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Spatially resolved rotational temperature of ground state hydrogen molecules desorbed from plasma-facing surface was measured in QUEST, LTX-β, and DIII-D tokamaks, and the increases of the rotational temperature with the surface temperature and due to collisional-radiative processes in the plasmas were evaluated. The increase due to collisional-radiative processes was calculated by solving rate equations considering electron and proton collisional excitation and deexcitation and spontaneous emission. The calculation results suggest a high sensitivity for the rotational temperature to electron and proton densities, but a negligible sensitivity to the electron, proton, and surface temperatures. In the three tokamaks with different plasma parameters and plasma facing surface materials, the spatial profile of the rotational temperature was estimated using Fulcher-α emission lines (600-608 nm). In QUEST, the spatial profile of the rotational temperature was estimated from spatially resolved spectra. In the other tokamaks, the rotational temperature was evaluated assuming a single point emission with a location determined from the Fulcher-α emission profile as measured with a filtered camera. In metal-walled devices QUEST and LTX-β, the rotational temperature increased with the surface temperature, and the calculated collisional-radiative increase is consistent with measured increase assuming that the rotational temperature at the surface is approximately 450 K higher than the surface temperature. In DIII-D with carbon walls, a larger collisional-radiative increase than the other tokamaks was observed because of the higher density leading to a large difference from the calculated increase compared to the other smaller tokamaks. Measurement of the Fulcher-α emission profile with higher spatial resolution in DIII-D may reduce the difference and reveal the effect of the surface temperature on the rotational temperature. These results shows the increases in the rotational temperature with the surface temperature and due to the collisional-radiative processes.

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.

In the GAMMA 10/PDX tandem mirror, plasma with strong ion-temperature anisotropy is produced by using the ion cyclotron range of frequency waves. This anisotropy of ion temperature causes several Alfven-Ion-Cyclotron (AIC) waves to spontaneously excite in the frequency range just below the ion cyclotron frequency. In addition, difference-frequency (DF) waves are excited in the radial inner region of the plasma by wave–wave coupling among the AIC waves. The radial density profiles were measured at multi-axial positions using a frequency-modulation reflectometer with an axial array of microwave antennas, and an axial variation of the density was found to be significant. In addition, a relative phase difference of the DF wave between axially separated two points was first obtained by finely choosing the probing frequency of the reflectometers with a maximum coherence used as a measure, indicating that the DF wave is a propagating wave, while the pump AIC waves are standing waves in the axial region of measurement.

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 high voltage DC power supply for the cathode of the 8.56 GHz CW klystron has been set up for electron cyclotron heating (ECH) in steady state tokamak operation on QUEST spherical tokamak. The power supply is equipped with an IGBT array and a reactor for fast shutoff of the voltage in 10 μs, where the influx of electric energy at the short circuit is limited to 5 J. AC switches also have been installed in the three-phase power lines. High voltage relays are useful to save electric energy consumption. Fast three-phase AC switching by IGBT-stack is applicable to reduce the electric load of the components of the klystron power supply.

A new transmission line from 28 GHz gyrotron to QUEST spherical tokamak (ST) has been developed for the highly efficient electron cyclotron heating and current drive (ECHCD) experiments. The ECHCD effect depends on the incident mode or polarization states of the injected beam. Although two corrugated-polarizer miter bends were developed and assembled to transmission line in the QUEST, arcing events were frequently detected at the polarizer miter bend when the high-power millimeter-wave were transmitted. A new quasi-optical (QO) concept for the polarizer system composed of two corrugated plate and a QO mirror was proposed to avoid the arcing at polarizer. The QO mirror was designed for coupling of input and output HE11 modes at the polarizer, its coupling property is discussed with Kirchhoff integral. Polarization-setting performance of the developed QO polarizer was examined in a low-power test system, and is discussed with grating-polarizer calculation.

The transition of a detached to attached plasma experiment has been performed in GAMMA 10/PDX by applying an additional plasma heating pulse of electron cyclotron heating (ECH). In a plasma detachment experiment, a short pulse (25 ms) of ECH was applied at the east plug-cell to examine the effects of electron heating on the plasma parameters in the D-module. It was found that the ion flux increased significantly during ECH injection. In the ECH injection period, the ion flux increased with the increasing impurity injection, which indicates the impurity ion components are enhanced by ionization in the D-module due to the application of ECH. During ECH injection period, the increase of the ion flux near the corner of the target plate shows a clear dependence on the gas species. The 2D visible emission has been captured by the high-speed camera and a bright emission near the corner of the target has been observed. The spectroscopic measurement in the D-module shows that the impurity ion emission increases remarkably during ECH injection. These results indicate the detached plasma change to the attached state. Keywords: GAMMA 10/PDX, Divertor simulation experiment, Plasma detachment, Detached to attached transition

In order to generate high-performance plasma for future fusion power generation, it is desirable to keep high quality vacuum during experiment. Mass spectrometer is commonly used to monitor the vacuum quality and to record the amount of atoms and molecules in the vacuum vessel. Leak is the most serious accident to avoid that can nullify an experiment and even harm researchers. Detecting leaks are ever more important since it can be easily overlooked, e.g., when the deterioration in the vacuum degree is modest. This forces the researcher to carefully observe the vacuum and mass spectrometer data. This article presents a way to suggest potential leaks in the vacuum vessel by analyzing mass spectrometer data. This is done by utilizing the Euclidean distance between composition ratios at different times for the clustering using the daily composition ratio. We show that our cluster analysis is an effective way of separating these two cases, which results in a semi-automatic determination of leaks is more efficient than the current norm, which is to check many measures to find a small abnormality in the data manually. We plan further model improvements for long-term evaluation.

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.

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.

As technology has improved in recent years, it has become possible to create new valuable functions by combining various devices and sensors in a network. This concept is referred to as the Internet of Things (IoT), and predictive maintenance is a new valuable function associated with the IoT. In large-scale experimental facilities with many researchers, it is not desirable that experiments cannot be performed due to sudden failure of equipment. For this reason, it is important to predict the failure in advance based on the measurement results of sensors and to perform repairs in a planned manner. On the Q-shu University experiment with steady-state spherical tokamak (QUEST) large experimental device, it is necessary to drive a large current of 50 kA, and the diagnosis of its power line deterioration is well performed as predictive maintenance through the evaluation of its contact resistances of several micro ohms order on the network. In addition, as an example of the IoT, mechanisms to assist safe operation, such as a sound alarm system and an entrance management system, are built by sharing experimental information between devices via the network., Computer Science; Electrical Engineering; Safety engineering; Internet of Things; System Diagnostics; Electric Power Transmission; System Fault Detection; Computer-Aided Engineering; Predictive maintenance; IoT; safety operation; FPGA; QUEST

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.

We have investigated the synergistic effect of a combination of various impurity gases and hydrogen gas on plasma detachment of high temperature plasma, equivalent to scrape-off layer plasma of tokamaks in the GAMMA 10/PDX end region, utilizing an open magnetic field configuration. A small puff of an impurity gas (N2, Ne, Ar, Kr, Xe) in combination with a puff of H2 gas is examined to evaluate their synergistic effect on the formation of detached plasma; the following results are obtained. (i) A combination of N2 and H2 puffs showed a clear decrease of electron density and ion flux; (ii) N2 and H2 puffs form a strong density gradient along the axial direction; and (iii) other noble impurity gases showed an insufficient synergistic effect. The new results indicate the possibility of achieving a reliable divertor operation scheme and the importance of a deeper understanding of the H2 and N2 assisted recombination process.

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.

A 28~GHz system with a high-power gyrotron tube has been used for the QUEST spherical tokamak to form an over-dense plasma for electron Bernstein wave heating (EBWH) and current drive with an 8.2~GHz-wave. Non-inductive high-density plasma ramp-up experiments with dual-frequency (dual$-f$) electron cyclotron (EC) [ 8.2~GHz and 28~GHz ] waves were conducted. A spontaneous density jump (SDJ) to an over-dense state was observed as a bifurcation phenomenon in the dual$-f$ wave experiment. The over-dense plasma on the 8.2~GHz-wave was non-inductively ramped up to 25~kA, and was maintained for 0.4~s under stable plasma equilibrium after two such jumps in one shot. Heating to mildly energetic electrons and bulk electrons was observed even in the over-dense region. The electrostatic EBWH effect on the mildly energetic electrons in the over-dense region is assessed following a dispersion analysis of the 8.2 GHz-wave. The bulk electron heating effect observed is explained as heat exchange from mildly energetic electrons heated by the electrostatic EBWH wave. Remarkably, a high hard-X-ray-radiation temperature ~ 500~keV was also observed in tangential viewing for current-carrying electrons in the over-dense core region. Synergetic heating from the overlap of different 28~GHz EC harmonic resonances as well as higher harmonic heating is discussed for maintaining the highly energetic electrons in the over-dense core region. In addition, the SDJ process and mechanism are considered based on the discussion of the electron heating effects with the 8.2 GHz-wave.

This paper describes the results of the experiments performed on Tandem Mirror device GAMMA 10/PDX mainly using a new “divertor simulation experimental module (D-module)” installed on one of the end mirror exits which is specially designed to investigate the physics of plasma detachment. The additional ICRF heating in the anchor-cells, connected to both ends of the central-cell, significantly increases the density in the both cells, which attained the generation of the highest particle flux up to 1023 particles/s m2 at the end-mirror exit. H2 and noble gas injection to enhance the radiation cooling in D-module was performed and a remarkable reduction of the electron temperature (from few tens eV to

Recent ICRF heating experiments performed in GAMMA 10/PDX are reported. Owing to antenna-phasing technique and four ICRF antennas installed in the mirror cells other than the central cell, the operational regime of GAMMA 10/PDX is significantly extended. Several ICRF heating schemes to increase the parameters (plasma density in the central cell, particle flux flowing to the machine end and its ion temperature parallel to the field lines) are demonstrated. These results present positive progress in the ongoing divertor simulation experiments on GAMMA 10/PDX.

In GAMMA 10/PDX, the divertor simulation experiment has been performed recently. Ion cyclotron range of frequency (ICRF) waves are used for plasma production and ion heating. It is necessary for obtaining better controllability and extending the operational regime to understand the excitation, propagation, and absorption of the waves. The density fluctuations accompanied by the ICRF waves propagating in the inner region of plasma has been measured by using a microwave reflectometer on the antenna-phasing experiments, where the propagation of the ICRF waves is actively controlled. The phase difference between two axially separated ICRF antennas remarkably affects the global plasma parameters. The density fluctuation caused by the interfered ICRF waves is shown to depend clearly on the phase difference between the waves excited from two antennas. The availability of a reflectometer for the evaluation of the internal wave propagation is shown.

In the GAMMA 10 tandem mirror, divertor simulation experiments that utilize particle flux toward the west end region (called end-loss flux) have been implemented. Since a positive correlation has b...

This paper describes the recent results of divertor simulation research toward the realization of the detached plasma using the end-mirror of a large tandem mirror device. The additional ion cyclotron range of frequency heating in the anchor-cells for higher particle flux generation significantly increases the density, which attained the highest particle flux up to 1.76 × 1023 particles/·m2 at the end-mirror exit. Massive gas injection (H2 and noble gases) into the divertor simulation experimental module (D-module) was performed, and a remarkable reduction of the electron temperature on the target plate was successfully achieved associated with the strong reduction of particle and heat fluxes in D-module. Two-dimensional images of Hα emission in D-module observed with a high-speed camera showed strong emission in the upstream region and significant reduction near the target plate. These results clarified the effect of radiation cooling and formation of detached plasma due to gas injection. It is a...

On the GAMMA 10 tandem mirror, divertor simulation experiments progress with high-temperature plasmas produced by waves in ion-cyclotron range of frequency (ICRF) and open magnetic fields. In these experiments, high-temperature and high-density plasmas are required to be produced and controlled. In order to enhance the ion heating in the anchor cell, phase-control experiments, which use two ICRF antennas installed in the central cell and the anchor cell, are introduced. In these experiments, ICRF waves in the same frequency (10 MHz) are excited. Ion heating in the anchor cell is performed more effectively with phase control. In order to analyze wave propagation in the phase-control experiments, a three-dimensional full wave code (TASK/WF), in which parallel processing has been implemented recently, is introduced. In this technical note, we discuss ICRF wave excitation and propagation in the phase-control experiments. It is clearly observed by the experiments and wave analysis that the enhancement ...

A launcher system for a round-trip interferometer, with a corrugated horn antenna and two quasi-optical mirrors, has been developed for the QUEST spherical tokamak experiment. The corrugated horn antenna has been designed using a three-dimensional (3D) electromagnetic simulator. At low power bench test, intensity and phase profiles at 0.1 m from antenna aperture show coaxial patterns as predicted by the 3D simulator, in agreement with the calculation results using an in-house developed Kirchhoff integral code. The designed waist sizes of the focused beam at a target position are 22 mm in the horizontal x direction and 43 mm in the vertical y direction. The beam reaches the target plate at QUEST's center-post and is reflected back along the line-of-sight round-trip. A well-focused beam of 32 and 36 mm in size (x, y) has been obtained at the target position. The HE11 mode excitation, beam transmission, and focusing properties are discussed for the prototype launcher system.

In the hot ion mode experiments of the tandem mirror GAMMA 10/PDX plasma, suppression of the flute like fluctuation which rotation direction was ion diamagnetic rotation direction was observed during the axially confined potential formation with applying the electron cyclotron heating (ECH) at the barrier and plug cells for the first time. In the previous experiments, the electron diamagnetic rotation type fluctuations were clearly suppressed with application of both barrier (B) and plug (P) ECH. We show the flute like fluctuation of ion diamagnetic rotation is also suppressed with application of B/P-ECH.

To investigate end-loss plasma control, several heating operations were carried out using ion-cyclotron range of frequency (ICRF) heating systems in GAMMA 10/PDX. In anchor region, one of the important heating positions to control the end loss particle and heat fluxes, excited ICRF waves are measured as the density fluctuations using an upgraded reflectometer system and the detailed radial profiles are observed. The efficiency of anchor heating was changed using the phase control between two antennas installed in the anchor and/or the central region. The interference of two waves during a phase-control ICRF heating has been directly measured inside the plasma. In addition, several phase dependencies were observed using absorption methods with photodiode in anchor region.

Effective ICRF heating creates a high ion-temperature plasma with strong ion-temperature anisotropy in the GAMMA 10/PDX central cell, where several Alfvén-ion-cyclotron (AIC) waves are spontaneously excited. It is clearly measured with a microwave reflectometer installed in the central cell that waves with the difference frequencies of the AIC waves are excited in a core region. In addition, it has been observed with an end-loss high-energy ion detector that high-energy ions of over 6 keV are axially transported along the magnetic field lines with the same frequencies as the difference frequencies of the AIC waves. In this study, in order to understand the related wave-particle interaction, another end-loss high-energy ion detector using a micro-channel plate has been developed. By changing the ion retarding grid voltage, which the new detector equips, it is found that an energy dependence exists in the loss mechanism; some frequencies that the axial transport includes disappear for high-energy ions of over 6 keV while they are significant for low-energy ions. In addition, by changing the radial position of the new detector, a radial localization of the axial transport to a core region, which is consistent with the profile of the difference frequencies waves, is indicated.

We used the additional Ion Cyclotron Range of Frequency (ICRF) heating in the anchor cell for the high density plasma production. When the ICRF waves are excited with antennas installed in the anchor cell, it is observed that the plasma potential increases significantly and forms non-axisymmetric potential distribution in the periphery of the central cell. Experiments with different frequencies were carried out to understand the physical mechanism of this potential change due to the ICRF waves. It is confirmed that the ion heating does not play an important role to the potential increase. The floating potentials on some tips set outside of the plasma in the anchor cell remarkably decrease during the additional heating and non-axisymmetric distribution of the floating potential in the central cell depends on the antenna location in the anchor cell. It is suggested that the radial transport of electrons near the antennas in the non-axisymmetric anchor cells with the ICRF antennas is a possible candidate of the potential increase in the central cell.

In the GAMMA 10/PDX tandem mirror, studies on divertor physics have been performed in the west end region by utilizing end-loss plasmas flowing from the confinement region. Since the plasma density in the end region is quite low (∼1016 m−3), an increase of the end-loss ion flux is required. The increase of the end-loss ion flux has been obtained by increasing the density in the confinement region on previous experiments. When an additional ICRF heating using the antennas in the anchor cells has been performed, a significant increase of the ion flux has been observed with the increase of the potential in the central cell although the change of line densities in the confinement region is little. The effect of the potential on the ion flux has been examined on the GAMMA 10/PDX using about 10,000 discharges. The ion flux increases almost linearly with the potential and the density increase. By comparing with a simple calculation, the increase of the ion flux is explained by the expansion of loss cone boundary of ions and the decrease of the transport time of the end-loss ions from the confinement region to the end region.

Beach heating using a slow Alfvén wave in ion cyclotron range of frequencies would be the first candidate for ion heating in a DEMO-relevant divertor testing linear plasma device if it is applicable to a high-density regime. To clarify its availability, the density dependence of a slow wave excitation is investigated using a full wave simulation with TASK/WF3D code in the GAMMA 10/PDX central cell configuration, where there is an extensive track record of a beach heating. A shielding effect is successfully demonstrated and well understood under a three-dimensional configuration in the limit of cold plasma approximation. As the density increases, excitable left-handed electric field, which contributes to ion cyclotron heating, degrades more and more from a core region, and resultantly the ion absorption region goes outwards with reducing its power. For core densities above 1020 m−3, the wave field exists only at a very edge, and ion heating becomes negligible unless the wave frequency is much increased with a correspondent magnetic field enhancement.

Slow Alfvén wave with a frequency just below an ion cyclotron frequency can be destabilized by the anisotropy of ion temperature and called as Alfvén-Ion-Cyclotron (AIC) wave. In the GAMMA 10/PDX central cell, several AIC waves simultaneously appear in an unstable frequency range. In addition, low-frequency waves with the difference frequencies (DF) between the AIC waves are observed, and their characteristics have to be clarified to reveal their unknown contributions to the ion transport suggested in GAMMA 10/PDX. In this study, simultaneous two-point measurements in the azimuthal and axial directions were conducted by using a two-channel microwave reflectometer with multi horn antennas. The azimuthal mode number (m) of the DF waves was confirmed at inner plasma region for the first time, and was zero, which satisfies azimuthal mode matching with the m = −1 AIC waves. In addition, it was shown that the DF waves were radially localized in an inner region, which is different from rather broad profiles of the AIC waves.

In the west end of GAMMA 10/PDX, the end-loss ion flux is utilized for divertor simulation experiments. It is important for controlling parameters of the end-loss ion flux to investigate the efficiency of ion cyclotron range of frequency (ICRF) heating in the west plug/barrier cell, which is next to the west end region. The ICRF heating experiments in the west plug/barrier cell have been performed. In this study, three-dimensional simulations of the ICRF wave propagation were carried out in the plug/barrier cell for the first time. Two types of antennas, a Double Half Turn (WB-DHT) and a Nagoya Type-III (WB-Type-III) antennas, are evaluated. As the results, it is clarified that the WB-Type-III antenna is more effective for the ion heating than the WB-DHT antenna when the frequency is near the ion cyclotron resonance frequency at the midplane of the plug/barrier cell. An additional experiment with the WB-Type-III antenna which is located near the midplane of the plug/barrier cell has been performed. The improvement of the power absorption has been observed. The WB-Type-III antenna located near the midplane is more effective for the ion heating. The experimental results are consistent with the simulation of the wave propagation analysis.