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Integrated physics analysis of plasma operation scenario of the compact helical reactor FFHR-c1 has been conducted. The DPE method, which predicts radial profiles in a reactor by direct extrapolation from the reference experimental data, has been extended to implement the equipartition effect. Close investigation of the plasma operation regime has been conducted and a candidate plasma operation point of FFHR-c1 has been identified within the parameter regime that has already been confirmed in LHD experiment in view of MHD equilibrium, MHD stability and neoclassical transport.

"A high density regime with an internal diffusion barrier (IDB) has been extended to the helical divertor (HD) configuration in the Large Helical Device (LHD). Avoidance of the local enhancement of neutral pressure is necessary to enable IDB formation, which is consistent with earlier works by using the Local Island Divertor (LID) with efficient active pumping. The central pressure reached 1.3 times atmospheric pressure, where ne(0) = 6 × 1020 m?3 and Te(0) = 660 eV. The plasmas with an IDB are located in the plateau collisionality regime. The significant impurity effect has not been observed throughout the discharges in spite of the existence of a negative radial electric field. A central pressure limiting event is observed in the plasmas with an IDB using the HD. During this event which is referred to as the core density collapse (CDC), particles are flushed out from the core on the time scale of a few hundreds of microseconds. The suppression of the Shafranov shift by vertical elongation (κ) is effective to mitigate CDC. At κ = 1.2, the central β value is increased up to 6.6% at 1 T."

A hydrogen pellet is injected into a plasma in the Large Helical Device (LHD). Strong radiation from a dense plasma formed around the pellet, the so-called 'pellet cloud', is observed and its spectrum is analysed. Emission lines of neutral hydrogen exhibit Stark broadening profiles and the electron density is evaluated through comparison with theoretical data (e.g. 2.1 ? 1023?m?3). The continuum radiation is dominated by two components which correspond to radiative recombination and radiative electron attachment, respectively, the latter of which yields negative ions. From the absolute intensity and its dependence on the wavelength of the continuum radiation, the electron temperature (1.02?eV), the atom density (1.7 ? 1025?m?3) and the observed plasma volume (1.6 ? 10?5?m3) are determined. These results indicate that the plasma state is close to complete LTE (local thermodynamic equilibrium). The Balmer-? line profile is deformed owing to the reabsorption effect. With the help of one-dimensional radiation transport calculation, the plasma thickness in the direction of the observation (2.4 ? 10?3?m) is estimated. The result suggests that the pellet cloud is extended anisotropically in the direction perpendicular to the observation.

In local island (m/n = 1/1) divertor discharges in the large helical device a stable super dense core plasma develops when a series of pellets are injected. A core region with a density as high as 4.6 × 1020 m−3 and a temperature of 0.85 keV is maintained by an internal diffusion barrier with a very high density gradient. In a study of island dynamics, we find that an externally imposed large island (m/n = 1/1) as large as 15% of the minor radius is healed when beta at the island exceeds a critical value.

One of the main objectives of the LHD is to extend the plasma confinement database for helical systems and to demonstrate such extended plasma confinement properties to be sustained in steady state. Among the various plasma parameter regimes, the study of confinement properties in the collisionless regime is of particular importance. Electron cyclotron resonance heating (ECRH) has been extensively used for these confinement studies of the LHD plasma from the initial operation. The system optimizations including the modification of the transmission and antenna system are performed with the special emphasis on the local heating properties. As the result, central electron temperature of more than 10 keV with the electron density of 0.6 x 10$^{19}$ m$^{-3}$ is achieved near the magnetic axis. The electron temperature profile is characterized by a steep gradient similar to those of an internal transport barrier observed in tokamaks and stellarators. 168 GHz ECRH system demonstrated efficient heating at over the density more than 1.0 x 10$^{20}$ m$^{-3}$. CW ECRH system is successfully operated to sustain 756 s discharge.

The utility of ICRF heating in the LHD was demonstrated in the third campaign carried out in 1999. This paper summarizes the investigations made in 2000 with a focus on the optimization of ICRF heating. The flexibility of the LHD magnetic configuration was fully utilized as a key factor in the investigations. The experiments include (a) scan of magnetic field intensity, (b) scan of aspect ratio, (c) scan of magnetic axis shift, (d) scan of quadrupole magnetic field, and (e) cancellation of magnetic island. The performance of the ICRF heating was thus optimized with respect to magnetic parameters, while optimization was achieved mainly with respect to heating regime and wall conditioning in the third campaign. Some of these parameters are directly related to the orbit of trapped particles. Therefore, the relation between particle orbits and the performance of ICRF heating is also addressed in this paper based on analyses of the high-energy tail of ions.

In the Large Helical Device (LHD), active edge plasma control using two types of divertor, helical divertor (HD) and the local island divertor (LID), is planned, respectively. The former is intrinsic in the heliotron-type magnetic configuration. In the latter case, large pumping efficiency is expected by making particle recycling localized toroidally using a divertor head and an externally produced m = 1, n = 1 island. In this paper, the divertor properties in the HD configuration are reviewed. The numerical result for design of LID is described. The results of the LID experiment performed in the Compact Helical System are also briefly reviewed.

This paper reports on the behaviour of high-energy ions created by ion cyclotron range of frequency (ICRF) heating on the Large Helical Device (LHD). In the third experimental campaign conducted in 1999, it was found that minority heating has good heating performance, and high-energy particles were observed. In the fourth campaign in 2000, the temporal behaviour of high-energy ions was investigated in the minority heating regime using turnoff or modulation of ICRF power. The time evolution of the high-energy particle distribution was measured using a natural diamond detector (NDD) and a time-of-flight neutral particle analyser (TOF-NPA). It was found that the count number of higher-energy particles declines faster than that of lower-energy particles after ICRF turnoff. In the modulation experiments, the phase difference of the flux of high-energy particles with respect to the ICRF power modulation increased with energy. These results were explained qualitatively by the Fokker-Planck equation with a simple model. The pitch-angle dependence of the distribution function was also measured in the experiment by changing the line of sight of the TOF-NPA, and an anisotropy of the high-energy tail was found. This anisotropy was reproduced by solving the bounce-averaged Fokker-Planck equation. The second harmonic heating was conducted successfully for the first time in the LHD in high-β plasma, and high-energy particles were also detected in this heating regime.

A power modulation experiment was conducted in the third campaign of the LHD. In a conventional analysis of the modulation experiments, the energy confinement time and heating efficiency are taken as constant, disregarding their dependence on the plasma parameters. In this paper, their dependence on the plasma temperature and heating power is taken into consideration to improve the analysis of the power modulation experiments. Several models, with differing dependence on the plasma parameters, have been examined. There have been several reports suggesting that the transport coefficients obtained from the dynamical method are different from those obtained from power balance analyses. This paper finally concludes that the energy confinement time obtained from the power modulation experiments well agrees with that obtained from the power balance analysis, made including temperature dependence both in energy confinement time and heating efficiency.

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.

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.

Radial electric field is a key physical quantity in determining the structure of toroidal helical plasmas, owing to the nonlinear dependence of transport on the radial electric field. It has been observed that the potential profiles show various interesting patterns in space and time in the compact helical system (CHS) plasmas. In particular, the phenomenon of electric pulsation, clearly demonstrates the nature of bifurcation of radial electric field in toroidal helical plasmas.

This paper describes the major objectives of the LHD divertor experiment which is proposed to produce currentless-steady-state plasmas with high performance and without any current disruption. Since further improvement in confinement is a common and general requirement for fusion research including the LHD project, it is also necessary to develop the edge plasma control techniques and to understand the physical behaviour in the LHD divertor, i.e. the newly developed continuous helical divertor and a local island divertor (LID) concepts. In order to achieve these objectives, there were several key issues in physics and technology, which had to be resolved through careful investigation before the LHD experiment could start. In this paper, we summarize the recent progress of the physics understanding of divertor functions, divertor plasma operation scenarios, and properties of the LHD magnetic field structure in addition to the experimental planning. We also describe the recent result of an LID experiment in the CHS device, which demonstrated the possibility of edge particle and heat control by the LID.

Impurity behavior has so far been investigated in steady state hydrogen plasmas in the Large Helical Device, which is a heliotron-type device and excellent for steady state operation. There was always found to be an impurity accumulation window, as observed before (Nakamura et al 2002 Plasma Phys. Control. Fusion 44 2121, Nakamura et al 2003 Nucl. Fusion 43 219). To clarify the boundary conditions, the dependences of impurity transport on edge plasma parameters are investigated with a database of steady state hydrogen discharges, and the boundary conditions for the impurity accumulation window are discussed. It is found that two different types of impurity screening effects are essential for preventing intrinsic impurities from entering the core plasma. One of them is due to positive radial electric field at the plasma edge on the low collisionality side and the other is impurity retention caused by friction force in the ergodic layer on the high collisionality side. The classification of steady state discharges on n–T space shows that the impurity behavior can be predicted by the impurity shielding criteria based on each empirical scaling.

Supersonic gas puffing (SSGP), where a high-pressure gas is ejected through a fast solenoid valve equipped with a Laval nozzle, has been applied to large high-temperature plasmas and its fueling characteristics have been investigated in the Large Helical Device. The fueling efficiency of SSGP depends on the target plasma density and decreases as the density increases. This is due to the fueling mechanism of SSGP, where the fuel particles are supplied to the plasma edge region and then transported to the core region by diffusion. SSGP locally supplies a large number of particles to the edge region within a short time on the order of milliseconds. A fueling efficiency of ∼20% can be achieved by SSGP at a low initial density of ∼1.5 × 1019 m−3, which is more than twice as high as that of ordinary gas puffing at a similar density. Furthermore, this property leads to the additional effect of edge cooling to SSGP that will be beneficial for divertor heat load reduction.

During hydrogen pellet injection experiments in the Large Helical Device (LHD), over-ablation caused by fast ions (due to tangential neutral beam injection (NBI) heating at 150–180 keV beam energy) is shown to dominate the measured penetration depths. The neutral gas and plasma shielding model, including the interaction of the pellet with fast ions, is applied, and its predictions are shown to agree well with the measured Hα emission profiles. The toroidal deflection of the pellet trajectories observed in the direction of beam injection is reproduced by the model when unbalanced ablation on the two sides of the pellet is included. The attenuation of the fast-ion and electron heat fluxes entering the ablation cloud is examined, showing that the high-energy part of the fast-ion distribution function is responsible for the high ablation rates, and that the thermal electron population is not the dominant ablating species. An analytical scaling for the balanced NBI condition is derived, which reasonably reproduces the measured penetration depths included in the LHD database. In the LHD, for given pellet parameters and beam energy, the fast-ion density is shown to be the major parameter affecting the experimental penetration.