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In order to realize an efficient ECRH and also to reduce stray radiation due to non-absorbed power during ECRH, it is necessary to excite a wave that is absorbed well near the electron cyclotron resonance. In the normal fusion magnetic field confinement machine and in the electron cyclotron frequency range, WKB approximation is valid almost all the way from antenna to the absorption region due to the large scale-length of the plasma density λn and the magnetic shear τs as compared with the local wavelength #955;0. In these situation, it is well known that the O/X mode propagates as O/X mode if τs ≫#955;0. Even in these situation, if τs and λn are comparable and ǀ1=#955;O - 1/#955;Xǀτs ≪1, there still remains the question from where "X"- or "O "- mode become "X"- or "O "mode at the peripheral region. In order to simulate this situation, one dimensional full wave calculation code which solve electromagnetic wave equation under arbitrary magnetic field configuration and arbitrary density profile for a given polarization state are developed and incorporated in the upgraded ray tracing code LHDGauss. It is tried to find the density and shear scale lengths region where the mode mixing effect is not negligible. [ABSTRACT FROM AUTHOR]

Recent progress in an ion cyclotron range of frequencies (ICRF) heating system and experiment results in a Large Helical Device (LHD) are reported. Three kinds of ICRF antenna pairs were installed in the LHD, and the operation power regimes were extended up to 4.5 MW; also, the steady-state operation was extended for more than 45 min in LHD at a MW power level. We studied ICRF heating physics in heliotron configuration using a Hand Shake type (HAS) antenna, Field Aligned Impedance Transforming (FAIT) antenna, and Poloidal Array (PA) antenna, and established the optimum minority-ion heating scenario in an LHD. The FAIT antenna having a novel impedance transformer inside the vacuum chamber could reduce the VSWR and successfully injected a higher power to plasma. We tested the PA antennas completely removing the Faraday-shield pipes to avoid breakdown and to increase the plasma coupling. The heating performance was almost the same as other antennas; however, the heating efficiency was degraded when the gap between the antenna and plasma surface was large. Using these three kinds of antennas, ICRF heating could contribute to raising the plasma beta with the second- and third-harmonic cyclotron heating mode, and also to raising the ion temperature as discharge cleaning tools. In 2014, steady-state operation plasma with a line-averaged electron density of 1.2 x 1019 m-3, ion and electron temperature of 2 keV, and plasma sustainment time of 48 min was achieved with ICH and ECH heating power of 1.2 MW for majority helium with minority hydrogen. In 2015, the higherpower steady-state operation with a heating power of up to 3 MW was tested with higher density of 3 x 1019 m-3. [ABSTRACT FROM AUTHOR]

In a high power Electron Cyclotron Resonance Heating (ECRH) system, a long-distance and low-loss transmission system is required to realize effective heating of nuclear fusion-relevant plasmas. A millimeter-wave beam position and profile monitor, which can be used in a high-power, evacuated, and cooled transmission line, is proposed, designed, manufactured, and tested. The beam monitor consists of a reflector, Peltier-device array and a heat-sink. It was tested using simulated electric heater power or gyrotron output power. The data obtained from the monitor were well agreed with the heat source position and profile. The methods of data analysis and mode-content analysis of a propagating millimeter-wave in the corrugated waveguide are proposed. [ABSTRACT FROM AUTHOR]

Until 2009, three high-power, over 1MW each, 77GHz gyrotrons have been installed and applied to the LHD experiment. In addition, a 154GHz gyrotron of 1MW was installed in 2012. The 77GHz gyrotrons suffer gradual increases of internal pressure during long-pulse operation delivering power to LHD. To mitigate the problem, quasi-steady operation by combination of on-off operations of the 77GHz gyrotrons was performed. Applying two 77GHz gyrotrons alternately with intervals of two minutes and an 84GHz gyrotron continuously, a 30 min. long-pulse discharge with the line average electron density ne_ave of 0.7x1019m-3 and the central electron temperature Te0 of 1.7keV was achieved by the time average injection power Pinj of 260kW in 2012, showing significant progress in sustained density from the former 65 min. discharge with ne_ave of 0.15x1019m-3 and Te0 of 1.7keV by Pinj of 110kW of 84GHz wave. In 2013, one of the 77GHz gyrotron was improved to furnish a sub-window to remove stray radiation inside the tube. And the new 154GHz gyrotron was applied to the long-pulse discharge experiment. Using three gyrotrons: 154, the improved and an existing 77GHz with Pinj of 340kW in total, higher temperature plasma having steep temperature gradient typical for internal transport barrier, with ne_ave of 1.1x1019m-3 and Te0 of 3.5keV was quite stably sustained for 325s. [ABSTRACT FROM AUTHOR]

The upcoming operation of Wendelstein 7-X (W7-X) will be supported by an Electron Cyclotron Resonance Heating (ECRH) system working at 140 GHz in second harmonic at the nominal magnetic field of 2.5T. Because the optimization of the plasma breakdown is crucial to ensure a successful plasma build-up, dedicated plasma start-up experiments were performed on three stellarator/heliotron devices: Heliotron J, LHD and WEGA. Start-up behavior and dependencies on ECRH injected power, neutral gas pressure and rotational transform were obtained in X2 heating. Plasma start-up delay time decreases with the increase in ECRH input power. However, this behavior saturates when low pre-fill neutral gas pressure conditions are met. Both the delay time and the electron density are an increasing function of the gas pressure. On Heliotron J and WEGA devices, the higher the rotational transform is, the faster the start-up and the higher the plasma density are. Analysis of the temporal evolution of the plasma start-up shows that plasma start-up on stellarators is a twostep process. In addition, off-axis heating experiments are characterized by a longer plasma start-up duration compared to on-axis heating discharges. Third harmonic in X-mode has been attempted on LHD for different neutral gas puffing settings but no plasma breakdown was achieved. This multi-machine study was useful to define ECRH start-up scenarios for W7-X. [ABSTRACT FROM AUTHOR]

In the large helical device (LHD), the theoretically predicted width of the ordinary-extraordinary-electron Bernstein wave (O-X-B) mode conversion (MC) window is comparable to the beam width and the power deposition is located in the off-axis region if the 77GHz fundamental electron cyclotron (EC) wave of is launched from an existing horizontal port antenna. In the experiment, the actual MC window location was looked for with changing the aiming. The effective aiming with that the increase of the stored energy was observed was two degrees apart from the location of the theoretical MC window at a maximum. Measurement of the waves originated from the thermally emitted EBW and radiated via the B-X-O mode conversion process is effective to improve the accuracy of the theoretical prediction with comparison between the theoretical and the experimental results. The theoretical prediction suggests that the width of the MC window of the fundamental 77GHz EC wave can be expanded if the lower port antenna is used. On the other hand, the MC window of the second harmonic 154GHz EC wave is blocked by horizontal port wall if another horizontal port antenna is used. It is required to move the final mirror of the quasi-optical antenna toward the plasma surface. Focusing of the beam at the plasma cutoff is (PC) also necessary for the effective mode conversion. [ABSTRACT FROM AUTHOR]

Enhancement of the output powEr pEr gyrotron has been planned in the Large Helical Device (LHD). Three 77-GHz gyrotrons with an output powEr of more than 1 MW have been opEraTed. In addition, a high powEr gyrotron with the frequency of 154 GHz (1 MW/5 s, 0.5 MW/CW) was newly installed in 2012 and the total injection powEr of ECRH reached 4.6 MW. The opErational regime of ECRH plasma on the LHD has been exTended due to the upgraded ECRH sysTem such as the central electron TempErature Te0 = 13.5 keV with ne = 1x1019 m-3. In the LHD, an electron-inTernaltransport barriEr (e-ITB) relaTed to the production of high Te plasmas has been realized by strongly centre-focused ECRH. The electron thErmal confinement clearly improved inside the e-ITB. The radial electric field was measured using the heavy ion beam probe. The formation of the positive Er was obsErved in the core region, which well agreed with the prediction of the neoclassical transport theory. The energy confinement characTeristics have been investigaTed in the ECRH plasmas. It was found that highEr plasma stored energy and lowEr radiation powEr was realized in the outward configuration. The plasma stored energy of 530 kJ with ne = 3.2x1019 m-3, which is the 1.7 times largEr than the previous record in the ECRH plasma in the LHD, has been successfully achieved. [ABSTRACT FROM AUTHOR]

doi: Long-pulse discharge with the electron density ne0 of 1 x 1019 m-3, electron temperature Te0 of 2.5 keV, discharge length tdis of 19 minutes and heating power Pinject of 1MW, is demonstrated using the HAS antenna and the PA antenna for ion cyclotron heating (ICH) and increasing in the power of electron cyclotron heating (ECH). The HAS antenna is designed to phase dipole and excite ideal fast wave with parallel electric field kept small, and low impurity generation and accumulation are achieved on the steady-state discharge by weak parasitic heating around antennas. On the long-pulse discharge, the radiation measured by bolometer is kept smaller than 20% for injection power, and the heat load to divertor is approximately 60 % with low energetic particle losses. The heat load ratio to divertor is not as a function of injection power around 1MW, and energy confinement has been kept during the steady-state discharge. [ABSTRACT FROM AUTHOR]

The central bulk electron temperature of more than 20 keV is achieved in LHD as a result of increasing the injection power and the lowering the electron density near 2x1018 m-3.Such collision-less regime is important from the aspect of the neoclassical transport and also the potential structure formation. The presences of appreciable amount of high energy electrons are indicated from hard X-ray PHA, and the discrepancy between the stored energy and kinetic energy estimated from Thomson scattering. ECE spectrum are also sensitive to the presence of high energy electrons and discussed by solving the radiation transfer equation. The ECRH power absorption to the bulk and the high energy electrons are dramatically affected by the acceleration and the confinement of high energy electrons. The heating mechanisms and the acceleration process of high energy electrons are discussed by comparing the experimental results and the ray tracing calculation under assumed various density and mean energy of high energy electrons. [ABSTRACT FROM AUTHOR]

In the large helical device (LHD), fundamental electron cyclotron resonance heating (ECRH) by the electron Bernstein wave (EBW) excited via the ordinaryextraordinary-- EBW (O-X-B) mode conversion process was performed with high power (∼1MW) launching. Profiles of increase of the electron temperature (Te) and the soft Xray signals during the power injection suggest power absorption in the core region. Effects of the local modification of the rotational transform l/2π(=1/q) by electron cyclotron current drive (ECCD) on the formation and sustainment of the electron internal transport barrier (e-ITB) was investigated for the first time. Co ECCD raised l/2π close to 0.5 in the core region and caused the flattening of the Te profile. Additional ECRH power is required to form the e-ITB. On the contrary, counter (cntr.) ECCD separates l/2pi from 0.5 in the core region and avoids the flattening of the Te profile. The e-ITB can be formed and sustained without additional ECRH. Analysis of the heat pulse transport with use of the modulation ECRH (MECH) shows the good confinement region extends to the ;/2π=0.5 rational surface in the case of cntr. ECCD. [ABSTRACT FROM AUTHOR]

To sustain plasmas with higher parameters and with longer pulse duration in LHD, ECH system has been upgraded by introducing newly developed 77 GHz gyrotrons. The designed output power and operation duration time are over 1 MW for several seconds and 0.3 MW for continuous operation, respectively. Owing to the upgrade of gyrotrons and improved power supply operation procedure, total injection power of EC-waves to LHD increased up to 3.7 MW at the last LHD experimental campaign in 2010. Application of the high-power 77 GHz EC-waves of 3.4 MW as focused beams to the center of plasma with low line-average electron density of ∼0.2×1019 m-3 causes highly steep electron temperature profile and the central electron temperature reached up to 20 keV, which highly exceeds the former record of 15 keV. At higher density region of 1×1019 m-3, central electron temperature reached 8.6 keV. Additional electron Bernstein wave heatings, O-X-B and slow X-B heatings, using a 77 GHz ECH system caused clear increase in plasma stored energy even for the high-density plasmas over plasma cutoff (>7.35×1019 m-3) sustained with NBI. For the O-X-B scenario, the 77 GHz EC-wave was obliquely injected from low-field side in O-mode polarization, aiming at the point where high mode-conversion efficiency was expected. For realizing slow X-B scenario, new inner-vessel mirrors were installed in LHD just close to a helical coil, that is, at the high-field side (HFS) region. Using the inner-vessel mirror, X-mode waves were injected from HFS, showing evident increase in plasma stored energy. Oblique injection of long-pulse 0.77 MW/8 s 77 GHz wave with various N∥ clearly demonstrated ECCD in LHD. The EC-driven current changes its direction with the sign of N∥, and the highest EC-driven current reached up to 42 kA. [ABSTRACT FROM AUTHOR]

The phased-array antenna system for Electron Cyclotron/Bernstein Wave Heating and Current Drive experiments has been developed in the QUEST. The antenna was designed to excite a pure O-mode wave in the oblique injection for the O-X-B mode conversion experiments, and its good performances were confirmed at a low power level. The plasma current (<∼15 kA) with an aspect ratio of 1.5 was started up and sustained by only RF injection in the low-density operations. The long pulse discharge of 10 kA was also attained for 37 s. The new density window to sustain the plasma current was observed in the high-density plasmas. The single-null divertor configuration with the high plasma current (<∼25 kA) was attained in the 17 s plasma sustainment. [ABSTRACT FROM AUTHOR]

Electron Cyclotron Resonance Heating (ECRH) has contributed to the achievement of high performance plasma production, high electron temperature plasmas and sustainment of steady-state plasmas in the Large Helical Device (LHD). Our immediate targets of upgrading the ECRH system are 5 MW several seconds and 1 MW longer than one hour power injection into LHD. The improvement will greatly extend the plasma parameter regime. For that purpose, we have been promoting the development and installation of 77 GHz/1–1.5 MW/several seconds and 0.3 MW/CW gyrotrons in collaboration with University of Tsukuba. The transmission lines are re-examined and improved for high and CW power transmission. In the recent experimental campaign, two 77 GHz gyrotrons were operated. One more gyrotron, which was designed for 1.5 MW/2 s output, was constructed and is tested. We have been promoting to improve total ECRH efficiency for efficient gyrotron-power use and efficient plasma heating, e.g. a new waveguide alignment method and mode-content analysis and the feedback control of the injection polarization. In the last experimental campaign, the 77 GHz gyrotrons were used in combination with the existing 84 GHz range and 168 GHz gyrotrons. Multi-frequency ECRH system is more flexible in plasma heating experiments and diagnostics. A lot of experiments have been performed in relation to high electron temperature plasmas by realization of the core electron-root confinement (CERC), electron cyclotron current drive (ECCD), Electron Bernstein Wave heating, and steady-state plasma sustainment. Some of the experimental results are briefly described. [ABSTRACT FROM AUTHOR]

We have investigated the transition to Core Electron-Root Confinement (CERC) using the 77 GHz ECRH system, which was newly installed in the Large Helical Device (LHD). The focal point scan of the ECRH was carried out shot by shot in the same poloidal surface along the magnetic field of 2.75 T, which is the fundamental resonance surface for 77 GHz ECRH. The formation of the steep gradient in the electron temperature and the change of the radial electric field from negative to positive value in the plasma core region were observed by the ECRH injection to ρ<0.5. The foot point of the peaked profile of the electron temperature appeared near the m/n = 2/1 rational surface, which was located at ρ∼0.5. These results imply that localized ECRH in the inner region within the lower order rational surface and/or the electron heat flux across the rational surface play important roles for the transition to CERC. We also confirmed that the calculated radial electric field from a neoclassical theory qualitatively agreed with the experimental results. [ABSTRACT FROM AUTHOR]

In the Large helical device (LHD), super dense core (SDC) regime [1] and high electron temperature regime with formation of the electron internal transport barrier (e-ITB) [2][3] have been studied strenuously. Electron cyclotron heating (ECH) and current drive (ECCD) in such regimes can be powerful tools for heating and control of the plasma confinement. In this paper, recent progress of ECH experiments aiming at further advanced operation in these regimes is reported. Study of fundamental ECH by electron Bernstein waves (EBWs) has been required in the SDC regime. Early experimental results of EBW-ECH by so-called O-X-B and X-B method are introduced. In a newly realized enhanced magnetic field configuration, the highest central electron temperature over 10 keV was obtained in electron cyclotron resonance (ECR) discharges. ECCD will be very important in both of high density and high temperature regimes. It has been progressed with the optimization of microwave injection and magnetic field configuration. Progress of ECCD experiment is shortly introduced. [ABSTRACT FROM AUTHOR]

It is very important to demonstrate the ability to sustain the plasma in a steady state on the Large Helical Device (LHD), which has external helical magnetic coils and is a superconducting device. The long pulse discharge experiment was carried out using the ion cyclotron range of frequencies (ICRF) heating mainly. The plasma discharge of 31 minutes and 45 seconds was achieved by a total injected heating energy of 1.3GJ. Swing of the magnetic axis to scatter the local heat load on the divertor plate was one of the key methods for the steady state operation. The repetitive hydrogen pellet injection was tried successfully to fuel the minority hydrogen ions for long pulse operation. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

A survey of linear mode-conversion characteristics between external transverse electromagnetic (TEM) waves and electron Bernstein waves (EBW) for various plasma and wave parameters has been presented. It is shown that if the wave propagation angle and polarization are adjusted appropriately for each individual case of the plasma parameters, efficient mode conversion occur for wide range of plasma parameters where the conventional ‘XB’ and ‘OXB’ scheme cannot cover. It is confirmed that the plasma parameters just at the upper hybrid resonance (UHR) layer strongly affect the mode conversion process and the influence of the plasma profiles distant from the UHR layer is not so much. The results of this survey is useful enough to examine wave injection/detection condition for efficient ECH/ECCD or measurement of emissive TEM waves for each individual experimental condition of overdense plasmas. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

Demonstration of a long pulse or a steady state operation of ECH and sustainment of non-collapsed plasma only by ECH is important in LHD from both the confinement device and the heating system engineering points of view. A gyrotron with a diamond output window is introduced and operated at the power level of 150 kW for more than 1 hour after modification of the cooling and evacuation system of the ECH transmission line. The power of about 110 kW injected into LHD is used to sustain the plasma with the electron density of 1.5 × 1018 m-3 and central temperature of more than 1.0 keV for 3900 sec. The gas puffing rate is carefully controlled so that the plasma density does not exceed the critical value above which the plasma collapsed for given injection power, magnetic configuration and wall condition of LHD. The results of gyrotron operation, transmission system modification for long pulse and optimizations of the magnetic field configuration of LHD and gas puffing for a given injection condition are discussed. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

(1) Physical process of initial formation of closed flux surface has been studied under the condition of steady external vertical fields by using a 2.45 GHz magnetron (5kW). After the injection of microwave power the plasma current increases gradually until up to 0.4 kA. Once the current reaches this value it quickly increases up to 1 kA within 2–3 msec. The magnetic measurements suggests that a small initial closed flux is formed at Ip=0.4kA and expands quickly to full closed flux in contact with the inboard limiter in accordance with the quick increase of the plasma current. (2) Plasma current up to 2 kA has been started-up by injecting microwaves in the range of 20–50 kW from a 2 GHz klystron and adjusting the external vertical field temporally. Magnetic measurement shows that last closed flux surface with a minor radius of a=15 cm and a major radius of R =25 cm (R/a=1.7) is formed. (3) A new ECH system using a 5 GHz klystron (200kW,100 msec) is under construction. With this system, better coupling to Bernstein waves from EM waves compared with the 2.0 and 2.45 GHz cases may be obtained. © 2003 American Institute of Physics [ABSTRACT FROM AUTHOR]

General properties of scattering matrix, which governs the mode conversion process between electron Bernstein (B) waves and external electromagnetic (EM) waves in the presence of steep density gradient, are theoretically analyzed. Based on the analysis, polarization adjustment of incident EM waves for optimal mode conversion to B waves is possible and effective for a range of density gradient near the upper hybrid resonance, which are not covered by the previously proposed schemes of perpendicular injection of X mode and oblique injection of O mode. Furthermore, the analysis shows that the polarization of the externally emitted EM waves from B waves is uniquely related to the optimized polarization of incident EM waves for B wave heating and that the mode conversion rate is the same for the both processes of emission and the injection with the optimized polarization. © 2003 American Institute of Physics [ABSTRACT FROM AUTHOR]