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Characteristics of compact laser plasma accelerators utilizing high‐power laser and thin‐target interaction were reviewed as a potential candidate of future spacecraft thrusters capable of generating relativistic plasma beams for interstellar missions. Based on the special theory of relativity, motion of the relativistic plasma beam exhausted from the thruster was formulated. Relationships of thrust, specific impulse, input power and momentum coupling coefficient for the relativistic plasma thruster were derived. It was shown that under relativistic conditions, the thrust could be extremely large even with a small amount of propellant flow rate. Moreover, it was shown that for a given value of input power thrust tended to approach the value of the photon rocket under the relativistic conditions regardless of the propellant flow rate.

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.

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..

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.