Optical Measurement of Cesium Behavior in a Large H− Ion Source for a Neutral Beam Injector

Optical emission in a negative hydrogen ion source for the Large Helical Device Neutral Beam Injector (LHD-NBI) has been measured to investigate the behavior of Cs. Two optical sight lines exist parallel to the plasma grid, in the discharge area and in the magnetic filter area near the plasma grid. In the discharge area, the spectrum intensity from Cs+ ions is considerably increased during 20 s of the beam extraction. This indicates a considerable increase in the Cs+ density inside the plasma due to the impact of back-streaming H+ ions. A strong neutral Cs spectrum is observed in the magnetic filter area, where the electron density is lower than in the discharge area. The rate of increase of neutral Cs is much enhanced after t = 30 s, probably because the Cs adsorbed on the cooled region inside the arc chamber evaporates because its temperature increases during the long pulse discharge

Atypical gaze patterns in children and adults with autism spectrum disorders dissociated from developmental changes in gaze behaviour

Eye tracking has been used to investigate gaze behaviours in individuals with autism spectrum disorder (ASD). However, traditional analysis has yet to find behavioural characteristics shared by both children and adults with ASD. To distinguish core ASD gaze behaviours from those that change with development, we examined temporo-spatial gaze patterns in children and adults with and without ASD while they viewed video clips. We summarized the gaze patterns of 104 participants using multidimensional scaling so that participants with similar gaze patterns would cluster together in a two-dimensional plane. Control participants clustered in the centre, reflecting a standard gaze behaviour, whereas participants with ASD were distributed around the periphery. Moreover, children and adults were separated on the plane, thereby showing a clear effect of development on gaze behaviours. Post hoc frame-by-frame analyses revealed the following findings: (i) both ASD groups shifted their gaze away from a speaker earlier than the control groups; (ii) both ASD groups showed a particular preference for letters; and (iii) typical infants preferred to watch the mouth rather than the eyes during speech, a preference that reversed with development. These results highlight the importance of taking the effect of development into account when addressing gaze behaviours characteristic of ASD

Pyrene is highly emissive when attached to the RNA duplex but not to the DNA duplex: the structural basis of this difference

Through binding and fluorescence studies of oligonucleotides covalently attached to a pyrene group via one carbon linker at the sugar residue, we previously found that pyrene-modified RNA oligonucleotides do not emit well in the single-stranded form, yet the attached pyrene emits with a significantly high quantum yield upon binding to a complementary RNA strand. In sharp contrast, similarly modified pyrene–DNA probes exhibit very weak fluorescence both in the double-stranded and single-stranded forms. The pyrene-modified RNA oligonucleotides therefore provide a useful tool for monitoring RNA hybridization. The purpose of this paper is to present the structural basis for the different fluorescence properties of pyrene-modified RNA/RNA and pyrene-modified DNA/DNA duplexes. The results of absorption, fluorescence anisotropy and circular dichroism studies all consistently indicated that the pyrene attached to the RNA duplex is located outside of the duplex, whereas the pyrene incorporated into the DNA duplex intercalates into the double helix. (1)H NMR measurements unambiguously confirmed that the pyrene attached to the DNA duplex indeed intercalates between the base pairs of the duplex. Molecular dynamics simulations support these differences in the local structural elements around the pyrene between the pyrene–RNA/RNA and the pyrene–DNA/DNA duplexes

Difference of co-extracted electron current and beam acceleration in a negative ion source with hydrogen-isotope ions

Improvement of the performance on a hydrogen/deuterium negative ion source for a nuclear fusion device is reported. In particular, the suppression of the co-extracted electron current, Ie, is an important issue to ensure the stable beam acceleration. Improvement of the Ie has been confirmed by optimizing the magnetic field of the electron deflection magnet in the extraction grid. Two other new methods for reduction of the Ie were validated. The first was an electron fence whose rods were set between the rows of apertures on a plasma grid. The electron and negative ion current ratio, approximately Ie/Iacc, was greatly improved from 0.7 to 0.25 in deuterium. The second was an outer iron yoke which enhanced the magnetic flux density 19% inside the arc discharge chamber. The Ie/Iacc using the outer yoke decreased by 0.1 compared with using a normal magnetic filter in a deuterium operation. These attempts have improved the total deuterium injection beam power of 8.4 MW by three negative ion based NBIs

Visualization of H? Dynamics in Extraction Region of Negative-Ion Source by Hα Imaging Spectroscopy

We developed a new imaging spectroscopy diagnostic tool for Hα emission and installed it on a negative hydrogen ion (H?) source to investigate the H? dynamics in the extraction region. During beam extraction, the Hα emission dropped; the same drop also appeared in the H? density (as measured by cavity ring-down spectroscopy). The reduction in the Hα emission results from the reduction in the excited hydrogen population caused by mutual neutralization processes between H+ and H? ions, which in turn are due to a decrease in the H? density. We find a reduction structure in Hα that is observed inside the plasma farther than 20 mm from the plasma grid (PG) surface. The result indicates that H? ions produced at the PG surface accumulate in the extraction region, so we conclude that they flow toward the PG apertures

LHDにおけるNBI用水素負イオン源とビーム生成の最新の研究成果

The state of hydrogen plasma in the extraction region in a hydrogen negative-ion (H−) source for NBI has been investigated. We clearly observe an improvement of H− density owing to the surface production effect with Cs seeding. H− ions are widely distributed in the extraction region which is obtained by movable cavity ring down (CRD). We confirm a negative ion rich plasma with a few electrons in the extraction region, which state is important for reduction of electron contamination in extracting beam. An extraction area is reached 30mm from the PG surface, which is measured by a 2D imaging diagnostic for Hα emission. We find the insensitive area for H− extraction at the PG surface between the apertures. Negative ions produced at the surface are considered tohave been supplied in the extraction region. The flow velocity of H− ions is obtained by a four-pin Langmuir probe using a photodetachment technique with an Nd:YAG laser. H− ion flows from the plasma grid surface, and its direction drastically changes at 20mm from the production surface. This flow behavior is considered to be an important characteristic for improving H− density in the extraction region

Measurement of electrostatic potential fluctuation using heavy ion beam probe in large helical device

Heavy ion beam probe (HIBP) for large helical device (LHD) has been improved to measure the potential fluctuation in high-temperature plasmas. The spatial resolution is improved to about 10 mm by controlling the focus of a probe beam. The HIBP is applied to measure the potential fluctuation in plasmas where the rotational transform is controlled by electron cyclotron current drive. The fluctuations whose frequencies change with the time constant of a few hundreds of milliseconds and that with a constant frequency are observed. The characteristics of the latter fluctuation are similar to those of the geodesic acoustic mode oscillation. The spatial profiles of the fluctuations are also obtained

Exploring deuterium beam operation and the behavior of the co-extracted electron current in a negative-ion-based neutral beam injector

The achievements of the deuterium beam operation of a negative-ion-based neutral beam injector (N-NBI) in the large helical device (LHD) are reported. In beam operation in LHD-NBIs, both hydrogen (H) and deuterium (D) neutral beams were generated by changing the operation gas using the same accelerator. The maximum accelerated deuterium negative-ion current () reaches 46.2 A from two beam sources with the averaged current density being 190 A m−2 for 2 s, and the extracted electron to accelerated ion current ratio () increases to 0.39 using 5.6 V high bias voltage in the first deuterium operation in 2017. An increase of electron density in the vicinity of the plasma grid (PG) surface, which is considered the main reason for the increase of co-extracted electrons in a beam, is confirmed by the half-size research negative-ion source in the neutral beam test stand at the National Institute for Fusion Science (NIFS). The deuterium negative-ion density is also larger than the hydrogen negative-ion density in the vicinity of the PG surface using the same discharge conditions. In the latest experimental campaign in 2018, increases to 55.4 A with the averaged current density being 233 A m−2 for 1.5 s using the shot extraction gap length. The low of 0.31 can be maintained by using high discharge power. The various parameters mentioned above are defined in detail below

Radial Transport Characteristics of Fast Ions Due to Energetic-Particle Modes inside the Last Closed-Flux Surface in the Compact Helical System

The internal behavior of fast ions interacting with magnetohydrodynamic bursts excited by energetic ions has been experimentally investigated in the compact helical system. The resonant convective oscillation of fast ions was identified inside the last closed-flux surface during an energetic-particle mode (EPM) burst. The phase difference between the fast-ion oscillation and the EPM, indicating the coupling strength between them, remains a certain value during the EPM burst and drives an anomalous transport of fast ions

Validation of the distribution of stripping loss neutrals in the accelerator of the negative ion source

The difference of the stripping loss between hydrogen and deuterium is examined using two approaches. The first is the measurement of the optical beam emission. The wavelength of beam emission spectrum reflects the energy distribution of beam particles by the Doppler effect. The low-energy stripping peak is observed in the energy band corresponding to the extraction voltage, and also a moderate shoulder is distributed in the lower energy region. Secondly, the spatial and the energy distribution in the accelerator is estimated by the attenuation calculation using the vacuum pressure distribution in the accelerator. Stripping neutrals are concentrated in the low energy region, and a peak is formed at 9 keV in the energy distribution due to stripping neutrals inside the extraction grid aperture. The total stripping loss inside the accelerator is 16% for hydrogen and 24% for deuterium. The calculated Doppler-shifted spectra for hydrogen and deuterium clearly show the peak with the moderate shoulder on the redshift side, which is consistent with the measured results