Kyoichiro Toki

Verification Tests of Carbon-Carbon Composite Grids for Microwave Discharge Ion Thruster

Anionbeam opticsfora10-cm-diam 400-W-classmicrowavedischargeion thrusterwasfabricatedanditsapplicabilityto along-termspacemissionwasdemonstrated.Theopticsconsistsofthree1-mm-thick e atcarbon ‐carbon composite panels with approximately 800 holes that were mechanically drilled and positioned with § 0:02-mmaccuracy.Whenmounted onanaluminum ring,spacingforthethreegridswaskeptat0.5 mm bythreesetsofspacers. The thruster produced an ion beam current of 140 mA with a microwave power of 32 W for plasma generation and a total acceleration voltageof 1.8 kV. Although thegrid is sputtered by the impingement of slow ions produced in charge-exchange collisions between fast beam ions and neutral atoms leaking from the engine, the grid showed only slight damage even after an 18,000-h endurance test. Also, other qualie cation tests including a mechanical test under launch conditions as well as a thermal vacuum test simulating the spacecraft thermal environment were successfully completed. Hence, the grid system was qualie ed for spacecraft propulsion.

Development of high-density helicon plasma sources and their applications

We report on the development of unique, high-density helicon plasma sources and describe their applications. Characterization of one of the largest helicon plasma sources yet constructed is made. Scalings of the particle production efficiency are derived from various plasma production devices in open literature and our own data from long and short cylinder devices, i.e., high and low values of the aspect ratio A (the ratio of the axial length to the diameter), considering the power balance in the framework of a simple diffusion model. A high plasma production efficiency is demonstrated, and we clarify the structures of the excited waves in the low A region down to 0.075 (the large device diameter of 73.8 cm with the axial length as short as 5.5 cm). We describe the application to plasma propulsion using a new concept that employs no electrodes. A very small diameter (2.5 cm) helicon plasma with 1013 cm−3 density is produced, and the preliminary results of electromagnetic plasma acceleration are briefly de...

Plasma Characterization of a 10-cm Diameter Microwave Discharge Ion Thruster

Plasma characterization was conducted for an electron-cyclotron-resonance (ECR) type ion thruster. For a 10-cm diameter microwave discharge ion source consisting of two samarium cobalt magnet rings surrounding a centered waveguide for launching microwaves, plasma profiles were found to have severely non-uniform distributions, with localized plasma found near the magnet rings. This localized plasma is mainly produced in the magnetic flux tubes between the two ring magnets, where electrons gain microwave energy as they pass the ECR line during the bouncing movement between magnetic mirrors. To obtain a low-cost microwave ion source, this type of ionization mechanism can be exploited. When introducing microwaves through a low magnetic field boundary, however, it is impossible to eliminate the accessibility difficulty related to the cutoff density, which results in a plasma below the cutoff density. Because of the accessibility difficulty, in this work, only a relatively small ion beam current density of 1.8 mA/cm2 was achieved.

An overview of challenges in modeling heat and mass transfer for living on Mars.

Abstract:  Engineering a life‐support system for living on Mars requires the modeling of heat and mass transfer. This report describes the analysis of heat and mass transfer phenomena in a greenhouse dome, which is being designed as a pressurized life‐support system for agricultural production on Mars. In this Martian greenhouse, solar energy will be converted into chemical energy in plant biomass. Agricultural products will be harvested for food and plant cultivation, and waste materials will be processed in a composting microbial ecosystem. Transpired water from plants will be condensed and recycled. In our thermal design and analysis for the Martian greenhouse, we addressed the question of whether temperature and pressure would be maintained in the appropriate range for humans as well as plants. Energy flow and material circulation should be controlled to provide an artificial ecological system on Mars. In our analysis, we assumed that the greenhouse would be maintained at a subatmospheric pressure under 1/3‐G gravitational force with 1/2 solar light intensity on Earth. Convection of atmospheric gases will be induced inside the greenhouse, primarily by heating from sunlight. Microclimate (thermal and gas species structure) could be generated locally around plant bodies, which would affect gas transport. Potential effects of those environmental factors are discussed on the phenomena including plant growth and plant physiology and focusing on transport processes. Fire safety is a crucial issue and we evaluate its impact on the total gas pressure in the greenhouse dome.

Electrode Configuration Effect on the Performance of a Two-Dimensional Magnetoplasmadynamic Arcjet

Thrust performance and internal plasma flowfield of a 1-MW class self-field magnetoplasmadynamic (MPD) arcjet were measured to evaluate their dependence on the cross-sectional geometry of the electrodes. A multichannel two-dimensional MPD arcjet in quasisteady operation was used to visualize the two-dimensional flowfield and reveal the correlation between the internal flowfield and the thrust performance. The experimental results for six different electrode configurations show that the thrust performance strongly depends on the thruster chamber cross-sectional geometries for the 7sp range of interest, 1000-3000 s. The cathode length determined the engine performance, regardless of the anode geometry. In particular, the convergent-divergent anode with a short cathode showed the best performance. The superior acceleration mechanism of the short cathode was explained on the basis of two-dimension al plasma distributions such as discharge current contours and plasma density obtained by Mach-Zehnder interferometry. A dense plasma region near the tip of the short cathode was observed and subsequent expansion guided by the diverging nozzle can enhance aerodynamic acceleration, which contributes to large thrust generation.

Numerical Analysis of a Two-Dimensional Magnetoplasmadynamic Arcjet

The effect of electrode cone guration on thrust characteristics of a two-dimensional magnetoplasmadynamic (MPD) arcjet was numerically investigated. A simple magnetohydrodynamics (MHD) model was developed and the numerical results were compared with the experimental data for several electrode geometries. To understand the features of the e owe eld, we introduced a magnetosonic Mach number, which is dee ned as local velocity divided by a propagation speed of the MHD disturbance. Based on the magnetosonic Mach number distribution of the e owe eld, the model can explain the thrust characteristics of the MPD arcjet, especially the superiority of a short cathode under various anode cone gurations. Because the electromagnetic thrust is unaltered for the same anode cone guration, the electrothermal component of thrust makes a difference between the long and the short cathodes. With a short cathode cone guration, the large heat deposition near the cathode tip, which is inevitable to MPD arcjets, can be cone ned in the submagnetosonic region where the local e ow is accelerated to magnetosonic velocity. Then the thermal deposition into the submagnetosonic region can be efe ciently recovered through transmagnetosonic acceleration, resulting in a large thrust generation.

Application of MPD thruster systems to interplanetary missions

Studies of space tests and applications of MPD thruster systems in the 1990s are described. They include the Advanced Space Experiment onboard the Space Flyer Unit (SFU) in low Earth orbit (LEO), the lunar polar orbiter mission in the early 1990s, and the asteroid rendezvous mission in the late 1990s. The SFU uses a 5 kw MPD thruster system (4 x 1.25 kW system) to assess the on-orbit performance and the potential impact on the LEO environment by thruster plasma injection. During the lunar mission, a 1.25 kW MPD thruster system lowers the orbiter from a high lunar polar orbit into a low orbit less than 100 kW in altitude. These early applications can be followed by an interplanetary asteroid rendezvous mission. Envisioning these missions, the MPD thruster system is required to have 10 X10 pulse lifetime and to generate 40 mN/kW at a specific impulse of 2000 s.

Optimal Control of Quasi-One-Dimensional Self-Field Magnetoplasmadynamic Arcjet Flowfields

The e owe eld of a self-e eld magnetoplasmadynamic (MPD) arcjet was analyzed to establish the optimum geometry that produces the highest possible thrust for specie ed operating conditions. A set of simplie ed assumptions, within a quasi-one-dimensional framework, was used to establish how the optimum e owe eld was coupled to the thruster geometry. The resultant distribution of discharge current was smooth without any prominent concentration along the electrodes. The approach employed a purely mathematical method of engineering optimal control to suggest design guidelines for MPD arcjet thrusters within the idealized constraints. The optimum was found to be a slowly convergent and quickly divergent geometry that maximized the exit velocity for a e xed electrical input power.

Thrust performance of a regeneratively cooled low-power arcjet thruster

A low-power arcjet thruster SAGAMI-I has been newly designed and fabricated based on the results of the finite element thermal analysis. Thrust performance was parametrically studied for several different constrictor configurations with submilimeter diameters using hydrazine simulative gas propellant (N2 + 2H 2). The SAGAMII exhibited significant performance advantage over our old-type thruster. Further reduction of a constrictor diameter was attempted, and the limit was found. The effect of regenerative cooling was also evaluated, and the obtained experimental data demonstrated that the energy gain from regenerative cooling resulted in the increase of specific impulse by 5-10%.

Quantitative Imaging of the Magnetoplasmadynamic Flowfield

Quantitative imaging of magnetoplasmadynamic (MPD) flowfields was made by nonintrusive measurements such as a magnetic-sensitive film for current, CCD images using interference filters for electron temperature, and far-infrared Mach-Zehnder interferometry for plasma density. A cathode jet carrying a high-density current into the cathode tip was observed with hydrogen propellant, and the associated wall-detached plasma confined in the self-induced magnetic field was found to improve the thruster performance. Ion-neutral momentum coupling is invoked to explain several phenomena, such as variation of the thrust performance, with a different mass flow rate, Af-shaped transition of thrust efficiency against specific impulse, and good-thrust efficiency with hydrogen propellant. These real-gas effects together with interelectrode geometry determine the thruster performance and its flow characteristics.