Hitoshi Kuninaka

The Arcing Rate for a High Voltage Solar Array : Theory, Experiment and Predictions

All solar arrays have biased surfaces which can be exposed to the space environment. It has been observed that when the array bias is less than a few hundred volts negative then the exposed conductive surfaces may undergo arcing in the space plasma. A theory for arcing is developed on these high voltage solar arrays which ascribes the arcing to electric field runaway at the interface of the plasma, conductor and solar cell dielectric. Experiments were conducted in the laboratory for the High Voltage Solar Array (HVSA) experiment which will fly on the Japanese Space Flyer Unit (SFU) in 1994. The theory was compared in detail to the experiment and shown to give a reasonable explanation for the data. The combined theory and ground experiments were then used to develop predictions for the SFU flight.

Ground studies of ionospheric plasma interactions with a high voltage solar array

While the efficient transfer and generation of high levels of electrical power require high voltages, such as the 160 V contemplated for the NASA Space Station, these exacerbate the effect of space-plasma interactions on spacecraft. The effects are especially severe with solar arrays, when interconnects among solar cells are exposed to the space plasma. A similarity law derived from the governing equations for solar array ion collection is used to define a scaled ground experiment, and the existence of interactions associated with the ion force and surface degradation by sputtering is confirmed. These phenomena must be factored into the projection of spacecraft dynamics and high voltage solar array endurance. 15 refs.

Numerical analysis of interaction of a high-voltage solar array withionospheric plasma

Three-dimensional analysis of the solar array that exposes the surfaces at a relatively high potential and flies in the ionospheric plasma was conducted. For the standard ionospheric conditions, the drain power was calculated at about 0.3% of the solar generated power, which is rather insignificant. On the other hand, the ion forces were found to be enhanced by the large electric potential difference between the solar array and the plasma and must affect significantly the motion of spacecraft with the neutral particle drag. Nomenclature c = unit cord vector = s x n D = characteristic length of solar array d = thickness of space charge sheath d = unit vector of ion drag = - u e - charge of electron F - vector of total ion force / = normalized charge density k = Boltzmann constant lie - ion-electron mean free path / = unit vector of ion lift = dxp M =ion molecular mass N,n = dimensional and nondimensional particle densities n =unit normal vector on conductive surface of solar array P, Q = collision and reflection momenta

Space Experiment on Plasma Interaction Caused by High-Voltage Photovoltaic Power Generation

A High Voltage Solar Array Experiment has been developed for space demonstration and will be operated onboard the Space Flyer Unit. The flight experiment should demonstrate the generation of high-voltage photovoltaic power with a maximum bus voltage of 260 V. In addition, the plasma interactions caused by the high voltage will be investigated during the flight of the satellite in low Earth orbit. Laboratory experiments and numerical simulations predict about 1 mA of ion collection and a maximum arc rate of several arcs per ten seconds during the Experiment. Nomenclature e = electric charge k = Boltzmann constant L = characteristic length of solar array M = ion mass Ne = electron density Nt = ion density NO = plasma density at background P = parameter for a scaling law Te = electron temperature U = orbital velocity V = electrical potential Va = absolute value of bias voltage v - ion velocity s = permeability of vacuum (/> = electrical potential normalized by ion kinetic energy, MU2/2e

Plasma coupling of microwave neutralizer

A microwave neutralizer was developed in Institute of Space and Astronautical Science. Three types of the microwave neutralizer were febricated and their operational characteristics were measured. A prototype neutralizer exhibited stalbe operation at low microwave power of 2W. A higher power operation was investigated using a short type neutralizer and showed a high electron current of over 500 mA. Plasma properties in the discharge chamber of the neutralizer were measured using Langmuir probe and a microwave connector called bias-T. Dense plasma over the cutoff density of 2.2xlO cm was observed at the electron extraction orifice.

Preliminary Study of High Power Hydrogen Electric Propulsion for the Space Exploration

High power electric propulsion system is strongly required for future orbital space transportation. MPD (Magneto-Plasma-Dynamic) thrusters and DC (Direct Current) arcjets with hydrogen as a propellant are promising candidates for the missions because of their high performance and adaptability to high power operation. However, to use hydrogen for long term orbital missions, its storage in orbit is crucial issue to be considered. Firstly, we proposed a hydrogen storage and feed system for electric thrusters by applying our technologies derived from the liquid hydrogen launch vehicles. Secondly, we present R&D activities of hydrogen MPD thruster and DC arcjet, especially focusing on the improvement of their performance and durability. Then, development strategy of hydrogen electric thrusters is also discussed. Finally, advantages of hydrogen electric thruster were shown compared with conventional xenon thrusters through mission analyses of lunar orbit insertion and GTO-GEO transportation.

Enabling Inexpensive Robotic Precursors with the MIU-10 Microwave Discharge Ion Engine System

A visit to a Near Earth Asteroid is a compelling mission for human spaceflight and is baselined as a destination on the way to Mars. T h e number of candidate targets and information about their properties must be increased prior to attempting such a mission. Small spacecraft launched from an EELV Secondary Payload Adapter with propulsion provided by an MIU-10 microwave ion engine system can serve as inexpensive robotic precursors. The Japanese Hayabusa mission has demonstrated the ability to carry out such a mission with this technology. Microwave discharge ion engines have a long life and high reliability because of electorodeless plasma generation. The first model, “μ10”, was installed on Hayabusa asteroid explorer as main propulsion and contributed to its successful return to Earth. The Hayabusa mission and its use of the μ10 Ion Engine System is reviewed. Based upon the flight heritage of Hayabusa, a versatile, upgraded microwave ion engine system, “MIU-10”, using an upgraded “μ10HP” thruster, is under development. The MIU-10 has improved reliability, operability and design flexibility to adapt to various kinds of missions and satellite buses. The characteristics of the MIU-10 system is given. It is an enabling technology for robotic precursors that essentially requires high specific impulse, long life, high technological maturity and low cost. The development of MIU-10 has been almost finished and flight models are under manufacture for Hayabusa-2, which is scheduled to launch in 2014. F i n a l l y , a n o t i o n a l s p acecraft design using the MIU-10 system to explore near Earth asteroids is proposed.

Numerical Lifetime Evaluation of Ion Thruster's Ion Optics using the JIEDI Tool

The JIEDI (JAXA’s Ion Engine Development Initiative) tool has been developed as a numerical tool for the lifetime qualification of ion thruster’s ion optics with high precision and accuracy. The numerical wear test from beginning-of-life to end-of-life (EOL) by the JIEDI tool was conducted for the ion optics of HAYABUSA’s microwave ion thruster (uf06d10 engineering model (EM)). It becomes clear that the erosion profiles for high ion beam current hole are most important to estimate the EOL of uf06d10EM ion optics. The ion optics of uf06d10EM ion thruster encounters its EOL by structural failure of the decelerator grid, which is mainly caused by sputtering of ions and neutrals scattered by elastic collisions. The estimated lifetime of the ion optics is 545 khours at the longest. Through the numerical wear test for uf06d10EM ion optics, the process for lifetime estimation of ion optics by the JIEDI tool was demonstrated.

Electric Field Measurement of ECR Ion Thruster u 10 with Optical Fiber Sensor

In order to reveal the internal phenomenon theoretically within the electron cyclotron resonance ion thruster μ 10, internal microwave electric field measurement is very important because it is closely related to plasma producing mechanism. We have established a technology of electric field measurement with an optical fiber sensor which uses an Electro-Optic crystal (EO probe). This technology enables electric field measurement in plasma source under beam acceleration without disturbing microwave electric field. In this study, first, validity of electric field measurement using the EO probe in the atmosphere was demonstrated by comparing experimental results with FDTD simulation. Then, we measured axial electric field distribution in the accelerated plasma. This experiments indicated that electric field distribution in the μ10 thruster was related to its beam current .

First CUDA FDTD simulation and experimental verification of the microwave transmission and distribution of the mu10 neutralizer

As part of the microwave suppression project of the mu10 ECR ion thruster head, the mu10 neutralizer was successfully modeled in a 3D FDTD approach to create a numerical simulation which gives access to the detailed field structure inside and outside of the device of interest. In order to meet the performance requirements of such a simulation, a massive parallel CUDA implementation was created which makes use of the parallel architecture of NVIDIA graphics cards. Compared to a heritage FORTRAN 3D FDTD code, a speed increase of 1700 is reported, enabling a new resolution and run-time performance, so far only seen on supercomputers. We present an experimental verification of the simulation results with a microwave transmission measurement to show the simulation precision and stability, concluding with the newly created simulation to be fit for scientific investigation and engineering optimization of the mu10 series microwave distribution.