Akimitsu Nishizawa

Measurement of Small-Scale Density Fluctuation in JIPP T-II Plasma by Millimeter and Sub-Millineter Wave Scattering

The small-scale density fluctuations have been studied by both millimeter (2 mm) and sub-millimeter (337 µm) wave scattering technique in JIPP T-II plasmas. Experiments were performed in two types of plasma: tokamak and stellarator plasmas. In both plasmas, the density fluctuations are localized in outer region of the plasma column, and show the characteristics of the drift wave mode turbulence. The density fluctuation level is \(\tilde{n}/n_{0}{\simeq}1{-}1.5\%\) around the plasma edge, and this value is inversely proportional to the mean electrondensity. Further, the dependence of the fluctuation level on the plasma parameters is studied and compared with the scaling law of the energy confinement time of the plasma. The diffusion coefficient estimated from the density fluctuation is about one third of the global diffusion coefficient.

LHD diagnostics toward steady-state operation

The large helical device (LHD) is the world largest helical system having all superconducting coils. After completion of LHD in 1998, six experimental campaigns have been carried out successfully. The maximum stored energy, central electron temperature, and volume averaged beta value are 1.16 MJ, 10 keV, and 3.2%, respectively. The confinement time of the LHD plasma appears to be equivalent to that of tokamaks. One of the most important missions for LHD is to prove steady-state operation, which is also significant to international thermonuclear experimental reactor (ITER) and to future fusion reactors. LHD is quite appropriate for this purpose based upon the beneficial feature of a helical system, that is, no necessity of the plasma current. So far, the plasma discharge duration was achieved up to 150 s. The plasma density was kept constant by feedback control of gas puffing with real time information of the line density. The issue for demonstrating steady-state operation is whether divertor function to control particle and heat flux is effective enough. Relevant to this, LHD diagnostics should be consistent with the following: 1) continuous operation of main diagnostics during long-pulse operation for feedback control and physics understanding; 2) measurement of fraction of H, He, and impurities in the plasma; 3) heat removal and measure against possible damage or surface erosion of diagnostic components inside of the vacuum chamber; 4) data acquisition system for handling real time data display and a huge amount of data. Although there are already some achievements on the above subjects, there remain still several issues to be resolved. On the other hand, the long-pulse operation of the plasma gives benefits to the diagnostics. For example, the polarizing angle of ECE emission can be changed during the discharge, and the intensity dependence on the polarizing angle has been obtained. The spatial scanning of the neutral particle analyzer and the spectrometer can supply the spatial profiles of the fast neutral particle flux and the specific impurity lines. In this paper, the present status of these issues and future plans are described.

Observation of Mode-Converted Ion Bernstein Wave by an HCN Laser Scattering

An HCN laser scattering technique is employed for detecting an electrostatic wave excited by ICRF heating on the JIPP T-IIU tokamak. The frequency spectra and wavenumber of the excited wave were observed during the heating. The measured wave is consistent with the theoretically estimated wave dispersion of the ion Bernstein wave which is mode-converted from the fast wave in the vicinity of the ion-ion hybrid resonance layer. The influence of MHD activity on the mode-converted ion Bernstein wave is examined. When the MUD activity grows and the plasma becomes unstable, the scattered signal from the ion Bernstein wave decreases, being accompanied with large fluctuations.

Increase of Lifetime of Thallium Zeolite Ion Source for Single-Ended Accelerator.

By multiple sintering, we have significantly increased the lifetime of the thallium (Tl) zeolite ion source used in single-ended electrostatic accelerators. The obtained lifetime of a small ion source (6.4 mm in diameter and 10 mm long) was about 4400 µ Ah (70 µ Ah/mm3 for Tl zeolite).

LHD diagnostics towards steady state operation

Summary form only given, as follows. The Large Helical Device, LHD, is the world largest helical system having all superconducting coils. After completion of LHD in 1998, 6 experimental campaigns have been carried out successfully. The maximum stored energy, electron temperature, and beta value are 1.2 MJ, 10 keV, 3.2 %, respectively. The confinement time of the LHD plasma appears to be equivalent to that, of tokamaks. One of the most important missions for LHD is to prove steady state operation, which is also significant to ITER and to future fusion reactors. LHD is quite appropriate for this purpose based upon the beneficial feature of a helical system, that is, no necessity of the plasma current. So far, the plasma discharge duration was achieved up to 127 sec. The plasma density was kept constant by feedback control of gas puffing with real time information of the line density. The issue for demonstrating steady state operation is whether divertor function to control particle and heat flux is effective enough.