A mode analysis technique of the millimeter-wave which propagates in the corrugated waveguide of the electron cyclotron resonance heating (ECRH) transmission line is proposed. The technique is based on the signals detected by beam-profile monitors (BPMs) developed and installed in the reflection plane mirror of the miterbends in the ECRH transmission line. The developed BPM consists of a mirror plate, Peltier-device array, and a heat-sink. The voltage signal of each Peltier device is related with the temperature of the mirror surface. The program which can analyze propagating-mode components using these signals is developed. The possibility of the propagating mode analyses using this code was confirmed using the artificially synthesized model data on the miterbend reflector.

From the viewpoints of radiation management and public acceptance, the monitoring of tritium concentration in the stack is one of the key issues for the deuterium plasma experiment in the Large Helical Device (LHD). Since 2012, an active tritium sampler was employed in the stack to monitor the background levels of tritium and discriminate between its chemical forms. However, the operation procedure became complicated, so we developed a simplified active tritium sampler to reduce the work after sampling. In this sampler system, there was no discrimination between tritium chemical forms, instead, all tritium chemical forms were collected in one process. To verify the performance of the simplified active tritium sampler, tritium concentration data was compared with the existing tritium active sampler and environmental tritium data at the NIFS site. The results revealed a correlation between these tritium data. Therefore, the simplified active tritium sampler was applied in the stack with a primary monitoring system for the deuterium plasma experiment in the LHD.

Tritium transport code in Li₂TiO₃ pebbles were developed in this study. In the simulation code, tritium transports in a grain, a closed-pore and an opened-pore were modeled and combined. The tritium transport in grain was modeled by the tritium diffusion, trapping/detrapping by defects, and the annihilation process of defects. The adsorption/desorption equilibrium of diffusing tritium gas molecule was modeled for the tritium transports in opened-pores. A vacant-core and shell model was used to model the tritium transport in closed-pores. The results by simulation code suggested that the tritium trapping by closed-pores would result in the shift of tritium release toward higher temperature side as dissociation of tritium gas molecule in closed-pores requires higher activation energy.

Divertor thermal load is one of the significant problems for a tokamak fusion device, and reduction of the thermal load is required. An application of a Cusp-Type Direct Energy Converter (CuspDEC), which is the device for D-³He fusion power generation to separate charged particles and convert their energy directly into electricity, was proposed to mitigate the heat flux of the divertor plasma. In order to install the CuspDEC in the narrow divertor region, a small-size CuspDEC simulator equipped with permanent magnets (PM-CuspDEC) was constructed to demonstrate divertor simulation experiment in GAMMA 10/PDX, and the performance of charge separation was examined. On one hand, development of measurement technology of heat quantity was also researched. A new calorimeter (CM) made for suppressing heat transmission has newly been manufactured to measure the small energy of end-loss flux in GAMMA 10/PDX experiment. The CM consists of a copper plate and thermocouples and can measure heat quantity and an inflow current simultaneously. The composed new CM is installed in the PM-CuspDEC, and its performance is investigated. A helium ion is used in the experiment of measuring heat quantity. The CM has successfully worked and measured the slight variation of the temperature.

The Large Helical Device has a helium subcooling system with two cold compressors for helical coils to enhance the magnetic fields and improve the cryogenic stability of the coils by lowering the coil temperature. The system was installed in 2006 and then it has stably supplied 3.2 K subcooled helium at the nominal mass flow rate of 50 g/s to the coils during ten plasma experimental campaigns. The running time of the cold compressors exceeds 30,000 hours and the total time of subcooling operations exceeds 20,000 hours. In the system, the supplied helium is subcooled in a heat exchanger of a saturated helium bath. The bath pressure and temperature are reduced by a series of two centrifugal cold compressors with gas foil bearing. In the steady state subcooling operation, the bath temperature is stabilized within range of 0.02 K with automatic flow control of helium gas through the cold compressors by a heater in the bath. The control method is also useful to protect the system by mitigating large disturbance of the pressure and the mass flow rate. In the present study, the thermal hydraulic behavior of the system and the operational performance during ten plasma experimental campaigns are reported.

A magnetized coaxial plasma gun (MCPG) is utilized to generate a compact toroid (CT). An MCPG-type CT injector had been developed as a particle refueling system for C-2/C-2U field-reversed configuration (FRC) plasmas. To inject CTs repetitively for a long-lived plasma, the injector has been upgraded. Iron-core bias coil system has been adopted to generate stationary bias magnetic field. Typical MCPG systems use excess neutral gas to produce a breakdown; therefore, the excess gas tends to flow into the confinement vessel and cool off the edge plasma as well as the target plasma. This negative effect is more serious for repetitive CT injection so that a pre-ionization (PI) system is required to reduce initial gas amount. By injecting the initial plasma using the PI system, amount of the neutral gas for the injector can be reduced. The combination of these systems also expands operating range of the injector. By moving the iron-core bias coil, the radial magnetic field can be controlled. The PI system can easily produce breakdown; therefore, the MCPG can be operated at lower gas pressure, reduced by approximately 40%. The optimum CT has higher velocity (>100 km/s) and ion temperature (>70 eV), increased by more than 40%.

We have developed a rapid sampling system for measuring the tritium in atmospheric water vapor. The system consists of a high-efficiency particulate air filter cartridge, an oil-free compressor, a water-vapor-separating module with hollow fiber membranes, two cold traps, and an oil-free rotary pump. Compressed air (0.4 - 0.7MPa) is introduced into the water-vapor-separation module, which consists of a stainless steel column containing polyimide membrane tubes. Water vapor permeates through the tubes and is collected by cold traps cooled with dry ice and ethanol. The module is heated with a flexible heater to control its temperature. We have determined the recovery yields under various sampling conditions and find that this system can collect atmospheric water vapor with a recovery yield of >99%. This system can thus be a useful tool for understanding short-term observations of tritium in atmospheric water vapor.

The Chinese First Quasi-axisymmetric Stellarator (CFQS) is a joint project of international collaboration. It is designed and fabricated by Southwest Jiaotong University (SWJTU) in China and National Institute for Fusion Science (NIFS) in Japan. In this work, a plasma boundary and modular-coil system for the CFQS have been designed and optimized via scan of the magnetic configurations with various aspect ratios. The present target parameters of the CFQS are as follows: toroidal periodic number, aspect ratio, magnetic field strength and major radius are 2, 4.0, 1.0 T, and 1.0 m, respectively. The VMEC and NESCOIL codes are employed to obtain the optimum coil design via minimizing the normal component of the magnetic field on the target plasma boundary surface generated by the modular coils. In order to estimate the accuracy of the filament coils, the shape of the plasma boundary, rotational transform, depth of magnetic well and main Fourier components of magnetic field strength produced by the modular-coil system are compared with the target plasma boundary, rotational transform, depth of magnetic well and main Fourier components, correspondingly. This comparison between properties of the coil induced configuration and the target configuration shows a good agreement.

Sacrificial anode methods are focused upon for suppression of the tritium fluoride corrosion of structural materials in molten salt blankets in fusion reactors. Stainless Steel 430 (SS430) is immersed into hydrogen fluoride (HF)-containing LiF-NaF-KF (FLiNaK) for 6hours at 773 K with Ti. It has been found that (1) the presence of Ti suppresses the corrosion of SS430 by HF. Further, by the electrical connection between SS430 and Ti, (2) Ti can sufficiently lower the potential of SS430, (3) the weight loss of Ti increases, and (4) the grain boundary corrosion of SS430 by Cr dissolution is mitigated. The applicability of sacrificial anode methods to molten salt blankets has been established.

In the transmission system of the high power Electron Cyclotron Heating (ECH), many mirrors have been installed in the injection antenna section in particular. In some cases, it is difficult to design a normal mirror due to the restriction of the boundary condition. A grating mirror is one of the solutions in such cases. Moreover, controlled power splitting or combining at high power can be realized by a grating mirror. Diffraction angle, efficiency, and distortion/conversion of the polarization can be controlled by the grating shape design. In this study, we developed a grating mirror to efficiently separate the second harmonics component from the fundamental high power beam at 82.7 GHz and higher harmonics. The performance of this grating is confirmed experimentally for the use of vortex beam formation. During this development, the design procedure of a grating mirror is established and generalized to apply for various purposes.

High power neutral beam injectors, like those for ITER, must satisfy very demanding parameters (40 A of negative ion current accelerated up to 1 MV for one hour). They are made of various components, which influence each other, so that the global performances eventually require the simultaneous control of several interacting parameters: thus the NBI is an example of a complex system. In the present paper, complex network theory is applied to verify the controllability conditions of the NIO1 experiment, a particle beam source operating at Consorzio RFX (Padova, Italy). Previous work on the subject is adapted to NIO1, the controllability conditions are assessed and the driver nodes are identified; first comparison between theoretical predictions and experimental data is also discussed.

SPIDER is the prototype beam source of the ITER Heating Neutral Beam injector. A movable diagnostic calorimeter will be used as a direct mean to obtain the beam footprint in short-pulses, while a fixed beam-dump is installed for steady-state operation. For the comparison between experiment and numerical simulations of ion beam extraction, measuring the beam emittance is extremely useful, being the most complete characterization for a particle beam. We discuss in this paper two proposals for beam-emittance measurements in SPIDER: at high beam energies, a fixed electric-sweep scanner is proposed for integration in the water-cooled beam-dump; at relatively low beam energies, a movable emittance scanner is proposed for the installation on the movable diagnostic calorimeter. The synthetic signals of the scanner are calculated considering the multibeamlet setup. The constraints given by the integration in high heat load components and the thermal design are discussed. The fixed ESS can be used to reconstruct the beam divergence, even if it detects only a limited section of the beamlet emittance, if identical single-beamlet optics is assumed. The movable emittance scanner is easy to integrate in the present design, and allows a full characterization of the single beamlet optics.

This study proposes a method for estimating plasma-emission transitions from plasma-emission videos using a hidden Markov model (HMM). The proposed method retrieves similar videos and learns model parameters from them. The plasma-emission characteristics that we have employed are color, brightness, position, shape, and the speed at which the brightness of a plasma emissions changes. Multiple HMMs based on these plasma-emission characteristics are employed to represent the plasma-emission patterns. The anticipated plasma-emission transitions are estimated using state-transition probabilities from the generated model. Experimental results are used to confirm that the proposed methods are effective in identifying similar plasma videos and estimating probable future states of the plasma.

Rhodium and ruthenium thin film coatings were conducted on tungsten samples by using a magnetron sputtering device; then, we irradiated He plasma to the samples in the linear plasma device NAGDIS-II (Nagoya Divertor Simulator). Fuzzy nanostructures were formed on rhodium and ruthenium samples when the surface temperatures were ∼950 and ∼1200 K, respectively. When the surface temperature was high, i.e., >1200 K, it was found that tungsten atoms diffused across the rhodium film and reached the film surface, and tungsten-fuzz was formed over the rhodium layer. From TEM analysis of ruthenium fibers, it was identified that there were thin parts in the fibers, and some fibers had no He bubbles. It was likely that fiber growth mechanism on ruthenium was different from the other metals.

We apply laser absorption spectroscopy to direct-current hollow-cathode helium plasma generated in a 0.3-mm-diameter cylindrical hole at gas pressures of 10 - 80 kPa. The measured transitions are 2³S-2³P and 2¹P-3¹D. We evaluate the helium metastable 2³S density and gas temperature using the former transition and the 2¹Patom density and electric field strength from the latter one. The observed lateral distributions of the 2³S and 2¹P atom densities depend on the gas pressure, which results from the sheath length estimated based on the spatial distribution of the evaluated electric field strength.

Fuzz nanostructure formation on metal by helium plasma irradiation has potential in industrial application due to the increasing in surface area. We have investigated the influence on the surfaces of platinum and tungsten carbide by helium plasma irradiation. On tungsten carbide, fuzz nanostructure was formed when the surface temperature was higher than 1000 K. However, when helium irradiation was conducted at the surface temperature of 940 K or higher, tungsten carbide changed to W possibly because of radiation enhanced sublimation. On platinum, fuzz nanostructure was formed when the surface temperature was in the range of 800 - 1000 K and the fluence was on the order of 10²⁶ m⁻². The nanostructure formation mechanism of platinum should be similar to that of tungsten, but incubation fluence of platinum was higher and the growth rate of fuzzy layer was likely to be lower than those of tungsten.

Herein, we investigated the characteristics of a backward wave oscillator using a dielectric discharge cold cathode (DDCC) in a weakly relativistic region with energies less than 100 keV. The DDCC comprises a copper disc electrode and a ceramic tube; the DDCC exhibited a circular burn pattern with a voltage less than about 30 kV. When a Marx generator was used as a high-voltage source, two microwave oscillations (first phase and second phase) were observed. Within 80 periods of the slow-wave structure, we observed microwave oscillation at the π-point with a voltage of about 30 kV and then detected a microwave signal of about 910 - 5500 mW.

A novel atmospheric pressure plasma jet (APPJ) device using argon and oxygen gases was utilized to synthesize and deposit silver particles into pure cotton fabrics. The main goal of the study was to test the efficiency of the novel APPJ system for materials processing for antibacterial application. With maintained electrode distance to obey Paschen's Law, silver electrodes were placed in a glass tube powered by 15 kV. Flow rates of argon and oxygen were kept constant at 15 LPM and 5 LPM, respectively. Plasma species and electron temperature were identified using the optical emission spectroscopy results. Pure cotton fabrics were exposed to plasma for 1 min, 3 mins and 5 mins. Scanning electron microscopy and x-ray fluorescence revealed that silver particles were successfully sputtered on fabric samples. Moreover, plasma treatment also enriched the cotton samples with antibacterial property against Escherichia coli and Staphylococcus aureus as determined by modified standard disk diffusion method. These results demonstrated the surface-immobilization of plasma-synthesized silver nanoparticles on cotton fibers and the promising performance in antibacterial applications.

Boron Neutron Capture Therapy (BNCT) is a new and promising cancer therapy. At present, development of accelerator based neutron source (ABNS) is underway to be utilized as a neutron source instead of nuclear reactor. However, it is known that the neutron fields formed with accelerators have different characteristics depending on kind of accelerators. It means we have to characterize the field before practical use. In the authors' group, various neutronics characterization devices have been developed for our BNCT machine named CSePT. It includes source intensity, epi-thermal neutron flux, neutron spectrum and so on. In the present paper, such measuring techniques were overviewed.

At the Japan Proton Accelerator Research Complex, a pulsed spallation neutron source has been providing neutron beams with high intensities and narrow pulse widths since 2008 for various materials science experiments. The neuron-pulse characteristics measured during early low-power operations indicated that this source is capable of providing the world's highest peak neutron intensities and pulse resolution at the 1-MW operation level, which is the goal of the facility. To achieve this operational goal, efforts have been underway to solve a critical issue affecting the target operation, i.e. mitigation of cavitation damages at the front end of the mercury target vessel, by injecting gas micro-bubbles and using a fast flow of the mercury through a narrow channel. Another issue is that the target vessel needs to be redesigned to ensure its robustness against the cyclic thermal stress produced by the temperature swings when the proton-beam trips because the water shroud surrounding the mercury target vessel failed during 500-kW operation in 2015.

The partitioning and transmutation (P-T) technology have promising potential for volume reduction and mitigation of degree of harmfulness of high-level radioactive waste. JAEA is promoting the development of the P-T technology by using an accelerator-driven system (ADS). To facilitate the development, JAEA has a plan to construct Transmutation Experimental Facility (TEF) as one of the experimental facilities of Japan Proton Accelerator Research Complex (J-PARC). TEF consists of two individual facilities: ADS Target Test Facility (TEF-T) and Transmutation Physics Experimental Facility (TEF-P). TEF-T equips with a liquid lead-bismuth spallation target bombarded by a 400 MeV - 250 kW proton beam in which candidate proton beam window materials of the ADS are to be irradiated. TEF-P equips with a critical/subcritical assembly to investigate physical and dynamic properties of the accelerator-driven system by using a low power (10 W) proton beam. Nuclear fuels containing minor actinide are to be loaded to the assembly. Comprehensive R&D activities to support the TEF construction have been conducted, and a technical design report of TEF-T was issued in March 2017.

In recent years, fusion research has become exceedingly popular in the Asian region. Many toroidal devices are currently operated for fusion research in universities and institutes. The long pulse operation of a high-performance plasma is also possible in four superconducting devices, which are EAST (Institute of Plasma Physics Chinese Academy of Sciences (ASIPP), China), KSTAR (National Fusion Research Institute (NFRI), Korea), LHD (National Institute for Fusion Science (NIFS), Japan) and SST-1 (Institute for Plasma Research (IPR), India). The world's largest superconducting tokamak, JT-60SA (National Institutes for Quantum and Radiological Science and Technology (QST), Japan), and middle-size tokamak, HL-2M (Southwestern Institute of Physics (SWIP), China), have now been constructed for the upcoming operation. Based on the progressive development of fusion research in Asian region an international collaboration on critical physics for the steady state operation of high-performance plasmas has been carried out among China, Japan and Korea during past five years through the A3 foresight program. Recent fusion research activities in the Asian region and the results of the collaboration are briefly reported with future prospects for international collaboration.

The successful operation of superconducting tokamaks is very challenging because of limitations arising out of a complex geometry, configuration and construction. The slow penetration of loop voltage because a continuous plasma chamber requires low loop voltage plasma initiation. Apart from these operational challenges, SST1 also has technical issues related to cooling down of the superconducting poloidal field (PF) coils, for obtaining advanced plasma operation and control. Furthermore, limits are imposed on the loop voltage by magnet power supplies and PF coil insulation. To improve our understanding of these issues, operations in SST-1 have been attempted with low loop voltages and will be discussed in this paper.

It is envisaged that designing and carrying out experiments in SST1, to understand above issues, may be very complex, time consuming and may not be feasible due to complexities in SST1 machine operational scenario. Hence Aditya machine upgradation was initiated with the understanding that solution to some of these problems may be obtained through experiments specially designed for this purpose in ADITYA-U, in parallel. Thus some of these experiments are also being planned to carry out in tokamak ADITYA-U, in particular, non-inductive current drive using lower hybrid waves, employing passive active multi-junction (PAM) antenna. The performance of the lower hybrid waves launched by the grill antenna is presented and the need for PAM antenna is presented. The design of the PAM antenna for ADITYA-U machine is also discussed.