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A diagnostic technique for detecting the wave numbers of electron density fluctuations at electron gyro-scales in an electron cyclotron frequency range is proposed and the validity of the idea is checked by means of a particle-in-cell (PIC) numerical simulation. The technique is a modified version of the scattering technique invented by Novik et al. [Plasma Phys. Controlled Fusion 36, 357-381 (1994)] and Gusakov et al., [Plasma Phys. Controlled Fusion 41, 899-912 (1999)]. The novel method adopts forward scattering of injected extraordinary probe waves at the upper hybrid resonance layer instead of the backward-scattering adopted by the original method, enabling the measurement of the wave-numbers of the fine scale density fluctuations in the electron-cyclotron frequency band by means of phase measurement of the scattered waves. The verification numerical simulation with the PIC method shows that the technique has a potential to be applicable to the detection of electron gyro-scale fluctuations in laboratory plasmas if the upper-hybrid resonance layer is accessible to the probe wave. The technique is a suitable means to detect electron Bernstein waves excited via linear mode conversion from electromagnetic waves in torus plasma experiments. Through the numerical simulations, some problems that remain to be resolved are revealed, which include the influence of nonlinear processes such as the parametric decay instability of the probe wave in the scattering process and so on. [ABSTRACT FROM AUTHOR]

This paper provides a software application of the sampling scope concept for fusion research. The time evolution of Thomson scattering data is reconstructed with a high temporal resolution during a modulated electron cyclotron resonance heating (MECH) phase. The amplitude profile and the delay time profile of the heat pulse propagation are obtained from the reconstructed signal for discharges having on-axis and off-axis MECH depositions. The results are found to be consistent with the MECH deposition. [ABSTRACT FROM AUTHOR]

In the Large Helical Device (LHD), evident electron Bernstein wave (EBW) heating was successfully performed. The experiment was carried out using the electron cyclotron heating (ECH) system that was upgraded by installation of high-power, long-pulse 77 GHz gyrotrons. The EBW heating was achieved by a mode conversion from injected EC wave to EBW by the so-called slow-XB technique where an X-mode wave is injected to the plasma from the high magnetic field side. The specific magnetic configuration of LHD provides a good opportunity to realize the slow-XB technique, which is generally difficult for tokamaks. With the slow-XB technique, increases in kinetically evaluated electron energy Wpe and electron temperature Te were observed in overdense plasmas. An electron heating in the so-called super dense core plasma in LHD, which is characterized with an internal diffusion barrier and a steep density gradient at the plasma core, was successfully demonstrated in the plasma core region where the central electron density ne0 of 17 x 1019 m-3 was about 1.2 times higher, at the beginning of the EC-wave injection, than the left-hand cut-off density of applied 77 GHz EC waves. [ABSTRACT FROM AUTHOR]

The latest development achievements in the University of Tsukuba of over-1MW power level gyrotrons required in present-day fusion devices, GAMMA 10, Large Helical Device (LHD), QUEST, Heliotron J and NSTX, are presented. The obtained maximum outputs are 1.9MW for 0.1 s on the 77 GHz LHD tube and 1.2MW for 1ms on the 28 GHzGAMMA10 one, which are new records in these frequency ranges. In long-pulse operation, 0.3MWfor 40 min at 77 GHz and 0.54MW for 2 s at 28 GHz are achieved. A new programme of 154 GHz 1MW development has started for high-density plasma heating in LHD. On the first 154 GHz tube, 1.0MW for 1 s is achieved. As a next activity of the 28 GHz gyrotron, an over-1.5MW gyrotron is designed and fabricated to study the multi-MW oscillation. The possibility of 0.4MWcontinuous wave and 2MWlevel output in operations of a few seconds, after the improvements of output window and mode converter, is shown. Moreover, a new design study of dual-frequency gyrotron at 28 and 35 GHz has started, which indicates the practicability of the multi-purpose gyrotron. [ABSTRACT FROM AUTHOR]

Collective Thomson scattering (CTS) system has been constructed at LHD making use of the high power electron cyclotron resonance heating (ECRH) system in Large Helical Device (LHD). The necessary features for CTS, high power probing beams and receiving beams, both with well defined Gaussian profile and with the fine controllability, are endowed in the ECRH system. The 32 channel radiometer with sharp notch filter at the front end is attached to the ECRH system transmission line as a CTS receiver. The validation of the CTS signal is performed by scanning the scattering volume. A new method to separate the CTS signal from background electron cyclotron emission is developed and applied to derive the bulk and high energy ion components for several combinations of neutral beam heated plasmas. [ABSTRACT FROM AUTHOR]

A control system for a fast steering mirror has been newly developed for the electron cyclotron heating (ECH) launchers in the large helical device. This system enables two-dimensional scan during a plasma discharge and provides a simple feedback control function. A board mounted with a field programmable gate array chip has been designed to realize feedback control of the ECH beam position to maintain higher electron temperature by ECH. The heating position is determined by a plasma diagnostic signal related to the electron temperature such as electron cyclotron emission and Thomson scattering. [ABSTRACT FROM AUTHOR]

Abstract: This article reviews 10 years of engineering and physics achievements by the Large Helical Device (LHD) project with emphasis on the latest results. The LHD is the largest magnetic confinement device among diversified helical systems and employs the world''s largest superconducting coils. The cryogenic system has been operated for 50,000h in total without any serious trouble and routinely provides a confining magnetic field up to 2.96T in steady state. The heating capability to date is 23MW of NBI, 2.9MW of ICRF and 2.1MW of ECH. Negative-ion-based ion sources with the accelerating voltage of 180keV are used for a tangential NBI with the power of 16MW. The ICRF system has full steady-state operational capability with 1.6MW. In these 10 years, operational experience as well as a physics database have been accumulated and the advantages of stable and steady-state features have been demonstrated by the combination of advanced engineering and the intrinsic physical advantage of helical systems in LHD. Highlighted physical achievements are high beta (5% at the magnetic field of 0.425T), high density (1.1×1021 m−3 at the central temperature of 0.4keV), high ion temperature (T i of 5.2keV at 1.5×1019 m−3), and steady-state operation (3200s with 490kW). These physical parameters have elucidated the potential of net-current free helical plasmas for an attractive fusion reactor. It also should be pointed out that a major part of these engineering and physics achievements is complementary to the tokamak approach and even contributes directly to ITER. [Copyright &y& Elsevier]