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The neutral beam (NB) shine-through profile is routinely monitored on the Large Helical Device (LHD) both to calibrate the port-through power of the NB and to evaluate the NB-deposition power to LHD plasmas. The profile is measured with a calorimeter (CM) array on an armor plate of the NB counter wall inside the LHD vacuum vessel. An infrared camera is also used to check the beam profile where CMs are not located, and measures a temperature increase of the armor plate due to the NB heat load. The measured beam profile is compared to the calculated NB profile at the armor plate. The measurement indicates that the beam is not uniform at the exit of the ion source and that the steering angle of the beam in the horizontal direction is not the same as the designed value. It is found that the monitoring of the NB shine-through profile is important to estimate the NB port-through power and the NB deposition power, especially when the neutral beam injector (NBI) is based on a large negative-hydrogen ion source.

The Large Helical Device (LHD) experiments have started after a construction period of eight years, and two experimental campaigns were performed in 1998. The magnetic field was raised up to 2.75 T at a magnetic axis position of 3.6 m at the end of the second campaign. In the third campaign, started in July in 1999, the plasma production with ECH of 0.9 MW and auxiliary heating with NBI of 3.5 MW have achieved an electron temperature of 3.5 keV and an ion temperature of 2.4 keV. The maximum stored energy has reached 0.75 MJ with an averaged electron density of 7.7×1019 m-3 by hydrogen pellet injection. The ICRF heating has sustained the plasma for longer than 2 s and the initial stored energy of the NBI target plasma has increased from 0.27 MJ to 0.335 MJ. The major characteristic of the LHD plasma is the formation of the temperature pedestal, which leads to some enhancement of energy confinement over the ISS95 scaling law. The confinement characteristic is gyro-Bohm and the maximum energy confinement has reached 0.28 s. The LHD has also shown its high potentiality for steady-state operation by realizing a 22 s discharge in the second campaign.

Electron cyclotron emission (ECE) can be used to determine the electron temperature profile in magnetized plasmas. The complex structure of the magnetic field configuration in the Large Helical Device (LHD) [M. Fujiwara et al., J. Fusion Energy 15, 7 (1996)] complicates the analysis of the ECE spectrum. The magnetic field is a nonmonotonous function of the radius and has a very large shear. The effects of the magnetic-field shear on the polarization of the ECE spectrum have been numerically and experimentally analyzed. For a high enough density (ne>1.0×1019 m−3) the polarization is found to rotate with the sheared magnetic field, yielding only a negligible mode conversion. Experiments on the polarization have been carried out at LHD. X-mode emission was detected under a specific angle, in agreement with the numerical predictions. The elliptically polarized ECE emission showed a very strong O-mode component (70% of X-mode), that can be attributed to the effect of wall reflections.

Recent results of energetic ion driven MHD instabilities observed in the heliotron/torsatron devices Compact Helical System (CHS) and Large Helical Device (LHD) are presented. Alfven eigenmodes (AEs) and fishbone-like burst modes (FBs) destabilized by energetic ions were observed in NBI heated plasmas of CHS. The AEs are toroidicity induced Alfven eigenmodes (TAEs) and global Alfven eigenmodes (GAEs), where the identified toroidal mode numbers are n = 1 and 2 for TAEs and n = 0 for GAEs. The frequencies of the FBs are less than, at most, half of the minimum TAE gap frequency and do not exhibit the obvious density dependence related to Alfven velocity. The modes have characteristic features of the energetic particle modes or the resonant TAEs excited by circulating energetic beam ions produced by NBI. Bursting amplitude modulation is observed in TAEs as well as in FBs. Rapid frequency chirping is observed in each burst, by a factor of 2-6 in FBs and about 25% in TAEs. In several shots, the power spectrum of the TAEs is split into multiple peaks having the same toroidal mode number through non-linear evolution of TAEs. A pulsed increase in energetic ion loss towards the wall is induced by m = 3/n = 2 FBs, but so far not by m = 2/n = 1 FBs, TAEs and GAEs, where m is the poloidal mode number. This research has been extended to LHD plasmas heated by neutral hydrogen beams with about 130 keV energy. Similar to CHS, TAEs and FBs were observed in relatively low density plasmas at low toroidal magnetic field (Bt = 1.5 T).

The Large Helical Device is the world's largest heliotron type helical system, with the plasma confining magnetic field being generated by only external superconducting coils. One of the main objectives of the LHD project is to sustain high temperature plasmas for a long time in steady state. The plasma vacuum vessel and the divertor are water cooled, and a heat load of 3 MW can be removed continuously. The NBI, ECH and ICRF heating systems, diagnostic instruments and data acquisition system are designed for long pulse operation. The present status of these systems and the recent experimental results of long pulse operation are reviewed. A steady state discharge with NBI was obtained for 35 s. The ECH discharge duration was extended to 120 s with a duty factor of 95%. Plasma sustainment by ICRF alone was achieved for 2 s. The performance of these long pulse operations is summarized.

The first experiment of the ion cyclotron range of frequencies (ICRF) heating in the Large Helical Device (LHD) was carried out at the end of 1998. The LHD is a large superconducting heliotron device and its first plasma was produced in March 1998. During the ICRF heating experiment, a maximum 300 kW/0.2 s of ICRF power was injected into the LHD plasma by using a pair of loop antennae. This paper reports on the installation of the loop antennae, the results of antenna coupling and the first heating experiments. The antennae are designed to operate in the steady state and to change their distance from the plasma by 0-15 cm. In the experiment, the antenna resistance coupled with the plasma was measured by changing the distance between the last closed flux surface and the launcher front from 9 cm to 5 cm. The resistance was almost doubled by decreasing the distance. The target plasma was produced by the second harmonic electron cyclotron heating (ECH) of 84 GHz gyrotrons at a magnetic field of 1.5 T and a low plasma electron density of less than 1 × 1019 m-3 . Therefore, the low coupling resistance limited the maximum injected power to less than 300 kW. The heating efficiency and heating species were varied by the minority ion gas-puffing rate. The heating characteristics were compared with a one-dimensional full-wave analysis code, and the experimental results were consistent with wave damping analysis. For the optimum condition of the minority hydrogen gas-puff ratio, the plasma internal energy increased from 13 kJ to 26 kJ by almost the same power as the ECH power.

Long-pulse neutral beam injection heating has been achieved in the large helical device (LHD). Two different confinement states are observed for different averaged densities in the long-pulse plasmas. A quasi-steady-state plasma was sustained for 21 s with an injection power of 0.6 MW, where the central plasma temperature was around 1 keV with a line-averaged electron density of 0.3 × 1019 m-3 . The discharge duration can be so extended as to keep the plasma properties in the short-pulse discharge. The energy confinement time is nearly the same as that of the short-pulse discharge, which is 1.3 times as long as the international stellarator scaling ISS95. At higher densities, a relaxation oscillation phenomenon, observed as if the plasma would breathe, lasted for 20 s with a period of 1-2 s. The phenomenon is characterized with profile expansion and contraction of the electron temperature. The density oscillation is out of phase with the temperature oscillation and is related to the density clamping phenomenon. The observed plasma properties are shown in detail for the `breathing' oscillation phenomenon. Possible mechanisms for the breathing oscillation are also discussed, with a view of the screening effect near the last closed magnetic surface and the power balance between the heating and the radiation powers. The long-pulse heating results indicate unique characteristics of the LHD where no special feedback stabilization is required due to absence of disruption and no need for current drive.

We have achieved long-pulse plasma heating using a negative-ion-based neutral beam injector (NBI) in the large helical device (LHD), where the confinement magnetic field is generated by only external superconducting coils. In the initial long-pulse experiments at lower power than that in short-pulse experiments, 80 keV–1.1 MW NBI heating lasted for 10 s with a little increase in the plasma density at the pulse end. Almost steady-state plasma heating was achieved for 21 s with 66 keV–0.6 MW NB injection. Plasma relaxation oscillation phenomena at a period of 1–2 s were also observed for 20 s. Above 1 keV plasma was easily sustained with a long-pulse NBI heating in LHD, without the current drive nor the disruption in tokamaks. Negative ion source operation was stable and the cooling water temperature rise of beam accelerator grids was nearly saturated with a temperature rise below 10 °C. For a higher power injection, the pulse duration is determined by the beam blocking, where the reionization loss is expon...

The initial experiments on the Large Helical Device (LHD) have extended confinement studies on currentless plasmas to a large scale (R = 3.9 m, a = 0.6 m). Heating by NBI of 3 MW produced plasmas with a fusion triple product of 8 × 1018m-3keVs at a magnetic field strength of 1.5 T. An electron temperature of 1.5 keV and an ion temperature of 1.1 keV were achieved simultaneously at a line averaged electron density of 1.5 × 1019 m-3. The maximum stored energy reached 0.22 MJ with neither unexpected confinement deterioration nor visible MHD instabilities, which corresponds to β = 0.7%. Energy confinement times reached a maximum of 0.17 s. A favourable dependence of energy confinement time on density remains in the present power density (~40 kW/m3) and electron density (3 × 1019 m-3) regimes, unlike the L mode in tokamaks. Although power degradation and significant density dependence are similar to the conditions on existing medium sized helical devices, the absolute value is enhanced by up to about 50% from the International Stellarator Scaling 95. Temperatures of both electrons and ions as high as 200 eV were observed at the outermost flux surface, which indicates a qualitative jump in performance compared with that of helical devices to date. Spontaneously generated toroidal currents indicate agreement with the physical picture of neoclassical bootstrap currents. Change of magnetic configuration due to the finite β effect was well described by 3-D MHD equilibrium analysis. A density pump-out phenomenon was observed in hydrogen discharges, which was mitigated in helium discharges with high recycling.

The Large Helical Device (LHD) has successfully started running plasma confinement experiments after a long construction period of eight years. During the construction and machine commissioning phases, a variety of milestones were attained in fusion engineering which successfully led to the first operation, and the first plasma was ignited on 31 March 1998. Two experimental campaigns were carried out in 1998. In the first campaign, the magnetic flux mapping clearly demonstrated a nested structure of magnetic surfaces. The first plasma experiments were conducted with second harmonic 84 and 82.6 GHz ECH at a heating power input of 0.35 MW. The magnetic field was set at 1.5 T in these campaigns so as to accumulate operational experience with the superconducting coils. In the second campaign, auxiliary heating with NBI at 3 MW has been carried out. Averaged electron densities of up to 6 × 1019m-3, central temperatures ranging from 1.4 to 1.5 keV and stored energies of up to 0.22 MJ have been attained despite the fact that the impurity level has not yet been minimized. The obtained scaling of energy confinement time has been found to be consistent with the ISS95 scaling law with some enhancement.

Plasma was successfully built up by neutral beams alone in LHD. After injecting 1.6 MW neutral beams through a low density neutral gas target of 10-3Pa for about 0.3 s, fully ionized plasma of 1018 m-3 was generated. Keeping the beams on, additional gas puff could increase the plasma density sufficiently for absorption of the beam, and a dense plasma of 5 × 1019 m-3 was sustained afterwards. This technique is specific to a helical system that has a vacuum confining magnetic field structure, and no other assisting heating tools are required such as ECH, which has been used for producing target plasma with NBI.

In the National Institute for Fusion Science, the large helical device (LHD) (R=3.9 m, ap=0.65 m, B=3–4 T) was constructed in Toki City on January, 1998. The LHD is a torsatron/heliotron type device with a superconducting helical winding of pitch parameters of l/m=2/10 and three sets of superconducting poloidal coils. A two color millimeter-wave interferometer has been developed for the measurement of line integral electron density on the horizontal midplane of the LHD. To overcome the effects of the change in the transmission path length (∼100 m) during long pulse plasmas (10–1800 s), we have applied two color interferometry at 140 and 285 GHz.

A millimeter-wave two-dimensional imaging array has been developed for diagnostics of magnetically confined plasmas. It is applied to the plug cell of the GAMMA 10 tandem mirror, and measures time evolution of radial and axial line-density and electron cyclotron emission (ECE) profiles in one plasma shot. The monolithic-type GaAs Schottky diode detector having an antenna and built-in amplifier on a GaAs substrate is newly fabricated using monolithic microwave integrated circuit (MMIC) technology. This novel detector is developed to attain high frequency response up to submillimeter-wave region and wideband heterodyne characteristics, and is expected to be applied to ECE imaging in a large helical device. The response of the detector is compared with that of the hybrid-type detector using beam-lead Schottky barrier diodes. The first experimental result of the ECE measurement is reported using a heterodyne radiometer with the MMIC detector.

A phase sensitive heterodyne polarimeter was installed on compact helical system (CHS). An HCN laser (337 μm) and Schottky barrier diodes are used for the radiation source and for detection, respectively. By using a phase sensitive heterodyne scheme, the Faraday rotation angle can be measured from the phase variations caused by the phase difference between the right- and left-handed waves in the magnetized plasma. The purpose of the polarimeter on CHS is for a stable density monitor which is free from fringe jump and for the measurements of the internal magnetic field induced by the plasma current. The present resolution of the Faraday rotation angle is 0.1° with 10 ms time resolution and 1° with 1 ms. These are mainly determined by the laser instability.

The first results on wall conditioning in the first and second campaign of LHD experiments are described. By confirming device integrity, operational reliability, and environmental safety, LHD has been successfully started up with intensive discharge cleaning without doing high-temperature baking. Evacuation of about 100 molecular layers of surface contaminants is found to be the key to start up the target plasma by ECH for NBI injection. Importance of combination between mild baking, GDC, ECR-DC and main shots is experimentally confirmed.

In order to determine the magnitude and the position of the plasma current in a long pulsed tokamak such as the International Thermonuclear Experimental Reactor (ITER), it is urged to establish a reliable method which is free from the zero-level drift of the integrator as well as the radiation damage for the steady-state magnetic field measurement. For this purpose, we have developed a hybrid system, a combination of a conventional magnetic probe for the measurement of fast varying magnetic field and a rotating coil magnetic probe for that of slowly varying field. The rotating coil is energized by an air turbine to avoid electromagnetic interference and the induce signal with a constant rotation frequency is picked up through a transformer to eliminate mechanical contacts. An automatic gain control circuit is also designed for the compensation of rotation speed fluctuation. The system is proved to achieve a flat frequency response with a proper choice of cross over frequency for high- and low-frequency systems. The rotating coil probe is tested for over 170 h without any trouble. The probe was applied to the poloidal magnetic field measurement on the TRIAM-1M long pulsed tokamak, and proved to work satisfactorily.

Extraordinary mode interferometry with the frequency lower than the electron cyclotron frequency has been applied to the core plasma of the JIPP TII-U tokamak. The system is operated as a reflectometer when the density exceeds the cutoff for the extraordinary mode. The measured fringe shift is in good agreement with the calculated fringe shift using the density profile obtained by a six-chord HCN laser interferometer. Advantages and disadvantages of extraordinary mode interferometry are shown numerically and experimentally. To overcome the disadvantages, a dual (ordinary and extraordinary) mode interferometer is proposed as a divertor diagnostic. There are two attractive choices for this scheme. One choice double the operational density range but does not compensate for changes in the length of the transmission line. The other can compensate for transmission line changes, but with a density range limited by the O-mode cutoff.

Intense submillimeter wavelength radiation (f=354 GHz, P=110 W) from Gyrotron FU II is converted into a well-collimated, linearly-polarized beam by a quasi-optical transmission line consisting of a quasi-optical antenna and several mirrors. The observed focusing of the beam agrees with calculations based on the Huygens equation. We have used this intense beam to study low frequency density fluctuations by plasma scattering in Compact Helical System (CHS) plasmas at the National Institute for Fusion Science.

Toroidal current penetration is studied in current ramp experiments on the JIPP T-IIU tokamak. The poloidal magnetic field profile in the peripheral region of a plasma (0.5 ≤ r/a ≤ 1.0) has been measured directly with a newly developed fast response Zeeman polarimeter. The experimental results indicate that a skin effect of the toroidal current density is clearly observed during both the current ramp-up (CRU) and the current ramp-down (CRD) experiments. The experimentally obtained toroidal current density profiles are well described by the profiles calculated on the assumption of neoclassical electrical resistivity, although the difference between the neoclassical electrical resistivity and the Spitzer electrical resistivity is not clearly distinguished because of the relatively collisional peripheral plasma region. Quasi-linear Δ' analysis of tearing modes for the measured current density profile is consistent with the time behaviour of the coherent magnetohydrodynamic (MHD) modes such as m = 4/n = 1 or m = 3/n = 1 (m is the poloidal mode number and n is the toroidal mode number) often observed during the CRU phase. An obvious enhancement of the current penetration by the MHD modes is not detected in this ramp-up experiment, where the relative amplitude of the poloidal field fluctuations is less than 0.8%.

In some plasma discharges, the phase measured by microwave reflectometry has many fringe ( radians) jumps. A new algorithm to detect and remove fringe jumps has been developed, and applied to the data in the JIPP TII-U tokamak. Using this algorithm, quantitative properties of fringe jumps, and their effects on the analysis of phase fluctuations are investigated. It was found that the occurrence of fringe jumps obeys a Poisson process, and the time scale of jumps is distributed over a wide range. Fringe jumps affect mainly the low-frequency components of phase fluctuations. Comparison of the phase corrected by the algorithm and the phase calculated from the time smoothed signals indicates that time smoothing (or frequency filtering) is an effective way to obtain information concerning the macroscopic density profile. Fringe jump and phase runaway can be phenomenologically explained by the distribution of the complex amplitude of the reflected wave.

Laser scattering signals that indicate the presence of small dust particles (diameter ≤ 2 μm) have been occasionally observed in the JIPPT-IIU tokamak chamber. This phenomenon was reproduced deliberately by spreading carbon dust from the top of the vacuum chamber. No noticeable effect on the plasma was observed for dust falls of up to at least 106 particles (10 μg) in 20 ms during discharges. Dust falling just before plasma startup seemed to be confined but it was soon ejected (in less than 30 ms)

Multiple and small sample volume measurements of the density turbulence and potential profile measurements in tokamak plasmas were conducted using a heavy ion beam probe. The obtained wavenumber-frequency spectrum S(k, omega ) shows that the cross-sections of neutral beam injection (NBI) heated plasmas are divided into three regions of different turbulence characteristics outside the layer where the direction of the density turbulence propagation reverses, a low frequency and low wavenumber mode travelling in the ion diamagnetic drift direction dominates. The region enclosed by this reversal layer is divided into two parts during nearly perpendicular NBI heating: a region where the propagation velocity is near the Etau /Bt poloidal rotation velocity and a bad curvature region of very small wavenumber and high propagation velocity. The region of high propagation velocity, found in NBI plasmas, disappears in the ohmic plasmas. In addition, a small component that propagates in the ion diamagnetic drift direction is observed in NBI plasmas

Copyright (c) 1997 Elsevier Science S.A. All rights reserved. Thomson scattering systems diagnosing three regions of the large helical device (LHD) are presented. (1) An obliquely back-scattered Thomson scattering system gives full profiles of electron temperature (T e ) and density (n e ) along the major radius on the mid-plane with a spatial resolution of 1.5−4.0 cm. A piezo-actuated mirror is dynamically oriented so that the beams from ten 50 Hz repetition rate Nd:YAG lasers propagate a common path along a major radius, giving T e and n e profiles every 2 ms. (2) A cavity Thomson scattering system will measure two-dimensional T e and n e profiles along the laser beam propagating off-axially right and left in an optical cavity. This will diagnose the island structures in the outer edge region. (3) A fiber Thomson scattering system, in which laser light is guided by an optical fiber, is freely movable and conveniently used to probe the diverted plasma. © 1997 Elsevier Science S.A.

A multi-channel Flit laser interferometer system is being constructed for the measurement of the spatial and temporal behaviour of the electron density profile on the Large Helical Device. The optical configuration is of the Michelson interferometer type with 13 vertical chords. A highly stable twin CH3OH laser pumped by a cw CO2 laser has been developed for the direct measurement of the phase shift. The resolution of the phase comparators is 1 100 fringes, and corresponds to a line averaged density of 5.6 × 1016 m−3 at the central chord.

Several types of reflectometer have been used on the JIPP TII-U tokamak to understand the phase runaway effect, which denotes the phenomenon in which the measured phase moves very fast in one direction. Homodyne, heterodyne, and ultra-narrow band reflectometers are used. Phase runaway and rapid phase variations are observed using all systems, except for the homodyne system which cannot measure phase. When the edge is measured, a relatively slow phase runaway is observed, that is caused by many fringe Jumps (2π phase jumps). In most cases, the amplitude of the reflected wave is well above the noise level during fringe jumps. Thus, the runaway is not a problem with the measurement system, but is a result of an interaction between the microwave and the plasma. An algorithm to remove fringe jumps has been developed, so that a phase without runaway is obtained, which probably represents macroscopic movement of the cut-off layer. The obtained phase shows that the effect of the fringe jumps on the power spectrum is serious at low frequency ranges.

Electromagnetic waves of medium power ( P = 110 W) in the submillimeter wavelength range ( f = 354 GHz) are generated in long pulses (τ = 0.6 s) by the GYROTRON FU-II and they have been applied to plasma scattering measurements in compact helical system plasma. Use of a gyrotron as a power source improves the S/N ratio of the measurement. The results of low frequency density fluctuations correspond to those of reflectometry. A new transmission line is designed for conversion of gyrotron outputs into a high quality beam. Analysis using Huygen's equation supports the view that this system can produce a high quality beam. The final goal is to improve the performance of the scattering measurement by applying an optimal probe beam.

A two color far-infrared (FIR) laser interferometer and polarimeter using 5.2- and 6.3-THz CH3OD lasers pumped by a 9R(8) CO2 laser has been developed for future plasma diagnostics. The pump 9R(8) CO2 laser has been stabilized by an external Stark-cell modulation. The performances are the power stability of 108 ± 0.6 W/hour and the frequency stability of ± 230 kHzp-p/hour at the line center.

The largest superconducting fusion machine, Large Helical Device (LHD), is now under construction in Japan and will begin operation in 1997. Design and construction of related R&D programs are now being carried out. The major radius of this machine is 3.9 m and the magnetic field on the plasma center is 3 T. The NbTi superconducting conductors are used in both helical coils and poloidal coils to produce this field. This will be upgraded in the second phase a using superfluid coil cooling technique. A negative ion source is being successfully developed for the NBI heating of LHD. This paper describes the present status and progress in its experimental planning and theoretical analysis on LHD, and the design and construction of LHD torus, heating, and diagnostics equipments.

Fast changes of electric potential with different polarities are observed during sawtooth oscil- lations in the core region of a tokamak plasma using a heavy ion beam probe. Near the inversion radius the polarity of the observed change of the potential is found to be dependent on the swift movement of the hot core at the crash and is consistent with the prediction of one- fluid MHD theory. Near the magnetic axis the change of the potentials is positive and outside the inversion radius the change is negative. This is in contradiction with the MHD prediction. A periodic crash of the central electron tempera- ture called a sawtooth oscillation is a common fea- ture of tokamak plasmas (l-61. After the crash, a flattening of the pressure throughout the q = 1 sur- face is observed. Kadomtsev proposed a theoretical model of the resistive reconnection (7). The theory predicted the flattening of the q profile as well as the pressure profile inside the q = 1 surface. The predic- tion was, however, found to be in contradiction with recent measurements of the current density in toka- mak plasmas (8). In addition, experiments in large machines such as JET and TFTR showed much faster crashes than those predicted by resistive reconnection (2-61. More refined theories, which take account of the kinetic effect and ergodicity on the reconnections, have now been proposed (9-121. The present Letter reports the first measurement of potential changes during sawtooth oscillations and discusses the impli- cations of the results. The experiment is performed with the JIPP T-IIU tokamak (13). Its major radius is 93 cm and the maximum toroidal field is 3 T. The experiment is conducted at a relatively low electron density of about 2 x 1013 cm3, so that good beam penetra- tion into the centre of the plasma could be obtained. A 450 keV singly ionized thallium beam is injected

CO2 laser based phase contrast interferometer was used previously on LHD to study plasma density microfluctuations. New two-dimensional 6×8 elements array of CdHgTe photoconductors was employed for detection. In addition to commonly measured intensity, frequency spectrum, and two-point correlation two-dimensional detector array allows determining the direction of propagation of waves in plasma. From the angular distribution it is possible to obtain localized along the viewing line properties of fluctuations using strong magnetic shear in LHD. As a result, the radial profiles of density microfluctuations can be obtained with 1 μs temporal resolution determined by detector bandwidth. The described scheme was tested using ultrasonic wave in air. New imaging diagnostics has been installed on LHD and first experimental results are reported.

We have installed a six channel ultrashort-pulsed radar reflectometer system on the large helical device and performed electron density profile measurements. The delay time of the reflected pulses from each cutoff layer in the plasma is measured by a time-of-flight measurement technique in order to avoid the mixture of radiation effects and spurious reflections. The electron density profile is reconstructed using an Abel inversion method from the profile of the delay time as a function of the probing frequency. The reconstructed density profile is compared with the profile measured with the far-infrared (FIR) interferometer. It is found that the arrival time of each reflected pulse differs from the estimated time measured with the FIR interferometer.

The effects of relativistic down shift frequency on the determination of the electron temperature profile from electron cyclotron emission (ECE) in a large helical device(LHD) have been studied by a numerical calculation. since the value of toroidal magnetic field has maximum at the magnetic axis along the sight line in the LHD, the emissivity at the magnetic axis decreases due to the relativistic effect. We found an apparent drop in electron temperature around the magnetic axis due to the relativistic effect. The parametric dependences of the relativistic effect on electron temperature measurement have been studied. When the electron temperature is higher and/or optical thickness is thinner, the gap from the nonrelativistic electron cyclotron frequency is wider. In the case of Te(0)=5 keV and ne=1×1019 m−3, the frequency gap is about 0.5 GHz. Calculation shows that the electron temperature at the central region (ρ

A new two color far infrared (FIR) laser interferometer under development for future fusion devices will be presented. The laser wavelength is optimized from the consideration of the beam refraction effect due to plasma density gradient and signal-to-noise ratio for an expected phase shift due to plasmas. Laser lines of 57.2 and 47.6 ?m are found to be suitable for the applications to high performance plasmas of Large Helical Device and future fusion devices such as the International Thermonuclear Experimental Reactor. The output power of 57.2 ?m CH_3OD laser is estimated to be ~1.6 W, which is the highest laser power in the FIR wavelength regime. The optical configuration of a new interferometer system using two colors will be proposed. In the system, one detector simultaneously detects the beat signals of the 57.2 and 47.6 ?m laser lines, and each interference signal can be separated electronically (1 MHz for 57.2 ?m and 0.84 MHz expected for 47.6 ?m). Mechanical vibration can be compensated by using the two color interferometer. The present status of the development of the system is also presented.

We have applied an ultrashort-pulse reflectometer to large helical device plasmas for density profile measurement. The frequency range of the impulse is up-converted into the R band, which corresponds to the cutoff frequency of edge plasma region in the ordinary mode. The reflected wave from plasma is received and directly recorded by a sampling scope. This system can be controlled and monitored from remote site (Kyushu University) by using ultrawideband science information network (super-SINET) that ideal bandwidth reaches up to 1 Gbps. As a result of the measurement, we have confirmed reflection wave from the plasma when the plasma density reaches steady state during the measurement.

High beta plasmas with a volume averaged equilibrium beta value of 2.1% were produced in CHS using tangential neutral beam injection. This beta value was achieved with the confinement improvement (reheat mode) observed after turning off strong gas puffing. Wall conditioning with titanium gettering was used to make high density operation (ne ? 8 ? 1019 m-3) possible for low magnetic fields (Bt = 0.6 T). The discharges start with the magnetic hill configuration (in vacuum) and finally achieve Mercier stable equilibrium owing to the self-stabilization effect given by the magnetic well which is produced by the plasma pressure. The Shafranov shift was about 40% of a plasma minor radius. Magnetic fluctuations did not increase with increasing plasma pressure when the beta value exceeded 1%. Dynamic poloidal field control was applied to suppress the outward plasma shift with increasing plasma pressure. Such operation gave an additional increase of beta value compared with the constant poloidal field operation

In the GAMMA 10 tandem mirror, the typical electron density is comparable to that of the peripheral plasma of torus-type fusion devices. Therefore, an effective method to increase Thomson scattering (TS) signals is required in order to improve signal quality. In GAMMA 10, the yttrium-aluminum-garnet (YAG)-TS system comprises a laser, incident optics, light collection optics, signal detection electronics, and a data recording system. We have been developing a multi-pass TS method for a polarization-based system based on the GAMMA 10 YAG TS. To evaluate the effectiveness of the polarization-based configuration, the multi-pass system was installed in the GAMMA 10 YAG-TS system, which is capable of double-pass scattering. We carried out a Rayleigh scattering experiment and applied this double-pass scattering system to the GAMMA 10 plasma. The integrated scattering signal was made about twice as large by the double-pass system.

Multi-line far-infrared (FIR) gas lasers from a single laser cavity have been development in order to apply ITER diagnostics. By using two pump CO 2 lasers with different wavelength and mixed gas of CH 3 OH and CH 3 OD, we succeeded in the simultaneous oscillations of three-color FIR lasers of 48, 57, 119 µm in wavelength.

By the use of simultaneous fast toroidal and slow poloidal sweeps of a heavy ion beam, plasma potential profiles in the JIPP T-IIU tokamak are measured at the rate of 120 spatial profiles per second. A new method to eliminate the error due to the change of out-of-plane entrance angle caused by large plasma current is successfully applied. One of the key factors is the very homogeneous characteristics of the energy analyser along the wide slit length. A shaped electrode system, instead of guard rings with a resistor chain, successfully increases the homogeneity of the analyser. Error due to the significant change of in-plane entrance angle during a poloidal sweep is carefully minimized and calibrated by a secondary beam ionized by neutral gas introduced in the vacuum vessel. The depth of the measured potential at the plasma centre reaches more than 1.5 keV in ohmic plasmas with an ion temperature of 600 eV. In some cases, a rather wide region of positive potential and a sharp decrease of the potential in the centre of the plasma are observed.

Characteristics of the H-mode transition observed in CHS are described. The transition is achieved with modification of the rotational transform profile by inducing a small ohmic heating current. The transition is often initiated by a sawtooth crash caused by a coherent mode with the poloidal mode number m=2 and toroidal one n=1.

Local impurity deposition in the n/m = 1/1 magnetic island, which is expanded by the external perturbation coils, has been obtained by means of the tracer-encapsulated solid pellet in the Large Helical Device. The differences of the local deposition of the tracer between inside the magnetic island and outside that were clearly observed by both the absolute extreme ultra violet silicon photodiode arrays and the ECE radiometer.

Responsivities have been measured by using 14 THz laser lines (1.63–6.29 THz) in order to apply a GaAs Schottky barrier diode (SBD) to beat signal detection of the 5–6 THz band. The performance was about 1.9–966 V/W for the THz frequency range. We succeeded in heterodyne beat signal detections in the two color (5.2 and 6.3 THz) THz laser interferometer of beat modulation type by using the GaAs SBD mixer.

This Letter presents the discovery of macroscale electron temperature fluctuations with a long radial correlation length comparable to the plasma minor radius in a toroidal plasma. Their spatiotemporal structure is characterized by a low frequency of {approx}1-3 kHz, ballistic radial propagation, a poloidal or toroidal mode number of m/n=1/1 (or 2/1), and an amplitude of {approx}2% at maximum. Nonlinear coupling between the long-range fluctuations and the microscopic fluctuations is identified. A change of the amplitude of the long-range fluctuation is transmitted across the plasma radius at the velocity which is of the order of the drift velocity.

Two color laser diagnostics using terahertz laser sources are under development for a high performance operation of the Large Helical Device and for future fusion devices such as ITER. So far, we have achieved high power laser oscillation lines simultaneously oscillating at 57.2 and 47.7 μm by using a twin optically pumped CH(3)OD laser, and confirmed the original function, compensation of mechanical vibration, of the two color laser interferometer. In this article, application of the two color laser diagnostics to the ITER poloidal polarimeter and recent hardware developments will be described.

A new electron cyclotron emission (ECE) diagnostics system was installed for the Second Korea Superconducting Tokamak Advanced Research (KSTAR) campaign. The new ECE system consists of an ECE collecting optics system, an overmode circular corrugated waveguide system, and 48 channel heterodyne radiometer with the frequency range of 110-162 GHz. During the 2 T operation of the KSTAR tokamak, the electron temperatures as well as its radial profiles at the high field side were measured and sawtooth phenomena were also observed. We also discuss the effect of a window on in situ calibration.

The square waveguide phased-array antenna system has been developed for Electron Bernstein Emission diagnostics. The broadband orthomode transducer was prepared to measure the wave polarization. The measured fields were in good agreements with those evaluated from the Kirchhoff integral.

An experiment in a new regime of ion Bernstein wave (IBW) heating was carried out using 130 MHz high power transmitters in the JIPP T-IIU tokamak. The heating regime utilized the IBW branch between the 3rd and 4th harmonics of the hydrogen ion cyclotron frequencies. This harmonic number is the highest one used in IBW experiments conducted previously. The net radiofrequency (RF) power injected into the plasma is around 400 kW and is limited by the transmitter output power. Core heating of ions and electrons was confirmed in the experiment and density profile peaking was found to be a special feature of IBW heating. Peaking of the density profile was also found when IBWs were injected into neutral beam heated discharges. An analysis, using a transport code with these experimental data, indicates that particle and energy confinement should be improved in the plasma core region upon application of IBW heating. It is also found that the ion energy distribution function observed during IBW heating has a smaller high energy tail than those observed in conventional fast magnetosonic wave ICRF heating regimes. The ion energy distribution function obtained during IBW heating is in reasonable agreement with that calculated using the quasi-linear RF diffusion/Fokker-Planck model

Millimeter wave imaging diagnostics such as electron cyclotron emission imaging (ECEI) and microwave imaging reflectometry (MIR) are advanced technology to investigate electron temperature/density profiles and their fluctuations in magnetically confined plasmas. We have installed an ECEI system in the large helical device (LHD) at National Institute for Fusion Science (NIFS). The optimization of a receiving antenna array is essential for the imaging diagnostics. We have designed and fabricated dual-dipole antenna array. In this paper, the design and fabrication process of dual-dipole antennas are described. The characterizations of the antennas are also reported by comparing simulation and experimental results.

We have developed an electron cyclotron emission (ECE) radiometer to measure the radial electron temperature profiles. Before installation of this system on KSTAR, we installed a part of this system on LHD, and confirmed that acquired signals are in good agreement with those provided by the Michelson spectrometer.

A two color far-infrared (FIR) laser interferometer of beat modulation type using 48- and 57-µm CH 3 OD lasers pumped by 9R(8) CO 2 laser has been developing for plasma diagnostics. The pump 9R(8) CO 2 laser frequency, 48- and 57-µm CH 3 OD laser outputs and FIR beat frequency have been stabilized preliminary. The power stability of pump CO 2 laser of 3 OD laser.