Yoshihiro Arakawa’s research while affiliated with Teikyo Heisei University and other places

A laser driven plasma wind tunnel was developed to generate hypersonic high enthalpy CO2 flows. At the plenum pressure of 0.34 MPa, a laser sustained plasma (LSP) was stably generated by focusing a 1000 W continuous laser beam. The LSP was expanded into a vacuum chamber through a convergent-divergent nozzle. In order to maximize the plume emission region, the LSP was set as close as possible to the throat. At 10 mm downstream from the nozzle exit, the flow characteristics were diagnosed by a Pitot tube and emission spectroscopy. As a result, the maximum Pitot pressure was 7.8 kPa, and the estimated Mach number was 10.5. The vibrational temperature estimated by the v = 0 transitions of the C2 Swan band ranged from 3500 K to 4200 K. The chemical composition and specific enthalpy were calculated using the measured pressure and temperature assuming a thermochemical equilibrium. The main components were 53% CO, 33% O and 10% O2. The estimated specific enthalpy was ranged from 10.1 MJ/kg to 15.3 MJ/kg.

The influences of comparably minor changes to the existing polytetrafluoroethylene (PTFE) propellant, including a layered design and inclusions of auxiliary material, are investigated on thruster performance. Four types of propellant were used in this study: standard pure PTFE, PTFE with layered metal sheets (S-PTFE), PTFE with metal sheets coated by vacuum deposition (D-PTFE), and PTFE with cylindrical inclusions of auxiliary material (C-PTFE). Zinc is used in both S-PTFE and D-PTFE, whereas the C-PTFE uses zinc, aluminum, and table salt (NaCl). All auxiliary materials require less energy to be ionized than PTFE and are thus expected to facilitate the ablation process and to reduce late-time ablation. The experimental investigation of ablative PPT using compound PTFE propellant revealed that, in general, thrust performance is influenced by even little modifications to the PTFE. The impulse bit per pulse is almost constant for D-PTFE and C-PTFE but reduces significantly for S-PTFE.

A study was conducted to apply cavity-enhanced absorption spectroscopy (CEAS) to a 750 kW arc-heated wind tunnel at the Japan Aerospace Exploration Agency (JAXA) for shock-layer measurements. The reflectivity was obtained by measuring a ring-down signal. The optical cavity was built up using highly reflective mirrors with a diameter of 1 in. and radius of curvature of 1500 mm. The signal from the photo multiplier tube was recorded using a digitizer. The arc-heated wind tunnel was operatedwith a current of 700 A and a mass flow rate of 10 g/s. The specific enthalpy was estimated at 21.8 MJ/kg by the energy-balance method.

In pulsed plasma thrusters (PPTs), a highly ionized plasma is created and accelerated by means of a pulsed triggered vacuum gap discharge with concurrently occurring ablation, ionization, and Lorentz-force-based acceleration of propellant. The plasma flow behavior during the PPT discharge is, therefore, highly dependent on time, space, and operation parameters. Experimental work is seen as the necessary key to further understand the processes involved and to determine future research needs. The application of mechanical probes to measure the Mach number of the plasma flow optically is discussed in this paper. Values of 1.6–3.2 were determined for a variation in time, space, and discharge voltage. An inclination of the PPT plasma flow toward the cathode was observed, which concurs with the previous research results.

A laser-driven plasma wind tunnel was developed as a simulator of high-speed continuous atomic-oxygen flows in the low Earth orbit (LEO) environment. A continuous-wave CO2 laser was used as the beam source, having a wavelength of 10.6 μm. The maximum output power was 2 kW, and the transverse electromagnetic wave (TEM) mode of the laser beam was TEM 10. The beam divergence was less than 2 mrad at the laser exit. The generator was connected to a water-cooled vacuum chamber having two quartz windows for access to an expanded plume by optical diagnostics. Laser diagnostics of the plume revealed the time-averaged flow velocity and translational temperature of the flows to be 6.1 ± 0.96 km/s and 1216 ± 139 K, respectively. The flux density for pure-oxygen flows was estimated as 1.3 × 1025 atom/m2s. An orbital velocity of 7.8 km/s could be achieved by increasing the helium mole fraction to 0.58.

Alumina reduction method using laser sustained plasma (LSP) is proposed for energy storage system using aluminum. In our previous study, the averaged reduction efficiency in the plasma was estimated as 5 %. To clarify the small efficiency, time resolved reduction efficiency was measured and the influence of plasma fluctuation along the flow axis on the efficiency was evaluated by emission intensity of Al I 396 nm. As a result, the ratio of maximum emission intensity to the averaged one increased with increase in F number of the focus lens. Since the fluctuation amplitude increases with the F number, this result shows that the reduction efficiency was drastically decreased when the LSP was located on the upstream of the throat because fed alumina powders passed through the relatively low temperature region between the LSP and the throat wall. Next, twin LSPs were produced by a dual focus lens to increase LSP size. As a result, the averaged reduction efficiency was increased to 11 times as much as the single focus lens case.

The authors have developed laser ignition micro solid rocket which is a propulsion system suitable for 10-kg-class microspacecrafts. Although the specific impulse is expected to increase by reducing the throat area, higher combustion chamber pressure with the smaller throat causes a couple of serious problems. One is throat erosion and the other is break of combustion chamber. In order to prevent these difficulties, we made three improvements in design of thruster. First, we reinforced the throat, and throat erosion did not occur. Secondly, we changed the ignition surface of the propellant. As a result, peak pressure reduced, but specific impulse also reduced. Thirdly, we made the propellant to encapsulate to constrain the combustion surface, and peak pressure reduced considerably. We designed thrusters which had smaller throat than conventional, and established the increase in specific impulse by 17%. We visualized the combustion chamber, and it was found that capsule could control the combustion surface of propellant.

Plasma flow behavior during the PPT discharge is highly dependent on time and operation parameters. Combined experimental and numerical work is seen as the key to understand further the processes involved, and to determine future research needs. The application of mechanical probes to measure the Mach number optically is discussed in this study. Values of around 2 were determined for a variation in time, space, and discharge voltage. Numerical efforts were executed concurrently to implement recent experimental results into a combined model. Ablation and ionization processes are implemented, and the final plasma numerically accelerated. In both experimental and numerical work, an inclination of the PPT plasma flow of about 8 deg towards the cathode was observed. Nomenclature I = discharge currenṫ m = ablation rate Ma = Mach number q = heat flux T = temperature t = time x = coordinate along the exhaust direction of the PPT µ = upper shock angle ϑ = probe angle of the wedge-type probe σ = lower shock angle

Influence of ambient pressure on energy conversion efficiency from a Nd : glass laser pulse (λ = 1.053 µm) to a laser-induced blast wave was investigated at reduced pressure. Temporal incident and transmission power histories were measured using sets of energy meters and photodetectors. A half-shadowgraph half-self-emission method was applied to visualize laser absorption waves. Results show that the blast energy conversion efficiency ηbw decreased monotonically with the decrease in ambient pressure. The decrease was small, from 40% to 38%, for the pressure change from 101 kPa to 50 kPa, but the decrease was considerable, to 24%, when the pressure was reduced to 30 kPa. Compared with a TEA-CO2-laser-induced blast wave (λ = 10.6 µm), higher fraction absorption in the laser supported detonation regime ηLSD of 90% was observed, which is influenced slightly by the reduction of ambient pressure. The conversion fraction ηbw/ηLSD≈90% was achieved at pressure >50 kPa, which is significantly higher than that in a CO2 laser case.

An air-breathing pulse laser powered launcher has been proposed as an alternative to conventional chemical launch systems. The trajectory from the ground to Geosynchronous Earth Orbit (GEO) by pulse laser propulsion is calculated by modeling the thrust during pulsejet, ramjet and rocket flight modes, and the launch cost is estimated. The results show that the pulse laser powered launcher can transfer 0.096 kg payload per 1 MW beam power to GEO, and 2,800 divisional launches of a payload are necessary to redeem the cost of its laser transmitter compared with conventional chemical launchers.