In H2 discharges of large helical device (LHD), H-mode-like transition has been obtained with edge localized mode (ELM)-like bursts in an outwardly shifted configuration of R ax = 4.00 m with an extremely thick ergodic layer, where the ι/2π = 1 position is located in the middle of the ergodic layer. This H-mode-like transition can be triggered by changing P NBI (<12 MW) from 3 beams to 2 beams in a density range (4–8) × 1013 cm−3. The spontaneous H-mode-like transition is also observed during a stable discharge with constant heating power and density, where a steep edge pressure gradient is formed at the ι/2π = 1 position. The ELM-like bursts occurred in the ergodic layer accompanied with density bursts. It is found that the density bursts triggered at the ι/2π = 1 position in the ergodic layer instantaneously propagate outward beyond the ergodic layer with positive density pulse, and a few ms later also begin to propagate inward from the ergodic layer with negative density pulse to recover the previous edge density profile. In the case of He discharges no H-mode-like transition and ELM-like bursts are observed. The frequency of the ELM-like bursts increases with heating power, e.g. 50 Hz (3 MW)–100 Hz (5 MW) at n e = 4 × 1013 cm−3. The external resonant magnetic field to generate a static m/n = 1/1 island inside the ergodic layer is supplied to the ELM-like bursts. It is also found that the externally supplied magnetic field makes a clear response even in the ergodic layer. As a result, the frequency of the ELM-like bursts decreases, e.g. from 60 to 45 Hz. The present study strongly indicates that those phenomena observed in LHD are quite different from the ones which have been observed in tokamaks until now.

Visible spectral emission lines from magnetic dipole transitions in Ar X, Ar XI, Ar XIV, and Ar XV are observed from plasmas heated with neutral-beam injection ͑NBI͒ in the Large Helical Device ͓O. Motojima et al., Phys. Plasmas 6, 1843 ͑1999͔͒. Orthogonal linearly polarized components of the emission line profiles are observed with a polarization separation optical system and high-resolution spectrometer. Zeeman split profiles reveal polarization characteristics of magnetic dipole transitions. Ion temperatures and emission locations are estimated from the profiles with the magnetic field information on the lines of sight ͑LOS͒. The spatially resolved emissions are observed by the array of absolutely calibrated views. The time histories of line profiles and emission intensities at the poloidal view are presented. The observed line profiles and the intensity distribution at the poloidal view indicate the localization of these charge states in the edge region just inside the last closed flux surface. The emission line of Ar X in the tangential observation indicates Doppler shifts of the Zeeman split profiles. The velocity components of Ar X ion flow along the LOS at the tangential view are 7.7 and 2.0 km/ s at the outer and inner edge plasmas, respectively, in the opposite direction to the NBI.

Articles you may be interested inRadial profile measurement of electron temperature in edge stochastic magnetic field layer of LHD using intensity ratio of extreme ultraviolet line emissionsa) Rev. Sci. Instrum. 83, 10E509 (2012); 10.1063/1.4732061 Two-dimensional measurement of edge impurity emissions using space-resolved extreme ultraviolet spectrometer in Large Helical Device Rev. Sci. Instrum. 83, 043503 (2012); 10.1063/1.4705290 Absolute intensity calibration of flat-field space-resolved extreme ultraviolet spectrometer using radial profiles of visible and extreme ultraviolet bremsstrahlung continuum emitted from high-density plasmas in Large Helical Device Rev. Sci. Instrum. 82, 113102 (2011); 10.1063/1.3658050 Space-resolved vacuum ultraviolet spectrometer system for edge impurity and temperature profile measurement in HL-2A Rev. Sci. Instrum. 81, 043503 (2010);A space-resolved 3 m vacuum ultraviolet ͑vuv͒ spectrometer has been developed to measure a radial ͑vertical͒ profile of impurity line emissions in the Large Helical Device ͑LHD͒. The system consists of a 3 m normal incidence spectrometer equipped with a space-resolved slit and a back-illuminated charge-coupled device ͑CCD͒ detector, view-angle adjustable mirrors, and a toroidal slit. A full vertical profile of the vuv line radiated from the LHD plasma is observed with a convex cylindrical mirror by extending the view angle. The vertical profiles of intrinsic impurities of carbon ͑C III-C V͒ and externally injected impurities of neon ͑Ne VIII͒, aluminum ͑Al XI͒, and argon ͑Ar VIII͒ are successfully obtained with a time interval of 125 ms in the VUV range of 500-3100 Å. The ion temperature profiles are also measured from the C IV line emitted in the ergodic layer of LHD.

A diode-pumped joule class in a 10 Hz output Nd:YLF ring amplifier has been developed. A phase conjugate plate was developed as a wavefront corrector for the residual wavefront distortion of an Nd:YLF rod. We have demonstrated a 0.46 J output of 10 ns pulse duration at 10 Hz repetition rate with 1.5 nJ of input energy. The effective gain of the ring amplifier system was 84.8 dB. To our knowledge, this is the highest magnification with joule-level output energy in a single-stage amplifier system that has ever been built. As a preamplifier system, this system contributed a demonstration of 21.3 J in a 10 Hz output diode-pumped Nd:glass zigzag slab laser system with a stimulated Brillouin scattering- phase conjugation mirror. We describe a robust and effective method of wavefront correction for high-energy laser systems.

Intrinsic impurities have been much reduced in toroidal fusion devices through the development of several wall-conditioning techniques as well as by the use of carbon materials in the first wall and divertor plates. Impurity elements useful for passive plasma spectroscopy have been then extremely limited. At present, only carbon is a subject for spectroscopic diagnostics in most discharges except for fuel atoms. The use of rare gas as a brighter light source is a method to overcome the present difficulty in passive spectroscopy. Recently, rare gases have also been used for edge cooling to reduce the divertor heat flux. Therefore, high-resolution spectra (∆λ ∼ 0.2 Å) from neon and argon in a 250 to 2300 Å wavelength range have been measured using a 3 m normal incidence spectrometer in Large Helical Device (LHD) and the measured spectra were precisely analyzed. The VUV spectra of carbon, neon and argon are presented for spectroscopic use and their wavelengths are tabulated with their relative intensities. The spectral profiles of almost all the spectral lines measured here are formed by the Doppler broadening and self-absorption processes. The Doppler broadening of neon and argon spectra are plotted against the ionization energies and Doppler spectra from carbon lines are presented. The self-absorption spectra of the hydrogen Lyman-α line, which are found in the LHD high-density discharge, are also presented and the neutral density is analytically estimated.