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A signal separation system is constructed on the multi-pass Thomson scattering system of Heliotron J to solve the problem of overlapping scattered light signals for the electron temperature anisotropy measurement. The phenomenon of overlapping scattered light signals is relieved by operating the signal separation system. A Raman scattering experiment is undertaken to verify the separation effect of the signal separation system. Two scattered light signals corresponding to two adjacent incidences of one laser shot were extended to 104 ns. Moreover, we applied the multi-pass Thomson scattering system with signal separation system to the electron temperature anisotropy measurement. No anisotropy was observed within the error bars in the initial experiment.

This paper proposes a design of dual scattering angles multi-path Thomson scattering system with a signal separation function to solve the overlapping phenomenon of scattered light signals and to increase the measurement accuracy for the investigation of anisotropic electron velocity distribution. Furthermore, an optical path design is proposed to demonstrate how overlapping scattered light signals can be separated by setting the optical path in a limited room with a compact layout, which makes the incident interval between two overlapping scattered light signals 1.7 times longer than that of our current system. The specific position of each optical component existing in the system is determined via a Gaussian beam analysis to avoid damage caused by overexpansion of spot size with the application of two cooperating image relay systems. Conversely, a polychromator is optimized by resetting the pass waveband of the interference filter combination to achieve high accuracy in electron temperature ( T e) measurement corresponding to two scattering angles simultaneously.

Here, we report on role of pre-ionization using non-resonant 2.45 GHz microwave heating (P 2.45 GHz < 20 kW) in plasma start-up of neutral beam injection (NBI) for heliotron configurations in low beam power (P NB) under non-resonant heating condition. A rapid electron heating towards burn-through of the low-Z impurities was observed experimentally in the early phase of beam injection when the seed plasma density produced by the non-resonant heating was enough for the plasma start-up. Beam heating time to the burn-through increased with decreasing the seed plasma density and a critical density condition of the seed plasma for successful start-up was observed experimentally. Proper timing of the gas fuelling is critical for plasma expansion because the beam fuelling is not significant. A 0-dimensional (0D) model analysis of the NBI start-up developed in this study well reproduces the experimental results. The 0D model clarifies the physical mechanism of the NBI start-up using pre-ionization described as follows: (1) the seed plasma produces sufficient beam ions immediately after beam injection, (2) the beam ions heat up electrons that promote the ionization/dissociation of the background neutrals, (3) this process acts as a positive feedback loop resulting in further electron heating towards burn-through. The 0D model analysis shows that the critical density corresponds to the state at which the electron heating by the beam ions is equal to electron power loss due to conduction and ionization/dissociation.

We report the development of a new interferometer with two stable, high-power, 320 GHz solid-state sources in Heliotron J. A heterodyne Michelson interferometer optical scheme is employed. Two solid-state oscillators are utilized as sources with a fixed frequency at 320 GHz and frequency tunable of 312–324 GHz. Quasi-optical techniques are used for beam transmission. The beam is elongated in the vertical direction with two off-axis parabolic mirrors and injected into the plasma as a sheet beam for the multi-channel measurement (>5 ch.). Passing through the plasma, the beam is reflected at a retroreflector-array installed at the vacuum chamber wall. The retroreflector-array is a bunch of retroreflector structures, which can suppress the beam refraction caused by plasma without much space inside a vacuum chamber unlike a single retroreflector and can facilitate the system design. The source, detectors, and the retroreflector-array are tested to evaluate their basic performance on a tabletop experiment.

The radial correlation length Lr of electron density fluctuations has been measured using a two-channel Omode reflectometer system in the helical-axis fusion plasma experimental device Heliotron J. The experimental results show that Lr is around 1.4 n 0.7mm in three different magnetic field bumpiness configurations (i.e. low, medium and high) for the low-density electron cyclotron heating (ECH) discharges. In high-density neutral beam injection (NBI) discharges with HIGP and pellet injection, Lr is found to be around 1 n 0.2 mm.

When realising future fusion reactors, their stationary burning must be maintained and the heat flux to the divertor must be reduced. This essentially requires a stationary internal transport barrier (ITB) plasma with a fast control system. However, the time scale for determining the position of the foot point of an ITB is not clearly understood even though its understanding is indispensable for fast profile control. In this study, the foot point of the electron ITB (eITB) was observed to be reformed at the vicinity of a magnetic island when the island started to form. In addition, the enhanced confinement region was observed to expand during the eITB formation according to the radial movement of the magnetic island toward the outer region. Compared to the time scales of the local heat transport, the faster time scales of the movement of the eITB foot point immediately after island formation (~0.5 ms) suggest the importance of the magnetic island for plasma profile control to maintain stationary burning., 核融合エネルギーの実現に必要な水素プラズマの超高温領域を瞬時に拡大することに成功 --ゼロエミッションエネルギー実現に向けて前進--. 京都大学プレスリリース. 2020-01-09.

Stray millimeter-wave radiation from a 70-GHz electron cyclotron heating and current drive (ECH/ECCD) system has been measured in the Heliotron J helical device. Two rotatable diode detectors located at the outboard side ports are used to pick up the stray radiation: one is installed at the ECH launcher port and the other is installed at a toroidal angle of 135 deg far from the ECH launcher port. Both detectors are rotated to measure the polarization of the stray radiation. The results show that at the toroidal position far from the ECH launcher, the polarization is not fully randomized before plasma breakdown, whereas the polarization is uniform after a quasi-stationary plasma is generated. The polarization near the ECH launcher is not uniform even in a quasi-stationary plasma. Plasma experiments scanning the electron density indicate that the EC power absorption estimated from the stray radiation at the toroidal position far from the ECH launcher is correlated with the single-pass absorption rate calculated by the TRAVIS ray-tracing code. These results indicate that the diagnostic using a simple diode detector can be used as a real-time monitor of EC power absorption only when the plasma with finite density is produced and the detector is placed far from the ECH launcher port.

A radial correlation ECE radiometer diagnostic has been developed for electron temperature fluctuation measurements in the helical-axis heliotron device, Heliotron J. The radiometer consists of two heterodyne detection systems. One system scans the frequency of a local oscillator from 52 to 64 GHz with a single intermediate frequency filter, and the second system has a fixed frequency, 56 GHz local oscillator with four intermediate frequency filters. This frequency range covers measurement positions spanning from the plasma core to the half radius. Laboratory tests indicate that each system has narrow intermediate frequency bandwidth and high-sensitivity over a large dynamic range. During plasma experiments with NBI heating, radiation temperature fluctuation measured by the CECE radiometer decrease with increasing ECCD commensurate with previous measurements of energetic particle driven modes on Heliotron J.

A simple near-infrared (NIR) spectrometer with a wavelength range of 898–2130 nm has recently been applied to diagnose Heliotron J plasmas. It adopts a symmetrical crossed Czerny–Turner mount equipped with a thermoelectrically cooled 512 channel InGaAs linear sensor. Reciprocal linear dispersion was deduced to 96.37 nm/mm at the center of the detector. External filters can be inserted into the path of the collection optics to reject second-order spectra, as needed. Absolute intensity calibration was performed together with a visible spectrometer using a tungsten halogen lamp, and the effect of the transmittance fringe in the visible region of the applied long-pass filter on the NIR calibration was investigated. The intended application of the NIR spectrometer includes extending the wavelength region of a spectral monitor to less contaminated regions for Heliotron J plasma studies. In preliminary measurements, we observed the Paschen series for the hydrogen pellet injection plasma and two atomic helium lines, i.e., 2S-2P singlet and triplet lines, in helium gas puffing experiments. A continuum spectrum in this regime that is attributable to black-body radiation from hot spots on the plasma-facing components was identified. In addition, this may also be used to monitor background radiation in the YAG-Thomson scattering signals near 1064 nm.A simple near-infrared (NIR) spectrometer with a wavelength range of 898–2130 nm has recently been applied to diagnose Heliotron J plasmas. It adopts a symmetrical crossed Czerny–Turner mount equipped with a thermoelectrically cooled 512 channel InGaAs linear sensor. Reciprocal linear dispersion was deduced to 96.37 nm/mm at the center of the detector. External filters can be inserted into the path of the collection optics to reject second-order spectra, as needed. Absolute intensity calibration was performed together with a visible spectrometer using a tungsten halogen lamp, and the effect of the transmittance fringe in the visible region of the applied long-pass filter on the NIR calibration was investigated. The intended application of the NIR spectrometer includes extending the wavelength region of a spectral monitor to less contaminated regions for Heliotron J plasma studies. In preliminary measurements, we observed the Paschen series for the hydrogen pellet injection plasma and two atomic helium lin...

In the medium-sized heliotron device Heliotron J, edge density fluctuation has been measured simultaneously with a high-speed video camera and a Langmuir probe. Poloidally propagating, parallel elo...

The formation of an electron internal transport barrier (eITB) has been observed for the first time with centrally focused electron cyclotron heating (ECH) microwaves injected into plasma in Heliotron J. When the heating power per electron density (${P}_{\mathrm{ECH}}/{\bar{n}}_{{\rm{e}}}$) exceeds a threshold of $250\times {10}^{-19}\,\mathrm{kW}\,{{\rm{m}}}^{3}$, transient increases of both the central Te and the core Te gradients are observed. A neoclassical (NC) calculation using the Sugama–Nishimura moment method predicts that the large positive radial electric field (Er) is formed in the core region. Heat transport analysis shows a significant reduction of the effective electron thermal diffusivity in the plasma with the eITB related to that without the eITB. The large gap between the experimentally obtained effective thermal diffusivity and the NC thermal diffusivity suggests that the suppression of anomalous transport contributes to the core improved confinement of the eITB plasma. The electron cyclotron emission measurement shows both the transient increase and the hysteresis phenomena during the eITB formation.

Experiments of suppressing fast-ion-driven MHD instabilities such as energetic particle modes (EPMs) and global Alfven eigenmodes (GAEs) have been made using a second harmonic x-mode electron cyclotron heating and current drive (ECCD) in the helical-axis heliotron device, Heliotron J. ECCD experiments show that the GAEs destabilized by fast ions of neutral beam injection with the observed frequency around 140 kHz are fully stabilized, and the EPMs with the observed frequency around 90 kHz are suppressed when the EC-driven plasma current flowing in the counter direction reaches approximately 0.7 kA. The low magnetic shear under the vacuum condition is modified into positive magnetic shear when counter-ECCD is applied, and the amplitude of GAEs and EPMs decreases with an increase in EC-driven plasma current. These results indicate that magnetic shear plays a key role in controlling GAEs as well as EPMs. The comparison of the calculation of shear Alfven spectra with experimental results shows that the increasing continuum damping rate with an increase in local magnetic shear by EC-driven current is important for both EPMs and GAEs. Moreover, the increase in plasma current leads to the inward movement of GAEs. This effect would also contribute to suppression of GAEs because the continuum damping rate increases more and more toward the core. Steady ECH is also found experimentally to be effective for controlling the amplitude of both GAEs and EPMs. The amplitude of EPMs, and especially for GAEs, decreases with an increase in the ECH power under fixed density conditions.

This paper describes the development study of the beam emission spectroscopy (BES) for the turbulent transport study in Heliotron J. Modification of the sightlines (10 × 4 for edge and 10 × 2 for edge) enables us to obtain 2-dimensional BES imaging. The cooling effect on the reduction in the electrical noise of avalanche photodiode (APD) assembly has been investigated using a refrigerant cooling system. When the temperature of the APD element has set to be -20 °C, the electrical noise can be reduced more than 50%. The measurement error of the phase difference in the case of low signal level has been tested by two light-emitting diode lamps. The APD cooling has an effect to improve the measurement error at the low signal level of APD.

A Faraday-cup type lost-fast ion probe (FLIP) has been designed and installed in Heliotron J for the purpose of the studies of interaction between fast ions and MHD instabilities. The FLIP can measure the co-going fast ions whose energy is in the range of 1.7–42.5 keV (proton) and pitch angle of 90∘–140∘, especially for fast ions having the injection energy of neutral beam injection (NBI). The FLIP successfully measured the re-entering passing ions and trapped lost-fast ions caused by fast-ion-driven energetic particle modes in NBI heated plasmas.

The 3D peripheral plasma and neutral transport code, EMC3-EIRENE was applied to the Heliotron J with a wide and flexible controllability of magnetic configuration. This code requires a three-dimensional (3D) grid with high resolution in the peripheral plasma region to reproduce the fine plasma structure. The grid generation tool, FLARE, was utilized to create the grid in conjunction with the code developed to arbitrarily set the outer boundary of the peripheral grid. After setting up the 3D grid, we carried out the EMC3-EIRENE calculation successfully for the first time in the Heliotron J's standard configuration. In addition, the convergence of the iterative calculation and the effects of different grid resolutions upon the calculation were investigated. A good numerical convergence was obtained, and the influence of resolution was observed in the electron density in the divertor region.

The optimization of gas fueling scenarios has been studied to improve the plasma performance in Heliotron J. Gas fueling effects on the plasma profile are discussed by comparing two fueling methods, a short pulse H 2 -beam fueling with a supersonic molecular-beam injection (SMBI) technique and a high intensity gas-puff fueling (HIGP) with a conventional gas puff technique. The maximum plasma stored energy W p after SMBI is about 20% higher than that after HIGP in this NBI-only sustained plasma experiment, where the line-averaged density is almost the same (∼3 × 10 19 m −3 ) for both cases. The electron and ion temperatures in the SMBI case are higher than those are in the HIGP case at the W p -peak timing. A peaked density profile is observed after SMBI, while it is flat or slightly hollow for the case of HIGP. These observations point out the importance of SMBI fueling for plasma density control to obtain better plasma performance.

A fast video camera is verified to be a powerful tool for observation of filaments/blobs near the last closed flux surface (LCFS). In order to extract the fluctuation component from the raw data of the fast camera, a pre-processing technique, sliding time window averaging subtraction (STWAS) has been developed to remove the background of slowly varying emission from the bulk plasma. By using this pre-processing technique, the fast camera data are analyzed. A method to identify the filaments in the pre-processed image is also discussed.

Since fueling technique is very important for maintaining fusion plasma, supersonic molecular beam injection (SMBI) was studied using mainly fast cameras, Hα measurement, Langmuir/magnetic probes, and electron density/diamagnetic measurement in Heliotron J. Using a fast camera with a tangential view a very bright stripe along the magnetic field line was observed during SMBI. Time-dependent FFT analysis of data from each pixel showed that the low frequency waves rotated around the magnetic field line in a left-handed sense at the initial stage of SMBI. After a few milliseconds they propagated towards the SMBI region along the magnetic field line, and their phase velocities were almost the same. Experimental evidence is consistent with the interpretation as the ion acoustic wave, and the peak frequency of these waves was the same as that of the power spectra of the magnetic probe signals. It suggests the slow magnetoacoustic wave may convert into the ion acoustic wave due to collisions with neutrals.

Fueling control with a supersonic molecular-beam injection (SMBI) technique is examined in Heliotron J comparing with conventional gas-puffing. The supersonic H2-beam is injected from a fast valve with a conic nozzle and the apparent fueling efficiency is estimated to be about 50–60% for NBI and 20–30% for ECH + NBI discharges with n ¯ e ∼ 1 × 1019 m−3. In a combination heating of ECH and NBI, the stored energy reached ∼4.5 kJ, which is about 50% higher than the maximum one achieved so far under the similar heating condition with conventional gas-puffing in Heliotron J. Although a short pulse of intense gas-puff can also produce a rapid increase of n ¯ e like the SMBI case, the n ¯ e –Wp dependence cannot overcome the saturation in higher density region.

Recent progress in plasma control studies on the improvement of plasma performance in Heliotron J is reviewed. The supersonic molecular beam injection (SMBI) fueling is successfully applied to Heliotron J plasma. A supersonic H2-beam is effectively injected to increase fueling efficiency and generate a peaked density profile. Local fueling with a short-pulsed SMBI can increase the core plasma density and avoid the degradation arising from edge cooling. Second harmonic electron cyclotron current drive (ECCD) experiments were conducted by launching a focused Gaussian beam with a parallel refractive index of −0.05 ≤ N|| ≤ 0.6. Results show that the electron cyclotron (EC) driven current is determined not only by N|| but also by local magnetic field (B) structure where the EC power is deposited. Detailed analysis of the observed N|| and B dependences is in progress with a ray-tracing simulation using the TRAVIS code. Fast ion velocity distribution was investigated using fast protons generated by ion cyclotron resonant frequency (ICRF) minority heating. For the standard configuration in Heliotron J, charge exchange neutral particle analysis (CX-NPA) measurements show higher effective temperature of fast minority protons in the on-axis resonance case compared to that in the HFS (high field side) off-axis resonance case. However, the increase in bulk ion temperature in the HFS resonance case is larger than that in the on-axis resonance.

A passive spectroscopic method that can measure spatially resolved emission spectra only by observing from a single diagnostic port without assuming plasma symmetry was applied to the stellarator-heliotron device Heliotron J. The emission spectra from the inboard and outboard sides were separated using the difference in their Zeeman effects. On the basis of the fact that in a spectrum as a function of the wavelength, the magnitude of the wavelength splitting by the Zeeman effect relative to the Doppler broadening increases with the wavelength, the method was applied to a near-infrared helium emission line (He I 1s2s 3S−1s2p 3P, 1083 nm), and it was confirmed that the uncertainty in the separated spectra was reduced compared with that in the application to visible emission lines. From the emissivity of the separated spectra, the local recycling flux was evaluated using the inverse photon efficiency method.

Electron cyclotron current drive (ECCD) experiments have been made in Heliotron J by using an upgraded EC launching system. A focused Gaussian beam is injected with the parallel refractive index, N||, ranging from -0.05 to 0.6. Ray tracing calculation shows that the EC power is more localized than that in the previous launching system. In the initial ECCD experiment, the EC injection angle has been successfully scanned as designed. Comparison between perpendicular and oblique launching experiments shows that for standard configuration, the EC current of 1 kA is driven at N� ∼ 0.3 in the Fisch-Boozer direction, and the bootstrap current flows up to 1 kA. A large increase in electron cyclotron emission (ECE) signals has been observed when the EC current was driven. c

A microwave reflectometer is developed for electron density profile measurement in Heliotron J. An amplitude modulation (AM) type system is adopted to reduce density fluctuation effects. The carrier frequency ranges from 33 to 56 GHz, and the modulation frequency is 200 MHz. The X-mode is selected as the propagation mode in order to measure a hollow density profile which is typically observed in ECH plasmas. A test-bench examination using an aluminum reflection plate shows that the measured phase shift agrees well with that expected from the change in the plate position. The initial measurement results show that the reconstructed density profile has hollow profile at low density and steep gradient at the edge in ECH plasma. The hollowness is weakened as the averaged density increases (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In Heliotron J, low magnetic shear configuration, instabilities with frequency chirping in the frequency range of Alfven eigenmodes have been observed in tangentially injected neutral beam plasmas. These modes are induced by energetic-particle driven magnetohydrodynamic (MHD) instabilities such as global Alfven eigenmode or energetic particle mode. A hybrid directional Langmuir probe system has been installed into Heliotron J to investigate the response of fast-ion fluxes to the MHD modes. A high coherent response of the ion flux to the bursting modes has been observed not only by the co-directed probe but also by the counter-directed one. A linear correlation between the response of the co-directed ion flux and the mode amplitude has been found. The radial profile of the response of the co-directed ions has decreased with the minor radius and has not been obtained significantly outside last closed flux surface. These results indicate that the fast-ion response is due to a resonant convective oscillation. The ion flux response of the counter-directed probe has appeared in the growth phase of the mode burst. Its phase relation is different from that of co-directed one and magnetic probe located at the Heliotron J vacuum vessel. Two candidates of the detected ion flux of the counter-directed probe have been discussed (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Gas fueling by supersonic molecular beam injection (SMBI) is successfully applied to ECH/NBI plasmas in Heliotron J. The stored energy reached ∼4.5 kJ for a heating combination of ECH (∼0.35 MW) and NBI (∼0.6 MW), which is about 50 % higher than the maximum one achieved so far under the conventional gas-puffing in Heliotron J. After an SMBI pulse for ECH+NBI plasma, two different kinds of perturbation in the radiation profile are caused simultaneously in the center and the peripheral regions. For ECH plasma, an SMBI pulse causes increase or decrease of electron temperature, which seems to depend on the target density (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The plasma confinement properties of Heliotron J plasmas and the toroidal current are investigated. A plasma energy of 2.5 kJ has been achieved by 70 GHz‐0.35 MW electron cyclotron heating (ECH). The energy confinement time is within the expected values as determined by the stellarator scaling law. In the high density region, however, better confinement plasmas are observed. The transition phenomena characterized by Hα signal drop are sometimes observed in such a region. The toroidal current generally affects plasma confinement since it generates a poloidal magnetic field. From this point of view, toroidal current control is studied in terms of field-component variation and electron cyclotron current drive. The zero current condition is found in the inner vertical field scan. Current control using the electron cyclotron wave is also demonstrated.

In order to extend the high-confinement reversed shear regime with an internal transport barrier (ITB) toward high radiation loss power, Ne and Ar injection have been investigated in JT-60U. With Ne injection, high confinement ( H 89PL >2.4, HH 98(y,2) >1.6) and high radiation loss power fraction (>80%) were simultaneously obtained at high density (>0.7 n GW ). The radiation loss power was enhanced both in the main plasma and in the divertor plasma. With divertor plasma detachment and an X-point MARFE, the ITB was pronounced and the H 89PL factor reached 1.8. However, Ar injection resulted in excessive radiation loss enhancement inside the ITB. The Ne and the C density profile were similar to the electron density profile, and their accumulation was not significant. On the other hand, Ar showed accumulation inside the ITB.

In ELMy H-mode plasmas of Japan Atomic Energy Research Institute Tokamak-60 Upgrade, extension of a high confinement regime toward high density and high radiation-loss-power fraction has been investigated by Ar injection. With injecting Ar and putting the outer strike point on the top of divertor dome, confinement improvement, electron density, and radiation-loss-power fraction reached the ITER relevant regime; the thermal energy confinement time was as high as that the International Tokamak Experimental Reactor ELMy H-mode scaling IPB98(y,2), the electron density is ∼80% of the Greenwald density, and the radiation-loss-power fraction is ∼0.8. Although electron density profile is slightly peaked with Ar injection, the confinement improvement is mainly attributed to the improvement of ion transport. Moreover, the maximum divertor heat load due to ELMs has been reduced by a factor of 3–5 with keeping high confinement. ELM activity can be controlled by injecting Ar and changing the strike point position.

Two-dimensional (2D) reconstruction software was developed for bolometer signals of the JT-60U tokamak. Radiative divertor plasma with emissivity of 10 MW m −3 was found previously to extend 10 cm from the target plates. When the density is increased further by larger gas puffing, the emissivity imbalance between both divertor legs becomes less significant. The radiative zones move upward along the divertor legs and eventually merge into a single bright peak near the X-point at density around 6×10 19 m −3 called X-point multifaceted asymmetric radiation from edge. Upward movement of the radiation peak during detachment is sometimes observed directly by the in-vessel bolometer. Total radiated power at the divertor saturates at a certain level while the core radiation continues to increase with the density. Although the 2D profile of the divertor radiation undergoes dramatic changes near the saturation, the energy confinement appears to degrade with density increase irrespective of these changes.

Heat and particle control has been studied under the reactor-relevant high-power heating in the large tokamak of JT-60U with an open divertor and progressively a W-shaped pumped divertor. Heat and particle control is crucial for reduction in heat load onto the divertor plates, control of density in the main plasma, effective exhaust of helium ash, and reduction in impurity contamination. For the reduction of heat load, radiative divertor concept was developed based on understanding of heat and particle transport in scrape-off layer and divertor plasmas, which contributed to establishment of divertor concept in ITER. With argon injection, the total radiation loss power reached up to 80% of the net heating power with high confinement of HH{sub y2} {approx} 1, where HH{sub y2} is a confinement enhancement factor over the IPB98(y,2) ELMy H-mode scaling, at high density of 80% of the Greenwald density in the ELMy H-mode plasma. For the density control, the dependence of particle confinement on plasma parameters was systematically studied with two confinement times for center- and edge-fueled particles, which enabled discussion of density controllability. Core fueling using a high-field-side pellet injection extended the operation range of high confinement (HH{sub y2} {approx} 1) from 60 to 70% ofmore » the Greenwald density in the high {beta}{sub p} ELMy H-mode plasma. Efficient helium ash exhaust of {tau}{sub He}*/{tau}{sub E} = 2.8 was demonstrated in the ELMy H-mode plasma with the pumping from the private flux region, which is the same pumping geometry as that in ITER design. Reduction in Z{sub eff} by puff-and-pump scheme was demonstrated, and chemical sputtering yields were estimated with the consideration of not only methane but also heavier hydrocarbons. Their sputtering yields showed strong dependence on the wall temperature and weak dependence on the particle flux. The measured profiles of C II and C IV line intensities were well reproduced by the Monte Carlo impurity transport simulation code (IMPMC code). The estimation of sputtering yields and development of the simulation code enabled reliable predictions for impurity behavior in a fusion reactor.« less

The diagnostic system of JT-60U (JT-60upgrade) is composed of about 50 individual diagnostic devices. Recently, the detailed radial profile measurements of plasma parameters have been improved, so ...

The significance of real-time feedback control is emphasized in this paper as an indispensable method to improve and sustain the improved plasma characteristics in JT-60U high fusion performance discharges as well as to operate the fusion reactor under the optimal divertor conditions with respect to the heat load and exhaust pumping. In accordance, substantial improvement in the equivalent fusion amplification gain of over unity has been reproducibly achieved at the JT-60U tokamak in the reversed shear mode of operation with the robust feedback controls, where the value of target density was deliberately optimized for the reliable internal transport barrier formation, and the magneto-hydrodynamic stability control was performed with the stored energy feedback. The feedback control techniques also demonstrated the effectiveness to produce quasi-steady-state high-performance plasmas. In addition, three major parameters associated with the fusion reactor instrumentations, namely the neutron production rate, operating density, and divertor radiation power, were simultaneously feedback controlled in the ELMy H-mode plasmas. Here, the matrix response function was evaluated to identify the limitations involved with the linear combination of independent controls. Other advanced feedback schemes, such as the feedback suppression of the neoclassical tearing mode required to sustain high plasma pressure in a steady-state, are alsomore » described. Finally, the controversial issues for the future intelligent plasma control necessary for the advanced steady-stated tokamak reactor are addressed.« less

Fusion plasma performance and confinement studies on JT-60 and JT-60U are reviewed. With the main aim of providing a physics basis for ITER and the steady-state tokamak reactors, JT-60/JT-60U has b...

A fluctuation analysis technique using analytic signals is proposed. Analytic signals are suitable to characterize a single mode with time-dependent amplitude and frequency, such as an MHD mode observed in fusion plasmas since the technique can evaluate amplitude and frequency at a specific moment without limitations of temporal and frequency resolutions, which is problematic in Fourier-based analyses. Moreover, a concept of instantaneous phase difference is newly introduced, and error of the evaluated phase difference and its error reduction techniques using conditional/ensemble averaging are discussed. These techniques are applied to experimental data of the beam emission spectroscopic measurement in the Heliotron J device, which demonstrates that the technique can describe nonlinear evolution of MHD instabilities. This technique is widely applicable to other diagnostics having necessity to evaluate phase difference.

A Nd:YAG Thomson scattering system has been developed for Heliotron J. The system consists of two 550 mJ 50 Hz lasers, large collection optics, and 25 radial channel (∼1 cm spatial resolution) interference polychromators. This measurement system achieves a S/N ratio of ∼50 for low-density plasma (n [e] ∼ 0.5 × 10{19} m{−3}). A time evolution of electron temperature profiles was measured with this system for a high-intensity gas-puff (HIGP) fueling neutral-beam-injection plasma. The peripheral temperature of the higher-density phase after HIGP recovers to the low-density pre-HIGP level, suggesting that improving particle transport in the HIGP plasma may be possible.

First results of the two-dimensionally reconstructed distribution of divertor radiation in JT-60U are presented. Hardware improvements of in-vessel divertor bolometer cameras to withstand severe electrical and thermal loads and the development of tomography software have made detailed and visual studies of divertor radiation possible. Line-integrated bolometer signals are successfully mapped onto the JT-60U geometry, indicating characteristic profiles for radiative divertor operation. A unique measurement of the radiating layer width at the target plate independently confirms the results of tomographic analysis. Radiation which is almost uniformly distributed along the separatrix flux line from the inboard to the outboard divertor is identified in the moderate density H-mode. Temporal evolution of radiative collapse due to heavy argon radiation is explored. Difficulties in the reconstruction associated with the local hot spot and neutrals are also discussed.

The divertor geometry of JT-60U was modified in 1998 to add a pumping slot near the outer strike point for improved pumping. In the both-side pumping divertor, the x-point MARFE onset density is ∼10% higher with pumping than without pumping. Radiated power near the x-point is decreased by pumping, and the divertor pumping is effective in suppression of the x-point MARFE. The effect of outer pumping is investigated in L-mode plasmas. The onset densities of the x-point MARFE are the same in both divertors. The fraction of total radiated power over absorbed power at the x-point MARFE onset is ∼20% higher in the both-side pumping divertor than in the inside pumping divertor, but it does not depend on the onset density and on a distance between the private dome and the outer leg. Carbon penetration from the divertor region to the main plasma has a weak dependence on the gap-out in the both-side pumping divertor.

Transport coefficients of impurities in reversed shear plasmas are evaluated and compared with those in ELMy H-mode plasmas. The diffusivity is significantly reduced (∼1/10) at the internal transport barrier. An inward pinch velocity of −3 m/s is also observed at the ITB. The inward pinch velocity increases as charge number of impurity increases. Effects of Ar seeding to the main plasma are investigated in ELMy H-mode. Argon seeding maintain improved confinement HITER89=1.4, the clear edge pedestal, the radiation fraction of ∼80% and the divertor detachment up to 70% of Greenwald limit. Moreover, argon seeding reduces type I ELM frequency less than half, but the heat load of each ELM on the outer target plate does not change. Therefore, the reduction of energy losses due to ELMs probably causes the improvement of edge confinement in Ar seeded cases, though the radiated power inside the separatrix increases.

In order to obtain improved confinement plasmas with high radiation at high density, Ar gas was injected into ELMy H mode plasmas in JT-60U. A confinement improvement of HH98(y,2) ≈ 1 was obtained with a high radiation loss power fraction (~80%) at an electron density of ~0.65nGW. The HH factor was about 50% higher than that in plasmas without Ar injection.