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For the preparation study of a future burning plasma, experimental information about the internal structure of energetic particle-driven MHD instabilities, such as toroidal Alfv?n eigenmodes, is important. In the Large Helical Device, three microwave reflectometer systems have been recently installed for measuring the density fluctuation in the wide radial region with a high spatial resolution. One of the three systems has three channels with fixed frequencies. The others are operated in a frequency-hopping mode. These systems are used for measuring the internal structures of an MHD mode and its temporal behaviors. Using these systems, strongly localized MHD instabilities driven by the energetic particles are measured in the plasma core region, and the measured values are consistent with the theoretical expectation.

In the TST-2 spherical tokamak, a non-inductive start-up plasma produced by electron cyclotron heating (ECH) at 2.45 GHz (3-5 kW) is sustained by RF wave heating at 21 MHz (10-24 kW). A current jump, which occurs at a certain time when only ECH is used, can be induced earlier by the RF heating, and a constant plasma current is sustained even after the ECH power is turned off. The plasma current is sustained in both hydrogen and deuterium plasmas, as long as the RF power exceeds a threshold level, which is lower for deuterium plasma. The maximum sustainment time by the the RF power alone was 40 ms, limited by the RF pulse length. According to CCD camera images of the total plasma light, the major radii are almost the same, but the RF-sustained plasma has a more peaked profile, compared with the distribution of an ECH-sustained plasma.

The poloidal eigenmode of the geodesic acoustic mode (GAM) is analyzed in the case of high aspect ratio circular plasmas, and an analytic representation for poloidal eigenfunction is derived. The m = ±1 and m = ±2 (m is the poloidal number) components of eigenfunction show up-down antisymmetry and up-down symmetry, respectively, in a torus. The mixing of the up-down symmetric and antisymmetric components becomes significant, when the ion gyroradius is large or when electron temperature is higher than ion temperature.

A compact Nd:YAG laser Thomson scattering system was constructed and applied to the TST-2 spherical tokamak in order to measure the electron temperature and the electron density. A large solid angle (∼35 msr) was achieved by use of a compact Newtonian mirror system for the collection optics. Absolute calibration was performed by Rayleigh scattering. An electron temperature increase from 140 eV to 210 eV was observed upon injection of 200 kW of RF power.

A reflectometer in Ka-band was designed, constructed and applied to the TST-2 spherical tokamak in order to detect density fluctuations induced by the radio frequency (RF) heating wave, and to measure the density profile. The optics of the reflectometer consists of two concave mirrors to make a small microwave beam spot inside the plasma. Using Kirchhoff integration, the parameters of the optics were optimized to achieve a high throughput (i.e. the ratio of the received to the launched powers) in various measurement conditions. Using the reflectometer, the density fluctuations in the frequency range of RF were detected and the electric field excited by RF (21 MHz/260 kW) was estimated to be about 1.3 kV/m.

In order to detect rf modulation of visible light emitted from plasma, a high-speed photodiode measurement system was developed. The system is located outside the vacuum vessel of the TST-2 spherical tokamak and measures visible light emissions through a quartz window. A dedicated amplifier for the photodiode was made. Care was taken to reduce the rf pickup noise. The frequency spectrum of the light signal detected during high harmonic fast wave (HHFW) heating showed modulation by HHFW. This is the first measurement of visible light modulation induced by HHFW, and shows promise for measurement of the rf electric field in the plasma core.

A frequency-modulated reflectometer with a frequency band of 26.5 to 40 GHz has been constructed and installed in the TST-2 spherical tokamak to measure fast density profile evolution. In order to calibrate the instantaneous frequency for various frequency sweep rates, a waveform matching method has been proposed and applied to the reflectometer. This method compares the interference patterns (i.e., waveforms) of a fixed target reflection, which do not depend on the sweep rate, and obtains the instantaneous frequency by fitting the target waveform time to the time for a reference sweep waveform. This allows evaluation of output frequency stability. The overall frequency error, including reproducibility, is around 0.1 GHz. Sweep rates up to 20 μs were used to measure the density profile evolution during rf heating.

Mode structures during MHD events in the TST-2 spherical tokamak are revealed by means of singular value decomposition. Soft X-ray and magnetic coil signals show growths of modes with toroidal mode numbers n = 1 and 2. Slow evolution of the soft X-ray profile indicates beginning of localized crash. Occurrence of these events is correlated with the spatial gradient of the soft X-ray profile, suggesting that these events are driven by the pressure gradient. Increases in impurity ion temperatures (CIII and OV) are observed. These are positively correlated with the increment of the plasma current during these events.

The differential phase (DP) method was applied to microwave interferometry for the first time. Its advantage over standard interferometry is that the greatly reduced phase shift enables the unique determination of phase. Such a system was implemented on the Tokyo Spherical Tokamak-2 (TST-2), and demonstrated that the correct phase shift can be obtained even when the probing frequency is close to the cutoff density, and that very fast time evolutions of the line-integrated density can be measured.

Neutral beam injection (NBI) into low-density plasmas can cause serious damage to the vacuum vessel wall. It is necessary to stop the NBI when the plasma terminates. This needs a reliable density monitor for NBI interlock. A three-channel homodyne reflectometer, installed on a large helical device was used for an NBI interlock. Microwaves of 28.5, 34.9, and 40.2 GHz were injected with O mode polarization. At present, a simple homodyne detection scheme is used. The reflected signal consists of a dc component due to local and reflected power and an ac component due to fluctuations in the position of the cutoff layer. Since the change in dc signal was very small, the root mean square value of the ac signal was used as the interlock signal. At present the 34.9 GHz O mode channel, whose O mode cutoff density is 1.5×10^19 m^?3, is used for the interlock. The system has been working since the first NBI experiments on LHD in 1999.

Impurity transport of carbon has been studied using a new method combined carbon pellet injection with high-spatial resolution bremsstrahlung measurement on the Large Helical Device [A. Iiyoshi et al., Fusion Technol. 17, 169 (1990)]. The carbon pellets are injected into a steady phase in neutral beam heated discharges with a standard configuration of Rax=3.6m. The particle transport coefficients (diffusion coefficient D and convective velocity V) are inferred using a diffusive/convective model. The results are compared between hydrogen and helium plasmas. As a result, it is found that the analyzed D has a constant radial profile with almost closed values of 0.2m2∕s in both plasmas. On the other hand, the inward V is required only at the plasma outer region (ρ>0.6) where the electron density gradient exits, and the inward V in helium plasmas (−0.4m∕s at ρ=0.8 and ne∼4.0×1019m−3) is nearly half as much as that in hydrogen plasmas (−0.7m∕s). This difference of the inward V between hydrogen and helium plasma...

A convergence study of bispectral analysis on toroidal plasma turbulence is presented with a theoretical estimate of bicoherence for the system of the drift wave and zonal flow. Examples are chosen from the analysis of floating potential fluctuation in the edge region of the JFT-2M tokamak. The convergence properties of the bispectral functions are clarified for an oscillatory branch of zonal flow (or geodesic acoustic mode) and drift waves. The total squared bicoherence or the biphase converges inversely proportional to the realization number or to the square root of the realization number, respectively. The numbers of realizations to qualitatively converge and obtain a quantitatively significant value of bicoherence are discussed for the drift wave fluctuations and for the zonal flow.

This paper presents experimental studies of nonlinear processes between meso-scale structures and turbulent fluctuations, by use of bispectral analysis, in the edge region of toroidal plasmas. In ohmically heated plasmas of the JFT-2M tokamak, the drift wave-zonal flow system is investigated by observation of the potential fluctuations. The total bicoherence is composed of two components, i.e., the peak at the geodesic acoustic mode (GAM) frequency and broadband distribution. The biphase is also obtained, which indicates the phase angle of coherent interaction between GAM oscillations and broadband fluctuations. Bispectral functions (e.g., magnitude and phase) give an order-of-estimate agreement for the drift wave-zonal flow systems. Bicoherence analysis is also applied to the edge plasmas of the high-confinement mode (H-mode) of the Compact Helical System (CHS). In the case of CHS plasmas, nonlinear couplings in a system composed of coherent global magneto-hydrodynamic (MHD) oscillations and broadband fluctuations (up to hundreds kHz range) have been observed. Bispectral analysis enhances our understanding of the self-regulation systems of various toroidal plasmas in which strong turbulence and meso-scale structures coexist.

Microwave interferometry is a useful diagnostic for measuring the line-integrated density of plasmas. To obtain a detailed profile of the electron density, multi chord interferometry is indispensable. Using a pin switch, we have successfully measured the density along two chords by alternating the beam with a time interval of 2 µs for each chord. This method is very cost advantageous, because it requires only one set of sources and detection components. The system has been applied to Tokyo Spherical Tokamak-2 (TST-2) to measure two vertical chords at different major radii. It is shown that from the densities measured along the two chords, profile changes can be deduced.

Linear mode conversion between ordinary- and extraordinary-mode microwaves in plasma with large magnetic shear is analyzed numerically. The purpose of the analysis is a basic-design study of a microwave reflectometer using the mode conversion available in the fusion reactor large helical device (LHD) with large magnetic field shear created by a pair of helical windings. We call this a mode conversion reflectometer. Analysis is performed assuming a simple one-dimensional model and practical plasma parameters. The results indicate that mode conversion is sensitively affected by the density gradient near the two cutoff points. A steep gradient results in a large degree of mode conversion. In some cases, the converted amplitude becomes larger than 10% that of the incident signal. It is shown that, for steep density profiles, the interference patterns obtained by two converted (daughter) signals are applicable to the measurement of the density profile without any conventional phase detection.

Effects of the net toroidal plasma currents on magnetohydrodynamic (MHD) modes have been investigated in neutral beam heated plasmas with the control of ohmic current. The characteristics of experimental observations are discussed in comparison with the calculated 3-D equilibria. The resonant modes are destabilized by the change of magnetic configuration due to the induced current in the co-direction. The modes withm/n=2/1 and 3/2 abruptly disappear at the specific current, which is consistent with the disappearance of the resonant surfaces. The behavior of these modes is phenomenologically consistent with that of the interchange mode.

The development of a high energy neutral particle measurement system for ion temperature measurements and high energy particle confin.-ment analysis in a Large Helical Device (LHD) is described here. We have improved the time-of-flight (TOF) neutral particle analyzer for the LHD which had been developed in ENEA Frascati. The mass rejection factor was improved up to 1 1000 by using a biased diaphragm in front of the stop detector.

Results are presented from experiments on the effects of glow discharge and carbonization on RFP plasma performance in REPUTE-1. With the proper choice of wall conditioning, control of plasma density behavior was demonstrated. Radiated power and carbon V intensity decreased by a factor of two after He glow. Using the carbonization technique, decreases in the plasma resistance and ion temperature as derived by the CV line and charge exchange neutral particles were observed, whereas the electron temperature did not change as much.

aNational Institute for Fusion Science, Toki 509-5292, Japan bResearch Institute for Applied Mechanics, Kyushu University, Fukuoka 816-8580, Japan cEuratom0UKAEA Fusion Association, Culham Science Centre, Abingdon OX14 3EA, United Kingdom dArt Science and Technology Center for Cooperative Research, Kyushu University, Kasuga 816-8560, Japan ePlasma Research Center, University of Tsukuba, Tsukuba 305-8577, Japan fGraduate School of Frontier Sciences, The University of Tokyo, Kashiwa 277-8561, Japan gInstitute of Plasma Physics, National Science Center, Kharkov Institute of Physics and Technology 1, Akademicheskaya St., Kharkov, 61108, Ukraine hFaculty of Engineering Science, Kansai University, Osaka 564-8680, Japan iTokyo Institute of Technology, Tokyo 152-8550, Japan jGraduate University for Advanced Studies, Toki 509-5292, Japan

The Large Helical Device (LHD) experiments [O. Motojima, et al., Proceedings, 16th Conference on Fusion Energy, Montreal, 1996 (International Atomic Energy Agency, Vienna, 1997), Vol. 3, p. 437] have started this year after a successful eight-year construction and test period of the fully superconducting facility. LHD investigates a variety of physics issues on large scale heliotron plasmas (R = 3.9 m, a = 0.6 m), which stimulates efforts to explore currentless and disruption-free steady plasmas under an optimized configuration. A magnetic field mapping has demonstrated the nested and healthy structure of magnetic surfaces, which indicates the successful completion of the physical design and the effectiveness of engineering quality control during the fabrication. Heating by 3 MW of neutral beam injection (NBI) has produced plasmas with a fusion triple product of 8 X 10^18 keV m^3 s at a magnetic field of 1.5 T. An electron temperature of 1.5 keV and an ion temperature of 1.4 keV have been achieved. The maximum stored energy has reached 0.22 MJ, which corresponds to = 0.7%, with neither unexpected confinement deterioration nor visible magnetohydrodynamics (MHD) instabilities. Energy confinement times, reaching 0.17 s at the maximum, have shown a trend similar to the present scaling law derived from the existing medium sized helical devices, but enhanced by 50%. The knowledge on transport, MHD, divertor, and long pulse operation, etc., are now rapidly increasing, which implies the successful progress of physics experiments on helical currentless-toroidal plasmas.

In the first four years of the LHD experiment, several encouraging results have emerged, the most significant of which is that MHD stability and good transport are compatible in the inward shifted axis configuration. The observed energy confinement at this optimal configuration is consistent with ISS95 scaling with an enhancement factor of 1.5. The confinement enhancement over the smaller heliotron devices is attributed to the high edge temperature. We find that the plasma with an average beta of 3% is stable in this configuration, even though the theoretical stability conditions of Mercier modes and pressure driven low-n modes are violated. In the low density discharges heated by NBI and ECR, internal transport barrier (ITB) and an associated high central temperature (> 10 keV) are seen. The radial electric field measured in these discharges is positive (electron root) and expected to play a key role in the formation of the ITB. The positive electric field is also found to suppress the ion thermal diffusivity as predicted by neoclassical transport theory. The width of the externally imposed island (n/m = 1/1) is found to decrease when the plasma is collisionless with finite beta and increase when the plasma is collisional. The ICRF heating in LHD is successful and a high energy tail (up to 500 keV) has been detected for minority ion heating, demonstrating good confinement of the high energy particles. The magnetic field line structure unique to the heliotron edge configuration is confirmed by measuring the plasma density and temperature profiles on the divertor plate. A long pulse (2 min) discharge with an ICRF power of 0.4 MW has been demonstrated and the energy confinement characteristics are almost the same as those in short pulse discharges.

A new spherical tokamak, TST-2, was constructed at the University of Tokyo and started operation in September 1999. Reliable plasma initiation is achieved with typically 1 kW of ECH power at 2.45 GHz. Plasma currents of up to 90 kA and toroidal fields of up to 0.2 T have been achieved during the initial experimental campaign. The ion temperature is typically 100 eV. Internal reconnection events are often observed. The internal magnetic field measured at r/a = 2/3 indicated growth of fluctuations up to the fourth harmonic, suggesting the existence of modes with several different mode numbers. In the presence of a toroidal field and a vertical field with positive curvature, non-inductively driven currents of the order of 1 kA were observed with 1 kW of ECH power. The driven current increased with decreasing filling pressure, down to 4 × 10-4Pa. A study of high harmonic fast wave excitation and propagation has begun. Initial results indicate highly efficient wave launching. The spatial distribution of the RF magnetic field was qualitatively consistent with the result of a full-wave calculation.

In the vacuum magnetic field configuration of CHS, the MHD stability depends on the position of magnetic axis (R{sub ax}). When R{sub ax}> 95 cm, the magnetic well in the central region and the strong magnetic shear in the boundary region give the MHD stability for the ideal interchange. For the inward shifted configuration, the Mercier unstable region comes out. The volume-averaged equilibrium beta 2.1 % was obtained in 1993 with R{sub ax} = 92 cm configuration. The self-stabilization effect of high-beta plasma gave Mercier stable equilibrium while the R{sub ax} = 92 cm configuration has the Mercier unstable region for low-beta plasmas. The measurements of magnetic fluctuations and the soft X-ray signals did not show strong instabilities in these discharges. In order to evaluate the stability boundary for ideal interchange instabilities, the efforts of producing high-beta plasmas have been made for more inward shifted magnetic axis configurations (89 cm< R{sub ax} < 92 cm). The strong MHD activities were observed for those discharges. 2 refs., 7 figs.