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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 assessment of a novel laser-assisted pulsed plasma thruster (PPT) 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 using only a laser beam. It was found that the discharge duration at low-voltage cases was as long as that of laser-induced plasma. However, in high-voltage cases, the discharge duration was much longer than that of laser-induced plasma. In this case, the laser-induced plasma should be leading the main discharge from a capacitor, where a certain amount of neutral components of vaporized propellant must be ionized through the discharge. 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 PPTs could be estimated. The impulse bit and the specific impulse linearly increased. On the other hand, the coupling coefficient and the thrust efficiency did not increase linearly. The coupling coefficient decreased with energy showing a maximum value (20.8 μN s/J) at 0 J, or in a pure laser ablation case. The 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, an impulse bit of 38.1 μN s, a specific impulse of 3791 s, a thrust efficiency of 8%, and a coupling coefficient of 4.3 μN s/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. 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.

The nature and mechanisms of explosions caused by the contact of hypergolic liquid propellants were investigated in detail for several combinations of fuels and oxidizers. It is shown that the explosion phenomena observed can be classified into three categories. 1) In the case of N2H4/NTO, sudden gasification of a superheated liquid layer formed at the boundary of two liquids occurs spontaneously and a detonation-like reaction proceeds in the reactive mixture produced. 2) In the cases of MMH/NTO and UDMH/NTO, the sudden gasification is caused by the shock of a local ignition, and a turbulent-combustion reaction proceeds in the reactive mixture produced. 3) In the cases of hydrazine type fuels/FNA, the sudden gasification occurs spontaneously as in the case of N2H4/NTO, but it is not augmented by chemical reaction, and in these cases the observed explosion is weak. Information on the vapor layer, which is formed between reactive fuel droplet and pool liquid plays an important role for the occurrence of explosion, is also given, that is based on the high-speed motion picture records.

An experimental study on coaxial laser-electromagnetic hybrid thrusters was conducted. The laser-electromagnetic hybrid thruster, consisting of a coaxial electrode configuration with an annular copper anode and carbon fiber rod cathode was used to produce laserinduced plasmas, which were further accelerated by electromagnetic force to improve thrust performance. Experimental measurement of impulse bit and mass shot was conducted. From the measurement, thrust performance showed impulse-bit of 2 ~ 45 μNsec, momentum coupling coefficient of 5 ~ 14 μNsec/J, specific impulse of 1000 ~ 1400 sec and thrust efficiency of 3 ~ 5 % for charge energies 0 ~ 8.6 J and a laser pulse energy of 120 mJ. In addition, a significant improvement of thrust performance, which were Ibit of 60 μNsec, Isp of 6,000 sec, and ηth of 20% at charge energy of 8.6 J, could be obtained with the use of alumina propellant.

Microfabrication of a 3 x 3 rectangular micro-nozzle array with each exit height of 500 μm using ultra-violet lasers and its operational tests were conducted. Slightly higher thrust and specific impulse were obtained at higher background pressure of 53 Pa. To evaluate thrust characteristics of the array-nozzle, thrust performance was compared with singlenozzles. Significant increases of the thrust and specific impulse with mass flow could be obtained with the array-nozzle case even in the lower background pressure of 4 Pa. A preliminary DSMC computation on internal and exhaust nozzle-flow characteristics of micro-nozzles of multi-nozzle array was also conducted. It was shown that the use of the multi-nozzle array was effective in suppressing expansion of each under-expanding jet, and in inducing axially confined jets, through the interactions of the jet boundaries, or multi-jet effects.

A preliminary study of combustion-augmented laser-ramjets was conducted, in which chemical propellant such as a gaseous hydrogen/air mixture was utilized and detonated with a focused laser beam in order to obtain a higher impulse compared to the case only using lasers. CFD analysis of internal conical-nozzle flows and experimental measurements including impulse measurement were conducted to evaluate effects of chemical reaction on thrust performance improvement. From the results, a significant improvement in the thrust performances was confirmed with addition of a small amount of hydrogen to propellant air, or in combustion-augmented operation.

CFD simulations using a time‐dependant Navier‐Stokes equation with finite‐rate chemistry on effects of a focused laser pulse for combustion and mixing enhancement characteristics were conducted for supersonic premixed hydrogen‐air streams. From the results, it was confirmed that, depending on the laser energy density, 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 in the recirculation zones were being stabilized in the supersonic stream for a while. Consequently, it was shown that laser‐induced plasmas could be effective in both combustion reaction and mixing enhancements for the supersonic combustion.

Fundamental investigations on plasma diagnostics of a forward laser plasma acceleration employing laser‐foil interactions were conducted for an Al‐foil target irradiated with an Nd:YAG laser of 1J/pulse with pulse‐width of 10nsec. A time‐of‐flight measurement was also conducted to evaluate ion speeds. In addition, temporal evolutions of electron temperatures and densities were evaluated with electrostatic probes and spectroscopic diagnostics. Moreover, a preliminary one‐dimensional particle‐in‐cell (PIC) simulation was conducted to elucidate acceleration mechanisms. From the results, it was shown that a speed of ions in a forward direction were about 135 km/sec, respectively. Also it was shown that the plasma temperature and density were about 2.5∼3 eV and 1010 cm‐3.

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.

A preliminary study on induction phenomena in a laser‐sustained scramjet was conducted. The induction processes include absorption process of a laser pulse by a reactive mixture, plasma formation, diffusion of active species, shock formation, thermalization process of ambient mixture, induction of local turbulence, etc. For observation of the initial phenomena, an experimental study on effects of a focused laser pulse (Nd:YAG, 335mJ/pulse, pulse width 5nsec) into a hydrogen‐air mixture was conducted. Temporal evolutions of typical line spectrum of a laser‐induced plasma of the mixture were measured with the photodiode or the photo‐multiplier‐tube through specific band‐pass filters for each spectrum for OH, O+, N+, H, and O. It was shown that the emission from O abruptly increased at 2 nsec, peaked at about 5 nsec, followed by an abrupt drop at 6 nsec. The emission from H atoms secondly increased. Other emissions of N+, O+, and OH peaked at about 17 nsec and continued for about 1 msec..

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.

A fundamental study on two types of novel laser-electric hybrid thrusters, a laser-assisted pulsed-plasma thruster and a laser-assisted electrostatic thruster, 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, was conducted to elucidate its discharge and acceleration characteristics. In the laser-assisted PPTs with 8.65 J discharge energy, the maximum current reached about 8000 A. Also, it was shown that the impulse bit and specific impulse linearly increased. While, coupling coefficient decreased with energy showing maximum value (20.8 μNsec/J) at 0 J, or in a pure laser ablation case. Thrust efficiency slightly decreased with energy ranging 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. In the laser-assisted electrostatic thruster, energy distributions of accelerated ions were measured by a Faraday cup. A time-of-flight measurement was also conducted for ion velocity measurement. It was shown that an acceleration electrode with positive potential was more effective than that with negative potential for positive-ion acceleration in laser induced plasma, or pulsed plasma, in which ions were induced with the Coulomb explosion following electrons.

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.

Effects of laser-induced plasmas on ignition and flame holding characteristics of a hydrogen diffusion flame in supersonic airstreams were investigated. Temporal observation down to nanosecond resolution for behaviors of the laser-induced plasma was conducted with an ICCD camera. It was confirmed that laser ignition processes were characterized into several time scales; (I) absorption of an incident laser pulse (10 -9 ~ 10 8 sec), (II) plasma formation process (10 -8 ~ 10 -7 sec), (III) ignition process (10 -6 ~ 10 -4 sec), and (IV) shock-flow interaction and (V) convective diffusion processes (10 -5 sec ~ ). From the numerical simulation, it was found that positions of plasma kernels induced by laser irradiation affected ignition and flame stabilization characteristics of the flame and also induction process of local turbulence augmenting a local mixing.

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.

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.

This paper reports capless annealing of Cd-implanted GaAs performed using a halogen and a mercury lamp, which emit infrared and ultraviolet rays, respectively. Sheet carrier concentrations depended on annealing time and the lamps. At the beginning of annealing, the implanted Cd was lost to a greater extent on the samples annealed by the mercury lamp than on the samples annealed by the halogen lamp. GaAs surfaces decomposed during prolonged annealing using both lamps. The carrier concentration with annealing was suppressed by the thermal decomposition of GaAs.

As the first step of the study of the combustion control of solid propellants by electrical discharges, the effects of an arc discharge, which flows along the burning surface, on the burning rate and on the increase of enthalpy of the combustion product were investigated. For specially devised composite propellants, which are composed of Al and Teflon powders, it was shown that the combination can be controlled by an arc discharge; the combustion continues when the arc discharge is applied and is interrupted when the arc discharge breaks. In the present investigation, it was also shown that an arc discharge coupled with a high-frequency electrical discharge has potential as an effective ignition method for solid propellants. For the application of this type of combustion control to an ignitor for a solid propellant rocket motor or to a control rocket motor, this method lacks flexibility in the configuration scale and needs relatively high electric power at the present stage.

Detailed falling droplet experiments (droplet: N2H4, pool: N2O4) were made, employing high-speed photography, pressure transducers and photodiodes to obtain information on the nature and the mechanisms of explosions. The probability and the strength of explosions increase with droplet size and impact velocity; and the strength has a tendency to saturate, when the impact velocity is increased beyond ≈4.0 m/sec. The magnitude of explosion overpressures observed and its dependency on droplet size could be explained based on the blast wave theory, using the source energy evaluated from high-speed motion picture records. The explosions of N2H4/N2O4 are always accompanied by simultaneous light emission. The time lag of the explosion fluctuates statistically for a fixed experimental condition. Introducing the concept of explosion probability in function of unit time, μ(t), it was shown that the time lag is composed of two parts: a first part of length to, where μ(t)=0, and a second part of length t1, where μ(t) is kept nearly constant (μ(t)=-μ). The length t0 was insensitive to droplet size and impact velocity (≈0.11 msec), although the value of -μ was influenced by them. High-speed motion picture records of ≈105 frames/second clearly showed a dispersed layer at the depressed N2O4 surface (the more volatile component), immediately before the beginning of the explosion. It is suggested, from considerations of the abrupt appearance of this layer and also the nature of the first period of explosion time lag, that the most likely explanation for the layer, which plays the role of a trigger for the explosion, is the sudden gasification of a thin surface layer of liquid N2O4, which was heated above the boiling point.

Current distribution on the electrodes of a conventional MPD arcjet without external magnetic field was calculated based on induction equation, with the assumption that the magnetic Reynolds number is small. The calculated result, which showed the influence of tensor conductivity, coincided fairly well with the corresponding experimental one. It was seen, however, that the current density near the cathode tip in experiment had a larger value than that in calculation. This discrepancy was explained tentatively as being the result of the occurrence of cathode spots. It is also suggested in this paper that the unfavorable current concentration at the cathode base can be suppressed by using a specially devised divided cathode.

Introduced is the activities of the Joint Research Project Team on Thorium Fuel organized by mainly university researchers in Japan and supported by the Ministry of Education, Science and Culture for seven years since 1980. Four major groups were organized; (1) nuclear data, reactor physics and design, (2) nuclear fuel, (3) down stream and (4) biological effects of thorium.The first group covered measurements and analysis on nuclear data of thorium related nuclides, experiment and analysis on nuclear characteristics of thorium containing cores, basic engineering on a thorium molten salt reactor, and designs of several types of reactors. Fabrication and irradiation tests of thorium oxide fuel, and basic studies on new type thorium fuels (e.g. carbide and nitride) were studied by the second group. The third group covered the use of solutions in reprocessing of spent fuel, behavior of fission products, immobilization of high level radioactive waste, and continuous reprocessing for a molten salt reactor. The fourth group performed the trace study for patients who had been intravascularly injected with thorotrast for diagnosis of war injuries during the Second World War.

The feasibility of ignition and combustion control of solid propellants by dc arc discharge was empirically examined. In this experiment Al/Teflon composite propellants and a conventional HTPB/AP/A1 composite propellant were studied. It was shown that the dc arc discharge coupled with a high-frequency-discharge arc initiator works as an efficient ignition method for solid propellants; that, for Al/Teflon propellants with appropriate O/F ratios, repeated ignitions and extinctions are possible by applying and interrupting the arc discharge; and that, for all the propellants tested, burning rates can be varied with the discharge intensity, although there exists a threshold current below which no effect on the burning rate is observed. It was observed that, for the combustion control of Al/Teflon propellants, the minimum discharge current required for sustaining combustion is lower than that required for ignition and that the former is influenced by the percentage of aluminum in the propellants (or by the heat of combustion of the propellants), while the latter has a nearly constant value. The solidpropellant system combined with arc discharges seems promising for the on-board reaction control engine when the satellite has an excess of electric power.

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.

A new type of target called a "cylindrical target" has been devised in which most of the rebounding particles leave the target radially through slits attached to its side face. It is expected that with this target the ambiguity inherent in target methods, which is caused by insufficient momentum accommodation of rebounding particles, will disappear. In this paper it is shown that, by using this type of target designed appropriately, impulse bits of pulsed plasma thrusters can be evaluated with sufficient accuracy.

In the measurement of burning velocities by the burner method, it is known that large values of burning velocities are obtained by the influence of atmospheric ixygen in the case of fuel-rich mixtures. In this respect, the effects of oxygen concentration in atmosphere, the size of burner and the flow rate of mixture on burning velocities have been examined, and it has been shown also that the value of the burning velocity of a fuel-rich mixture measured on the spherical flame seems to agree with that measured on the Bunsen flame in nitrogen atmosphere. Furthermore, in this investigation, the estimation of the value of activation energy in the combustion of propane from the correlation between burning velocities and temperatures of reaction zone has been done.

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.

Spark ignition of flowing gases is investigated experimentally at room temperature and atmospheric pressure. Under laminar condition the inflammability of flowing gases increases with gas velocity. This is due to the tendency of initial flames to escape from the quenching effect of electrodes under the influence of gas flow. Under turbulent condition the inflammability of flowing gases decreases with increasing gas velocity. This is ascribed to the increased eddy diffusivity and is in accordance with the result of Swett's experiment under turbulent condition at reduced pressures.

The lattice disorder in GaAs produced by fast neutrons with a fluence of 7 × 10 17 n cm −2 has been investigated with 1.5-MeV 4 He + channeling and electron spin resonance (ESR) measurements. The slight change in the 〈100〉-aligned yield for irradiated crystals indicates that each primary knock-on (PKO) produces approximately 10 3 displaced atoms. Detailed angular scans suggest a spread distribution of randomly-located defects imbedded in the lattice structure. On the other hand, As Ga 4 antisite defects per PKO estimated from ESR measurements are ~ 100. It is suggested that neighboring atoms of As Ga 4+ antisite defects are slightly distorted after the PKO event.

The annealing behavior of As implanted Si-layers is investigated by measuring the atom concentration profile with radioactive tracer technique and the carrier concentration profile with Hall effect measurements in the same sample. Silicon substrates have been implanted at room temperature with a low (4×1013/cm2) and high (5~ 6×1014/cm2) dose of 75As ions together with radioactive 76As ions at 45keV and subjected to annealing at 500°C and 600°C for 20min. After annealing at 600°C, the carrier concentration profile corresponds closely to the implanted As ion profile. The carrier mobility observed for 600°C annealing is almost the same as that expected for comparably doped bulk silicon, except near the peak region for the high dose implantation. The results suggest that the recovery of the carrier and mobility takes place from inside toward the surface.

As implanted Si was annealed using halogen and mercury lamps whose lights primarily have, respectively, infrared and ultraviolet regions. Annealing time necessary for fully electrically activating the implanted As in the mercury lamp annealing was shorter than that in the halogen lamp annealing, when the light emitted from the two lamps was of the same intensity. This was due to the difference in intensity of the light absorbed into the implanted Si between the lamps used at the beginning of annealing.

The influence of thin native oxide films (∼25 Å) on the annealing behavior of As and Ga implanted in Si has been studied. Although no implanted As evaporated from the solids into the ambient atmosphere during annealing, the amount of implanted As diffusing from the Si into the native-oxide films increased with the annealing temperature to 2×1014 cm-2 at a dose of 1×1016 cm-2 at an annealing temperature of 1100°C. On the other hand, the amount of implanted Ga evaporating from the solids into the ambient atmosphere increased exponentially with the annealing temperature; the amount of implanted Ga remaining in the Si was only 7% of the dose of 1×1015 cm-2 at an annealing temperature of 1100°C.