Kenya Akaishi

The large helical device (LHD) helical divertor

The Large Helical Device (LHD) now under construction is a heliotron/torsatron device with a closed divertor system. The edge LHD magnetic structure has been studied in detail. A peculiar feature of the configuration is the existence of edge surface layers, a complicated three dimensional magnetic structure which does not, however, seem to hamper the expected divertor functions. Two divertor operational modes are being considered for the LHD experiment-high density, cold radiative divertor operation as a safe heat removal scheme and high temperature divertor plasma operation. In the latter operation, a divertor plasma with a temperature of a few keV, generated by efficient pumping, is expected to lead to a significant improvement in core plasma confinement. Conceptual designs of the LHD divertor components are under way

Progress summary of LHD engineering design and construction

In March 1998, the LHD project finally completed its eight year construction schedule. LHD is a superconducting (SC) heliotron type device with R = 3.9 m, ap = 0.6 m and B = 3 T, which has simple and continuous large helical coils. The major mission of LHD is to demonstrate the high potential of currentless helical-toroidal plasmas, which are free from current disruption and have an intrinsic potential for steady state operation. After intensive physics design studies in the 1980s, the necessary programmes of SC engineering R&D was carried out, and as a result, LHD fabrication technologies were successfully developed. In this process, a significant database on fusion engineering has been established. Achievements have been made in various areas, such as the technologies of SC conductor development, SC coil fabrication, liquid He and supercritical He cryogenics, development of low temperature structural materials and welding, operation and control, and power supply systems and related SC coil protection schemes. They are integrated, and nowadays comprise a major part of the LHD relevant fusion technology area. These issues correspond to the technological database necessary for the next step of future reactor designs. In addition, this database could be increased with successful commissioning tests just after the completion of the LHD machine assembly phase, which consisted of a vacuum leak test, an LHe cooldown test and a coil current excitation test. These LHD relevant engineering developments are recapitulated and highlighted. To summarize the construction of LHD as an SC device, the critical design with NbTi SC material has been successfully accomplished by these R&D activities, which enable a new regime of fusion experiments to be entered.

Analysis of the resonance characteristics of a cantilever vibrated photothermally in a liquid

The resonance characteristics of stainless‐steel cantilevers vibrated photothermally in a liquid have been investigated using an optical detection system. Resonance frequencies of vibrating cantilevers in liquids are lower than those in air as a result of the action of reaction forces in the liquid. It is shown that experimental values of the resonance frequencies for several types of cantilevers in water agree well with those calculated. The half power width at resonance broadens with increasing viscosity of the liquid. The possibility for using this photothermal vibration method as an optical sensing system for the density or viscosity of liquids is described.

Ion and electron heating in ICRF heating experiments on LHD

The ICRF heating experiments conducted in 1999 in the third experimental campaign on LHD are reported, with an emphasis on the optimization of the heating regime. Specifically, an exhaustive study of seven different heating regimes was carried out by changing the radiofrequency relative to the magnetic field intensity, and the dependence of the heating efficiency on H minority concentration was investigated. It was found in the experiment that both ion and electron heating are attainable with the same experimental set-up by properly choosing the frequency relative to the magnetic field intensity. In the cases of both electron heating and ion heating, the power absorption efficiency depends on the minority ion concentration. An optimum minority concentration exists in the ion heating case while, in the electron heating case, the efficiency increases with concentration monotonically. A simple model calculation is introduced to provide a heuristic understanding of these experimental results. Among the heating regimes examined in this experiment, one of the ion heating regimes was finally chosen as the optimized heating regime and various high performance discharges were realized with it.

Review of initial experimental results of the PSI studies in the large helical device

The large helical device (LHD) is the largest heliotron type superconducting device. Its operation was started on 31 March 1998. Three experimental campaigns have been completed until the end of 1999. Wall conditioning mainly by cleaning discharges using ECRF or glow discharges worked well even without high temperature baking. The plasma production with ECRH and auxiliary heating with NBI and/or ICRF in the LHD configuration equipped with open helical divertor were well performed. The divertor material was SS316L in the first and second campaigns, and was replaced by the graphite in the third campaign. The influences of the different divertor materials were investigated. Our understanding of the edge and the divertor plasma has progressed. Long-pulse discharges 80 and 68 s heated by NBI (0.5 MW) or ICRF (0.9 MW) have been achieved, respectively. No severe limitation of the duration has appeared.

Boronization study for application to large helical device

An experimental device named SUT (SUrface modification Teststand) was constructed for a boronization study. An ultra high vacuum (UHV) condition, a changeable high temperature liner and in situ AES are three distinctive feature of the SUT device. Saturation density of oxygen atoms was as large as 1.2 × 10 17 /cm 2 on a boronized surface, whereas 1.5 × 10 16 /cm 2 on a bare stainless steel surface. It is found by AES analysis that the oxygen-contained layer was as thick as 50 nm from the top surface of the boron film. From such a large oxygen-saturation density, we expect that the oxygen-gettering ability of the boronized surface is likely to be maintained during one-day experiment of LHD. The oxygen-saturation behavior was quite similar between the boronized surfaces obtained with decaborane and diborane, which indicates that, as a working gas of the boronization, the decaborane works well compared with diborane, as far as oxygen gettering is concerned

A study of high-energy ions produced by ICRF heating in LHD

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.

Ion cyclotron range of frequency heating experiments on the large helical device and high energy ion behavior

Ion cyclotron range of frequency (ICRF) heating experiments on the Large Helical Device (LHD) [O. Motojima et al. Fus. Eng. Des. 20, 3 (1993)] achieved significant advances during the third experimental campaign carried out in 1999. They showed significant results in two heating modes; these are modes of the ICH-sustained plasma with large plasma stored energy and the neutral beam injection (NBI) plasma under additional heating. A long-pulse operation of more than 1 minute was achieved at a level of 1 MW. The characteristics of the ICRF heated plasma are the same as those of the NBI heated plasma. The energy confinement time is longer than that of International Stellarator Scaling 95. Three keys to successful ICRF heating are as follows: (1) an increase in the magnetic field strength, (2) the employment of an inward shift of the magnetic axis, (3) the installation of actively cooled graphite plates along the divertor legs. Highly energetic protons accelerated by the ICRF electric field were experimentally...

Innovative divertor concepts for LHD

Abstract We are developing various innovative divertor concepts which improve the LHD plasma performance. These are two divertor magnetic geometries (helical and local island divertors), three operational scenarios (radiative cooling in the high density, cold boundary, confinement improvement by generating high temperature divertor plasma and simultaneous achievement of radiative cooling and H-mode like confinement improvement) and technological development of new efficient hydrogen pumping schemes.

ISS studies on sputtering of chemisorbed gases by low-energy ions

Abstract We measured the desorption cross sections of chemisorbed gases, such as H2, CO, O2 and segregated S on single crystalline Mo(llO) and polycrystalline Ni due to He+, Ne+ and/or Ar+ ion bombardment, by using the low-energy ion scattering spectrometer (ISS). By making a comparison between the measured and the theoretical values, the underlying mechanism of ion induced desorption was clarified. Furthermore it was found that the theory always gives larger values within a factor of 3 than the values measured in this work. The reason for this behavior was discussed. In particular, in case of the desorption of weakly bound gases due to light ion bombardment, it was empirically found that the theoretical values are in good agreement with the experimental results. On the basis of these empirical results, we calculated the desorption cross sections of adsorbed H atoms on Mo and Ni due to H+ ion bombardment, in anticipation of the simulation of hydrogen recycling process in fusion devices.