Ken-ichiro MAKI

Toward the orbital demonstration of a solar power satellite (SPS), a breadboard model of the SPS has been developed for preliminary experiments including wireless power transmission. The model forms a thin panel structure with a thickness of 6 cm, and the panel consists of three layers being functionally assigned to a transmission antenna, microwave amplification and control, and thermal radiation, respectively. Microwave amplified up to 160 W is emitted from a large-scale phased array antenna with 256 microstrip elements. In order to evaluate the effect of multiply arranged antennas to the emission, the measurement of the radiation pattern is conducted for different relative positions of the antenna panels. Additionally, the steering of the transmission beam is achieved as an initial demonstration for retrodirective control. © 2012 IEEE.

We are developing a bread board model for a microwave wireless power transmission (WPT) from a satellite in orbit to the ground. WPT using microwaves is one of the critical and important technologies toward a solar power satellite (SPS). We have carried out a conceptual design and proposed a space experiment using a scientific small satellite which has been developing by JAXA. The purposes of the space experiments are to demonstrate a precise directional control of WPT technology for SPS, which includes the direct detection of rectenna sites, and the microwave beam control skill with high angular precision, and to clarify the propagation characteristics of the microwave power in the ionosphere. Performances of the breadboard were measured and evaluated. Thermal transient characteristics of the breadboard model were simulated. © 2012 IEEE.

Terahertz (THz) beam steering is demonstrated based on the phased-array antenna principle without using actual phase shifters. A periodically-poled lithium niobate crystal is pumped with infrared radiation generated from a dual-wavelength optical parametric oscillator. The THz radiation is generated from the crystal via difference-frequency mixing and the emission angle could be varied by tilting one of the incident pump beams. This effect is equivalent to using a phase-shifter array for tilting the phase front of the THz radiation. This technique has the advantage of high speed, wide angular range, and compactness compared to conventional beam steering methods. (C) 2009 The Japan Society of Applied Physics

Terahertz (THz) beam steering is demonstrated based on the phased-array antenna principle without using actual phase shifters. A periodically-poled lithium niobate crystal is pumped with infrared radiation generated from a dual-wavelength optical parametric oscillator. The THz radiation is generated from the crystal via difference-frequency mixing and the emission angle could be varied by tilting one of the incident pump beams. This effect is equivalent to using a phase-shifter array for tilting the phase front of the THz radiation. This technique has the advantage of high speed, wide angular range, and compactness compared to conventional beam steering methods. (C) 2009 The Japan Society of Applied Physics

We report on terahertz generation by Cherenkov-type optical rectification in lithium niobate using an actively controlled femtosecond pumped enhancement cavity. In this way a much higher pump power is available inside the cavity and an increased terahertz output power is obtained. The advantages of terahertz generation in the Cherenkov geometry are verified by comparing it with other types of emitters by means of electro-optical detection as well as by bolometer measurements. (C) 2008 American Institute of Physics.

We demonstrate a terahertz (THz) beam steering method using difference frequency generation that is based on the principle of phased array antennas. A strip-line photoconductive antenna was illuminated by two spatially dispersed beams produced from an ultrafast laser. THz radiation with a bandwidth of 65 GHz was generated from the overlapping area of the two beams, between which the frequency difference was approximately constant. We confirmed that the THz beam can be steered by tilting one of the incident pump beams so as to change their relative phase relation. The steering range of the THz beam was 29 degrees when the angle between the incident pump beams was only varied within a range of 0.155 degrees, that is, 187 times less. In addition, by laterally shifting one of the pump beams, the frequency of the THz radiation could be tuned from 0.3 to 1.7 THz. This technique can be applied to high-speed terahertz imaging and spectroscopy systems. (C) 2008 Optical Society of America.

We report on terahertz generation by Cherenkov-type optical rectification in lithium niobate using an actively controlled femtosecond pumped enhancement cavity. In this way a much higher pump power is available inside the cavity and an increased terahertz output power is obtained. The advantages of terahertz generation in the Cherenkov geometry are verified by comparing it with other types of emitters by means of electro-optical detection as well as by bolometer measurements. (C) 2008 American Institute of Physics.

We demonstrate a terahertz (THz) beam steering method using difference frequency generation that is based on the principle of phased array antennas. A strip-line photoconductive antenna was illuminated by two spatially dispersed beams produced from an ultrafast laser. THz radiation with a bandwidth of 65 GHz was generated from the overlapping area of the two beams, between which the frequency difference was approximately constant. We confirmed that the THz beam can be steered by tilting one of the incident pump beams so as to change their relative phase relation. The steering range of the THz beam was 29 degrees when the angle between the incident pump beams was only varied within a range of 0.155 degrees, that is, 187 times less. In addition, by laterally shifting one of the pump beams, the frequency of the THz radiation could be tuned from 0.3 to 1.7 THz. This technique can be applied to high-speed terahertz imaging and spectroscopy systems. (C) 2008 Optical Society of America.

Microwave emissions due to hypervelocity impacts have been detected by heterodyne receivers in the ground experiments using an accelerator. We aim to establish the detection system of space debris impacts on a space structure via microwave. The emitted powers at several frequencies due to the impact velocity of 10km/sec are estimated from the characteristics at the experimental impact velocity. Considering the emitted power, the receiving antennas and the microwave frequencies, the maximum distances for detecting the microwave emission are 85m at 2GHz-band and 24m at 22GHz-band. The type and location of receiving antennas which can detect the impact on all habitation and experiment modules on the International Space Station are discussed. It is concluded that the impact can be sufficiently detected via a low gain antenna at 2GHz band.

It was formerly confirmed by experiment that hypervelocity impacts on aluminum plates cause microwave emission. In this study, we have carried out experiments in order to clarify the mechanism of the emission. The microwave is detected by heterodyne detection scheme at 22 and 2 GHz with an intermediate frequency bandwidth of 500 and 120 MHz, respectively. A nylon projectile is accelerated using a light-gas gun to impact a target. First, aluminum plates with ten different thicknesses ranging from 1 to 40 mm were used as a target, and microwave signals were detected. The experimental results are statistically analyzed assuming a Gaussian distribution of the emitted power. The standard deviation of pulse voltage is calculated to show the existence of two kinds of signals: sharp pulse and thermal noise. It is shown that the emitted energy and the dispersion have a relation with the extent of the target destruction. Next, nylon projectiles are impacted on different metals such as aluminum, iron, and copper. These results suggest that microcracks are essential to microwave emission. Finally, in order to clarify the mechanism of charging and discharging across the microcracks, the experimental results are compared with this model for the following factors: (1) the thermally excited electrons and the emitted power, and (2) the bond dissociation energy of target material and emitted power. The analytical results suggest that electrons are excited thermally and by transition from a crystalline state to an atomic state. (c) 2007 American Institute of Physics.

It was formerly confirmed by experiment that hypervelocity impacts on aluminum plates cause microwave emission. In this study, we have carried out experiments in order to clarify the mechanism of the emission. The microwave is detected by heterodyne detection scheme at 22 and 2 GHz with an intermediate frequency bandwidth of 500 and 120 MHz, respectively. A nylon projectile is accelerated using a light-gas gun to impact a target. First, aluminum plates with ten different thicknesses ranging from 1 to 40 mm were used as a target, and microwave signals were detected. The experimental results are statistically analyzed assuming a Gaussian distribution of the emitted power. The standard deviation of pulse voltage is calculated to show the existence of two kinds of signals: sharp pulse and thermal noise. It is shown that the emitted energy and the dispersion have a relation with the extent of the target destruction. Next, nylon projectiles are impacted on different metals such as aluminum, iron, and copper. These results suggest that microcracks are essential to microwave emission. Finally, in order to clarify the mechanism of charging and discharging across the microcracks, the experimental results are compared with this model for the following factors: (1) the thermally excited electrons and the emitted power, and (2) the bond dissociation energy of target material and emitted power. The analytical results suggest that electrons are excited thermally and by transition from a crystalline state to an atomic state. (c) 2007 American Institute of Physics.

We report on the generation of broadband terahertz (THz) pulses using Cherenkov-type generation in magnesium oxide-doped lithium niobate (MgO:LN). The efficiency of the output coupling process of THz radiation at higher frequencies into free space is considerably increased by the use of a properly cut silicon prism. The achieved spectrum is broader compared to the normal Cherenkov-cut geometry. Due to a considerably reduced propagation length in the absorbing MgO:LN, the effective application of longer crystals is possible. Thus, the measured spectral intensity is much higher and the spectrum broader.

We report on the generation of broadband terahertz (THz) pulses using Cherenkov-type generation in magnesium oxide-doped lithium niobate (MgO:LN). The efficiency of the output coupling process of THz radiation at higher frequencies into free space is considerably increased by the use of a properly cut silicon prism. The achieved spectrum is broader compared to the normal Cherenkov-cut geometry. Due to a considerably reduced propagation length in the absorbing MgO:LN, the effective application of longer crystals is possible. Thus, the measured spectral intensity is much higher and the spectrum broader.

Microwave emission was found when materials were destroyed by a hypervelocity impact or by a static pressure. This paper describes the experimental setup to observe the phenomena, the obtained terms and results, and the possibility to apply the phenomena to geophysical explorations. As this field may not be familiar to most readers and the special techniques to receive and measure impulsive microwaves are required, the overall understanding is pursued instead of detailed description. In the receiving system, microwave signal is first amplified by a low noise amplifier, digitized in a sampling frequency high enough for the observed frequency, and then stored as data. The observed frequency was 22GHz, 2GHz, 300MHz and 1MHz. If the data storage capacity is too small to keep the data, namely at 22GHz and 2GHz, the signal is converted to a lower frequency by a heterodyne receiver and then processed to data.<br> In the impact experiment, the velocity is 7 km/sec at maximum. Target material was selected from metal such as aluminum or iron, ceramic, brick or rubber. In the destruction experiment due to a static pressure, four kinds of rocks were pressed with a compressor. The observed microwave is intermittent quite narrow pulses in every destruction mode. In the rock destruction due to a static pressure, 22 GHz was detected only from quartzite. As the waveforms thus obtained are almost sinusoidal in shape, we can calibrate the power through the receiving system. As a result, the average emitted power at 2 GHz was 2.7x10^-5mW and 2.7x10^-8mW in the hypervelocity and static pressure experiments, respectively. The cause of microwave emission is inferred to be the dissociation of atoms or molecules, but is not yet completely confirmed. Currently, the phenomena are expected to be applied to geophysical explorations in the following fields:<br> (1) Research of material characteristics: celestial body impacts, material science, space debris issues.<br> (2) Change of the underground structure: rock crush.<br> (3) Earthquake detection.<br>

Electromagnetic emissions observed in a series of rock fracture tests are described. Four kinds of rocks, basalt, gabbro, granite and quartzite were pressed by uniaxial compression to fracture, for all of which many signals were detected at two microwave bands (2GHz and 300MHz). These detected signals consist of intermittent pulses of a short duration. Comparing the microwave records and the observation with a high-speed digital video camera, we found that the pulse signals were generated after the decrease of the axial load, and even after the macroscopic fracture (deformation) was completed. This differs from the occurrence of lower frequency emissions (0.3-300kHz) monitored as well, which became active and was strongest during the load decrease. The occurrence of signals at the two microwave bands did not always coincide, but a signal at 300MHz often followed a signal at 2GHz with a short interval of 50-100ns. An additional detector at 22GHz picked up emissions only for quartzite, which occurred exclusively during the decrease of axial load.

Microwave emissions due to hypervelocity impacts and their dependence on the target material are described. Microwave signals were measured for four kinds of target materials: aluminum, alumina ceramic, red brick, and polyurethane rubber. The signals were composed of two kinds of wave form: intermittent sharp pulses and white noise. The pulse signals were emitted strongly, especially with the aluminum target. The energy emitted from each target was estimated from the signal detected after calibrating the measuring system. The energy of the pulses was greater for conductors than for insulators. We hypothesized that the microwaves were emitted from a discharge along a microcrack in the target. The signals detected in the experiment agreed well with theoretical results. (c) 2005 American Institute of Physics.

Microwave emissions due to hypervelocity impacts and their dependence on the target material are described. Microwave signals were measured for four kinds of target materials: aluminum, alumina ceramic, red brick, and polyurethane rubber. The signals were composed of two kinds of wave form: intermittent sharp pulses and white noise. The pulse signals were emitted strongly, especially with the aluminum target. The energy emitted from each target was estimated from the signal detected after calibrating the measuring system. The energy of the pulses was greater for conductors than for insulators. We hypothesized that the microwaves were emitted from a discharge along a microcrack in the target. The signals detected in the experiment agreed well with theoretical results. (c) 2005 American Institute of Physics.

A hypervelocity impact causes the emission of radio-waves in the microwave frequency range. In order to understand the features of the phenomena and eventually to clarify the mechanism of the radio-wave generation, two kinds of experiments were carried out. The first one is the simultaneous observation of the phenomena by the micro-wave detection and the optical imaging method. The second one is to investigate the microwave emission in relation to the speed of a projectile. This paper describes the experimental results, and compares the microwave and optical methods from the viewpoints of impact detection. (C) 2004 COSPAR. Published by Elsevier Ltd. All rights reserved.

A hypervelocity impact causes the emission of radio-waves in the microwave frequency range. In order to understand the features of the phenomena and eventually to clarify the mechanism of the radio-wave generation, two kinds of experiments were carried out. The first one is the simultaneous observation of the phenomena by the micro-wave detection and the optical imaging method. The second one is to investigate the microwave emission in relation to the speed of a projectile. This paper describes the experimental results, and compares the microwave and optical methods from the viewpoints of impact detection. (C) 2004 COSPAR. Published by Elsevier Ltd. All rights reserved.

Microwave emission due to hypervelocity impacts on metallic plates has been found. The targets used in the experiment are aluminum plates with various thicknesses. The projectile, a nylon cylinder with metal screw of 0.21 gm, was accelerated up to the velocity of 4 km/s; a heterodyne receiver detected the microwave at 22 GHz. The emission is a random sequence of pulses with several nanosecond width, which lasts more than 10 mus. The phenomenon seems to be dependent on the extent of target destruction through the formation of impact craters or penetration. If so, we could use the characteristics of the phenomena to better understand the mechanical destruction process. We propose several models for the cause of this microwave generation and study them on the basis of timing relation of observed events. (C) 2002 American Institute of Physics.

Microwave emission due to hypervelocity impacts on metallic plates has been found. The targets used in the experiment are aluminum plates with various thicknesses. The projectile, a nylon cylinder with metal screw of 0.21 gm, was accelerated up to the velocity of 4 km/s; a heterodyne receiver detected the microwave at 22 GHz. The emission is a random sequence of pulses with several nanosecond width, which lasts more than 10 mus. The phenomenon seems to be dependent on the extent of target destruction through the formation of impact craters or penetration. If so, we could use the characteristics of the phenomena to better understand the mechanical destruction process. We propose several models for the cause of this microwave generation and study them on the basis of timing relation of observed events. (C) 2002 American Institute of Physics.