Technologies in the far infrared region are developing rapidly and utilize in many aspects of human life. The far infrared wavelength region is defined from 25 to 1000µm based on the spectrum of molecular rotating in this issue. The radiation sources are many types of molecular laser, free electron laser, and gyrotron with high power. The detection techniques include an antenna receiving, a hot-electron- and a sophisticated quantum- detector. This special issue compiles modern technologies with excellent applications in the far infrared region.

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A powerful short-wavelength far-infrared (FIR) laser from 40µm to 100 µm in wavelength is required for the optical source of diagnostics of high density and large volume plasmas, and a production of γ-ray by inverse Compton scattering. In order to design the optical system, we have measured the optical constants (refractive index and absorption coefficient) of crystal quartz, CVD-diamond and silicon for the short-wavelength FIR lasers.

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Slot antenna coupled thin-film warm carrier devices were fabricated, and the detection properties were investigated at 94 and 700 GHz. Two-dimensional 8 × 3 slot antenna arrays fed by a coplanar waveguide were used as the antennas. The thin-film antennas were fabricated with a photolithographic method, and the warm carrier detectors, which have the contact area of 8 × 10^-10 cm², were fabricated with an electron beam lithographic method. The experimental results indicate that the fabricated devices receive the far-infrared radiation with antenna and rectified it with non-linear I-V characteristic. By changing the antenna from the single slot antenna to the two-dimensional 8 × 3 slot antenna array, the detected sensitivity improved by 11 dB.

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A prototype of a compact Free Electron Maser (FEM) has been designed for the operation in a usual small laboratory which does not have electric source capacity available enough. The electron energy is 60-120 keV. As it is lower, stronger guiding magnetic field is necessary in addition to a wiggler field. To fulfil this condition a solenoid-induced helical wiggler is applied from the viewpoint of saving the electric power of the restricted source capacity. The wiggler, for example, with the period of 12mm creates the field of 92G in the guiding field of 3.2kG. The whole system of FEM has been just constructed in a small scale laboratory. It is so small to occupy the floor area of 0.7×2.9 m².

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We have successfully developed low-noise, low-power cryogenic readout electronics (CRE) for sensitive far-infrared detectors operated at low temperatures. The CRE must be mounted besides of the detector, and thus, it must be operated at cryogenic temperatures. The reasons of that are to avoid electrical interferences to the high-impedance portion between the detector itself and the CRE, and to minimize the stray capacitance that may decrease the read-out gain. The goals of the CRE performance are the operation temperature can be down to 2K, the noise level is 2µV/√Hz at 1Hz, the power consumption is 10µW/channel, and the open-loop gain of differential amplifier is over 1000. We have so far manufactured the CRE four times, and evaluated the performances at 4.2K. The present performance achieved is nearly acceptable for the far-infrared sensor of the next Japanese infrared astronomical satellite, ASTRO-F.

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High frequency, high power, high harmonic gyrotrons which cover a wide frequency range in millimeter and submillimeter wavelength region are being developed as far-infrared radiation sources and applied for new far-infrared technologies, for example, plasma diagnostics, electron spin resonance (ESR) spectroscopy and so on. The present status of the development and applications of the high frequency gyrotrons are summarized.

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A large orbit gyrotron (LOG) operating at higher harmonics of electron cyclotron frequency has been constructed by use of a permanent magnet system whose intensity is around 1 T. The operation test was already performed successfully up to the fourth harmonic. The operation efficiency was extremely high even at the third and the fourth harmonics, because of high efficient interaction between beam electron and high frequency electric field in LOG. The test results of the LOG are summarized.

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Transmission properties of 2-dimensional dielectric and metallic photonic crystals with trigonal hole arrays have been investigated in the terahertz region. The transmission spectra are quite different between the dielectric and metallic photonic crystals, i. e. the characteristics of the former is explained by folding of the wave-guide mode of the slab into the first-Brillouin zone by the periodic structure and that of the latter is explained by the transmission through the isolated metallic wave guides and by the Wood’s anomaly.

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Metal foil eddy currents induced with flat (stacked spiral) induction coils are calculated numerically using an almost exact approach. Edge bake phenomena of metal foils are investigated to develop a new all-over heat adhesion method. As our aim, this approach is constructed with basic formulas for theoretically analyzing the induction heating phenomena. Fundamental equations in electromagnetic theory are used and a calculation method of self-inductance is developed using the Neumann’s formula. These results can be applied to certification of various computer simulations by software on the market concerned with the eddy current.

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For a needle-plane electrode system with a barrier, which establishes the electric field across the axis of a groove, creeping discharge characteristics in N₂ gas under µs pulse voltage applications have been investigated. The distance h between the barrier surface and the needle tip as well as the distance M between the groove center and the needle tip were changed. In the case of h=0.3mm, when the needle tip is located near the far-side groove edge from the plane electrode (M=0.6mm), the flashover voltage has the maximum value. At that time, a growth of a corona is suppressed near the groove edge. These unique characteristics should associate with a field relaxation.

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Variation of the electromagnetic (EM) field distribution in the anechoic box is calculated by finite difference time domain (FDTD) method. It is confirmed that quantitative relation between the performance of the wave absorber and the variation of EM field distribution under the condition that two types of the absorbers are installed with the wall of the box. Moreover, the effectiveness of the measurement of the anechoic box by using FDTD method is confirmed because the tendency of the analytical result of the magnetic field distribution is similar to its experimental result.

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This paper proposes the quantitative estimation method of negative polarity sparkover voltage and time (V-t) characteristics based on the optical observation of the discharge development process. The optical observation result was obtained in SF₆ at 0.1MPa under quasi-uniform field gap (g=80mm) with steep-front square impulse and fast-oscillating impulse voltages. We modeled the discharge development processes with streamer development, leader conversion and leader development and evaluated parameters by the electric field respectively. The streamer development process is different between non-oscillating impulse (square impulse) and fast-oscillating impulse based on optical observation result, we took into consideration the influence of voltage oscillation. As a result, it was obtained that calculation methods of the V-t curve in SF₆ are proposed and their applicability to the evaluation of actual V-t characteristics is verified. These methods can be used to predict V-t curves for other voltage waveforms having an arbitrary rise time.

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In order to improve the NOx removal efficiency by ozone injection method, three kinds of experiment, namely oxygen injection, HC addition and direct discharge were carried out. It was found that oxygen injection can remove more NO than the case without oxygen injection and hydrocarbon addition to ozonizer deteriorated the NO removal efficiency. It was also found that air injection into exhaust gas improves the energy efficiency even in direct discharge method.

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An automatic NOx control device was developed for NOx removal system by silent discharge targeting diesel engine generator. A new algorithm of controlling the exit NO concentration at specified values was developed. The control system was actually made in our laboratory and it was confirmed that exit NO concentration could be controlled in the specified value.

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In order to understand dc breakdown characteristics of a polar polymer at the room temperature, a space charge study on ethylene-vinyl acetate copolymer (EVA) film was carried out using the pulsed electroacoustic method. Both fast negative carrier and slow positive carrier were injected. The positive carrier became dominant several tens of seconds after the voltage application, and increased the anode field. The breakdown did not take place when the electrode field reached the maximum value, but took place after it had fallen down due to the injection of the counter charge. It is concluded that the generation of injected and/or dissociated carriers increase the conduction current, and would bring the final breakdown. The temperature distribution of film surface until the electric breakdown was also measured by thermograph. Partial heating was observed before the electric breakdown. This suggested thermal electric breakdown in EVA film at room temperature region.

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Charge accumulation on thick solid dielectrics may seriously influence surface discharge and insulation characteristics in SF₆, oil, or vacuum, which are the leading insulators for the recent high voltage equipment. Nevertheless, charge measurement technique in such environment has not been fully developed in practical level. One of the authors once proposed a probe method based on multipoint measurement. The procedure is as follows, (i) represent the relationship between charge distribution σ and the multipoint probe-outputs W_p as Aσ=W_p, (ii) calculate the matrix A by a numerical field calculation faithfully simulating the experimental arrangement, and (iii) estimate σ by an inverse calculation of (i) from the measured outputs. In recent papers, we have reported the improved accuracy and stability of (ii) and (iii), respectively, through numerical simulation studies. In this paper, we experimentally measure accumulated charges on a model insulator in order to confirm our numerical improvements. The effects of the following items are investigated, (a) difference of calculation methods of the matrix A, (b) distance between the probe and the insulator surface, (c) length of the guard electrode, and (d) thickness of the insulator. In all cases, we have successfully estimated the charge distribution in practical accuracy.

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The shielding effect (SE) of a magnetically shielded walk-in room (MSR) consisting of one or more layers of Permalloy for biomagnetism, testing of the spacecrafts and calibration of magnetometers, is often estimated using the Mager and Kalha (MK) formula. This was derived ideally, assuming closed continuous shells and did not take account of the existence of practical necessities such as holes, gaps and doors etc. and usually based on the assumption that each layer of Permalloy has a constant scalar permeability (µ_s) of about 30, 000. In this paper, 3-D static magnetic field analyses of the SE of cube type MSR’s composed of two layers and multiple layers and having a door, are described. The analyses are carried out by the finite element method and take account of the nonlinear magnetization property of Permalloy which was measured with using a ring sample. The following results are obtained. Firstly, the method of analysis is verified by comparing computed and measured results of the spatial magnetic field in MSR. Secondly, the relationship between the SE and the spacing between each layer, calculated by the 3-D magnetic field analysis, roughly agrees with that obtained by the MK formula, assuming µ_s of the two layers is a constant scalar. The µ_s of outer layer is higher than that of inner layer. Thirdly, in the case of the MSR of three and four layers, the calculated SE is smaller than that obtained using the MK formula. As µ_s depends on the amplitude of the magnetic flux density B, the µ_s of the outer layer is about 100, 000, which is much greater than 30, 000, but the µ_s of the inner layers is less than 60, 000. This indicates that the MSR of three and four layers actually consists of one layer of very high µ_s and the rest very low. This suggests that the discrepancy between the actual SE and the pre

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Polymer, common insulating material, can be used with the metal layer. In order to investigate the reliability of its insulation performance, it is important to measure space charge by the pulsed electro-acoustic (PEA) method. However, it is difficult to measure space charge correctly because of the acoustic characteristic difference between the polymer and the metal layer. Therefore, we have proposed the numerical analysis model on PEA method for generation and propagation of acoustic wave in order to discuss the error factors in space charge measurement. In this paper, the results from the numerical analysis on the metal-polymer laminate material have proved that the metal material could have some influences on the results of space charge measurement.

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