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All the hydrogen isotope (H, D, T) simultaneous TDS (Thermal desorption spectroscopy) measurement system (HI-TDS system) was newly designed to evaluate all hydrogen isotope desorption behavior in materials. The present HI-TDS system was operated under Ar purge gas and the H and D desorptions were observed by a quadruple mass spectrometer equipped with an enclosed ion source, although T desorption was evaluated by an ionization chamber or proportional counters. Most of the same TDS spectra for D and T were derived by optimizing the heating rate of 0.5 K s−1 with Ar flow rate of 13.3 sccm.Using this HI-TDS system, D and T desorption behaviors for implanted or DT gas exposed tungsten samples installed in LHD (Large Helical Device) at NIFS (National Institute for Fusion Science) was evaluated. It was found that major hydrogen desorption stages consisted of two temperature regions, namely 700 K and 900 K, which was consistent with the previous hydrogen plasma campaign and most of hydrogen would be trap...

Magnetic flux surface measurements have been carried out in the Large Helical Device (LHD) in the standard magnetic field configuration with toroidal magnetic field strength up to 2.75 T. An electr...

Wall conditioning in the Large Helical Device (LHD) has been conducted successively since the first experimental campaign in 1998. The effects of wall conditioning on the vacuum condition, the plas...

A global particle balance study has been investigated in the Large Helical Device (LHD) in which the first wall and the divertor tiles are made of stainless steel (SUS-316L) and carbon, respectivel...

Easy access to the high-density regime without fatal disruptive phenomena is one of the important characteristics of the Large Helical Device (LHD). The operational density is considerably higher t...

The Large Helical Device (LHD) has been equipped with movable- and fixed-type material probe systems. Characterization studies of surface modifications on plasma-facing components (PFCs) ha...

To achieve an improvement of plasma confinement by an effective edge plasma control, the local island divertor (LID) was originally proposed in the National Institute for Fusion Science in the earl...

An interesting high-density operational regime with an internal diffusion barrier (IDB) has been extended to the helical divertor configuration in the Large Helical Device. The IDB is characterized...

Transport characteristics of the stochastic magnetic boundary of the Large Helical Device (LHD) are investigated, based on three-dimensional Monte-Carlo Braginskii-type fluid model code, EMC3, coup...

The Large Helical Device (LHD) is the world’s largest heliotron-type device with l = 2, m = 10 continuous superconducting helical coils and three pairs of superconducting poloidal coils. The major ...

The divertor performance of LHD is studied for the two configurations, LID and HD. It is shown that the both divertor configurations play important roles for obtaining high performance plasmas in LHD : the large pumping capability of the LID to keep the low edge density in the IDB-SDC plasma, the large wetted area and the flexibility of strike point sweep of HD to reduce the power load on the divertor plates in long pulse operations. The possible effect of the ergodic layer on impurity retention in divertor is discussed by using the 3D edge transport modelling. It is found that the drag force exerted by the plasma flow can dominate over the thermal force, providing the impurity retention effect. The further changes needed to improve the current divertor configurations are discussed. New divertor designs for the future upgrade of LHD and for a LHD-type reactor are presented.

One of the characteristics of the heliotron-type magnetic configuration is that it has an intrinsic divertor structure (helical divertor). Particle control using a helical divertor configuration, to achieve improved confinement and sustainment of steady-state high-performance plasmas, is a major experimental goal in the Large Helical Device (LHD), the largest heliotron-type superconducting device, and it needs to be demonstrated on the route to the design of the heliotron-type fusion reactor. The LHD scrape-off layer (SOL) in the intrinsic helical divertor configuration has a unique magnetic field line structure consisting of stochastic regions, residual islands, whisker structures, and laminar layers contrasting with the "onion-skin"-like magnetic field line structure in poloidal divertor tokamak SOLs. Since the first experimental campaign in LHD in 1998, studies aiming at understanding the edge plasma properties in the "open" helical divertor configurations have been conducted experimentally and theoretically. In this paper, the helical divertor studies in the LHD are reviewed, and the future experimental plan is shown.

The long-pulse plasma discharge experiment is an important experiment in the Large Helical Device, which has a superconducting coil system and the capability of steady-state operation. The experiment of long-pulse plasma discharge was carried out using mainly ion cyclotron range of frequencies heating. The maximum plasma duration is 31 min and 45 s, and the total injected heating energy reached 1.3 GJ. Swing of the magnetic axis is adopted as an effective method to scatter the local heat load on the dive rtor plate during the discharge. The plasma was terminated abruptly by the influx of metallic impurities accompanied by a spark in the vacuum vessel.

Understanding the reason for the divertorflow asymmetry in heliotron/torsatron-type fusion devices is important for the safety of the in-vessel components, such as divertor plates subject to the direct impact ofthe plasma or the electrical probes and thermocouples for measurements of the particle and energy fluxes to the divertor plates. In previous work, the divertorflow distributions were studied mainly with focusing on the up-down asymmetry in a heliotron-type fusion device, Heliotron E. This paper analyzes the in-out asymmetry of divertor flows and discusses the effects on this asymmetry of the magnetic axis position (the horizontal shift due to variation of the vertical magnetic field or plasma pressure) and power of neutral beams.

Long-pulse operation and high-energy particle confinement properties were studied using ion cyclotron range of frequency (ICRF) heating for the Large Helical Device. For the minority-ion mode, ions with energies up to 500 keV were observed by concentrating the ICRF heating power near the plasma axis. The confinement of high-energy particles was studied using the power-modulation technique. This confirmed that the confinement of high-energy particles was better with the inward-shifted configuration than with the normal configuration. This behavior was the same for bulk plasma confinement. Long-pulse operation for more than 2 min was achieved during the experimental program in 2002. This was mainly due to better confinement of the helically trapped particles and accumulation of fewer impurities in the region of the plasma core, in conjunction with substantial hardware improvements. Currently, the plasma operation time is limited by an unexpected density rise due to outgassing from the chamber materials. The temperature of the local carbon plates of the divertor exceeded 400 deg, C, and a charge-coupled device camera observed the hot spots. The hot spot pattern was well explained by a calculation of the accelerated-particle orbits, and those accelerated particles came from outside the plasma near the ICRF antenna.

A local island divertor (LID) experiment has begun in the Large Helical Device (LHD) to demonstrate improved plasma confinement, and fundamental LID functions were demonstrated in the sixth experimental campaign in 2002-2003. It was clearly shown that when an m/n = 1/1 island is generated by adding a resonant perturbation field to the LHD magnetic configuration, the particle flow is guided along the island separatrix to the backside of the island, where carbon plates are located on a divertor head. The particles recycled there are pumped out efficiently so that the line-averaged core plasma density is reduced by a factor of {approx}2 at the same gas puff rate, compared with non-LID discharges. Obvious improvement of the global plasma confinement was, however, not observed yet, because the discharge could not be optimized, due to a large amount of outgas from the divertor head to the core plasma. The size of the divertor head was found to be larger than the optimum one; hence, the core plasma impacted slightly on the core plasma-facing portion of the divertor head with which the core plasma was not expected to collide.

A comprehensive study of the divertor plasma flow (DPF) distribution in the Heliotron-E helical device has been carried out by means of collector arrays under various experimental conditions. A strong inhomogeneity of DPF distribution is observed in all the investigated regimes. This inhomogeneity is characterized by a significant asymmetry of plasma flows recorded by symmetrically displaced collectors. The degree of asymmetry in horizontal (in-out) and in vertical (up-down) directions depends on characteristics of magnetic configuration as well as on experimental conditions: method of plasma heating, injected power and power absorbed by the plasma, plasma density, gas puffing time, pellet injection, etc. A certain degree of asymmetry also remains after termination of the active phase of discharge, i.e ., at the stage of decaying plasma. By comparison of asymmetry indices at the active stages of discharge and at the stage of afterglow plasma, the contribution of trapped particles into the asymmetry of DPF...

A new diagnostic method to measure ion energy distribution using ion reflection on the solid surface, one of the processes of plasma-wall interactions, is proposed. It is well known that the energy distribution of reflected particles is related to the energy distribution of incident ions. This method is used in relatively Iow ion temperature region such as edge plasma in fusion experimental devices. Incident ions to target plate which is inserted in the edge plasma are mainly reflected as neutral particles, and time-of-flight method is utilized to measure energy distribution of reflected particles. Energy distribution of incident ions is deduced by fitting with calculation using Monte Carlo simulation code TRIM.SP. The sheath potential in front of the target can also be obtained with this method.

In devices such as Stellarator/Heliotron, nested magnetic surfaces are surrounded by the so-called separatrix layer, which consists of open field lines with various connection length to the vacuum chamber wall and/or divertor plates. In this paper, we study the strike point distribution of field lines on Compact Helical System chamber wall, especially under divertor configuration. Then, by using these distribution as the measure of neutral recycling source, Monte Carlo simulation of 3-D neutral transport is done. The results are compared with those obtained for different magnetic configuration.

Open field line region plays the key role for steady state operation of the Large Helical Device (LHD) and greatly contributes to the high-performance plasma confinement in the LHD. Chaotic field line region, produced by high magnetic shear and nonaxisymmetry of the magnetic field, is present in open field line region outside the last closed flux surface (LCFS) of the LHD. The chaotic field line layer can sustain ambient plasma due to the long connection length of lines of force, presence of the embedded magnetic islands and mirror confinement effect of helical ripple nature of the magnetic field. This ambient plasma plays a role of an impregnable barrier for the core plasma, which suppresses both the MHD instabilities and the cooling of the core plasma due to charge exchange processes. Slow and small periodic sweeping of magnetic axis position can control the deconcentration of divertor heat flux in the LHD.