Search

The vertical neutron camera (VNC) has been developed to measure neutron emission profile in deuterium plasmas of the large helical device (LHD). The experiment on spatial resolution evaluation for VNC was carried out in November 2016 by using a 252 Cf neutron source of 800 MBq. The neutron source was introduced into the LHD vacuum vessel through an aluminum pipe from an upper diagnostics port to the equatorial plane. The source was placed at the following two positions. The first position is just on the cylindrical collimator axis, and the second position is in the middle of two neighboring collimator axes. The stilbene scintillation detectors are aligned radially at the collimator end to detect unscattered fast-neutrons generated in a deuterium plasma. The 3-D neutron transport calculations by using a general-purpose Monte Carlo N-particle code 6 reproduced the experimentally obtained neutron counting rate, indicating that VNC has sufficient spatial resolution and the crosstalk between two neighboring collimators is a fairly small fraction. It is expected that the VNC on LHD will function as a tool for the study on the radial transport of energetic ions.

Dilution refrigerators are an important tool used in solid state and quantum fluid physics for cooling to temperatures below 0.3 K. Conventional dilution refrigerators consume a lot of liquid helium, which has to be recharged in a helium bath every few days. Cryogen-free dilution refrigerators, however, do not use liquid helium and then automatic operation by electricity can be possible from room temperature to the mK region. In near future, therefore, most conventional dilution refrigerators will be replaced by cryogen-free refrigerators because they are easy to operate, do not require maintenance and do not consume helium. We have developed two types of cryogen-free dilution refrigerator. One is directly cooled by a pulse tube refrigerator in the same cryostat using copper thin wires as a thermal link, and the other is cooled by a separate Gifford McMahon refrigerator using circulating helium gas through a flexible syphon tube. The latter has been developed as a vibration-free cryogen-free dilution refrigerator. These two types of cryogen-free dilution refrigerator are compared considering several key points: base temperature, precooling time, minimum temperature and vibration amplitude.

The concept for a compact DEMO reactor named 'SlimCS' is presented. Distinctive features of the concept are low aspect ratio (A = 2.6) and use of a reduced-size centre solenoid (CS) which has the function of plasma shaping rather than poloidal flux supply. The reduced-size CS enables us to introduce a thin toroidal field coil system which contributes to reducing the weight and perhaps lessening the construction cost. Low-A has merits of vertical stability for high elongation (κ) and high normalized beta (βN), which leads to a high power density with reasonable physics requirements. This is because high κ facilitates high nGW (because of an increase in Ip), which allows efficient use of the capacity of high βN. From an engineering aspect, low-A may ensure ease in designing blanket modules robust to electromagnetic forces acting on disruptions. Thus, a superconducting low-A tokamak reactor such as SlimCS can be a promising DEMO concept with physics and engineering advantages.

In order to support the operation of ITER and the planned experimental programme an extensive set of plasma and first wall measurements will be required. The number and type of required measurements will be similar to those made on the present-day large tokamaks while the specification of the measurements—time and spatial resolutions, etc—will in some cases be more stringent. Many of the measurements will be used in the real time control of the plasma driving a requirement for very high reliability in the systems (diagnostics) that provide the measurements. The implementation of diagnostic systems on ITER is a substantial challenge. Because of the harsh environment (high levels of neutron and gamma fluxes, neutron heating, particle bombardment) diagnostic system selection and design has to cope with a range of phenomena not previously encountered in diagnostic design. Extensive design and R&D is needed to prepare the systems. In some cases the environmental difficulties are so severe that new diagnostic techniques are required. a Author to whom any correspondence should be addressed.

We have fabricated vertically-stacked interface-treated Josephson junctions using YbBa/sub 2/Cu/sub 3/O/sub 7-x/ (YbBCO) as a counter electrode for improving the uniformity. We used YBCO for the base electrode. Most of the junctions fabricated on one chip exhibited resistively-shunted-junction (RSJ) characteristics. The properties of the junctions at 4.2 K were as follows: J/sub c/=1.0/spl times/10/sup 3/ A/cm/sup 2/ with a 1/spl sigma/ spread of 6.9%, and I/sub c/R/sub n/=0.5 mV with a 1/spl sigma/ spread of 5.4%. These spreads were improved compared with those of junctions with YBCO as a counter electrode. Furthermore, the normal conductance G/sub n/ of the junctions with the YbBCO counter electrode was independent of the temperature. Such characteristics have not been obtained for YBCO vertically-stacked interface-treated junctions. We speculate that this difference is attributed to the lattice mismatch between base-and counter-electrodes.

The current from copper electrodes treated by four different types of surface cleaning procedures was measured under DC high field gradient condition. The best results were obtained by using the electrode rinsed with ultra-pure water after diamond turning. A field gradient of 47 MV/m was achieved with dark current at the level of 1 nA, and the microscopic field enhancement factor was estimated to be a very low value of 56. The dark current from this electrode was dependent only on the field gradient at the cathode and not affected by the total voltage applied to the gap. In this case the surface would be covered by a Cu2O layer, which creates few secondary ions by the electron bombardment. On the other hand, a large total voltage effect and a large vacuum increase were observed for the electro-polished electrode. Cu(OH)2, which would be formed at the copper surface during the electro-polishing process, would emit H2O molecules during the electron bombardment.

Polarized electron beam with sub-nanosecond multi-bunch structure has been "rst produced from an In-GaAs}AlGaAs strained-layer superlattice photocathode with heavy surface doping. The extracted charge of0.6]1010e~/bunch is determined by the space charge limit of the 50keV gun. The multi-bunch e!ect due to the surfacechargelimitphenomenon,whichispeculiartoanegativeelectrona$nitysurfaceofsemiconductorphotocathode,isnotobserved in the second bunch separated by 2.8ns. We conclude that a superlattice photocathode with a heavily dopedsurface is the best photocathode for the future linear colliders. ( 2000 Elsevier Science B.V. All rights reserved. 1. IntroductionPolarized electron source based on semiconduc-torphotocathodeis expectedto play a very impor-tant role in future linear colliders [1,2]. In order toachieve a high luminosity of &5]1033cm~2s~1,about 80 micro-bunches with a separation time of2.8nsmustbeproducedat150HzrepetitionrateatJapan Linear Collider (JLC). Each bunch mustcontainmorethan2]1010e~(each)()1ns width)at the source [3].The generation of such a high-intensity multi-bunch beam is not easy due to a surface chargelimit (SCL) phenomenon [4,5]. This phenomenonis peculiar to a negative electron a$nity (NEA)surface, which is a key technique for the semicon-ductorphotocathodestoextracttheopticallyexcit-edelectronsintheconductionbandtothevacuum.When a photocathode is illuminated with a high-intensity laser light, the electrons which cannotescape to the vacuum are trapped at the surfaceband bending region. The accumulated surfacecharge prevents electrons in the later portion ofthe pulse from escaping to the vacuum.In order to overcome the SCL problem, weinvestigated the multi-bunch generation using the70keVpolarizedelectronsourcesystemat NagoyaUniversity. It was demonstrated that a super-lattice photocathode (for example, GaAs}AlGaAs

The research to develop the polarized electron source required by future linear colliders has been conducted by our collaboration. Recent advances in settling three difficult problems to realize the high polarization and high quantum efficiency, the long cathode lifetime and the multi-bunch beam generation are briefly described in this article.

The article provides a comprehensive overview of the history of how signal-processing researchers have been effectively transforming signal-processing algorithms into efficient implementations. Starting from the early days of analog circuits for signal processing, to digital signal processors (DSPs), to application specific DSPs and programmable DSPs, and to the trend of integrating a complete system on a single chip, this article provides a thorough coverage of the past and the present of design and implementation technology for signal processing systems. Moreover, it presents the exciting challenges faced by the study of the design and implementation of current and future signal processing applications. Topics covered include milestones in signal-processing integrated circuits, the past and future of the signal processor, signal processing in consumer applications, and design automation for signal processing.

Detailed plasma-physics investigations by the use of x-ray-tomography data supported by the fundamental theoretical studies of x-ray-detector responses enhance the importance of x-ray diagnostics for fusion-plasma analyses. However, degradation in responses of semiconductor x-ray detectors after fusion-produced neutron exposure still remains one of the most serious problems in recent fusion experiments even at this time. For the purpose of investigating and characterizing neutron effects on semiconductor x-ray detectors, detection characteristics of n-type silicon semiconductor detectors which are similar to those utilized for x-ray-tomography detectors in the Joint European Torus (JET) tokamak, are studied by the use of synchrotron radiation from a 2.5-GeV positron storage ring at the Photon Factory. The fusion neutronics source (FNS) of Japan Atomic Energy Research Institute is employed as well-calibrated deuterium-tritium (D-T) neutron source with fluences from 10 13 to 10 15 neutrons/cm 2 onto these semiconductor detectors. Degradation in x-ray responses with increasing neutron fluences has been reported; however, our recent detailed investigations of detector responses show nonlinear dependence as a function of the neutron fluence.

The development of a 275 kV gas-cooled, gas-insulated power transformer with a sound level ideal for an urban area, which depends on SF/sub 6/ gas alone for both insulation and cooling, is described. The design philosophy, technical aspects, rating, and features of a 275 kV 30 MVA transformer are described, and the performance and long-term energization test results are discussed. >

Recent lower hybrid current drive (LHCD), and heating (LHH) experiments on JT-60 are reported. The current drive product of neRpIRF approximately 12.5*1019 m-2 MA was achieved at the LH power of approximately 4.5 MW, and the CD efficiency, the energy confinement, the global power balance and the heat load on divertor plates were investigated in high power LHCD plasmas. Nearly steady state H-mode discharges were found during LHCD with two different frequency injections. Sawtooth suppression in NB heated plasmas by LHCD have shown an improvement in confinement near the plasma center. Parametric instabilities in LH heating experiments were significantly reduced by increasing the plasma current, and the stored energy increased linearly with heating power of up to approximately 9 MW at ne approximately 7*1019 m-3 and Ip=2.75 MA. Parametric instabilities near the plasma edge in the ion heating regime were also reduced in peaked density plasmas produced by pellet injection and LH waves increased the central plasma pressure at ne(0) > 1.4*1020 m-3.

For escaping energetic ion measurements on magnetic fusion devices it is very important to get information on spatial distributions (poloidally and toroidally), pitch angle and energy distributions of escaping energetic ions. This paper presents two kinds of optical measurements of escaping energetic ions. The first is a scintilator-type probe which can resolve simultaneously both pitch angle and gyroradius of lost ions. The second is ceramic scintilator plates fixed on the first wall directly to survey spatial distribution of lost ions. There are difficulties in escaping energetic ion measurements in large scale magnetic fusion devices. For example, signal transportation is severe because loss points of fast ions are away from a port of a fusion device, especially on recent superconducting coils (SC) devices. We have studied these issues on Large Helical Device (LHD) and the signal was successfully transported by approaching an optical fiber close to a scintilator. Plans of lost alpha particle measurement on International Thermonuclear Experimental Reactor (ITER) using these optical techniques are also discussed in this paper.

Maximum radiation loss without disruption is essential to extend the maintenance intervals of ITER and following fusion reactors. The strong radiation distribution at the divertor must be known in fine detail to control such discharges. The corresponding reference diagnostic uses metal resistor bolometer cameras for detailed tomographic reconstructions of the radiation distribution2. The radiation hardness of the detector had been identified as the main question concerning the reference design. We report on the results of a Japanese/European collaboration (under ITER task T492) achieving the first in situ neutron irradiation tests of present day bolometers. A neutron dose equivalent to 0.1 dpa was reached, corresponding to the fluence between two maintenance periods at the bolometer locations behind the first wall elements or in the divertor under full load conditions of ITER-FEAT.

We have tested two kinds of spin-polarized electron photocathodes, the GaAs-GaAsP strained layer superlattice and the GaAs-AlGaAs superlattice with distributed Bragg reflector. The experimental results of these photocathodes are briefly reported.

Our collaboration group have continued to develop the polarized DC-gun system for future electron-position linear colliders [1]. In parallel, the feasibility study to develop the polarized RF-gun is planned. As the first step, the test of Cs2Te photocathode was performed.

A 200 keV polarized electron gun system with a load lock mechanism has been developed to produce the high-intensity and low-emittance beam required by Japan Linear Collider. The construction of this system has been completed and the performance tests are in progress. Up to now, DC high voltage of 150 kV could be successfully applied to the accelerating electrodes with an extremely low dark current of

Our collaboration group has been conducting the R&D works on the polarized electron source for Japan Linear Collider. The experimental results to produce the high-intensity multi-bunch beam from the superlattice photocathodes are summarized in this report.

The field emission dark currents from stainless steel and copper electrodes were measured under DC high field gradient condition. The stainless steel electrode was made from special material called Clean-Z and was polished by electro-chemical buffing. The electrodes achieved 34 MV/m with the dark current of 90 pA at the gap separation of 1 mm. The copper electrodes were treated by four different types of surface cleaning procedures. The best results were obtained by using the electrode rinsed with ultra-pure water after diamond turning. A field gradient of 47 MV/m was achieved with dark current at the level of 1 nA, and the microscopic field enhancement factor was estimated to be a very low value of 56. The dark current from this electrode was dependent only on the field gradient at the cathode and not affected by the total voltage applied to the gap. This fact suggests that the surface fabricated by diamond turning method with ultra-pure water rinsing creates few secondary ions by the electron bombardment.

We are constructing a new polarized electron source for Japan Linear Collider. It is designed to operate the gun at 200 kV. The “load-lock” mechanism is employed to avoid the dark current due to the Cs accumulation on the electrodes and to exchange the activated NEA photocathode quickly. We have developed superlattice photocathode which has advantages of high spin polarization, high quantum efficiency and high resistance against surface charge limit phenomenon. However, it seems difficult to clean the surface of such a thin layer photocathode by the normal etching procedure without destruction of its delicate structure if it has no As caplayer. Atomic hydrogen is expected to clean the surface of superlattice effectively. We have introduced these techniques to the new source design.

The creep behaviour under various combinations of superimposed tensile and hydrostatic loading are quantitatively investigated using nonlinear-viscoelastic cellulose nitrate heated at 65° C. The creep strain and the creep strain-rate are seriously affected by the effect of hydrostatic pressure. The trends of behaviour under superimposed tensile and hydrostatic loading cannot be predicted by the concepts which apply to uniaxial loading. The stress—strain relation of the creep behaviour under superimposed loading is deduced from the invariant theory using an hypothesis of creep potential. The creep data obtained on the cellulose nitrate at 65° C under superimposed loading, as well as uniaxial loading, are found to fit to the deduced relation well.

The stress concentration during the loading and unloading processes of transient creep is investigated for a strip with semi circular notches by means of the photo-viscoplasticity, using specimens of celluloid at 65dC. The relations between the stress concentration factor, the geometries of strips, and time during the loading and unloading processes are derived from the time dependent variations of isochromatic fringe orders. Such trends cannot be analysed by the ordinary photoplasticity in which the time effect is not taken into account.

The behaviour of polymers is known to be significantly influenced by the hydrostatic pressure in creep deformation or elastic-plastic deformation. The effect of the third stress invariant on the nonlinear viscoelastic deformation is much smaller than that of the hydrostatic pressure. In this paper, a constitutive equation for transient creep is proposed, which includes the effect of the hydrostatic pressure on the yield function. The creep and plastic strains or the creep strain rate converge to zero with increasing hydrostatic pressure. The proposed constitutive equation is in good agreement with the actual creep data of cellulose nitrate and cellulose acetate, under various combinations of superimposed tensile and hydrostatic loadings.

The creep behaviour in unloading process under various combinations of superimposed tensile and hydrostatic loading are quantitatively investigated using non-linear-viscoelastic cellulose nitrate heated at 65° C. The creep strain and the creep strain-rate in the unloading process are quite influenced by the effect of hydrostatic pressure, but are not so influenced as those in the loading process mentioned in a previous paper. The deformation properties in the unloading process are also discussed with experiments for proportional loading (namely, uniform rate of stress increases or decreases with time). The stress-strain relation in the unloading process of the creep behaviour under superimposed loading is deduced by using the invariant theory. The deduced relation gives good agreement with the actual observations under the superimposed loading.