Hideyuki Horisawa

Review: Laser-Ablation Propulsion

LASER ablation propulsion (LAP) is a major new electric propulsion concept with a 35-year history. In LAP, an intense laser beam [pulsed or continuous wave (CW)] strikes a condensedmatter surface (solid or liquid) and produces a jet of vapor or plasma. Just as in a chemical rocket, thrust is produced by the resulting reaction force on the surface. Spacecraft and other objects can be propelled in this way. In some circumstances, there are advantages for this technique compared with other chemical and electric propulsion schemes. It is difficult to make a performance metric for LAP, because only a few of its applications are beyond the research phase and because it can be applied in widely different circumstances that would require entirely different metrics. These applications range from milliwatt-average-power satellite attitude-correction thrusters through kilowatt-average-power systems for reentering near-Earth space debris and megawatt-to-gigawatt systems for direct launch to lowEarth orbit (LEO). We assume an electric laser rather than a gas-dynamic or chemical laser driving the ablation, to emphasize the performance as an electric thruster. How is it possible for moderate laser electrical efficiency to givevery high electrical efficiency? Because laser energy can be used to drive an exothermic reaction in the target material controlled by the laser input, and electrical efficiency only measures the ratio of exhaust power to electrical power. This distinction may seem artificial, but electrical efficiency is a key parameter for space applications, in which electrical power is at a premium. The laser system involved in LAP may be remote from the propelled object (on another spacecraft or planet-based), for example, in laser-induced space-debris reentry or payload launch to low planetary orbit. In other applications (e.g., the laser–plasma microthruster that we will describe), a lightweight laser is part of the propulsion engine onboard the spacecraft.

Control of soot emission of a turbulent diffusion flame by DC or AC corona discharges

The effects of DC or AC (14 kHz) corona discharges, formed between tips of opposed needle electrodes, on soot emission of a propane turbulent diffusion flame were investigated experimentally. It is shown that when a DC corona discharge (e.g., 3.6 W; 0.06% of the combustion energy released by the flame) or a discharge system composed of three AC coronas (e.g., 25.5 W in total; 0.43% of the combustion energy) is applied across the lower part of the flame, with a gap width such that the electrode tips are located outside the reaction zone, a marked reduction in soot emission is observed, without noticeable change in the shape of flame luminous region. When corona discharges are applied, increases of the density of charged species and/or charged soot particles are observed in the flame over the whole length downstream of the corona application. It is suggested that, in the case of DC corona application, additional air and inorganic charged species and electrons, produced in the air near the tip of the positive electrode, are carried into the flame mainly by corona winds, whereas in the case of the AC corona application the inorganic charged species and electrons are carried into the flame by diffusion processes. The charged species and electrons carried into the flame may influence the state of charging of incipient soot particles and also reduce the concentration of growing ions, i.e., soot precursors, which directly relate to the soot emission of the flame. In TEM photographs it was found that separate soot particles, or those forming chains in the flame, decrease in mean size with the application of corona discharges. Smaller size soot particles burn faster than larger size particles in the high-temperature oxidizing atmosphere at the flame top region.

Fundamental Study on Laser Plasma Accelerator for Propulsion Applications

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.

Influence of constrictor size on thrust performance of a very low power arcjet

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.

OPTIMIZATION OF ARC CONSTRICTOR SIZES IN LOW POWER ARCJET THRUSTERS

It is known that the performance of a low power arcjet is influenced by the geometry of the constrictor. The merging of the arc with the propellant gas in the constrictor is a complex process involving electrical and non-equilibrium chemical phenomena with high energy transport rate to the wall. An experimental investigation was performed to investigate the effect of the constrictor geometry on the characteristics of arc discharges and the heat transport rate to the wall, using quartz glass constrictors with different geometries. Following results were obtained. 1) The discharge voltage of the arc increases with the constrictor length (/con) almost linearly, though no distinct effect of the constrictor diameter (cfcon) is observed on it. 2) The thermal efficiency, defined as the ratio of the power of gas ejected from the constrictor (Peject) to the input electrical power (Pin), increases with increasing gas pressure (Peon) and with decreasing /con generally, while with decreasing rfcon the efficiency takes a maximum value at a middle length of dcon. 3) The diameter of arc column decreases with increasing Peon, with increasing /con, and with decreasing rfcon by the thermal pinch effect. 4) The main feature of the variation of constrictor gas temperature with /con and fifcon coincides with that of ejected power and thermal efficiency. It is suggested that the existence of the maximum in the thermal efficiency for the change of cfcon, at a fixed Peon and a fixed mass flow rate of propellant, is due to the coexistence of two type processes, one suppresses heat transport to the constrictor wall and another promotes it.

Experimental Simulation of Magnetic Sails

In order to simulate the interaction between the artificially deployed magnetic field produced around a magnetic sail spacecraft and the solar wind, a laboratory simulator in a space chamber was designed. As a solar wind simulator, a high-power magnetoplasmadynamic arcjet was operated in a quasisteady mode of about 0.8 ms duration to provide a high-speed hydrogen plasma plume of about 0.7 m in diameter, which is accelerated to above 20 km/s with high plasma densities around 10 17 -10 19 m -3 . Into this high- density and high-velocity plasma jet, a small coil of 2-cm-diameter was immersed to obtain 1.9-T magnetic field at the center of the coil. These devices are operation in a large 2-m- diameter space chamber, and the formation of a magnetic cavity was observed around the coil. From the analysis of scaling parameters, it is found that the laboratory experiment of the plasma flow around the coil of the magnetic sail corresponds to a sub-Newton-class magnetic sail.

Multi-Jet Effects of Micro-Nozzle Array in Very Low-Power DC Micro-Arcjets

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.

Fundamental study of laser cutting using high-speed photography

Transient action of molten metal during laser cutting process in a kerf using different assist gases was experimentally observed. In the experiments, we used a typical mild steel sheet with a thickness of 4.4 mm. A nearly dross-free condition with oxygen as an assist gas is at a scanning speed of 1000 mm/minute, CO2 laser power of 600 W pressure of an assist gas 0.15 MPa, while with air a scanning speed of 300 mm/minute, CO2 laser power of 800 W and assist gas pressure of 0.4 MPa were used. Motions of molten metal in a kerf were captured with a high-speed digital CCD camera at a maximum frame rate of 18000 frames per second. As results, we found very bright portion (presumably higher temperature portion) in an erosion front moving toward bottom of a kerf. For oxygen as an assist gas, the portion was moved with a periodic action with a time interval of every 2.22 ms. While air was used, the motion was observed to be every 18.6 ms corresponding to ten times as long as obtained with oxygen as an assist gas.

Microfabrication of Quartz Micro-Arcjet Nozzles with a Fifth-HG Nd:YAG Laser

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 %.

IGNITION CHARACTERISTICS OF LASER-INDUCED PLASMAS IN SUPERSONIC COMBUSTION

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.