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Iron (Fe) nano-polycrystalline metal obtained by sintering reduced iron nanoparticles from iron oxide was applied to an axial flux generator. When the thickness of the core material is 1/10, it has the same ability of the power generation as when Fe bulk is used as the core. We can also reduce the iron loss (hysteresis loss and eddy current loss), which enables the construction of thin and axial flux generators with light weight. We investigated the magnetization property of the core inductor at the direct current and found that Fe nanoparticles were produced from iron oxide particles by high voltage pulse or laser ablation in liquids. Core inductors with these materials were fabricated. It has been clarified from measurements of the core inductor magnetization that the relative permeability of the sintered Fe nanopolycrystalline was one million.

We adopted a single-mode, single-wavelength volume holographic grating (VHG) wavelength-stabilized wavelength laser diode (LD) as a pumping LD for an end-pumped microchip Nd:YAG and Nd:YVO4 lasers we developed during CW and pulse operations. Higher optical-optical and slope efficiencies during CW operation have been obtained than when using a VHG LD experimentally. Output laser power is insensitive to the temperature of the LD when using a wavelength-stabilized LD and can remain stable and almost constant until the temperature of LD increases up to 40°C. The improved optical-optical conversion efficiency of 58% for the Nd:YVO4 laser has been obtained and calculated the output laser power during CW operation and compared it with the experimental results. We found that the output laser power of the Nd:YVO4 laser using the VHG wavelength-stabilized LD was more than twice as high as that using an LD without VHG. When the ambient temperature increases, the difference in output laser power should be large. In the future, a low-cost end-pumped microchip laser that does not require a temperature control should be developed.

Metal nanoparticles have become attractive as original materials for nano-inks and nano-pastes, which are used in printed electronics. Synthesizing various metal nanoparticles has been researched. We investigated the possibility of reducing metal oxide in a metal by using high-voltage pulses in this paper. This method should save electrical consumption power compared with conventional methods that use high-temperature and high-pressure plasma such as arc discharge. Reduced Al nanoparticles were obtained by applying high-voltage pulses to solutions. By analyzing elements and the composition of reduced Al nanoparticles by STEM and EDX, a large amount of reduced Al nanoparticles with diameters of a few 100 nm and thin oxide film of around 1 nm on metal surfaces were produced in experiments for reducing Al oxide. It was found from hydrogen generation using reduced Al nanoparticles by applying high-voltage pulses to solutions to evaluate reduction rate that a high reduction efficiency of 97% was obtained at maximum. We concluded that using high-voltage pulses for reduction can be suitably applied to printed electronics because the oxide film on reduced Al nanoparticles is thin. Also, because this method is similar to laser ablation in liquids using pulse lasers, the similarities were discussed in this paper.

An output peak power of over 1 kW for a Nd/Cr3+:YAG ceramic multi-amplifier system with white-light pumping was obtained. A four-stage active-mirror amplifier system was adapted for obtaining kW laser peak power. Output spectrum of pumping light used was close to that of solar light. Enhancement of saturated output laser power at high temperatures was observed owing to the phonon-assisted cross-relaxation effect. An output laser peak power of 1.6 kW were achieved at a ceramic disk temperature of 100 °C. The evaluated optical-optical conversion efficiency for volume of ceramics through which laser passed was 63% for the maximum output laser peak power of 1.6 kW. The laser system will be applicable to solar-pumped lasers with high optical-optical conversion efficiency.

We investigated the optimized sensitizing effect of Ce/Cr/Nd:YAG material for solar-pumped solid-state lasers using the sol–gel synthetic YAG powder method. After a parameter mapping survey on Ce 3+ and Cr 3+ additives in Nd:YAG for the sensitizing effect on sunlight, we found that the most efficient composition of Ce/Cr/Nd:YAG is where Ce 3+ and Cr 3+ were 0.9 and 8.0 mol% respectively, when the Nd 3+ concentration was 1.0 mol% and the sensitizing effect of the optimized Ce/Cr/Nd:YAG was increased four times more than the Cr/Nd:YAG one. This result indicates that Ce 3+ can work as an effective sensitizer to Nd 3+ and optimized Ce/Cr/Nd:YAG ceramics can be useful to construct a space solar power system using a laser in space.

We fabricated core inductors with an aluminum nano-polycrystalline structure, which is a ferromagnetic body, for the first time. Al nano-polycrystalline structure using Al nanoparticles synthesized by pulse laser worked as inductor cores at 5-MHz frequency.

Spectral broadening of the fluorescence of a Nd/Cr:YAG ceramic at the 1064 nm lasing wavelength was observed, and the amplification properties at a high temperature were investigated by considering cross-relaxation. These ceramics are promising for use as a solid-laser material pumped with solar or lamp light. It has been found that whenever the temperature of a laser medium is high and a spectral shift occurs, a high small-signal gain remains owing to the broad spectral band and the cross-relaxation. This optical property is remarkably different from that of a Nd:YAG laser. For a conventional Nd:YAG laser, the bandwidth at 1064 nm is 0.45 nm, and a reduction in small-signal gain occurs at a temperature of 373 K because the spectral peak shift is 0.005 nm/K. However, for the Nd/Cr:YAG ceramic, the bandwidths are 1.2 and 1.9 nm in the case of 0.1% Cr ion doping and 3% Cr ion doping, respectively, owing to the existence of excited Cr ions and the shortening of the effective Nd ion coherence time. It is prospected that the laser medium can be used at a high temperature of 600 K.

Pulse power sources can produce high temperature or high-density extreme conditions within a short time. They have been introduced to various fields such as those in laser, fusion research, the production of plasma, shockwaves in water, water treatment, and exhaust gas treatments. Various high voltage pulse sources using insulated-gate bipolar transistor (IGBT) semiconductors with high voltage resistivity have been used. Also, Marx bank circuit is well known as an instrument that produces voltage pulses with low repetitive rates. These instruments have several advantages. However, their problems are a complex structure, high cost, and excessive weight. A simple method of producing high voltage pulses with short rising times based on electrostatic induction in external capacitor used for pulse power applications is proposed. The circuit has a simple structure and contains a minimum number of parts, which makes the instrument small and light weight. In fact, the generation of sawtoothed high voltage pulses with short rising times and low repetitive rates of a few 100 Hz was successfully conducted in experiments. Theoretical analysis was simultaneously undertaken. The numerically calculated results for generating high voltage pulses were goodly consistent with the experimental ones. Moreover, it has been confirmed that amplification of the output voltage by electro-hydrodynamics (EHD) electricity generation using a jet flame resulted in higher voltage pulses, lower electricity consumption, and high repetition rates.

We examined luminescence properties of highly chromium co-doped Nd:YAG powders fabricated by the sol–gel method. Though the maximum luminescence intensity of Cr:YAG was at 3.0 mol% of Cr3+ concentration, we found that the enhancement effect on Nd:YAG by co-doping Cr3+ was continued until 6.0 mol% of Cr3+ concentration. This phenomenon is well-explained by a simple energy transfer model derived from the rate equations between Cr3+ and Nd3+. The enhancement of sensitization for Nd3+ activator by high co-doped Cr3+ is very effective for making a miniaturized or thin film-type solar laser.

We propose a convenient method for pre-evaluating the spectroscopic properties of luminescent impurity doped YAG by sol–gel synthetic powder. Three synthetic methods, the sol–gel, normal-strike, and reverse-strike methods, are examined for their ability as a predictor and the sol–gel method proved the most effective of the three in experiments on crystallinity, particle size, and spectroscopic properties. The sol–gel Nd:YAG powder agrees well with that of a single crystal or ceramic YAG regarding the luminescent spectrum, the intensity, and the lifetime, even though the powder sintered at a low temperature of 1100 °C in comparison with the YAG crystal melting point of 1970 °C. These facts suggest that the sol–gel synthetic powder is the most effective and the easiest technique for evaluating spectroscopic properties of bulk YAG materials before fabricating the final materials.

Laser pulses with high repetitive rate generated from a Q-switch microchip Nd:YAG oscillator were amplified by Active mirror composed of Nd/Cr:YAG ceramic pumped by CW arc-lamp. The laser pulses were amplified, and saturation of averaged output laser power was observed. Repetitive ratio of injected laser pulses was changed from 20 to 100 kHz. Averaged output laser power and gain were measured, and gain coefficient and power-extraction efficiency were evaluated. Output laser power was calculated and compared to experimentally measured one, and the calculated results are consistent with the experimental ones. For amplification of laser pulses with high repetitive ratio, this laser material can realize very high laser gain with low pumping power density and high optical–optical conversion efficiency under CW-lamp-light pumping.

Laser pulses generated in a laser oscillator were amplified by Nd/Cr:YAG ceramic amplifiers. The saturation of output-laser energy was experimentally observed. We found that the stimulated emission cross-section of Nd/Cr:YAG ceramics was effectively increased and the saturation fluence was effectively reduced to 0.1 J/cm 2 due to the energy transfer between excited Cr and Nd ions. The output-laser energy we calculated was compared with the experimental ones and the extractable stored energy in the Nd/Cr:YAG ceramics was evaluated. This laser material is suitable for high-repetition-rate operations because of the high laser gain with low pumping intensity and low effective saturation fluence.

The concept of an Active mirror with transparent Nd/Cr:YAG ceramics is proposed. A new ceramic material pumped by an arc-metal-halide lamp has been experimentally implemented. CW-laser light generated in a 1064 nm Nd:YAG micro-tip oscillator was amplified by an Active mirror made of Nd/Cr:YAG ceramic and pumped by CW arc-lamp light. The saturation power density of the Nd/Cr:YAG ceramic was lower than that of Nd:YAG because the stimulated-cross-section and lifetime were effectively enhanced by the sensitization of Cr ions. The maximum output laser power while keeping spatial beam quality was up to 3 W with an input lamp power of 9 W; the resulting optical–optical conversion efficiency of 33% was close to the theoretical prediction of nearly 50%.

Remarkable improvements in the lifetime of the Nd upper level and in the effective stimulated emission cross-section of Nd/Cr:YAG ceramics have been theoretically and experimentally studied. Until recently, it had been thought that the long energy transition time from Cr ions to Nd ions of Nd/Cr:YAG adversely affects laser action, degrading optical–optical conversion efficiency under CW and flash lamp pumping. However, current research showed that high-efficiency energy transition has a positive effect on laser action. The effective lifetime is increased from 0.23 to 1.1 ms and the emission cross-section is effectively increased to three times for that of the conventional Nd:YAG. A small signal gain is significantly improved, and the saturation power density is reduced to 1/10 that of the Nd:YAG for the same pumping power density. A CW laser light generated in a laser diode (LD)-pumped 1064 nm Nd:YAG laser oscillator was amplified, and the measured output power was saturated. The output laser power calculated using theoretical saturation power density was consistent with the experimental results.

We observed laser oscillations of rod-type Nd/Cr:YAG ceramics experimentally pumped using an arc-metal-halide lamp having a similar spectrum to solar light. An optical–optical conversion efficiency of 43% was obtained by chromium-ion codoping of Nd:YAG with an efficiency of 26%.

We observed the lasing of highly sensitized Nd/Cr:YAG ceramic disks that use artificial solar radiation pumping. The disk material can be used for high power multi-stage amplifiers pumped by lamplight or sunlight because of its scalability and ability to handle high power densities. A maximum output power of 86 mW was experimentally obtained, and this value correlated well with the calculated results. A small signal gain of 1.9 was obtained, and the estimated stored energy was 0.6 J/cm 3 , which is three or four times higher than that from a Nd:YAG medium.

The multi-pass laser amplifier has been often used as the final power amplification stage in the high peakpower laser systems. The Amplified Spontaneous Emission (ASE) at such a strongly pumped laser generally results in a parasitic oscillation that limits a high gain operation of the amplifier. The three dimensional propagation of the finite-band-width ASE from laser glasses has been calculated for a 30-m long Cassegranian 3-pass disk amplifiers of 35-cm clear aperture. The computational code includes the temporal behavier of flashlamp pumping, the gain reduction in a laser disk due to ASE generation, the long-path back-and-force propagation of ASE between Cassegranian mirrors, and the reduction effect on ASE of spatial filter installed after the amplifier. The ASE energy and its profile have been compared for the on-axis and off-axis arrangement of Cassegranian optics, and also z-pass geometries for three pass configuration. The off-axis alignment of the mirror pairs is very preferable to reduce ASE to one hundredth level than that appeared in the on-axis geometry.

Thermal dependence on the small signal gain of Nd/Cr:YAG (yttrium aluminum garnet) ceramics was observed experimentally. Usually, Nd:YAG crystal and ceramics have remarkable gain reduction when optical pumping is performed and the temperature of the laser media is upped to 373 K. A CW laser light generated in a 1064 nm Nd:YAG laser oscillator was amplified by Nd/Cr:YAG ceramic amplifier, and the output power was measured at non-saturation level. Laser small signal gain of the ceramic disk was kept to 470 K. This property was remarkably different from one of Nd:YAG crystals or ceramics. The peak shift of the fluorescence was observed experimentally when the temperature is high.

We calculated ASE propagation in a Cassegrain-type amplifier. The code used in the calculations is applicable to a multipass amplification system. Output fluence of the ASE at the exit of the system can be decreased by optimization of design parameters. Ratio of diameter of main mirror hole to sub mirror diameter is the most sensitive parameter, and should be larger than 1.2. It is suitable to set gain medium (laser glasses) as near to the main mirror as possible. We found optimal values for a new amplifier chain in GXII laser facility which can deliver ultrashort and high power pulses for the basic research on fast ignition (Fujita et al., CLEO '97, CFK 3, Baltimore MD, 1997; [1]).

The upgrade of the actual large laser facilities (GEKKO XII in our case) implies an increase of filling-factor, but avoiding the generation of spatial high frequencies. To reach that purpose we intend to construct the beams using serrated apertures with an optimised shape. The performances of our system led after simulation to a 70% filling factor with less than 2% fluctuation on the laser pattern intensity map. The optimisation was also checked from a financial point of view. The usual serrated apertures are already giving good results, so we attached ourselves to seek for financial efficiency.

Marked increase in output laser power of a high-power Nd/Cr:YAG ceramic amplifier based on the cross-relaxation effect at high temperature under quasi-solar light pumping was observed. The solar-pumped laser is applicable to renewable energy production.

It has showed in experiments that aluminum nano-polycrystalline substances are ferromagnetic. A magnetic hysteresis curve measured by SQUID suggested the ferromagnetism. Al bulk is normally thought to be non-magnetic body. Authors also fabricated core inductors using Al nano-polycrystalline substances and measured the inductances of cored inductors with frequency dependence. Al nano-polycrystalline substance was fabricated by sintering Al nanopaste with Al nanoparticles. Al nanoparticles were prepared by using laser ablation in liquid. The structures and components of the sintered Al nano-polycrystalline substances were analyzed by SEM and EDX. It has been shown that inductor using Al nano-polycrystalline substance with low volume resistivity, which is the same order of metal, works at low frequencies below 500 Hz, while inductor using substance with high volume resistivity works at high frequency of 5 MHz. Our analysis of Al nano-polycrystalline substance with high volume resistivity revealed relative permittivity of 7 at frequency of 1 MHz. It has been expected that these inductors to work at frequency of a few GHz because the magnetic resonance frequency of the Al nano-polycrystalline substances were evaluated to be 5.6 GHz and the high volume resistivity results in suppressing the eddy currents.

Efficient lasers as energy transfer devices from broadband solar light to coherent laser light have been developed recently. Research on efficient amplification of Nd3+/Cr3+:YAG ceramics based on cross-relaxation pumped by white light was performed.

We report total-reflection active-mirror laser experiments by using a cryogenic Yb:YAG composite ceramic. The composite ceramic has no high reflection coating on the bottom surface, and is cooled with liquid nitrogen directly. We obtained 273 W output power with optical efficiency of 65% and slope efficiency of 72% against the absorbed pump power. The laser power and optical efficiency will be improved more when the pump power increases further. To investigate thermal effects of the laser material in more detail, we have measured the thermal lens focal length and the temperature of Yb:YAG. We observed thermal lensing effect of f ~ 2000 mm, and the maximum temperature of 150 K for 400 μm-thick Yb:YAG sample. We have also studied the theoretical analysis of thermal distribution in the composite ceramic.

We report on a multiple total-reflection active-mirror laser with cryognic Yb:YAG ceramics using zigzag optical path. A 214W output power with 62% slope efficiency has been demonstrated.

With the rapid development of high transparent ceramics, it is possible to achieve high laser output powers. Currently, high average power lasers with high efficiencies and good beam qualities are desired for many applications. To develop such an ideal optical source, it is necessary to avoid thermal effects. Giesen et al. [1] proposed a thin disk laser concept. This concept used a face cooled thin, disk-shaped active medium. This face cooling minimized the temperature gradient and the phase distortions transversal to the direction of the beam propagation, and allowed realization of a laser with high output power (≫ 5.3 kW), high efficiency (≫ 65%), and good beam quality (M2 ≪ 24), simultaneously from a single Yb:YAG disk. The laser was pumped up to about 4 kW/cm2. Alternatively, Tsunekane et al. [2] reported a diode edge pumped microchip laser, and 414 W CW output powers were obtained at incident pump powers of 946 W from a 200-µm thick microchip laser. In their work, the pump density was about 8.8 kW/cm2 and the slope and optical-to-optical efficiency were 52% and 44%, respectively. However, the temperature rise at the high-reflection coating was considerable due to its low thermal conductivity. In fact, the temperature of the opposite surface which was non-cooled was measured to be 228 µC at the input of 946 W. Without the high-reflection coating and the heat sink, the temperature rise can be suppressed, leading to higher operation.

We report on the efficient operation of directly liquid-nitrogen-cooled Yb:YAG active-mirror laser. A CW output power of 273 W with slope efficiency of 72% was demonstrated. High heat removal ability was ensured at 743 W/cm2.

We report on the efficient high-power laser operation by using a cryogenic Yb:YAG composite ceramics with a total-reflection active-mirror arrangement. A 273W CW output power was demonstrated at 65% optical efficiency and 72% slope efficiency.

We observed laser oscillations of rod-type Nd/Cr:YAG ceramics experimentally pumped using an arc-metal-halide lamp having a similar spectrum to solar light. An high optical-optical conversion efficiency of 43% was obtained by Cr ions co-doping.

Electro hydro dynamic (EHD) and electrostatic propulsion devices were developed in the 1920s by Thomas Townsend Brown. One such device, called a “lifter”, has no moving parts and, in the air, operates on electrical energy. It is a fashionable device and has a very simple structure, basically consisting of a narrow wire electrode and a large, flat one. However, it has a low ratio of propulsion force to unit electrical input power. According to theory, the propulsion force it generates depends on the interaction between the ion density of the ionized air and the charges on the surface of the large electrode. EHD and electrostatic propulsion models using the polarization effect are proposed to improve the ratio of the propulsion force to unit electrical input power. The propulsion device generates propulsion force through the use of an asymmetrical metal structure with charges generated by the polarization effect. The propulsion force the new devices generated for the same electric energy was 5.7 times higher than that of a basic type lifter owing to additional propulsion force being generated by the maximum polarization effect in the experiments. It was found that combining other effects with this polarization effect results in the ratio of generated propulsion force to electric power being close to 100N/kW when the electric power is high. This value is as high as that of a helicopter. We also performed numerical analysis was also performed for capacitances and charges for various kinds of EHD and electrostatic propulsion devices. An optimized system was developed and is discussed in this paper.

Propagation and amplification of partial coherent light (PCL) is simulated with a 3-D propagation code and fast Fourier transformation. The PCL is useful to realize highly uniform irradiation on a target in internal confinement fusion. On the other hand, the PCL has speckle structure in space and time owing to partial coherence. The intensity modulation due to speckles is enhanced by self-focusing in laser glasses due to nonlinear refractive index. Calculations on the laser propagation for the PCL show that the speckles of cm scale self-focus to 10 mm scale. As a result, average fluence of the PCL is limited at lower level comparing with that of the coherent laser. Experimental results are also coincident with the simulation results.

Reduced iron, aluminum, copper, and magnesium nanoparticles were produced from iron oxide (Fe3O4), aluminum oxide (Al2O3), copper oxide (CuO), and magnesium oxide (MgO) powders by using laser ablation in liquids, and nanopastes were synthesized with the reduced iron, aluminum, copper, and magnesium nanoparticles. The nanopastes were sintered by using a continuous-wave fiber laser in air atmosphere. The laser-sintered nanopastes consist of polycrystalline metal. The structures of the laser-sintered metal nanopastes were analyzed by SEM and EDX, and their resistivities were evaluated by four-terminal method. The metal nanopastes sintered by hot plate have 2.5- to 11-times-higher resistivities than those of common metals fabricated in blast furnaces. Moreover, the laser-sintered metal nanopastes have 9.5- to 45-times-higher resistivities than those of common metals fabricated in blast furnaces.

The upgrade of the actual large laser facilities (GEKKO XII in our case) implies an increase of filling factor avoiding generation of high spatial-frequencies. To reach that purpose we intend to construct the beam using serrated aperture with optimized shape. The performances of our system lead after simulation to 70% filling factor with less than 2% fluctuation on laser pattern intensity map. As only a very few articles were written on that subject although it is not very new, this paper includes a theoretical approach of the serration effects. It presents mainly results from simulation and some experimental data.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

An increase in the effective fluorescence lifetime of Nd/Cr:YAG ceramics with temperature dependence was observed. The dependence on the doped Cr ion density for the increased effective fluorescence lifetime was also investigated. The effective lifetime of fluorescence in the ceramics increased from 1.1 to 1.8 ms owing to the phonon-assist cross-relaxation induced by the excited Cr ions, which is commonly observed in Tm:YAG or glass lasers. The increase in effective fluorescence lifetime can be explained by spontaneous emissions occurring from the excited Nd ions and excitations of the Nd ions from the lower level to the upper level at the same time. Additionally, an experiment for laser oscillations with the temperature of the laser material controlled was performed, and a remarkable increase of the output laser energy owing to cross-relaxation was observed when the temperature increased. The obtained maximum laser energy output was near twice that without cross-relaxation. The ceramics are considered promising lamp- or solar-pumped solid-laser materials owing to the efficient laser action based on this cross-relaxation.

We report on a novel amplifier configuration concept for a 10 kW laser system using a zig-zag optical path based on a cryogenic Yb:YAG Total-Reflection Active-Mirror (TRAM) laser. The laser material is a compact composite ceramic, in which three Yb:YAG TRAMs are combined in series to increase the output power. Output powers of up to 214 W with a slope efficiency of 63% have been demonstrated for CW operation, even at a quite low pump intensity of less than 170 W/cm2. Further scaling could achieve output powers of more than 10 kW.

We constructed a theory to explain the mechanism of laser generation with a high optical–optical conversion efficiency for Nd3+- and Cr3+-doped yttrium aluminum ceramics when sunlight or lamplight sources are used for pumping. As a result, a unique mechanism of laser action was found where the solar or lamp-light power could be converted to laser power with a high efficiency close to 80%, which has not previously been observed. The high conversion efficiency was not only considered to be based on one-to-one photon conversion but on two-photon excitation by a single photon with phonon assistance. Thus, the mechanism of lasing action should include a process where thermal energy is converted to photon energy. The theoretical results we obtained were consistent with those of the experiments.

A passively Q-switched Nd3+-and Cr3+-doped yttrium aluminum garnet (YAG) ceramic laser using Cr4+:YAG crystal as a Q-switch element is reported for the first time. Unique temporal waveforms of the output laser pulses were observed. A temporal waveform of a Q-switched laser pulse was compared with one in a free-running pulse. Laser pulses of 130–300 ns duration were generated. A maximum output laser energy of 1.1 J and an average peak power of 0.1 MW were obtained for multiple Q-switched pulses by a single shot, and a maximum output laser energy of 80 mJ was obtained for a single pulse. The evaluated maximum peak power was 0.6 MW as determined from measured pulse energy and duration. The effective saturation fluence and maximum stored energy were lower than those of Nd:YAG. However, a high peak power can be obtained using an output coupler with a low reflectivity owing to the high gain as in the case of Nd:YAG. The threshold of the Nd/Cr:YAG ceramic laser oscillator is reduced to less than 10% of that of a Nd:YAG oscillator. It was determined by calculation that the output laser energy of the Nd/Cr:YAG ceramic laser was close to 4 times that of Nd:YAG.

The authors propose an energy cycle based on a renewable fuel. Magnesium is chosen as an energy carrier and is combusted with water to retrieve energy using many power devices. MgO, the combustion residue, is reduced back to Mg by laser radiation generated from solar and other renewable energy sources. They have achieved an energy recovery efficiency of 42.5% for converting MgO to magnesium, using a laser. Combined with a demonstrated 38% efficiency for converting an artificial sunlight source (metal halide lamp) into laser output energy indicates that the proposed energy cycle is already in a feasible range for practical use.