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Microfabrication of a microarcjet with Fifth-harmonic generation Nd:YAG pulses (wavelength 213 nm) and its plasma acceleration characteristics was investigated. The microarcjet with a rectangular exit of 0.44 mm in height was machined in a 1.2 mm thick quartz plate. For electrodes, Au films of 100 nm in thickness were coated on a divergent part of the nozzle for anode and an inner wall surface for cathode. In operational tests, a stable discharge was observed for mass flow of 1.0 mg/s, discharge current of 6 mA, discharge voltage of 600 V, or 3.6 W input power. With the 3.6 W input power, plasma speed of 1441 m/s and an energy efficiency of 7% were obtained.

Investigations on optimum operational conditions of very low-power DC micro-arcjet, or micro-plasmajet, were conducted. It was shown that with an electrically insulating material for a constrictor stable discharges below 5 W could be achieved. Also, microfabrication of micro-arcjet nozzles with fifth-harmonic Nd:YAG pulses (wavelength 213 nm) were conducted. Micro-arcjet nozzles were machined in a 1.2 mm thick quartz plate. Sizes of the nozzle exit were 0.44 mm in height and constrictor height of 0.1 mm. For an anode, a thin film of Au (∼100 nm thick) was coated in vacuum on divergent part of the nozzle. As for a cathode, an Au film was also coated on inner wall surface. In discharging operational tests, a stable discharge was observed for mass flow of 1.0 mg/s, discharge current of 6 mA, discharge voltage of 600 V or 3.6 W input power. In this case, plenum pressure of the discharge chamber was 80 kPa. Thrust obtained was 1.4 mN giving specific impulse of 138 s with thrust efficiency of 24% with 3.6 W input power.

An investigation on effects of a focused laser pulse for combustion enhancement characteristics was conducted for supersonic airstreams with a transverse hydrogen-fuel injection. In order to estimate behaviors of a laser-induced plasma, temporal variations of its emission distribution were measured with an ICCD camera and photodiodes. From these measurements, characteristic time scales of an absorption process of an incident laser pulse and a plasma formation process were also estimated. Numerical simulations using a time-dependant Navier–Stokes equation with finite-rate chemistry were also conducted to elucidate laser-induced ignition and mixing enhancement characteristics in supersonic airstreams with a transverse hydrogen injection. From the results, it was confirmed that, depending on the laser energy density and focal points, radicals, a shock wave, and shock-induced local turbulences were being formed through laser irradiation. Then, recirculation zones with flamelets were induced. Moreover, it was shown that these flamelets were growing in moving upstream and downstream from the focal point up to 1 ms. Consequently, it was shown that laser-induced plasmas could be effective in both combustion reaction and mixing enhancements for the supersonic combustion.

Recent advances of laser plasma accelerators that promise very high gradients of the accelerating fields and enable small sized accelerators were reviewed, and a feasibility study of the utilization of these accelerators for space propulsion was conducted. Forward plasma acceleration through a foil target with a low-power laser pulse was observed, and resultant impulse was measured for various targets. The significant merit of this thruster was discussed in the case of operation of this type of thruster on earth and/or solar orbit, in which solar power is available. Related issues on this point were also discussed.

Studies on the mechanism of ignition and flame-holding by upstream-injection plasma jets were conducted for H2 diffusion flames in high speed air streams. The plasma jets were operated with Ar gas involving active species such as O2, N2 and H2, in the range of relatively low power. It was observed that for ignition and flameholding, the concentration of active species in feedstock is more important than plasma jet power. This fact suggests the effectiveness of active species produced by plasma jets, rather than that of plasma-jet thermal energy, and also the importance of radical concentration supplied to the ignition-point of a fuel jet, compared to the total quantity of radicals produced. It was also found that the effectiveness of additives is in the order, O2≥N2>H2, and the results of numerical simulation on combustion-reaction promotion by the radicals added agreed qualitatively with the experimental ones.

A fundamental study on a laser‐electric hybrid thruster was conducted, in which laser‐induced plasmas were generated through laser beam irradiation on to a solid target and accelerated by electrical means instead of direct acceleration using only a laser beam. As two typical cases of the hybrid propulsion systems, a feasibility study on electrostatic acceleration mode and electromagnetic acceleration mode of the laser ablation plasma were conducted including thrust performance tests with a torsion‐balance, ion current measurements, and ICCD camera observations. It was confirmed that the thrust performances could be improved with electric energy inputs.

Microfabrication of micro-arcjet nozzles with fifth-harmonic Nd:YAG pulses (wavelength 213 nm) and their operational tests were conducted. Micro-arcjet nozzles were machined in a 1.2 mm thick quartz plate. Sizes of the nozzle exit were 0.44 mm in height and constrictor height of 0.1 mm. For an anode, a thin film of Au (~ 100 nm thick) was coated by DC discharge PVD in vacuum on divergent part of the nozzle. As for a cathode, an Au film was also coated on inner wall surface. In operational tests, a stable discharge was observed for mass flow of 0.4 mg/sec, input power of 6 W. In this case, plenum pressure of the discharge chamber was 50 kPa. With 6 W input power, thrust obtained was 1.2 mN giving specific impulse of 147 sec with thrust efficiency of 7 %.

A fundamental study of a forward laser accelerated plasma thruster was conducted. In order to evaluate thrust performances of the thruster, a time‐of‐flight measurement was conducted for an Al‐foil target irradiated with an Nd:YAG laser of 1J/pulse with pulse‐width of 10nsec. From the measurement, the average plasma speed was about 53 km/sec. Time‐gated imaging of the plasma with an ICCD camera was also conducted. From the observation, rapid plasmas were observed on both sides of the target. Each image from the ICCD camera was processed by an image processing software into an emission intensity distribution of the plasma at every 10nsec. Axial velocity of the plasma was estimated from the temporal evolution of the plasma edge. The average and maximum plasma expansion velocities in a forward direction were estimated about 40 km/s and 160 km/sec, respectively.

CFD simulations on effects of a focused laser pulse for combustion enhancement characteristics were conducted for supersonic airstreams with a transverse hydrogen-fuel injection. In order to estimate behaviors of a laser-induced plasma, temporal variations of its emission, and laser absorption processes were measured with photodiodes. From these measurements, characteristic time scales of the absorption process of an incident laser pulse and a plasma formation process were estimated. Numerical simulations using a time-dependent Navier-Stokes equation with finite-rate chemistry were also conducted to elucidate laser-induced ignition and mixing enhancement characteristics in supersonic airstreams with a transverse hydrogen injection. From the results, it was confirmed that, depending on the laser energy density and focal points, radicals, a shock wave, and shock-induced local turbulences were being formed through the laser irradiaton. Then recirculation zones with flamelets were induced. Moreover it was shown that these flamelets were growing in moving upstream and downstream from the focal point up to 1 msec. Consequently, it was shown that laser-induced plasmas could be effective in both combustion reaction and mixing enhancements for the supersonic combustion.

An assessment of a novel laser-electric hybrid propulsion system was conducted, in which a laser-induced plasma was induced through laser beam irradiation onto a solid target and accelerated by electrical means instead of direct acceleration only by using a laser beam. A fundamental study of newly developed rectangular laser-assisted pulsed-plasma thruster (PPT) was conducted on discharge characteristics and thrust performances with increased peak current compared to our previous study to increase effects of electromagnetic forces on plasma acceleration. At 8.65 J discharge energy, the maximum current reached about 8000 A. With a newly developed torsion-balance type thrust stand, thrust performances of laser assisted PPT could be estimated. It was shown that the impulse bit and specific impulse linearly increased. On the other hand, coupling coefficient and the thrust efficiency did not increase linearly. The coupling coefficient decreased with energy showing maximum value (20.8 μNsec/J) at 0 J, or in a pure laser ablation case. Thrust efficiency first decreased with energy from 0 to 1.4 J and then increased linearly with energy from 1.4 J to 8.6 J. At 8.65 J operation, impulse bit of 38.1 μNsec, specific impulse of 3791 sec, thrust efficiency of 8%, and coupling coefficient of 4.3 μNsec/J were obtained.

An investigation on effects of a focused laser pulse for combustion enhancement characteristics was conducted for supersonic airstreams with a transverse hydrogen‐fuel injection. Numerical simulations using a time‐dependant Navier‐Stokes equation with finite‐rate chemistry were conducted to elucidate laser‐induced ignition and mixing enhancement characteristics. From the results, it was confirmed that, depending on the laser energy density and focal points, radicals, a shock wave, and shock‐induced local turbulences were being formed through the laser irradiation. Then recirculation zones with flamelets were induced. Moreover it was shown that these flamelets grew in moving upstream and downstream from the focal point up to 1 msec. From these results, it was shown that laser‐induced plasmas could be effective in both combustion reaction and mixing enhancements for the supersonic combustion.

Compact accelerators employing laser‐foil interactions which have significant potentials for space propulsion applications were reviewed. A feasibility study was conducted for the use of this phenomenon to primary propulsion applications. In order to evaluate thrust performance of this accelerator, time‐gated imaging measurement with an ICCD camera was conducted for an Al foil target irradiated with an Nd:YAG laser set 1J/pulse with pulse width of 10nsec and wavelength of 1064nm. From the measurement, the maximum plasma speed was ∼ 100 km/sec.

An assessment of a novel laser‐electric hybrid propulsion system was conducted, in which a laser‐induced plasma was induced through laser beam irradiation onto a target and accelerated by electrical means instead of the direct acceleration only by using a laser beam. A fundamental study of newly developed rectangular laser‐assisted pulsed‐plasma thruster (PPT) was conducted. Inducing a short‐duration conductive plasma between electrodes with certain voltages, short‐duration switching or a discharge was achieved. At low‐voltage conditions (∼ 100 V), applied to electrodes or charged to a capacitor, it was confirmed that electric discharge can be achieved even under low voltage conditions. From the results, it was found that discharge duration at the low‐voltage case was as long as that of laser‐induced plasma. Therefore, the discharge in the low‐voltage case must be controlled with an incident laser pulse, or a laser‐induced plasma. While in high‐voltage cases (∼ 2000 V), the discharge duration was much longer than that of laser‐induced plasma. In this case, the laser‐induced plasma should be leading main discharge from a capacitor, where some amount of neutral components of vaporized propellant must be ionized through the discharge. Considering ratios of the laser energy to the discharge energies, the discharge process in the high‐voltage mode cases must be defined as the laser‐assisted electric discharge, or the laser‐assisted electric propulsion mode, while in the low‐voltage mode case with smaller electric energy, as the electrically‐assisted laser‐induced process, or the electric‐assisted laser propulsion mode. Moreover, plasma behaviors emitted from each thruster in various cases were observed with the ICCD camera. It was shown that the plasma behaviors were almost identical between low and high voltage cases in initial several hundred nanoseconds, however, plasma emission with longer duration was observed in higher voltage cases. Canted current sheet structures were also observed in the higher voltage cases using a larger capacitor.

Potential concepts of laser‐accelerators which can be applied to the spacecraft propulsion using a conventional technique were reviewed. Maximum ion energy of order tens of MeV through the acceleration was reported in recent studies. As for a proton beam accelerated up to 58 MeV, which was achieved in a recent study, its speed corresponds to 33 % of the speed of light and the specific impulse of order 107 sec. Also, a feasibility study of the utilization of these accelerators, based on the special theory of relativity, for space propulsion was conducted. It was shown that significantly high specific impulse can be obtained through this propulsion system for primary propulsion and attitude control applications. In addition, a preliminary experiment was conducted with an ultrashort‐pulse laser system. Induced impulse in the forward acceleration mode was observed using a 12‐μm thick Al foil as a target.

Recent advances of laser plasma accelerators which promise very high gradients of the accelerating fields and enable small sized accelerators, or the table top accelerators, were reviewed, and a feasibility study of the utilization of these accelerators for space propulsion was conducted, A preliminary experiment on laser plasma acceleration was conducted. In the experiment, both penetrative and reflective plasma acceleration processes through laser irradiation on a target were observed, and resultant impulse was measured for various targets. Also, possibilities for induction of impulse through the laserinduced plasma formation were confirmed in the penetrative plasma acceleration case. In addition, the size of the crater on the bottom surface was larger than the top surface, indicating a lager removal rate, in the penetrative acceleration case.

An experimental study was performed to evaluate the feasibility of arcjet operation at very low power levels ranging 5 W 35 W. The very low power arcjet was run using nozzles with different material and geometry. Nitrogen gas was used as the propellant. Both propulsive performances and thermal characteristics at the constrictor exit were investigated for conventional nozzles which consist of an assemble of tungsten nozzle parts, and modified nozzles, consisting of an assemble, an insulator and a tungsten anode. In the modified nozzles, a ceramic material or an insulator was used as a part of a constrictor to allow an arc column penetrate further downstream of the constrictor or to maintain the high-voltage mode discharges and to reduce the electrode losses. Stable operations with the specific impulse levels of ~ 270 sec at very low power levels ranging about 5 W 35 W with the constrictor diameter of 0.3 mm or 0.5 mm were confirmed at efficiencies between 30 and 40 percent, except a singular case, glow discharge, in which little effect in propulsive performance was observed with the expense of electrical power. At higher specific powers the specific impulse was relatively independent of mass flow rate. At lower specific powers, the specific impulse for the lower mass flow rate was slightly above that for the higher mass flow rate. The constrictor diameter was found to have significant effect on the thermal characteristics (heavy particle temperature and thermal efficiency) of the internal gas flow and the performance of the device. With partially insulated nozzles the specific impulse and thrust efficiency were significantly increased compared to conventional nozzles.

In this study we propose a novel laser‐assisted plasma thruster, in which plasma is induced through a laser beam irradiation onto a target, or a laser‐assisted process, and accelerated by electrical means instead of a direct acceleration only by using a laser beam. Inducing the short‐duration conductive plasma between electrodes with certain voltage, the short‐duration switching or a discharge is achieved, in the laser‐assisted thruster. Also, reductions of energy losses to electrodes, electrodes erosion, and an improvement of specific impulse through the intense current caused by the short duration discharge can be expected. Here, a fundamental study of newly developed two‐dimensional laser‐assisted pulsed‐plasma thruster (PPT) and coaxial laser assisted PPT is conducted. A DC power supply (10 ∼ 600 V) was used for the power source, and an Nd:YAG laser (wave length: 1.06μm, maximum pulse energy: 1.4J/pulse, pulse width: 10 nsec) was utilized. With this system, the peak current of about 500A with its duration of 3 μsec (FWHM) was observed in a typical case.

A time-of-flight technique and iron-filtered neutrons were applied to neutron radiography and to computer tomography. By making use of iron-filtered neutrons, the existence of 1 mm thick copper within an iron block of 20 cm thickness could be clearly distinguished. To investigate the possibility of neutron computer tomography using iron-filtered neutrons, two cylindrical metallic samples made of CuFePb and CuFeAl were used. From the reconstructed image of the samples, we could clearly see the internal structures.

Careful and detailed falling droplet experiments were made (droplet: unsymmetrical dimethylhydrazine,pool: fuming nitric acid), to obtain information on the mechanisms of the generation of a sequence of pressure waves (observed reproducibly) and explosions (observed occasionally), which is useful for safe-handling practices. The change of the shape of the droplet at the instant of impact, caused by the oscillation,has a definite influence on the impact overpressure value and on the possibility of occurrence (as well as the strength) of explosion. When the droplet is elongated, the value of impact overpressure (observed just after the contact of two liquids), is small and the possibility of an explosion is large, compared when the droplet shape is spherical or flattened. The experimental results showed that impact overpressure increases with V 2∼2.5 and D ∼2.5 (V: impact velocity, D: droplet diameter) and is considerably higher for large droplets with high impact velocities. The tendency of the impact overpressure to increase is explained satisfactorily based on simple acoustic source theory. It was shown that the occurrence of explosion and its strength are influenced considerably,not only by the droplet shape but also by the impact velocity, and, furthermore, explosion tends to become stronger when the time lag of occurrence is long. These experimental results are qualitatively consistent with the elucidation of the explosion phenomenon by Friedman et al, in which the role of turbulence is emphasized. In the present report, the influence of the thickness of the vapor layer formed between two liquids on the occurrence of the explosion is proposed, based on newly obtained experimental data.

In electric arcs, there generally exists a considerable streaming of plasma caused by Lorentz forces generated by self-magnetic fields. The method of measurement of such streaming velocity is described. The arc used is a d-c thermal arc of 105 amp in argon at atmospheric pressure. The temperature of the arc was measured by a spectroscopic method and by a thermocouple, and a temperature distribution ranging from 16,000 to 1500 deg K was obtained. The streaming velocity was evaluated by measurement of the drag of a small plate swept across the arc, using the result of the temperature measurement, the drag coefficient of the plate, and the calculated density and viscosity coefficient of the plasma. At a section 2 mm from the cathode, the evaluated stream velocity was 135 m/sec at the center and 25 m/sec at the edge of the arc column from cathode to anode. The error in measured temperature is estimated at less than 10%, and the error in the measured stream velocity is considered to be less than 17.5%, excluding the error due to some change of the arc discharge caused by inserting the drag measuring plate. (auth)

The energy dependent cross-section for the 115 In( n, n′ ) 115 m In reaction was measured in the neutron energy region from 3·37 to 4·89 MeV with a 2 MeV van de Graaff accelerator. A pair of semiconductor proton recoil counters composed of a polyethylene radiator and a silicon detector was used for the absolute measurement of the neutron flux at the indium sample. A comparison of the average cross section, 175 ± 6 mb, which was measured with a fission plate, with the average cross-section calculated with the energy dependent cross-section and the Maxwellian fission neutron spectrum is presented and discussed.

A 1024 channel high speed time analyzer was made by using a plated wire as a memory element. This time analyzer has two operation modes, TOF mode (add-one mode) and MS mode (multi-scaler or add-N mode), both of which use a 133 MHz crystal oscillator. The channel width of the former mode can be selected from 15 ns to 3.84 μs (9 stages) and that of the latter from 1 μs to 65.536 ms (9 stages). For background counting, the last eight channels have longer channel widths and selective delays. These channel widths and delay times were tested with random pulses from a liquid scintillation counter and a weak radioactive source, and periodic pulses from a pulser. The results agreed satisfactorily with the designed values. The dead time for the TOF mode was found to be about 270 ns and that for the MS mode was about 19 ns in counting and about 250 ns in memory cycle. These values are drastically shorter than previous time analyzers, such as TMC-211. This time analyzer has been applied to fast neutron time-of-flight experiments with an electron linear accelerator, and pulsed neutron experiments in beryllium-oxide piles. In this case, no correction for count losses by dead time of a time analyzer is required.