Transient Coaxial Helicity Injection (CHI) in NSTX has generated toroidal plasma currents up to 300 kA. When induction from the central solenoid is then added to these discharges they maintain up to 300 kA additional current compared to discharges initiated by induction only. Standard inductive discharges in NSTX require 50% more solenoid flux than a CHI started discharge to reach 1 MA. The CHI-initiated discharges have low plasma density and normalized internal plasma inductance of 0.35 through the inductive ramp, typical of advanced scenarios planned for future STs. The Tokamak Simulation Code (TSC) has been used to understand the scaling of CHI generated toroidal current with variations in the external toroidal field and injector flux. These simulations show favorable scaling of the CHI start-up process with increasing machine size, consistent with theory. Scaling based on the analysis of experimental results and TSC simulations indicates the possibility for substantial current generation by CHI in the planned upgrade to NSTX. These results demonstrate that CHI is a viable solenoid-free plasma startup method for future STs.

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Predicting and controlling disruptions is an important and urgent issue for ITER and impacts the designs for reactors based on the ST and Tokamak concepts. Disruptions have been such a ubiquitous feature of tokamak operations for decades that, while work is in progress to develop reliable methods to avoid disruptions, some may be unavoidable. For these cases, a fast discharge termination method is needed to minimize the deleterious effects of the disruption, particularly the generation of large populations of runaway electrons. Massive Gas Injection (MGI) is one approach to addressing this difficult issue for ITER. NSTX-U plans to compare MGI from different poloidal locations to assess the gas penetration efficiency. We are starting to model gas penetration using DEGAS-2 and it appears that the scrape-off-layer plasma may place limits on the achievable gas penetration fraction to the separatrix.

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The Helicity Injected Torus with Steady Inductive Helicity Injection (HIT-SI) has achieved a breakthrough in the development of a new method of toroidal plasma startup and current drive. HIT-SI accomplishes helicity injection current drive on a spheromak of major radius 0.3 m with two injectors driven sinusoidally at 14.5 kHz. Results include the first sustainment of toroidal plasma current of over 50 kA at up to 3 times the injected currents added in quadrature. Separatrix toroidal currents—currents not linking the helicity injectors—are sustained at up to 40 kA. Toroidal currents persist for up to 0.65 ms after the injectors are turned off. Results are achieved after helium operations condition the alumina plasma-facing surface. The conditioned alumina walls then act to pump the deuterium, thereby reducing the spheromak density. High performance discharges (I_tor/I_inj ≥ 2) are characterized by a decrease in the n = 1 mode activity measured by surface probes near the midplane of the confinement volume. Suppression of internal magnetic fields (measured by an internal probe) below the predicted Taylor equilibrium indicates that the magnetic fields in HIT-SI can no longer be described by a zero pressure, constant λ equilibrium.

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In order to identify plasma shape, there is a way to represent the plasma current profile with several parameters, and adjust these parameters with least-square technique in order for calculated magnetic values to accord with measured ones. Here, the plasma shape parameters such as minor radius, elongation, and triangularity are chosen as the fitting parameters to represent plasma shape more directly, and the applicability to the control of the plasma shape are described by evaluating its calculation time. In order to find minimum of an objective function with least-square technique, two methods are compared, namely a linear approximation method and a downhill simplex method. While high accuracies of the measured magnetic signals are required, the good reproducibility is obtained, and the plasma shape identification can be done within several milliseconds in both methods.

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Non-inductive plasma current start-up experiments using RF power in the lower hybrid frequency range is being conducted on the TST-2 spherical tokamak. Plasma currents of up to 15 kA have been achieved. The effect of direct current drive can be seen by comparing the cases with co-drive and counter-drive. X-rays in various energy ranges were measured to investigate the interaction between the wave and the electrons. Soft X-ray (SX) measurements revealed that the perpendicular SX emission increased significantly as the plasma current increased, and that the tangential SX emission in the direction of RF drive was enhanced more strongly in the co-drive case compared to the counter-drive case. These observations imply that the fast electrons accelerated by the lower hybrid wave contribute to the plasma current. However, RF amplitude modulation experiments showed that the confinement time of these fast electrons are very short (less than 0.05 ms), much shorter than the collisional slowing down time. Hard X-ray spectral measurements showed that the radiation temperature of fast electrons in the co-direction for current drive was higher than that in the counter-direction. These observations are consistent with the existence of RF-driven fast electrons.

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Recent progress towards SSO of a spherical tokamak (ST) on QUEST is introduced. Zero-dimensional calculations of particle balance for QUEST experiment, refresh wall, and hot wall were performed showing that a SSO time of more than 1000s is required to obtain reliable results in regard to particle balance. Basic features of blobs were investigated in electron cyclotron resonance (ECR) heated plasmas. Formation and maintenance of a single-null divertor configuration was successfully obtained. Plasma duration in the divertor configuration was limited by out-gassing from a non-water-cooled first wall. Design studies for a hot wall are introduced.

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This paper presents initial results from experimental observation of edge turbulence structures in ohmically heated plasmas on Tokyo Spherical Tokamak-2 (TST-2). The experiment was conducted using two-dimensionally movable Langmuir probes. With monitoring floating potential V_f and ion saturation current I_is, the radial and poloidal structures of the edge turbulence are obtained. The high coherence in fluctuations at 10-100kHz frequency was observed by the use of the Langmuir probes 90° toroidally and ∼75° poloidally distant from each other, suggesting that the fluctuations have long range correlation along a magnetic field line. To investigate nonlinear structures of the turbulence, bispectral analysis was used, and the observed auto-bicoherence pattern suggests the presence of the coupling and energy transfer between different modes.

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The power deposition profiles were analyzed with a multiple ray tracing code for the Electron Bernstein Wave Heating and Current Drive (EBWH/CD) experiments in QUEST. In the EBWH/CD experiments in QUEST, the OXB mode conversion scenario was selected for the plasma current sustainment in the rather low-density case. The algorithm for the wave penetration through evanescent layer beyond a O-mode cutoff position was developed for the multiple-ray analysis. The launching antenna positions were considered to obtain the significant wave absorption in the specific propagating direction for current drive experiments using the developed ray-tracing code.

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Electron Bernstein Wave Heating and Current Drive (EBWH/CD) experiments have been conducted to sustain a spherical tokamak configuration in a steady state in QUEST. In the EBWH/CD experiments on QUEST, the O-X-B mode conversion scenario was selected for plasma current sustainment in rather low-density plasmas. A new phased-array antenna system was proposed to launch a pure elliptically-polarized O-mode in the oblique injection. The prototype antenna system was designed and its performance was checked at low power test facilities. Good focusing and steering properties were confirmed in the low power test.

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For stabilization of neoclassical tearing mode (NTM), non-resonant helical field (NRHF) is investigated. The time variation of magnetic island is described by modified Rutherford equation. In this work, plasma parameter change due to NTM is analyzed using 1.5-dimensional transport code TOTAL. In ST plasma, magnetic island at 3/2 mode grows by bootstrap current and the central temperature decreases. If NRHF is added, the effect of bootstrap current decreases and NTM is stabilized.

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TOKASTAR-2 device can be operated to generate the magnetic surface without plasma current by using additional helical field coils. Since magnetic surfaces depend on the location of additional field coils, the setup angle of them was determined using magnetic field tracing analysis. And we investigated the relation between varying coil current ratios and the shift of magnetic surface. In the experiment we tried to induce plasma current in pre-ionized plasma. Only 0.1 kA plasma current was produced and horizontal displacement of plasma was observed.

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The 1-GWe D-T normal-conducting spherical tokamak (ST) reactor is designed using PEC (Physics-Engineering-Cost) system code, and is compared with conventional superconducting tokamak reactor (TR) and other fusion reactors. Especially two reference ST designs and one reference TR design are defined, and their design and environmental parameters are compared. It is found that the cost of electricity (COE), the life-cycle CO₂ emission rate and the energy payback ratio (EPR) strongly depend on plasma aspect ratio in ST designs, in contrast to TR designs which are insensitive to plasma aspect ratio. The clarification of plasma beta and confinement improvement factor in ST plasmas is crucial for searching for optimized reactor designs.

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To reduce reactor construction cost and to get good economic efficiency, it is desired to find out innovative magnetic configurations with compactness and high capability of confining fusion plasmas. We proposed 6 types of the compact spherical tokamak-stellarator hybrid configurations called TOKASTAR, which consists of a few simple spherical or cylindrical coil systems. We carried out magnetic flux surface analysis and equilibrium analysis for them by computational simulation with HSD and VMEC codes to understand their features of confinement capability. As the result, we obtained the TOKASTAR configuration with low-aspect-ratio (average value <A_p>=1.2∼3.1) and high ellipticity, which leads to high equilibrium beta limit. Their flux surfaces form deep well depth (∼50%) but the average rotational transform is relatively low (< 0.1).

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The energy payback ratio (EPR) is the index evaluating how a power plant produces the higher output energy effectively from the lower input energy. We used the system code and evaluated the EPRs of three reactor models; spherical tokamak (NC-ST), low aspect ratio tokamak reactor (LATR), and standard tokamak reactor (TR). The NC-ST and LATR systems have no center solenoid at the inboard side of plasma. In the case of NC-ST, normal conducting coils (NC) are used for toroidal field system. In LATR and TR superconducting coils (SC) are adopted. After we studied the relationship between the EPR of each fusion reactor and the aspect ratio in the range from 1.2 to 4.0, we evaluated the EPR of three typical models; ARIES-ST like, VECTOR like, and ARIES-RS like. The input parameters of aspect ratio, elongation, and normalized beta are as same as those of each original reactor. It was found that the EPR of LATR is the highest. Since LATR has no power loss due to ohmic-heating of normal conducting coils, the input energy of construction, operation and replacement is the lowest.

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We applied the TOTAL (toroidal transport analysis linkage) simulation code for the analysis of the operational scenario of D-³He spherical tokamak reactors with high beta values and high bootstrap current fractions. Several technical elements, such as the control of the fuel ratio or selective exhaust of the α particle, need to be developed to establish steady-state burning. Negative magnetic shear configuration is a candidate for the high bootstrap current fraction operation.

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Ambient fluctuations and the generation of coherent convective structures are investigated as a function of vertical to toroidal magnetic field ratio (B_z/B_t) in the spherical tokamak QUEST [K. Hanada et al., Plasma Fusion Research 5, S1007]. Fluctuation signals are obtained at a high spatial and temporal resolution using the fast visible camera. The coherent structures (blobs) were seen to be apparent only beyond a critical B_z/B_t ratio (∼ 10^-2). This clearly indicates a shift from the drift to the interchange mode with the increase in vertical magnetic field. The blobs are propagated radially at a high speed ∼ 10³ m/s which is ∼ 1/10^th of the ion acoustic speed in QUEST. Lifetime of the blobs is about 700 μs. Second part of the study comprises of the investigation of the finer structures in the current ramp-up phase which is characterized by lower levels of fluctuations. Singular value decomposition (SVD) and principal component analysis (PCA) techniques are applied on the fast camera images to reveal the structures which are otherwise embedded in the overall intensity image. Electron banana orbits were seen to form from the slab annular plasma and grow wider to eventually shape the plasma current front.

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This paper presents a fusion reactor concept named “JUST (Japanese Universities’ Super Tokamak reactor)”. From the plasma confinement system to the power generation system is evaluated in this work. JUST design has features as follows: the superconducting magnet, the steady state operation with high bootstrap current fraction, the easy replacement of neutron damaged first wall, the high heat flux in the divertor, and the low cost (or high β). By winding the OH solenoid over the center stack of toroidal field coil, we have the low aspect ratio and the 80cm thick neutron shield to protect the superconducting center stack. JUST is designed by using the 0-D transport code under the assumption that the energy confinement time is 1.8 times of the IPB98(y,2) scaling. Main parameters are as follows: the major radius of 4.5m, the aspect ratio of 1.8, the elongation ratio of 2.5, the toroidal field of 2.36T, the plasma current of 18MA, the toroidal beta of 22%, the central electron and ion temperature of 15keV and the fusion thermal power of 2.4GW. By using the mercury heat exchanger and the steam turbine, the heat efficiency is 33% and the electric power is 0.74GW.

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To investigate confinement and transport characteristics in the low aspect-ratio torus plasmas which are produced and maintained solely by electron Bernstein waves in the LATE device, we are developing an ion beam probe system for potential measurement. After some component tests, the injection and the detection beam lines are installed on LATE, and the ion beams are injected in vacuum with the toroidal field, where the beam positions and profiles are observed by matrix plates detectors. It is confirmed that the beam positions are controlled by the sweeper and the deflector voltages correctly. However, the beam spreads in the toroidal direction widely and focussing of the beam is necessary. The basic design of the energy analyzer is completed to detect a potential difference of ∼1 V.

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A novel two dimensional Thomson Scattering (TS) Measurement System has been developed By adopting the time-of-flight (TOF) difference in the signals scattered from the three parallel laser beams formed by multiple reflections. New ideas are: (1) a YAG laser beam is reflected by 6 mirrors so that covers the whole r-z plane of the Spherical Tokamak (ST) plasma; (2) the observation position is determined by the time delay on the scattered light which helps to largely reduces the necessary numbers of spectrometers and detectors. Using a torus plasma merging device TS-4. we successfully measured the electron temperature at nine points with showing the basic theory of this novel TS system.

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Helium microwave-induced plasma (He-MIP) for nonmetal elements analysis was generated by a 2.45 GHz microwave power (∼1 kW) with the Okamoto cavity of the surface wave mode at atmospheric pressure. Effects of the main parameters of the cavity such as a thickness (direction of the electric field) of the cavity, the width of a ring-slot and the position of a discharge tube (torch) on an excitation temperature and the analytical signals were investigated. The excitation temperature and an electron density of the plasma as a parameter of the microwave power, and the plasma and carrier gas flow rates were measured. With 600 watt input microwave power, the excitation temperature of high energy part of 7000 K and the electron density of 2.4×10¹⁴/cm³ were obtained. An aqueous solution of NaF was introduced into the center of the annular He-MIP and the F I (685 nm, 14.5 eV) signal was detected, and the effects of microwave power and the gas flow rates on the signal were studied. The detection limit of 60 ppb for fluorine was obtained.

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In recent years, inverter drive machines such as a hybrid vehicle and an electric vehicle are operated under high voltage pulse with high repetition rate. In this case, inverter surge is generated and affected the machine operation. Especially, the enameled wire of a motor is deteriorated due to the partial discharge (PD) and finally breakdown of the wire will occur. In order to investigate a PD on a resistant enameled wire, characteristics of PD in the twisted pair sample under bipolar repetitive impulse voltages are investigated experimentally. The relationship between the applied voltage and discharge current was measured at PD inception and extinction, and we estimated the repetitive PD inception and extinction voltages experimentally. The corresponding optical emission of the discharge was also observed by using an ICCD camera. Furthermore, ozone concentration due to the discharge was measured during the life-time test of the resistant enameled wires from a working environmental point of view.

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Downsizing has become essential in the development of motors recently in order to meet requirements related to environmental issues such as resource depletion and energy shortages. To fulfill these demands, we propose downsizing techniques for a conventional motor where a solid insulation system is hired and a large amount of insulation resin was employed to replace structural metal materials used in conventional motors.
This paper deals with a feasibility study of a solid insulation system applied downsized motor. The optimal heat design, mechanically stable structure, and proper insulation resin are proposed and discussed. A prototype motor was manufactured, and results of a basic insulation test, temperature rise test and noise test are presented.

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The authors proposed a novel chemical vapor deposition system, a new evaporation supply method by using flash boiling spray, to improve several kinds of problems such as the decomposition of the precursor at supply line and evaporator. In this paper, the relation between film surface condition and injection quantity was investigated. Tetraethyl methyl amino hafnium and n-pentane were used as the mixed solution, and HfO₂ film was deposited on Si substrate by using this method. As a result, the film surface roughness and grain size are increased as increasing injection quantity.

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