Search

In recent years, with the rise of global environmental protection activities, it is becoming necessary to reduce CO2 emissions in social activities. In the field of power generation, it is very important to capture CO2 emissions and increase the conversion efficiency from chemical energy to electrical energy. Because the increased electrical efficiency can lead to the increased power, it is likely to reduce the consumption of fossil fuels for power generation, resulting in reduced CO2 emissions. In our previous report, the effect of a fuel regenerator in a two-stage solid oxide fuel cell (SOFC) module on the electrical efficiency was studied. The results showed that the electrical efficiency was considerably improved by removing H2O and CO2 from the anode off-gas by the fuel regenerator. The highest electrical efficiency was obtained when 100% of H2O and CO2 were removed from the anode off-gas, and it was estimated to be a net alternating current (AC) of 71.8% (lower heating value (LHV)). Although it is necessary to maintain the module at a high temperature in order to realize the system, a 4.8 kW two-stage SOFC module with a fuel regenerator was developed and confirmed to have the electrical efficiency equivalent to a net AC of 66.0% (LHV) with the thermal self-sustainability. In 2020, a demonstration test using a highly efficient two-stage SOFC system has started. In this study, we focused on an anode off-gas recycling SOFC module with a fuel regenerator. The reason why we studied the module is that it can generate highly efficient power and capture a large amount of CO2 at the fuel regenerator. Therefore, the effect of the fuel regeneration on both the electrical efficiency and capture of CO2 was examined. The authors are presently not aware of any reports of examining the effect of the fuel regenerator, which removes CO2 and/or H2O extensively from the anode off-gas, on the electrical efficiency and CO2 capture in an anode off-gas recycling SOFC module. The calculation was carried out under the following conditions. CH4 was used as a fuel, and the ratio of steam to carbon of the inlet fuel was 2.5 or more in order to avoid the carbon deposition. The temperature of both the reformer and SOFC stack was set at 700°C. The area specific resistance and current density of the SOFC stack were assumed to be 0.383 ohm cm2 and 0.25 A cm–2, respectively. The lowest available fuel concentration, which is assumed to be the ratio of the partial pressure of hydrogen to water vapor (= p(H2) / p(H2O)), was set by calculating the outlet fuel concentration when operating a single-pass SOFC stack with a maximum fuel utilization rate of 80%. For estimating the system performance, the inverter efficiency and auxiliary device loss rate were assumed to be 95% and 6%, respectively. The Nernst voltage was determined from the inlet and outlet gas composition of the stack which is calculated by a process simulation software. The electrical efficiency was estimated from the cell voltage and fuel utilization rate. The highest electrical efficiency was obtained when both the fuel utilization rate and recycle rate were 100%, and the removal rates of CO2 and H2O at the fuel regenerator were 100% and 44.4%, respectively. The reason to remove only 44.4% of H2O from the anode off-gas is to keep the ratio of steam to carbon of the inlet fuel at 2.5. The electrical efficiency was a net AC of approximately 75.5% (LHV) with 100% of CO2 capture at the fuel regenerator. In addition, the CO2 removal rate was studied to reduce to 63.9% because there is a margin in the fuel concentration of the SOFC stack. Even if the removal rates of CO2 and H2O at the fuel regenerator were 63.9% and 44.4%, respectively, the electrical efficiency was estimated to be a net AC of approximately 74.9% (LHV), and the CO2 capture at the fuel regenerator was 100%. These results show that the anode off-gas recycling SOFC module can generate highly efficient power as well as the two-stage SOFC module when using the fuel regenerator. Furthermore, the anode off-gas recycling SOFC module has advantage of capturing CO2 at the fuel regenerator. Although it is not easy to generate highly efficient power with the thermal self-sustainability, it is expected that the anode off-gas recycling SOFC module with the fuel regenerator is developed and put into practical use in the future.

A power-supply system was developed for Ohmic heating (OH) to double × 1018 the amount of change magnetic flux in the primary central solenoid (CS) on the QUEST spherical tokamak. Two power supplies are connected with stacks of insulated-gate bipolar transistors, and sequentially operated to generate positive and negative CS currents. This bipolar power-supply system is controlled via a field-programmable gate array, which guarantees the safety of the entire system operation. The new OH system, assisted by electron cyclotron heating, enables the stable generation of plasma currents exceeding 100 kA. Moreover, the achieved electron density over the wide range in the major radial direction exceeds the cut-off density for one of the high-power microwave sources in QUEST. This strategy yields target plasmas for future experiments with the electron Bernstein wave.

Curtain grouting for dam foundation treatment is one of the most crucial work items in dam construction to secure the impermeability of the foundation rock. Some decades ago, the Grouting Intensity Number (GIN) Method developed in Europe has been frequently applied to relatively simple geotechnical structures. On the other hand, the Conventional Method, which requires phased mix proportion and water pressure tests through a sequence of the works, is as yet reliable for inhomogeneous geology. This paper presents the development of a modified curtain grouting method and its application to the Nam Ngiep 1 Hydropower Project in Lao PDR, which has an inhomogeneous geology of sedimentary rock with weak layers affected by fold movement. The method has been dubbed as “hybrid” because it garners both the economical superiority of the GIN Method in that it enables the use of a single mix proportion, and the technical superiority of the Conventional Method in that the individual design pressure in each stage is based on water pressure tests.

Magnetic interaction between a tokamak reactor and its reinforced-concrete building has been analyzed using the analytical method and ANSYS electromagnetic code. Although it is better to use the non-magnetized material for the foundation of the air core tokamak building, it is interesting to study whether the magnetic material can be used in order to save the total cost of a fusion reactor. This study is motivated from the present ITER under construction in France. Assuming the iron plate approximation instead of discrete rebars, we analyzed the effect of the magnetized material on the magnetic field null regime in the plasma break down phase, x point location in the divertor operation, the magnetic force applied to the floor, and the induction effect on the iron floor. Although the effect of the magnetic material on the plasma performance is found to be not so crucial, the mechanical soundness of the floor due to the magnetic attractive force is most concern for machine safety.

Electron spins in solids constitute remarkable quantum sensors. Individual defect centers in diamond were used to detect individual nuclear spins with nanometer scale resolution, and ensemble magnetometers rival SQUID and vapor cell magnetometers when taking into account room temperature operation and size. NV center spins can also detect electric field vectors, despite their weak coupling to electric fields. %even that of an isolated fundamental charge, despite their weak coupling to electric fields. Here, we employ ensembles of NV center spins to measure macroscopic AC electric vector fields with high precision. We utilize low strain, $^{12}$C enriched diamond to achieve maximum sensitivity and tailor the spin Hamiltonian via proper magnetic field adjustment to map out the AC electric field strength and polarization and arrive at refined electric field coupling constants. For high precision measurements we combine classical lock-in detection with aspects from quantum phase estimation for effective suppression of technical noise. Eventually, this enables $t^{-1/2}$ uncertainty scaling of the electric field strength over extended averaging periods, enabling us to reach a sensitivity down to $10^{-7}$ V/$\mu$m.

Tokyo Gas has developed a highly efficient solid oxide fuel cell system by increasing the fuel utilization rate using two-stage stacks and fuel regeneration. We focused on the separation membrane, which removes both steam and carbon dioxide from the anode off-gas of the first-stage stack. In this study, we evaluated the gas separation properties of the membrane module. We connected the membrane module to our fifth-generation hot module and evaluated the power generation performance. The results show DC efficiency of 74.8%LHV at a gross power output of 4.89 kW. In addition, we applied both the separate membrane module and steam condenser to our fifth-generation hot module. This system demonstrated 67.4%LHV DC efficiency at a gross power output of 4.93 kW.

This research investigated fuel particle balance during long duration discharge in an all-metal plasma facing wall (PFW) through intensive QUEST execution. A simple wall model including the plasma-induced deposition layer that creates hydrogen (H) barriers, called the H barrier model, was established. A simple calculation, based on a combination of H state rate equations and the H barrier model, was applied to real plasma in the early phase of its longest discharge. The model accurately reconstructed the evolutions of electron density and wall-stored H over time, proper values are chosen for the parameters that are difficult to determine experimentally. Comparative calculations that used the H barrier and a fully reflective models, predicted significant impacts of wall models on the plasma density time response and value of electron density, indicating that a proper wall model should be developed for all-metal PFW devices. Keywords: Fuel particle balance, steady state operation, Hydrogen barrier model, QUEST

We demonstrate an integrated and miniaturised magnetic field sensor based on the negatively charged nitrogen-vacancy centres (NV−) in diamond. The compact device includes all optical components, both for the optical excitation path and for the detection of the emitted fluorescence signal. We experimentally verify that it enables optically detected magnetic resonance (ODMR) measurements and we specify noise and sensitivity of the magnetometer. The minimal detectable magnetic field of the device is ≈ 1 μT for a given integration time of 1 ms, which is approximately one order of magnitude larger than its photon shot-noise limit. It has the significant advantage over traditional setups using NV− centres (including a laser and a complex optical system) that the specific construction volume is about 2.9 cm3 with a total power consumption of ≈ 1.5 W, which enables the device for a wide range of industrial sensing applications.

Plasma shape reconstruction is an important part of tokamak data analysis. An off-line plasma shape reconstruction, based on the EFIT code in Linux system, can be carried out in Q-shu Univ. Experiment in Steady-State Spherical Tokamak (QUEST). In this paper, the algorithm about equilibrium reconstruction is described in detail. Magnetic reconstruction in QUEST, which is calculated by the EFIT code according to the external magnetic measurements, is introduced and the results are presented in detail. It is known that the eddy current effect is large in QUEST experiment, the eddy current effect is first considered into equilibrium reconstruction. Fourier expansion is applied to reconstruct the vessel current distribution in EFIT code. To test the accuracy of the result, a comparison with the result of Cauchy condition surface method has made, which suggests the reliability of the result. A visualization interface is made, which is convenient for physicists and operators to analyze the behavior of plasma. At the same time, the operation space of elongated discharges is discussed based on the QUEST database. The real time EFIT will be applied to the plasma control system of QUEST in the future.

QUEST (Q-shu university experiment with steady state spherical tokamak) is a midsize spherical tokamak capable of steady-state operation, comprising all-metal plasma-facing walls and a hot wall (HW) to address issues pertaining to fuel particle balance. The HW was installed summer 2014. Quantitative analysis pertaining to the HW at 373 K is carried out, and clarify the quantitative impact of shot history that obviously appears in wall stored hydrogen just before the discharge at the wall temperature. The model indicates the plasma-induced deposition layer play an essential role in fuel particle balance. A clear temperature dependence of fuel recycling was observed using outgassing just after plasma termination and played an essential role in regulation of particle balance. Consequently, long duration discharges lasting more than 1 h has been obtained at wall temperature, TW

A spectroscopic system with multiple viewing chords was developed for QUEST (Q-shu University Experiment with Steady-State Spherical Tokamak) to measure the spatial distribution of ion toroidal velocities in discharges sustained by electron cyclotron resonance heating (ECH). Twenty-four viewing chords were aligned in the midplane and C III emission line spectra were measured for three types of ECH discharge under different magnetic field configurations. By applying an inversion method to the measured spectra, we evaluated the radial distributions of C2+ ion emissivity, temperature, and toroidal velocity. The error in the evaluated velocity was estimated to be less than 5 km/s. It was found that the velocity depends on the magnetic field configuration.

In modern-day quantum metrology, quantum sensors are widely employed to detect weak magnetic fields or nanoscale signals. Quantum devices, exploiting quantum coherence, are inevitably connected to physical constants and can achieve accuracy, repeatability, and precision approaching fundamental limits. As a result, these sensors have shown utility in a wide range of research domains spanning both science and technology. A rapidly emerging quantum sensing platform employs atomic-scale defects in crystals. In particular, magnetometry using nitrogen-vacancy (NV) color centers in diamond has garnered increasing interest. NV systems possess a combination of remarkable properties, optical addressability, long coherence times, and biocompatibility. Sensors based on NV centers excel in spatial resolution and magnetic sensitivity. These diamond-based sensors promise comparable combination of high spatial resolution and magnetic sensitivity without cryogenic operation. The above properties of NV magnetometers promise increasingly integrated quantum measurement technology, as a result, they have been extensively developed with various protocols and find use in numerous applications spanning materials characterization, nuclear magnetic resonance (NMR), condensed matter physics, paleomagnetism, neuroscience and living systems biology, and industrial vector magnetometry. In this chapter, NV centers are explored for magnetic sensing in a number of contexts. In general, we introduce novel regimes for magnetic-field probes with NV ensembles. Specifically, NV centers are developed for sensitive magnetometers for applications where microwaves (MWs) are prohibitively invasive and operations need to be carried out under zero ambient magnetic field. The primary goal of our discussion is to improve the utility of these NV center-based magnetometers.

Quantum sensors are keeping the cutting-edge sensitivities in metrology. However, for high-sensitive measurements of arbitrary signals, limitations in linear dynamic range could introduce distortions when sensing the frequency, magnitude and phase of unknown signals. Here, we overcome these limitations with advanced sensing protocol that combines quantum phase-sensitive detection with heterodyne readout. We present theoretical and experimental investigations using nitrogen-vacancy centers in diamond, showing the ability to sense radio signals with a 98 dB linear dynamic range, a 31 pT/Hz$^{1/2}$ sensitivity, and arbitrary frequency resolution. Further, we perform the quantum-assisted distortion-free audio signal (melody, speech) sensing with high fidelity. The methods developed here could broaden the horizon for quantum sensors towards applications in telecommunication, where high-fidelity and low-distortion at multiple frequency bands within small sensing volumes are required., Comment: 30 pages, 7 figures

Physical review / B 103(3), 035307 (2021). doi:10.1103/PhysRevB.103.035307, Published by APS, Woodbury, NY

The spatial distribution of the hydrogen atom density was evaluated in a spherical tokamak (ST) plasma sustained only with 28 GHz electron cyclotron heating (ECH). The radially resolved H&delta, emissivity was measured using multiple viewing chord spectroscopy and Abel inversion. A collisional-radiative (CR) model analysis of the emissivity resulted in a ground-state hydrogen atom density of 1015&ndash, 1016 m&minus, 3 and an ionization degree of 1&ndash, 0.85 in the plasma.

In tokamaks, the temperature of the plasma-facing wall is an important parameter for achieving particle balance and therefore steady-state operation. QUEST, which is a middle-sized spherical tokamak, has hot walls that act as plasma-facing walls. They can be actively heated with sheath heaters and actively cooled with water. To control the wall temperature, heating and cooling systems have been developed. These systems adjust the power of the sheath heaters and the motor valves of the cooling system, respectively. The two systems communicate via Ethernet through UDP and control the hot-wall temperature cooperatively. The plasma control system (PCS) in QUEST has also been modified, especially with respect to gas fueling, in order to enable long-duration plasma sustainment. A feedback controller has been installed in the PCS, together with a mass flow controller, allowing Hα emission from the plasma which is used as a reference signal, to be well controlled. Plasma density calculations using a field-programmable gate array are proposed for the feedback control system.

Fracture toughness of the divertor armor in ITER needs to be elucidated for prediction of the response of armor materials to intense heat loading. The present study has been performed to establish a method for inserting a well, defined fatigue precrack into the test specimens and measure the reliable fracture toughness values of tungsten (W) materials that exhibit low temperature brittleness. Sub-sized, single edge notched bend (SENB) specimens with two orientations, L -T and T-L, taken from a commercially available pure W plate manufactured in accordance with the ITER material specifications (ITER specification W, ALMT-grade), were precracked in two-step fatigue loading: fully uniaxial compression fatigue to provoke a crack from the notch tip, and subsequent 3-point bend (3 PB) fatigue to extend the crack length for the minimization of influence of the residual tensile stresses generated by compression fatigue. During compression and 3 PB fatigue loading the fatigue crack was closely monitored by continuous in-situ observations on the notch tip. Continuous tracing of the crack tip position has enabled capture of the fatigue cracking behavior as a kinematic response in a rolled W material, disclosing a marked difference in crack inducement and extension between the two orientations. 3 PB fracture testing of the precracked specimens at room temperature (RT) yielded ~10 MPa√m for the T-L and 12~13 MPa√m for the L -T orientation as fracture toughness: anisotropic variation in fracture toughness of the W plate is comparatively small. Fracture surface observations reveal that a crack path is entirely transgranular for the L -T, while crack propagation involves intergranular fracture to a large extent for the T-L orientation. These results obtained are discussed and compared with the literature data.

Recently, the desire of decarbonization is increasing on the part of the general public worldwide as many companies take part in RE100 initiative. Tokyo Gas Co., Ltd. declared to reduce 10 million tons carbon dioxide emission by 2030 in “Compass 2030” which is company vision of Tokyo Gas group in November 27th, 2019. We drive CO2 net-zero including emission from customer and lead the transition to a decarbonized society. To achieve CO2 net-zero, we will take two actions. The first one is expansion of use of renewable energy. Second is development of decarbonization technologies for gaseous energy, which include effective use of natural gas, use of carbon capture, utilization and storage (CCUS). Fuel cell system is expected to use natural gas effectively. In Japan, residential fuel cell system has already installed extensively since the first model was sold in 2009. Tokyo Gas have also studied fuel cell technologies for over three decades. These days, we developed and reported mono generation system which had AC-power generation efficiency of over 65.0%LHV and output of 5kW. In this system, anode lines of solid oxide fuel cell (SOFC) stacks are connected in series. A fuel regenerator is placed between two-stage SOFC stacks, which remove H2O from the anode off-gas of first-stage SOFC stack. Then, the regenerated anode off-gas is used as a fuel of the second-stage SOFC stack. Hence, this system can be operated at high total fuel utilization (Uf) value of over 90% and enable each SOFC stack to be operated at safe Uf condition for each, simultaneously. We could make easy flow system by using condenser as fuel regenerator. This system can reduce much CO2 than existing power resource because of its higher efficiency, but still exhausts some amount of CO2 gas. To achieve decarbonized society, further reduction of CO2 emission for fuel cell system will be required. Therefore, we focused on CO2 separation membrane as a fuel regenerator which has high selectivity for both H2O and CO2. In previous work, we performed DC-power generation of 75.5%LHV by connecting membrane module as the fuel regenerator of the two-stage SOFC system. CO2 recovery rate from membrane reached to about 40%. To improve the CO2 recovery rate, we invent the new system as figure 1. The new system recycles regenerated anode off-gas which was removed H2O and CO2 by CO2 separation membrane. The membrane has high selectivity of H2O and CO2. In addition, a separation at the membrane was driven on the differential pressure by decompressing without sweep gas. As a result, high concentration of CO2 was recovered. In ideal situation, the new system will be operated under Uf of 100% and recovered CO2 of 100%. In this study, we demonstrated this system as shown in figure 1. Membrane was put in fuel cell module. It was operated at a total Uf of 97.1%, and power generation efficiency reached DC 77.3%LHV at power output of 5.65 kW under the thermal self-sustainable condition without the electric heater. Assuming the efficiencies of the auxiliary machines and the power conditioner to be respectively 94% and 95%, the estimated AC-power generation efficiency of the hot module was 69.0%LHV and CO2 emission factor reached to 294 g-CO2/kWh. At the same time, CO2 recovery rate was 85%. We achieved high efficiency and low in this system. In the future, we have to discuss about the use of recovery CO2. Figure 1

An upgraded Ohmic heating (OH) system with a bipolar circuit is designed to achieve plasma current over 100 kA ideally without the effect of eddy current in the QUEST spherical tokamak. IGBT (Insulated Gate Bipolar Transistor) stacks work as switches to change the polarity of OH primary current up to ±8 kA. A main target of the new OH is to generate over-dense plasmas for the 8.56 GHz electron cyclotron (EC) wave. Heating with electron Bernstein waves can be conducted in such over-dense plasmas. The effect of eddy current flowing in the vacuum vessel, which is generated by the current swing through the central solenoid, on null point is investigated by analysis of magnetic field and flux surfaces. In the initial phase, eddy current flowing through the vacuum chamber shifts the null point to the high magnetic field side where multiple EC resonance layers are located.

A solid oxide fuel cell (SOFC) module using two-stage stacks and a fuel regeneration process between them was developed in this study for the first time, to the best of the authors' knowledge. Upon configuring the first-stage and second-stage stacks and a steam reformer between them in the SOFC module, a gross output power of DC 2.27 kW was generated with gross power generation efficiency of DC 69.2% (lower heating value (LHV)), at a total fuel utilization rate of 86.3%. This technology enables operation at a very high total fuel utilization rate even while operating the stacks at a moderate fuel utilization rate (below 70%). Considering an auxiliary device loss (6%) and inverter loss (5%), the net power generation efficiency is estimated to be AC 61.8% (LHV); hence, the module is considered to exhibit a high power generation efficiency. Further increases in the power generation efficiency could be realized in the future by removing the CO2 from the anode off-gas during the fuel regeneration process and/or operating the stacks at higher temperatures by decreasing heat leakage from the module.

Enhancing the power generation efficiency, which is the main advantage of solid oxide fuel cells (SOFCs), can have valuable benefits to introduce SOFC systems into business and industrial markets of Japan, where power demands are higher than thermal demands. In this study, we examined a high-efficiency SOFC system with an off-gas regenerating technique. A two-stage SOFC stack configuration was employed. For off-gas regeneration, we used a CO2 absorber and a H2O condenser. A total of 92.0% of the fuel was successfully used with an electrical efficiency of 77.8% (DC, LHV). However, there existed the heat loss from the fuel cell system due to the thermal insulation performance. To compensate the heat loss, additional electric heaters were used to keep temperatures high, therefore heat sustainability remained an issue.

Sensing vector magnetic fields is critical to many applications in fundamental physics, bioimaging, and material science. Magnetic-field sensors exploiting nitrogen-vacancy (NV) centers are particularly compelling as they offer high sensitivity and spatial resolution even at nanoscale. Achieving vector magnetometry has, however, often required applying microwaves sequentially or simultaneously, limiting the sensors' applications under cryogenic temperature. Here we propose and demonstrate a microwave-free vector magnetometer that simultaneously measures all Cartesian components of a magnetic field using NV ensembles in diamond. In particular, the present magnetometer leverages the level anticrossing in the triplet ground state at 102.4 mT, allowing the measurement of both longitudinal and transverse fields with a wide bandwidth from zero to megahertz range. Full vector sensing capability is proffered by modulating fields along the preferential NV axis and in the transverse plane and subsequent demodulation of the signal. This sensor exhibits a root mean square noise floor of about 300 pT/Hz^(1/2) in all directions. The present technique is broadly applicable to both ensemble sensors and potentially also single-NV sensors, extending the vector capability to nanoscale measurement under ambient temperatures.

Ensembles of nitrogen-vacancy (NV) centers in diamonds are widely utilized for magnetometry, magnetic-field imaging and magnetic-resonance detection. They have not been used for magnetometry at zero ambient field because Zeeman sublevels lose first-order sensitivity to magnetic fields as they are mixed due to crystal strain or electric fields. In this work, we realize a zero-field (ZF) magnetometer using polarization-selective microwave excitation in a 12C-enriched HPHT crystal sample. We employ circularly polarized microwaves to address specific transitions in the optically detected magnetic resonance and perform magnetometry with a noise floor of 250 pT/Hz^(1/2). This technique opens the door to practical applications of NV sensors for ZF magnetic sensing, such as ZF nuclear magnetic resonance, and investigation of magnetic fields in biological systems.

Magnetic field sensors that exploit quantum effects have shown that they can outperform classical sensors in terms of sensitivity enabling a range of novel applications in future, such as a brain machine interface. Negatively charged nitrogen‐vacancy (NV) centers in diamond have emerged as a promising high sensitivity platform for measuring magnetic fields at room temperature. Transferring this technology from laboratory setups into products and applications, the total size of the sensor, the overall power consumption, and the costs need to be reduced and optimized. Here, a fiber‐based NV magnetometer featuring a complete integration of all functional components is demonstrated without using any bulky laboratory equipment. This integrated prototype allows portable measurement of magnetic fields with a sensitivity of 344 pT Hz−1/2.

The plasma current is ramped up primarily by a 28 GHz electron cyclotron wave (ECW) in the Q-shu University experiment Steady-State Spherical Tokamak (QUEST), with multiple harmonic resonance layers from the second to the fourth stay in the plasma core. A steering antenna comprising two quasi-optical mirrors enhances the power density of ECWs. The ECW beam is injected obliquely from the low-field side where the parallel refractive index is N ∥ = 0.75 at the second-harmonic resonance layer. Analysis of the resonance condition has found that energetic electrons moving forward along the magnetic field resonate more effectively than those moving backward. Such symmetry breaking is consistent with the results of the current ramp-up experiment. The peak plasma current reaches I p > 70 kA, constantly injecting a beam of radio frequency power of 100 kW. Ray-tracing by the TASK/WR code demonstrates that the power of the 28 GHz extraordinary mode is absorbed by energetic electrons via single-pass cyclotron absorption.

The plasma control system (PCS) of QUEST is developed according to the progress of QUEST project. Since one of the critical goals of the project is to achieve the steady-state operation with high temperature vacuum vessel wall, the PCS is also required to have the capability to control the plasma for a long period. For the increase of the loads to processing power of the PCS, the PCS is decentralized with the use of reflective memories (RFMs). The PCS controls the plasma edge position with the real-time identification of plasma current and its position. This identification is done with not only flux loops but also hall sensors. The gas fueling method by piezo valve with monitoring the Hα signal filtered by a digital low-pass filter are proposed and suitable for the steady-state operation on QUEST. The present status and prospect of the PCS are presented with recent topics.

In the present RF-driven divertor plasma of QUEST, it has been observed that orbit-driven current flows in the open magnetic surfaces outside of the closed magnetic surfaces. To observe this phenomenon and the characteristics of the orbit-driven current, current density profiles have been calculated on two different equilibrium conditions. We calculated current density profiles from particle guiding center orbits both for the fundamental and the second harmonic resonances for the 8.2 GHz electron cyclotron current drive. From this calculation, hollow current density profiles have been obtained with significant characteristics on both conditions. Only positive current distribution has been observed in the open magnetic surfaces outside of the closed magnetic surfaces.

Fully non-inductive plasma start-up was successfully achieved by using a well-controlled microwave source on the spherical tokamak, QUEST. Non-inductive plasmas were maintained for approximately 3–5 min, during which time power balance estimates could be achieved by monitoring wall and cooling-water temperatures. Approximately 70%–90% of the injected power could be accounted for by calorimetric measurements and approximately half of the injected power was found to be deposited on the vessel wall, which is slightly dependent on the magnetic configuration. The power distribution to water-cooled limiters, which are expected to be exposed to local heat loads, depends significantly on the magnetic configuration, however some of the deposited power is due to energetic electrons, which have large poloidal orbits and are likely to be deposited on the plasma facing components.

In order to evaluate the magnetic surface current density distributions, the electron guiding center orbits were calculated with the equilibrium fitting plasma equilibrium analysis. The current density distributions in electron cyclotron-heated plasma were estimated from the orbit analyses in the equilibrium plasma shaping for the fundamental and the second-harmonic resonant electrons separately. The current density distribution profiles on the equatorial plane were obtained for the electrons with initially positive and negative velocities parallel to the magnetic field direction as well as the trapped electrons. The surface averaged current density profiles of the closed flux surfaces were also evaluated. A significant amount of the positive current density distributions appeared outside the last closed flux surface (LCFS), while all the negative current density distributions were evaluated inside the LCFS. The trapped electrons being the second-harmonic electron cyclotron resonant contributed the negative current inside the LCFS. The positive current density distributions outside the LCFS were significant for the driven current density for both the fundamental and second-harmonic resonances.