strength when the energy ratio of magnetic field to plasma is greater than 4.3, and the magnetic field pushes back the plasma to generate a thrust, as previously suggested by numerical simulation and experiments. (c) 2018 The Japan Society of Applied Physics

Laser fusion rocket is one of the candidate propulsion devices for Mars exploration. It obtains thrust from the interaction between plasma and magnetic field and this propulsion system is called magnetic thrust chamber. We constructed a spectrometer with high wavelength resolution of 35 pm to obtain plasma parameters by measuring ion feature of laser Thomson scattering from a laser-produced plasma in a magnetic thrust chamber. We obtain the plasma parameters such as electron temperature, electron density, and velocity as well as the plasma density structure showing the stagnation of the plasma by magnetic field. (C) 2018 The Japan Society of Plasma Science and Nuclear Fusion Research.

We report an experimental demonstration of controlling plasma flow direction with a magnetic nozzle consisting of multiple coils. Four coils are controlled separately to form an asymmetric magnetic field to change the direction of laser-produced plasma flow. The ablation plasma deforms the topology of the external magnetic field, forming a magnetic cavity inside and compressing the field outside. The compressed magnetic field pushes the plasma via the Lorentz force on a diamagnetic current: j x B in a certain direction, depending on the magnetic field configuration. Plasma and magnetic field structure formations depending on the initial magnetic field were simultaneously measured with a self-emission gated optical imager and B-dot probe, respectively, and the probe measurement clearly shows the difference of plasma expansion direction between symmetric and asymmetric initial magnetic fields. The combination of two-dimensional radiation hydrodynamic and three-dimensional hybrid simulations shows the control of the deflection angle with different number of coils, forming a plasma structure similar to that observed in the experiment.

We demonstrate a magnetic thrust chamber system, in which an expanding plasma is controlled by an external magnetic field to produce a thrust. The plasma structure and energy dependences are discussed in terms of the drive laser energy and magnetic field strength. The density distribution from two different experiments show identical structure despite the laser energy is different by two order of magnitude when the ratio of magnetic field to plasma energy is more or less same. The experimental results indicate that this ratio is one of the essential factors to extrapolate the plasma dynamics for much larger energy such as inertial confinement fusion plasmas.

<p>Technological innovations in recent years have enabled the downsizing of high-altitude balloon systems to be realized. The purpose of this study was, therefore, to utilize these technologies to develop a small high-altitude balloon operating system. Using this novel system, it will be possible to design an inexpensive experimental plan with a flexible schedule. This operating system is expected to be used for general observational purposes, such as the measurement of acoustic waves, sampling of air-particulate matter, and optical observations at high altitude. In this report, we introduce a novel mobile operational system for a small high-altitude balloon and experimental data obtained via a collaborative flight experiment.</p>