Koji Tanaka

We have made a conceptual design of a long-distance wireless power transmission (WPT) experiment using a small scientific satellite, which is developed by ISAS/JAXA. The purposes of the WPT experiments are to demonstrate a precise directional control of microwave beam, which includes the direct detection of rectenna sites, and to clarify the propagation characteristics of intense microwaves in the plasma of the ionosphere. The sub-recurrent orbit at an altitude of 370 km is a prime candidate. Typical microwave power radiated from the transmission antenna with frequency of 5 GHz band is around 2kW. This level of microwave power injection will generate a power density above 1,000 W/m^2 within 40 m in the ionosphere. The onboard instrument that consists of plasma probes and wave receiver can measure effects of interaction between high power microwaves and plasma in ionosphere. The ground system is designed to evaluate the microwave beam characteristics such as the radiation pattern and microwave power. Wireless power transmission efficiency from space to the ground will be evaluated by analyses of the results of space demonstration experiments.

Toward the in-orbit demonstration of a solar power satellite (SPS) being investigated as one of the future power generation system, the breadboard model of the wireless power transmission via microwave has been developed for the preliminary demonstration. The precise transmission beam control is a principal technology required for a long-distance transmission between space and the ground. The directivity of the transmission antenna has experimentally evaluated by utilizing the model. The breadboard model chiefly consists of the phased array antenna having 256 array elements, and planar microwave circuits including both functions of amplification and phase control, forming a thin panel with double layers. This report is intended to clarify the effect of the relative phase errors in the antenna to the beam directivity, and evaluate the validity of the phase compensation.

Light-weight thin plate structure is expected to play an important role in space development in the near future. Thin film solar array, solar sail, planar antenna, and inflatable structure are the typical examples. We have carried out hyper-velocity impact experiments on thin plate targets using a light gas gun, to study propagation of the impact plasma associated with an interference with electrical parts on the planar plate. Propagation of the luminous cloud and generated plasma were observed by a high-speed video camera and an array of plasma probes, respectively. It was found there existed a high-dense plasma propagating along the surface of the plate at a high-speed more than the projectile velocity. The characteristic behaviors of the plasma propagation were explained by the half-range Maxwellian plasma uniformly ejected from the collision area.

The experiment of charged particle collection experiment by bare electrodynamic tape tether was performed by using JAXA's sounding rocket S-520-25 on August 31, 2010. This was the first experiment of bare tether in space in the world. This paper shows the outline of the experimental apparatus and the initial experimental results of the verification of bare electrodynamic tether operation and the charged particle collection theory. As experimental results, Valuable data of electron collection by the induced electromotive force and ion collection by a space potential probe in space were obtained, although charged particle collection data by tether and boom were not obtained by a trouble of high voltage power source.

Series connections of energy-storage modules such as electric double-layer capacitors (EDLCs) and lithium-ion batteries result in voltage imbalance because of the nonuniform properties of individual modules. Conventional voltage equalizers based on traditional dc-dc converters require numerous switches and/or transformers, and therefore, their costs and complexity tend to increase. This paper proposes a novel single-switch equalization charger using multiple stacked buck-boost converters. The single-switch operation not only reduces the circuit complexity but also contributes to increasing the reliability. The fundamental operating principles and design procedures of key components are presented in detail. An experimental charge test using a 25W prototype of the proposed equalization charger was performed for four series-connected EDLC modules whose initial voltages were intentionally imbalanced. Experimental results demonstrated that the proposed equalization charger could charge the series-connected modules preferentially in the order of increasing module voltage and that all the modules could be charged up to a uniform voltage level.

Electric double-layer capacitors (EDLCs) offer several advantages over traditional batteries, such as long cycle life, high power capability, good low-temperature performance, etc. However, their major drawbacks, such as low specific energy and large voltage variation due to charge/discharge cycling, necessitate the use of high-efficiency power conversion electronics that can be used to efficiently discharge EDLCs and thus completely utilize the precious stored energy. In this study, we propose a novel discharger for EDLCs; this discharger uses cascaded switched capacitor converters (SCCs) and selectable intermediate taps. Although the voltage conversion ratio of SCCs is fixed, the load voltage can be maintained within a desired voltage range by the selectable intermediate taps. The circuit configuration, operating principle, and procedure for designing SCCs and selectable intermediate taps are presented. Experimental tests were performed using an EDLC module and a 200W prototype of the discharger. The obtained results showed that the 60V EDLC could be discharged to 30V with an average efficiency of 96% when the load voltage was maintained within the range 30-40V.

When electric double-layer capacitors (EDLCs) are connected in series, cell voltage imbalance that results due to non-uniform cell properties is observed. Cell voltage imbalance should be minimized to prolong cycle lives and maximize the available energy of cells. In this study, we propose a series-parallel reconfigurable cell voltage equalizer that is considered suitable for energy-storage systems using EDLCs instead of traditional secondary batteries as main energy-storage sources. The proposed equalizer requires only EDLCs and switches as its main circuit elements, and it utilizes EDLCs not only for energy storage but also for equalization. An equivalent circuit model using equivalent resistors that can be regarded as an index of equalization speed is developed. Current distribution and cell voltage imbalance during operation are quantitatively generalized. Experimental charge-discharge tests were performed for EDLC modules to demonstrate the performance of the cell voltage equalizer. All the cells in the modules could be charged/discharged uniformly even when a degradation-mimicking cell was intentionally included in the module. The resultant cell voltage imbalances and current distributions were in good agreement with those predicted by mathematical analyses.

Power conversion electronics for photovoltaic (PV) systems are desired to operate as efficiently as possible to exploit the power generated by PV modules. This paper proposes a novel PV system in which a dc-dc converter is partially connected to series-connected PV modules. The proposed system achieves high power-conversion efficiency by reducing the passing power and input/output voltages of the converter. The theoretical operating principle was experimentally validated. Resultant efficiency performances of the proposed and conventional systems demonstrated that the proposed system was more efficient in terms of power conversion though the identical converter was used for the both systems.

Selenological and Engineering Explorer is composed of a lunar orbiter 'Kaguya', and two small sub-satellites: Relay Satellite 'Okina' and VRAD Satellite 'Ouna'. It was launched in September 2007, and operated until June 2009. Ground stations for their operations were GN stations of JAXA, Usuda Deep Space Center (UDSC), and foreign stations. Among them, 64m antenna of UDSC executed the downlink of mission data from Kaguya, and the first experiments of 4-way Doppler measurements of lunar orbiter relayed by Okina. We report about the outline of these operations, and the new technology to achieve the 4-way relay link.

Switching power conversion electronics, such as dc-dc converters and inverters, generate ripple current components. The generated ripple current is superimposed to an average dc current of a fuel cell, so that the potential of the cell inevitably varies at high frequency due to the induced ripple current. One of the major degradation factors of proton exchange membrane fuel cells (PEMFCs) is a loss of electrochemical surface area (ESA) of Pt/C catalyst that is generally accelerated by potential variations. This study investigates a qualitative influence of high-frequency potential variation produced by an interaction between power conversion electronics and PEMFC on Pt/C catalyst degradation. PEMFCs were operated and their potentials were varied at high frequency in order to investigate ESA degradation trends. The ESAs at low frequencies deteriorated significantly, while those at high frequencies showed comparable trends with those at dc conditions. The experimental results obtained in this study are expected informative to determine operating frequencies of switching power conversion electronics and to regulate ripple frequencies.

Unreacted fuel expelled form the fuel cell output is recirculated back to the inlet by mechanical pumps in proton exchange membrane fuel cell (PEMFC) systems aiming for high-efficiency performance. Mechanical pumps generally not only consume electrical power but also generate vibration and noise, so that the major benefits of fuel cells, such as high efficiency, good tranquility, and lack of vibration, are neutralized to some extent by the mechanical pumps. On the basis of electrical-fluid analogy, this paper derives the pumpless recirculation system originating from a switched capacitor circuit that is commonly used in electrical circuits. Experimental tests were performed for the derived pumpless recirculation system, and its operating characteristics were evaluated based on comparisons against simulation results of the switched capacitor circuit. A PEMFC single cell was operated with the derived recirculation system over 10 h in anode recirculation mode, and showed stable performance.

Conventional cell/module voltage equalizers or equalization chargers based on traditional dc-dc converters require numerous switches or transformers as the number of series connections increases; therefore, their cost and complexity tend to increase and their reliability decreases as the number of connections increases. This paper proposes a novel voltage equalization charger that consists only of passive components such as capacitors, diodes, and a transformer. The fundamental operating principle, major features, and derivation of equivalent dc circuits are presented. A symmetrical configuration is also proposed to mitigate the RMS current flowing through energy storage cells in the charging process. Simulations and experimental charging and cycle tests were performed on series-connected electric double-layer capacitor modules to demonstrate the equalization performance. The experimental and simulation results were in good agreement, and the voltage imbalances were gradually eliminated as time elapsed even during charge-discharge cycling.

Conventional voltage equalizers or equalization chargers, which are used for series-connected energy storage cells to eliminate cell voltage imbalance, consisting of a number of switches or transformers tend to be complex with the number of series connection of the energy storage cells. A novel voltage equalization charger consisting only of capacitors, diodes, and an ac power source is proposed, and its dc equivalent circuit expressed by resistors and a dc power source is derived in this paper. Experimental charge tests demonstrated that series-connected EDLCs could be charged up to the uniform voltage by the proposed equalization charger.

Series-connected energy storage cells require cell equalizers in order to mitigate cell voltage imbalance. Conventional cell equalizers, however, consist of a number of switches or transformers that are considered not preferable in the viewpoint of circuit complexity and reliability. This paper proposes an equalization charger that consists of a single switch and passive components. Experimental performance tests showed that series-connected EDLCs could be charged up to the uniform voltage level by the proposed equalization charger though the initial voltages of the EDLCs were imbalanced.

The Japan Aerospace Exploration Agency (JAXA) will make the solar power sail craft demonstrate for both its photon propulsion and thin film solar power generation during its interplanetary cruise. The spacecraft deploys and spans its membrane taking the advantage of the spin centrifugal force. The deployment system of large membrane was developed. The overall deployment experiment of solar power sail using a balloon was planned. However the balloon was not launched due to the bad weather. In this paper, the contents and significance of the experiment are reported.

The primary objectives of the sounding rocket experiment, S-520-25, are to deploy an electro-conductive bare tape tether in space and to study the electron current-collection by the tape tether when biased positively. Before the space experiment, we have carried out laboratory experiments on the plasma collection by the tape tether in a large space science chamber at ISAS/JAXA. This paper presents the major results on the currentvoltage characteristics of the tape tether in the plasma environment which simulates the ionospheric plasma It was found that the tether current approached to the prediction by the Orbit Motion Limit (OML) theory as the tether voltage was increased up to 500 V. In the high voltage region more than 200 V, we often observed the discharge at the surface of the tether that damaged the tether material.

Unreacted fuel expelled from the outlet of fuel cells should be recirculated back to the inlet if wastage of the fuel must be minimized. Conventional recirculation systems employ mechanical pumps that consume electrical power and generate vibration and noise. This paper derives the pump-less recirculation system based on the analogy between electrical and fluid circuits. Concept of a switched capacitor circuit which is commonly used in electrical circuits was applied for recirculation in the fluid circuit. Operating characteristics and long-term stability of the proposed fuel cell system were experimentally demonstrated.

SELENE (Selenological and Engineering Explorer) is a Japan's lunar probe which was launched and injected into the lunar polar orbit in 2007. The Main Orbiter of SELENE named Kaguya has separated the Relay Satellite: Rstar (Okina). We have executed four-way Doppler measurements which determined the orbit of Kaguya aviating above the lunar far side. The ground station up-links ranging signals, and the relay satellite transponder on Rstar (RSAT-1) relays the carrier waves to Kaguya. Then the transponder on Kaguya (RSAT-2) receives the signals and returns to Rstar, and down-linked to UDSC. Thus, our system has achieved the first case to track two fully moving links between the lunar orbiters and carried out Doppler measurements.

The Japanese lunar explorer SELENE (KAGUYA), which has been launched on Sep. 14th, 2007, utilizes VLBI observations in lunar gravimetry investigations. This can particularly improve the accuracy of the low degree gravitational harmonics. Combination of ground based VLBI observations and Doppler measurements of the spacecrafts enable three dimensional orbit determinations and it can improve the knowledge of the gravity field near the limb. Differential VLBI Radio sources called VRAD experiment involves two on-board sub-satellites, Rstar (Okina) and Vstar (Ouna). These will be observed using differential VLBI to measure the trajectories of the satellites with the Japanese network named VERA (VLBI Exploration of Radio Astrometry) and an international VLBI network.<BR>Two new techniques, a multi-frequency VLBI method and the same-beam VLBI method, are used to precisely measure the angular distance between the two sub-satellite radio sources Okina and Ouna. The observations are at three frequencies in S-band, 2212, 2218 and 2287 MHz, and one in X-band, 8456 MHz. We have succeeded in making VLBI observations of Okina/Ouna with VERA and the international network, and have also succeeded in correlating of signals from Okina/Ouna, and obtained phase delays with an accuracy of several pico-seconds in S-band.

This paper describes the on-orbit results and lessons learned of the small scientific satellite "INDEX" (REIMEI) for aurora observation and demonstration of advanced satellite technologies. REIMEI is a small satellite with 72kg mass, and is provided with three-axis attitude controlled capabilities for aurora observation. REIMEI was launched into a nearly sun synchronous polar orbit on Aug. 23^<rd>, 2005 (UT) from Baikonur, Kazakhstan by Dnepr rocket. REIMEI satellite functions satisfactorily on the orbit. Three axis control is achieved with accuracy of 0.05 deg. Multi-spectrum images of aurora are taken with 8Hz rate and 2 km spatial resolution to investigate the aurora physics. REIMEI is a small scientific satellite for aurora observation and advanced satellite technologies, and was launched into a nearly sun synchronous polar orbit on Aug. 23^<rd>, 2005 (UTC) from Baikonur, Kazakhstan by Dnepr rocket. REIMEI satellite functions satisfactorily on the orbit. The three-axis attitude control is achieved with accuracy of 0.05deg. REIMEI is performing the simultaneous observation of aurora images as well as particle measurements. REIMEI indicates that even a small satellite launched as a piggy-back can successfully perform the unique scientific mission purposes.

The current-voltage characteristics of the array electrodes on the dielectric material in laboratory plasma have been studied in association with the interaction of high-voltage solar array with the ambient space plasma. It has been found that the charging effect and secondary electrons of the dielectric material surrounding the electrodes play an important role in collection of the electrode current. The charging effect suppresses the electrode current, while the secondary electrons enhance the electron current to the electrode. In the electrode array (3 × 3 electrodes separated by 0.5~10 cm) , the current of each electrode was generally smaller than that of single electrode, but a rapid enhancement of the electrode current was observed when spacing of the electrodes was less than 1.5 cm. These results indicate a possibility that we can reduce the risk of electric discharge if we use the dielectric material with a low secondary-emission yield and select the distribution of the solar array voltage minimizing the high voltage-plasma interaction.

We are studying the problems associated with high voltage power systems in space. Especially we are interested in the potential distribution of the solar array that is resistant to the electrical discharge. We have carried out experiment on the interaction between the space plasma and the high voltage solar array. An array of electrodes distributed on a dielectric material was used to simulate the inter-connectors of the solar array panel in space environment. One of major concerns in the usage of the high voltage solar array in space is the arc discharge on the array. Based on the plasma sheath theories, there is a possibility to control or to prevent the discharge by selecting a potential distribution of the electrode array. As the first step to find the potential distribution that is tolerate to the discharge, we measured the current to the electrodes, changing the spacing of the electrodes. This paper presents the experimental results suggesting that we can control the discharge by selecting a proper potential distribution of the high voltage solar array.

This paper describes the initial on-orbit results of the small scientific satellite "INDEX" (REIMEI) for aurora observation and demonstration of advanced satellite technologies. REIMEI is a small satellite with 72kg mass, and is provided with three-axis attitude control capabilities for aurora observation. REIMEI was launched into a nearly sun synchronous polar orbit on Aug. 23^<rd>, 2005 (UT) from Baikonur, Kazakstan by Dnepr rocket. REIMEI satellite functions work satisfactorily in the orbit. The first imaging observations of aurora were successfully performed above the southern polar region in Sep. 16^<th>, 2005.

We are studying the problems associated with high voltage power systems in space. Especially we are interested in the potential distribution of the solar array that is resistant to the electrical discharge. We have carried out experiments on the interaction between the high voltage solar array and the ambient plasma. In the experiment, an array of electrodes distributed on the insulation panel was used to simulate the inter-connectors of the solar array. An electrode array without the insulator panel was also used for comparison to study the effect of the insulator. One of major concerns in the usage of the high voltage solar array in space is the arc discharge on the array. Based on the plasma sheath theories, there is a possibility to control or to prevent the discharge by selecting a potential distribution of the electrode array. As the first step to find the potential distribution that is tolerate to the discharge, we measured the distribution of space potential surrounding an array of electrodes and measured the current to the electrodes. This paper presents the experimental results suggesting that we can control the discharge by selecting a proper potential distribution of the high voltage solar array.

A laboratory experiment on the interaction between the high voltage solar array and the ambient plasma has been conducted.In the experiment, an array of electrodes distributed on the honeycomb structure CFRP panel was used to simulate the inter-connectors of the solar array. One of major concerns in the usage of the high voltage solar array in space is the arc discharge on the array. Based on the plasma sheath theories, there is a possibility to control or to prevent the discharge by selecting a potential distribution of the electrode array. As the first step to find the potential distribution which tolerates the discharge, we measured the distribution of space potential surrounding an array of electrodes and measured the current to the electrodes. This paper presents the preliminary results of the experiment suggesting that we can control the discharge by selecting a proper potential distribution of the high voltage solar array.

In this report, fabrication and results of layered active integrated antenna array for application to a spacetenna of a solar power satellite are shown. First, a design method of the Push-Pull amplifier with two output ports was demonstrated, and the fabrication was performed. As a result, the output with 180-degree phase difference was obtained from each output port. Moreover, the two-element layered active integrated amplifier antenna array by directly connecting the output of the Push-Pull amplifier with a patch antenna was fabricated. From this, fundamental data of the active integrated antenna for the spacetenna were obtained.

In this paper, prototypes of a retorodirective active integrated antenna are reported for microwave power transmission in the Space Solar Power Satellite (SSPS) system. As one of the important function required spacetenna, the retrodirectivity was examined. Using a heterodyne mixing method, the authors proposed the concept of retrodivective active integrated antenna with the parallel feed back oscillator and the patch antenna. Using the two types of power divider, the retrodivective active integrated antenna for the response signal was made and its fundamental principle was confirmed. Further, by making the prototype of the retorodirective array for the question and response signals, the circuit configuration and measurement results of the simple retrodirective active integrated antenna are described.

We propose space experiments in order to verify significant technologies and to promote public awareness of the Solar Power Satellite(SPS). Some technical and engineering problems were still remained toward SPS. The main missions we proposed is wireless power transmission(WPT)experiment from Low Earth Orbit(LEO)to the ground using one satellite that can be launched by only one large launch vehicle such as the HII-A. The WPT technologies for SPS, which are the direct detection of rectenna sites, and the microwave beam control skill with high angular precision, will be verified and established. The large phased array antenna system that radiates the microwave with a power of around 400 kW was considered.

In conceptual study of SPS2000, which was conducted by ISAS Solar Power Satellite (SPS) working group in Japan, technical and engineering problems were identified. Especially, microwave power transmission (WPT) and high power operation of solar arrays are key features that are required space experiments for SPS. In order to verify these technologies and to promote public awareness of the SPS, we propose a demonstration of WPT experiment from Low Earth Orbit (LEO) to the ground and a high power operation in the space environment using one satellite that can be launched by only one large launch vehicle such as the H-IIA or Ariane V. Solar arrays, which are consisted of thin flexible solar cells, on the satellite generate more than several ten kW. The life span and the influence of space debris in LEO concerning the solar arrays are tested. The microwave generator and transmitter are installed on the satellite. The frequency of the microwave is 5.8 GHz in order to make the transmitter small. The microwave power of more than 100 kW is radiated from the phased array antenna system. Propagation characteristics of the microwave in the ionosphere and the atmosphere are examined by receiving the transmitted microwave.

We propose space experiments in order to verify significant technologies and to promote public awareness of the Solar Power Satellite(SPS). Some technical and engineering problems were still remained toward SPS. The main missions we proposed is wireless power transmission(WPT) experiment from Low Earth Orbit(LEO) to the ground using one satellite that can be launched by only one large launch vehicle such as the HII-A. The WPT technologies for SPS, which are the direct detection of rectenna sites, and the microwave beam control skill with high angular precision, will be verified and established. The large phased array antenna system that radiates the microwave with a power of around 400 kW was considered.

We numerically analyze a linear dispersion relation for quasi-TE modes propagating in a sinusoidally corrugated metal wall slow wave structure. Numerical result for quasi-TE_<11> mode is compared with that for the conventional TM_<01> mode. Experiments on a high power backward wave oscillator with an intense electron beam are carried out based on the numerical results near 2π-mode condition. The microwave output power, frequency and efficiency are measrued. The TE_<11> mode in addition to TM_<01> mode microwave radiation is observed far from π-mode region of operation. It is confirmed that oscillation property of quasi-TE_<11> and TM_<01> modes agrees fairly well with numerically predicted values. The study of quasi-TE mode oscillation is required for an optimum design of high power backward wave oscillators which work in a single frequency ofer a wide range of operating conditions.