The new era of Japan, Reiwa, began in May 2019, and it has been almost a year since then. In the first half of the year, big typhoons in autumn have caused enormous damage in eastern Japan, and in the last six months, we have suffered with the spread of coronavirus infection. These disasters had a great impact on our daily lives. Due to the coronavirus, meetings, especially involving the movement of people, have been restricted by the declaration of emergency and many conferences have been cancelled and/or postponed. The 27th Annual IAPS Annual Meeting, IAPS 2020, scheduled to be held in Xi'an in March, has been postponed until the end of September 2020. Nowadays, people is suffering from natural disasters including virus disaster. This is changing our daily lives. We should start to
prepare for it as soon as possible. IAPS members need to create a new era, Reiwa, for our further progress.
In Hall thruster R&D, high thrust, high thrust efficiency and long operating lifetime are required for future space missions of In-Space Missions such as manned Mars exploration and transportation in the Solar System, and 1 GW-class solar power satellite construction. However, current Hall thrusters do not satisfy all requirements. Therefore, Hall thrusters are required to improve their performances. In this study, the performances of Magnetic-Layer-Type (Stationary Plasma Thruster: SPT-type) Hall thrusters were investigated in high electric-power and high specific-impulse operational ranges. Performance measurements were performed at Osaka Sangyo University (OSU) and Japanese Aerospace eXploration Agency (JAXA). The SPT-type Hall thrusters THT-VI, developed at OSU, achieved thrust efficiencies above 60% and specific impulses of 2,000-3,500 sec with discharge voltages of 300-1,000 V and input electric powers of 1-5 kW. The performances could satisfy the final target of performance. In addition, in order to improve performance, it is necessary to understand plasma features inside the discharge channel and to predict the performance. Therefore, numerical calculation using two-dimensional Hybrid-PIC model was carried out. The calculated performances roughly agreed with measured ones. The optimum plasma acceleration process was also suggested.
Phosphoric acid was used to generate single bubble sonoluminescence (SBSL). The time history (especially time width) of the bubble emission intensity was acquired by a streak camera. Since the single emission of SBSL was very dim, the time width was estimated by accumulating the multiple emissions of SBSL. As a result, the emission time width of SBSL was estimated as 90- 680 ps. The emission time increases with increasing the concentration of phosphoric acid. And, when the any gas dissolved in liquid, the emission time also increases with increasing the ionization tendency.
This special issue was dedicated to the experimental and numerical researches of the discharge phenomena induced by a high-energy laser and millimeter-wave beam. Regarding the laser supported detonation phenomena, although there was no quantitative agreement between the one-dimensional analyses and the measurements, the experiments clearly showed a unique correlation between the local laser intensity and the local discharge propagation velocity, suggesting the influence of the twodimensionality of laser intensity distribution. As for the millimeter-wave discharges, it was concluded that the rapid evolution of the discharge wave-front and the spatially non-uniformly generated plasma structure form a pair and conserve thermodynamic quantities. For both types of discharges, measurement techniques and physical models have been studied and improved to the level where the spatial distribution of electron temperature, plasma density, etc. can be compared in detail.
Optical emission spectroscopy was conducted for measuring vibrational and rotational temperatures of atmospheric millimeter-wave discharge plasma. Measured spectra were fitted to the second positive system of nitrogen molecules with a theoretical scheme assuming Boltzmann distribution. The results showed that vibrational temperature was constant at about 6000 K and on the other hand, rotational temperature decreased from about 5500 K to 3000 K with the increase of peak intensity. The comparison between millimeter-wave absorption time and
required time for satisfying equilibrium in vibrational–translational modes, revealed that the non-equilibrium is attributed to ionization front propagation velocity. Finally, the structure of millimeter-wave supported detonation was proposed and the pressure and temperature behind a detonation wave is thought to depend on ionization front propagation velocity.
We propose a high-power millimeter-wave multiplexer which utilizes a ring resonator technology for a beam station of Microwave Rocket. The multiplexer consists of multi-staged ring resonators. The multiplexer switches and distributes RF power from the power source to several antenna elements. RF power of each port is determined by resonant condition as relation of each resonator circuit length and the RF wavelength. Theoretical analysis of the multiplexer which has 5 staged ring resonators is represented at the frequency of 170 GHz for single output port selection, equally power distribution, and Gaussian power distribution assuming active-phased-array antenna operation. By changing the circuit length of each ring resonator stage, all the operations are well represented in success. The performance of equally power distribution and Gaussian power distribution are less dependent on reflection coefficient of beam splitters of resonators. The multistaged ring resonator type multiplexer can be utilized for RF power control in the beam station of Microwave Rocket.
In laser detonation thrusters, laser energy is converted directly into thrust through the Laser Supported Detonation phenomenon. To predict their thrust performance, it is necessary to know the propagation velocity of the LSD wave and pressure rise behind the wave. In this study, LSD wave propagation velocity and blast-wave energy conversion efficiency were measured by experiments. As a result, the measured LSD wave propagation velocity was found much lower than the calculated velocity in CFD. The blast-wave energy conversion efficiencies measured in air and Ar at 1.0 atm were 45% and 48%, respectively, and the laser energy transfer efficiency is considered to be approximately equal to those efficiencies. On this background, a CJ detonation solution can exist in the Hugoniot analysis when approximately 85% of the transferred laser energy is assumed to flow out in the radial direction on the center axis of the beam. In addition, it was suggested that a twodimensional analysis was needed to have a quantitative agreement in LSD propagation velocities between measurements and CFD analyses.
Laser-supported detonation (LSD), a type of Laser-supported wave (LSW) accompanied by a supersonic flow, is regarded as a very important phenomenon in the field of Beam propulsion systems, because it can generate the required high pressures and temperatures. In this study, the unsteady performances are investigated by one-dimensional simple geometry and variable laser intensity using a computational fluid dynamics (CFD) analysis with the thermochemical non-equilibrium model for inert gas plasma.The periodic irradiative laser intensity is instantly followed by LSD propagation velocity and the history is negligible, therefore the velocity can be evaluated from the laser intensity.
As a preliminary study for the laser cutoff modeling on the laser supported detonation (LSD), we investigated the gird resolution dependency of 1-D computational fluid dynamics analyses of complete and incomplete air LSD. As a result, the grid convergences of maximum physical values behind the laser absorption layer front were unattainable in the present investigation range of the grid width. In the complete LSD case, this reason is that the finer grid results in the stronger isochoric heating because the grid width is still too large to resolve the laser absorption length. On the other hand, in the incomplete LSD case, if we use more finer grid, the grid convergence will be
attainable because the present gird width is much larger than the absorption length. Moreover, the computation found that the LSD propagation speed has no grid resolution dependency because the pressure behind the laser absorption layer is independent of the grid resolution.