Kazumichi Narihara

Effects of global MHD instability on operational high beta-regime in LHD

In the Large Helical Device (LHD), the highest operational averaged beta value has been expanded from 3.2% to 4% in the last 2 years by increasing the heating capability and exploring a new magnetic configuration with a high aspect ratio. Although the magneto-hydrodynamic (MHD) stability properties are considered to be unfavourable in the new high aspect configuration, the heating efficiency due to neutral beams and the transport properties are expected to be favourable in a high-beta range. In order to clarify the effect of the global ideal MHD unstable mode on the operational regimes in helical systems, especially the beta gradients in the peripheral region and the beta value, the MHD analysis and the transport analysis are performed in a high-beta range of up to 4% in LHD. In a high-beta range of more than 3%, the maxima of the observed thermal pressure gradients at a low order rational magnetic surface in the peripheral region are marginally unstable to the low-mode-number ideal MHD instability. Though a gradual degradation of the local transport in the region has been observed as beta increases, a disruptive degradation of the local transport does not appear in the beta range up to 4%.

Experimental study of impurity screening in the edge ergodic layer of the Large Helical Device using carbon emissions of CIII to CVI

Four resonance transitions of CIII (977.03u2002A:2s2u2009S10-2s2pu2009P11), CIV (1550u2002A:2su2009S2-2pu2009P2), CV (40.27u2002A:1s2u2009S10-1s2pu2009P11), and CVI (33.73u2002A:1su2009S2-2pu2009P2) have been observed in vacuum ultraviolet and extreme ultraviolet regions to study the edge carbon impurity transport in the Large Helical Device ergodic layer. Here, CIII and CIV indicate the carbon influx at the outside boundary of the ergodic layer and CV and CVI indicate the ions in higher ionization stages, which have already experienced the transport in the ergodic layer. The intensity ratio of CV+CVI to CIII+CIV, therefore, represents the degree of impurity screening, which has been analyzed with different edge plasma parameters and ergodic magnetic field structures. The ratio decreases by two orders of magnitude with an increase in electron density ne in the range of 1–8×1019u2002m−3. The CV and CVI emissions tend to decrease with ne, whereas the CIII and CIV emissions monotonically increase with ne. The result suggests an enhancement of the impurity scr...

Observation of the low to high confinement transition in the large helical device

The low to high confinement transition has been observed on the large helical device [A. Iiyoshi, A. Komori, A. Ejiri et al., Nucl. Fusion 39, 1245 (1999)], exhibiting rapid increase in edge electron density with sharp depression of Hα emission. The transition occurs in low toroidal field (Bt=0.5–0.75T) discharges and are heated by high power neutral beam injection. The plasma thus has a relatively high value (∼1.5%) of the volume averaged β value. The electron temperature and density profiles have steep gradients at the edge region which has high magnetic shear but is at a magnetic hill. Formation of the edge transport barrier leads to enhanced activities of the interchange type of modes with m=2∕n=3 (m,n are the poloidal and toroidal mode numbers) in the edge region. At present, these magnetohydrodynamic activities limit the rise of the stored energy; the resultant increment of the stored energy remains modest.

Materials selection for the in situ mirrors of laser diagnostics in fusion devices

When mirrors for the laser scattering diagnostic for large fusion devices need to be inside the vacuum chamber, they are subjected to irradiation by multiple high-energy laser pulses and bombardment by charge exchange atoms. Both of these assaults are known to degrade and eventually damage metal laser mirrors given sufficient time and flux. Our aim in this article is to use current data on these damage mechanisms to make design selections of metal mirror materials for application in fusion device diagnostics. We identify tradeoffs between low sputtering rates and multipulse laser damage resistance in candidate metals. The data for multipulse laser damage are incomplete and extend to a maximum of only 104 shots for a few metals. However, there is a clear trend of decreasing laser-damage threshold with increasing number of shots, and damage threshold fluences can fall to 10% of the single-pulse damaging laser fluence. Further experiments up to 106 or 108 laser shots need to be conducted on the likely mirror...

Measurement of profiles of the space potential in JIPP T-IIU tokamak plasmas by slow poloidal and fast toroidal sweeps of a heavy ion beam

By the use of simultaneous fast toroidal and slow poloidal sweeps of a heavy ion beam, plasma potential profiles in the JIPP T-IIU tokamak are measured at the rate of 120 spatial profiles per second. A new method to eliminate the error due to the change of out-of-plane entrance angle caused by large plasma current is successfully applied. One of the key factors is the very homogeneous characteristics of the energy analyser along the wide slit length. A shaped electrode system, instead of guard rings with a resistor chain, successfully increases the homogeneity of the analyser. Error due to the significant change of in-plane entrance angle during a poloidal sweep is carefully minimized and calibrated by a secondary beam ionized by neutral gas introduced in the vacuum vessel. The depth of the measured potential at the plasma centre reaches more than 1.5 keV in ohmic plasmas with an ion temperature of 600 eV. In some cases, a rather wide region of positive potential and a sharp decrease of the potential in the centre of the plasma are observed.

Raman calibration of the LHD YAG Thomson scattering for electron-density measurements

We have calibrated the LHD yttrium aluminum garnet (YAG) Thomson scattering system by using anti-Stokes rotational Raman scattering from air for the measurements of absolute electron-density profiles of LHD plasmas. The air Raman calibration was carried out at near atmospheric pressure and room temperature. Total uncertainty in measured electron densities is estimated to be ±15% or less. Electron densities obtained with the calibrated YAG Thomson scattering show good agreements with those measured by the LHD millimeter-wave interferometer.

Experiment of magnetic island formation in Large Helical Device

Magnetic island formation is experimentally investigated in the Large Helical Device. The (m, n) = (1, 1) vacuum magnetic island is generated by using the local island diverter field, where m and n are the poloidal and toroidal mode numbers, respectively. The island width depends on plasma parameters (the electron temperature and the β) and the magnetic axis position. In the case of Rax = 3.53u2009m the magnetic island in the plasma is larger than that in the vacuum field. Here, Rax is the major radius of the magnetic axis. In the case of Rax = 3.6u2009m the magnetic island is not generated when the error field is less than the threshold which is increased as the β is increased. Evidence of island current is obtained when the magnetic island is formed due to a small error field. However, the mechanism that generates the island is not yet known.

Initial results of local island divertor experiments in the large helical device

Abstract 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 ~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.

Spatial resolved high-energy particle diagnostic system using time-of-flight neutral particle analyzer in Large Helical Device

The time-of-flight-type neutral particle analyzer has an ability of horizontal scanning from 40 to 100° of the pitch angle. The information from the spatially resolved energy spectrum gives not only the ion temperature but also the information of the particle confinement and the electric field in plasmas. We have been studying the energy distributions at various magnetic configurations in the neutral beam injection (NBI) plasma. The spatially resolved energy spectra can be observed during long discharges of the NBI plasma by continuous scanning of the neutral particle analyzer. The shape of spectra is almost similar from 44° to 53°. However, the spectra from 55° are strongly varied. They reflect the injection pitch angle of the beam. The pitch angle scanning experiment during the long discharge of NBI plasma has also been made under the reversal of the magnetic field direction. NBI2 becomes counter injected with the reversal. We can easily observe the difference between co- and counter injections of NBI. During the electron cyclotron heating in the low-density plasma for the formation of the internal thermal barrier, large neutral particle increase or decease can be observed. The degree of the increase/decrease depends on the energy and the density. The reason for the variation of the particle flux is that the orbit of the trapped particle changes due to the electric field formed by the strong electron cyclotron heating.

Construction of a 100-Hz-Repetition-Rate 28-Channel Thomson Scattering System for the JIPPT-IIU Tokamak.

An ASDEX-type Thomson scattering system (YAG Thomson) with an improved performance (100 Hz repetition rate and 1.5 cm spatial resolution) was designed and constructed for the JIPPT-IIU tokamak. Efforts were concentrated on realizing a large solid angle of collection optics (0.08 sr), high throughput of polychromators ( >64%) and high quantum efficiency of detectors (80% at 900 nm), which, together, yield high-quality data even with a small laser energy (0.4 J) delivered by a 100-Hz-repetition-rate Nd:YAG laser and with a short scattering length (1.0 cm). The compactness of the polychromators also contributes to achieving a high spatial resolution.