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Emissions of ion-cyclotron-range-of-frequency (ICRF) waves have been observed in a solely electron-cyclotron-resonance-heated plasma, namely no fast-ion plasma, on the JT-60U tokamak for the first time. The mode frequencies are around the ion cyclotron frequency at the low-field-side plasma edge. The waves are considered to be electromagnetic and standing waves from poloidal and toroidal phase differences measured with ICRF antennas. Wave characteristics of the ICRF emissions such as frequency, polarization and propagation are similar to those of edge-ion cyclotron emissions (ICEs) driven by neutral-beam-injected fast ions although driving sources are completely different. From analysis of the resonance conditions, this ICRF wave emission is considered to be driven by the electron-cyclotron-resonance-heated fast electrons via the Cherenkov resonance condition. When ICE is used for fast ion diagnostics, such fast-electron-driven ICRF wave emission may complicate its interpretation during electron cyclotron resonance heating.

量研では、全8機のITERジャイロトロン実機の製作を完了し、5機の性能試験を実施し、出力1MW/効率50％/300秒発振の高繰り返し運転を90%以上の運転信頼度で成功させている。また、ITERの幅広い運転領域に対応するために複数周波数ジャイロトロンの開発研究も進めている。開発試験では、狙い通りに170GHzと137GHz発振では1MW/300秒、104GHz発振では0.9MW/300秒という単一ジャイロトロンによる3周波数での高出力連続運転を成功させた。, 核融合エネルギー連合講演会

ITERをはじめとする大型トカマク装置で運用される1MW級電子サイクロトロン加熱電流駆動システムでは、あらゆる磁場配位に対して入射モード(O/X)を制御する目的で低損失のマイターベンド型偏波器が用いられる。本研究では、偏波器の応答関数を抽出し、偏波器を擁する伝送路から真空容器への出力偏波の計算手法を確立した。実際の伝送系を用いて検証を実施した結果、計算と実測偏波の差は十分小さく、モード純度98.5%以上を達成できることが分かった。本手法はITER水平ランチャー内部の可動ミラーの反射角を考慮した偏波計算にも適用できる。, 第14回核融合エネルギー連合講演会

JT-60SAの初期研究フェーズで用いられるECH/CD装置（138GHz/110GHz/82GHz）について伝送路のレイアウト設計と評価を行った。新たに設計したレイアウトでの伝送効率を評価するために、これまで考慮されてこなかった、伝送路を構成する導波管の接合部におけるミスアライメントによる傾きや、伝搬するミリ波の偏波角も考慮した、従来よりも現実に即した伝送損失の計算を行ってきた。今回、本計算の妥当性を評価するため、既存の伝送路を使用して、伝送効率を測定し、計算値と比較した。その結果、計算結果が実験結果とよく一致し、計算が現実の伝送効率をよく表すことが明らかとなった。新レイアウトの伝送効率は本計算によると、入射ポートまでの距離の長い系統で84%、より短距離の系統では87%（いずれも110GHz）であり、目標値である伝送効率80%以上を見込めるレイアウトとなっていることが分かった。また、本伝送路では2台の偏波器を用いた偏波制御を行うが、伝送路上に配置する偏波器の位置に依存して3周波数の一部に生成できない偏波が存在する。そこで、新レイアウト上での偏波器の位置に対する出力可能な偏波状態を総当たりで計算し、プラズマ実験で必要となる偏波状態と比較、その差が最小となる偏波器の最適な位置を求めた。これにより、新レイアウトにおいて3周波数いずれもプラズマ入射時のモード純度99%以上が達成可能な偏波器の配置に成功した。, 第14回核融合エネルギー連合講演会

On JT-60SA an integrated commissioning has been conducting. During the integrated commissioning phase, measurements of the transmission efficiency and the conditioning of the ECRF TLs have been carried out. In order to evaluate the transmission efficiency, the RF power was calorimetrically measured at the dummy loads (DLs) installed at two different positions close to the gyrotron and the end of each TL. As a result, the transmission efficiency was evaluated to be 70% for Unit 1, and 81-85% for three frequencies of Unit 2. The commissioning and conditioning of the ECRF systems has also been conducted. The progress of the commissioning is also reported., Korea-Japan Workshop on Physics and Technology of Heating and Current Drive

JT-60SAの統合コミッショニング期において使用される二系統のECRF装置について、ジャイロトロンを用いた高パワーでの伝送効率の評価を行った。また、通常のコンディショニングに使用しているジャイロトロン近傍のダミーロード位置、および伝送効率測定に使用しているランチャー直近の二カ所に加え、MOU出口と伝送路上二カ所の計五カ所においても仮設の短パルスダミーロードを用いてパワー測定を行った。本会議ではパワー測定および伝送路中の各要素における損失評価の結果について報告する。, 第38回 プラズマ・核融合学会年会

In many tokamak devices in the world, electron cyclotron wave heating plays an important role in plasma control, and polarization control by polarizers is essential to select an appropriate injection mode. Generally, a polarizer internally decomposes the input electric field into two orthogonal components (fast polarization and slow polarization) and generates a phase difference between those components to produce a polarization different from the input. Therefore, the response of a polarizer is characterized by two parameters: the angle at which the electric field is decomposed and the generated phase difference. In Stokes space, each polarization is represented as a vector, and it is known that a rotation operator represents the response of a polarizer. In this presentation, we propose the new method to determine polarizers' response parameters by converting the input and output polarizations into Stokes vectors to identify the rotation operators from a necessary and sufficient dataset. In the conventional method, the measured electric field components are analyzed in real space, and the equation for determining the response parameters of the polarizer tends to be complicated. However, by considering the polarizer's response geometrically in Stokes space, the response parameters can be determined more prospectively. In order to confirm the analysis method, a polarization measurement were carried out using a horn antenna detector and two polarizers for 110/138 GHz transmission lines. The method to determine the response parameters of one polarizer and to simultaneously determine the response parameters of two polarizers were verified. In both cases, it was confirmed that the output polarizations calculated from the determined response parameters agreed with the measured polarizations with a purity of more than 99%., Korea-Japan Workshop on Physics and Technology of Heating and Current Drive

現在JT-60SAのECRF設備のコミッショニングが進められている。今回統合コミッショニング期において使用される二系統のECRF設備について、伝送路の損失と伝送効率を評価するための熱量測定を行った。計測の結果、小口径導波管を用いている1号機では60%の、大口径導波管を用いている2号機では三周波数いずれに対しても81-85%の伝送効率を得た。本会議では熱力測定および伝送路中の各要素における損失評価の結果と、現在までに進められているECRF設備のコミッショニングの進捗について報告する。, Korea-Japan Workshop on Physics and Technology of Heating and Current Drive

ITERにおける電子加熱/電流駆動(ECH/ECCD)用の水平ポートランチャーは量研機構により調達される予定であり、2021年の最終設計レビューに向けて設計が進められている。これに対して量研機構では昨年度、ITER機構が指定する導波管の配置に準拠すると同時に加熱/電流駆動特性の要求値を満足させるための光学設計を最終化させることに成功した。本報告では、トカマク配位安定化のための電流分布制御に関する計算の進捗を含む、水平ランチャーの現設計から想定されるプラズマの加熱/電流駆動特性を報告した。, 第37回 プラズマ・核融合学会 年会

Presence of a large population of fast particles may qualitatively modify the tokamak equilibrium from the one that can be described by the standard magneto-hydrodynamics (MHD). The kinetic modification of the MHD equilibrium was studied for a lower-hybrid wave driven plasma on the TST-2 spherical tokamak. The analysis was performed using an equilibrium reconstruction method based on an extended MHD model that considers a two-component plasma of bulk MHD fluid and kinetic fast electrons. Scrape-off-layer current appeared naturally in the extended MHD analysis because of the finite electron orbit excursion from the flux surfaces. This reduced the bulk pressure contribution to the toroidal current substantially. The resulting poloidal flux profile was more consistent with that inferred from the Thomson scattering measurement.

Based on the latest optical design of ITER ECH&CD equatorial launcher, the power absorption at the plasma breakdown is evaluated with considering reflections by the 3-dimensional first wall structure. The beam diffusion is simulated by a bunch of spreading rays and the reflections on the first wall are analyzed for each ray. The power absorption is evaluated by the optical thickness when the rays pass across the resonant surface. The frequency of EC wave is 170 GHz and the density of concern is from 2.5 × 10 15 m − 3 . As a result, at least 2 MW is absorbed out of 6.7 MW injection from the Top row in the case of fundamental O-mode injection even at the breakdown. In the case of 2nd harmonic X-mode injection, the total absorbing power is 200 kW at the electron density of 10 18 m − 3 and the electron temperature of 5 eV, which is proportional to the electron density and the electron temperature. It is found that a certain injection angle should be excluded in order to prevent intense stray RF injection up to 4 MW propagating toward specific ports within a few reflections.

A multi-energy soft x-ray pinhole camera has been designed, built, and deployed at the Madison Symmetric Torus to aid the study of particle and thermal transport, as well as MHD stability physics. This novel imaging diagnostic technique employs a pixelated x-ray detector in which the lower energy threshold for photon detection can be adjusted independently on each pixel. The detector of choice is a PILATUS3 100 K with a 450 μm thick silicon sensor and nearly 100 000 pixels sensitive to photon energies between 1.6 and 30 keV. An ensemble of cubic spline smoothing functions has been applied to the line-integrated data for each time-frame and energy-range, obtaining a reduced standard-deviation when compared to that dominated by photon-noise. The multi-energy local emissivity profiles are obtained from a 1D matrix-based Abel-inversion procedure. Central values of Te can be obtained by modeling the slope of the continuum radiation from ratios of the inverted radial emissivity profiles over multiple energy ranges with no a priori assumptions of plasma profiles, magnetic field reconstruction constraints, high-density limitations, or need of shot-to-shot reproducibility. In tokamak plasmas, a novel application has recently been tested for early detection, 1D imaging, and study of the birth, exponential growth, and saturation of runaway electrons at energies comparable to 100 × Te,0; thus, early results are also presented.

A compact hard X-ray camera has been developed to study the behavior of fast electrons accelerated by lower hybrid waves in the TST-2 spherical tokamak. The camera consists of a pinhole made of tungsten alloy, a thin resin window, a thin LYSO scintillator disk, 5 fiber optic light guides and 5 photomultiplier tubes and shields. The camera is installed on a tangential port, and two-dimensional images are obtained by rotating the system around the axis of the port on a shot by shot basis. The typical energy range is 10 - 250 keV, and the energy resolution is about 100% (FWHM) at 100 keV. The measured images of a lower hybrid wave driven plasma show bright region at the inboard side (i.e., high field side).

A multi-energy soft x-ray pinhole camera has been designed and built for the Madison Symmetric Torus reversed field pinch to aid the study of particle and thermal-transport, as well as MHD stability physics. This novel imaging diagnostic technique combines the best features from both pulse-height-analysis and multi-foil methods employing a PILATUS3 x-ray detector in which the lower energy threshold for photon detection can be adjusted independently on each pixel. Further improvements implemented on the new cooled systems allow a maximum count rate of 10 MHz per pixel and sensitivity to the strong Al and Ar emission between 1.5 and 4 keV. The local x-ray emissivity will be measured in multiple energy ranges simultaneously, from which it is possible to infer 1D and 2D simultaneous profile measurements of core electron temperature and impurity density profiles with no a priori assumptions of plasma profiles, magnetic field reconstruction constraints, high-density limitations, or need of shot-to-shot reproducibility. The expected time and space resolutions will be 2 ms and

A new tool has been developed to calculate the spectral, spatial, and temporal responses of multi-energy soft x-ray (ME-SXR) pinhole cameras for arbitrary plasma densities (ne,D), temperature (Te), and impurity densities (nZ). ME-SXR imaging provides a unique opportunity for obtaining important plasma properties (e.g., Te, nZ, and Zeff) by measuring both continuum and line emission in multiple energy ranges. This technique employs a pixelated x-ray detector in which the lower energy threshold for photon detection can be adjusted independently. Simulations assuming a tangential geometry and DIII-D-like plasmas (e.g., ne,0 ≈ 8 × 1019 m-3 and Te,0 ≈ 2.8 keV) for various impurity (e.g., C, O, Ar, Ni, and Mo) density profiles have been performed. The computed brightnesses range from few 102 counts pixel-1 ms-1 depending on the cut-off energy thresholds, while the maximum allowable count rate is 104 counts pixel-1 ms-1. The typical spatial resolution in the mid-plane is ≈0.5 cm with a photon-energy resolution of 500 eV at a 500 Hz frame rate.

Removal of the central solenoid is essential to realize an economical spherical tokamak fusion reactor, but non-inductive plasma start-up is a challenge. On the TST-2 spherical tokamak, non-inductive plasma start-up using lower-hybrid (LH) waves has been investigated. Using the capacitively-coupled combline (CCC) antenna installed at the outboard midplane, fully non-inductive plasma current ramp-up up to a quarter of that of the typical Ohmic discharges has been achieved. Although it was desirable to keep the density low during the plasma current ramp-up to avoid the LH density limit, it was recognized that there was a maximum current density that could be carried by a given electron density. Since the density needed to increase as the plasma current was ramped-up, the achievable plasma current was limited by the maximum operational toroidal field of TST-2. The top-launch CCC antenna was installed to access higher density with up-shift of the parallel index of refraction. Numerical analysis of LH current drive with the outboard-launch and top-launch antennas was performed and the results were qualitatively consistent with the experimental observations.

In TST-2 Ohmic discharges, local current is measured using a Rogowski probe by changing the angle between the local magnetic field and the direction of the hole of the Rogowski probe. The angular dependence shows a peak when the direction of the hole is almost parallel to the local magnetic field. The obtained width of the peak was broader than that of the theoretical curve expected from the probe geometry. In order to explain this disagreement, we consider the effect of sheath in the vicinity of the Rogowski probe. A sheath model was constructed and electron orbits were numerically calculated. From the calculation, it was found that the electron orbit is affected by E × B drift due to the sheath electric field. Such orbit causes the broadening of the peak in the angular dependence and the dependence agrees with the experimental results. The dependence of the broadening on various plasma parameters was studied numerically and explained qualitatively by a simplified analytical model.

AC Ohmic coil operation experiments with frequencies up to 10 kHz were performed on the TST-2 spherical tokamak device, and the pre-ionization process was studied in detail. The minimum loop voltage for pre-ionization was 0.4 V, which corresponds to 0.5 V ${\rm m}^{-1}$ at the inboard limiter. Dependences of growth rate and saturation level of the process on various parameters were obtained, and they are compared with a time-dependent 0-dimensional model based on Townsend avalanche and loss along field lines. Most of the dependences are reproduced qualitatively by the model, and quantitative differences are within a factor of several. However, the external vertical field dependence of the appearance time, which is defined as the time to observe a plasma, and the isotope effect cannot be reproduced by the model. An ambipolar diffusion state which is predicted theoretically but mitigated experimentally is discussed. It was found that secondary electron emission at the limiter surfaces is a candidate mechanism to mitigate the state.

Non-inductive plasma start-up is a critical issue for spherical tokamaks since there is not enough room to provide neutron shielding for the center solenoid. Start-up using lower hybrid (LH) waves has been studied on the TST-2 spherical tokamak. Because of the low magnetic field of a spherical tokamak, the plasma density needs to be kept at a very low value during the plasma current ramp-up so that the plasma core remains accessible to the LH waves. However, we have found that higher density was required to sustain larger plasma current. The achievable plasma current was limited by the maximum operational toroidal field of TST-2. The existence of an optimum density for LH current drive and its toroidal field dependence is explained through a numerical simulation based on a ray tracing code and a Fokker–Planck solver. In order to access higher density at the same magnetic field, a top-launch antenna was recently installed in addition to the existing outboard-launch antenna. Increase in the density limit was observed when the power was launched from the top antenna, consistently with the numerical predictions.

Measurements of edge plasma parameters such as current density, electron density, and electron temperature were performed during internal reconnection events in TST-2 Ohmic plasmas. The measured current density consists of two components: a slowly varying component and a spiky bipolar component. The magnitude of the slowly varying component is comparable to the mean current density averaged over the poloidal cross section, and it seems to represent the global transport from the core to the edge. The spiky bipolar component is about an order of magnitude larger than the slowly varying component, but the spatial structure seems to be localized and its effect on plasma confinement is not catastrophic.

Thomson scattering (TS) diagnostics have been widely used in fusion studies to measure profiles of electron temperature Te and electron density ne. In order to measure the low-density plasmas (ne ≤ 1018 m−3) in TST-2, which is sustained by lower hybrid wave power, the signal-to-noise ratio in TS measurement has been improved by various means. For instance, optimization of the detecting system, accumulation of TS data obtained from reproducible discharges, and application of a coaxial multi-pass scheme were carried out. As a result, the profiles have been measured successfully and a peaked ne profile and a hollow Te profile were obtained. Additionally, isotropy of Te near the plasma center was confirmed by coaxial double-pass TS measurement.