Experimental study on chorus emission in an artificial magnetosphere.

Wave particle interaction plays an important role in geospace and space weather phenomena. Whistler mode chorus emissions, characterized by non-linear growth and frequency chirping, are common in planetary magnetospheres. They are regarded as the origin of relativistic acceleration of particles in the radiation belts and pulsating aurora. Intensive theoretical investigations and spacecraft observations have revealed several important features of chorus emissions. However, there is a need to conduct high-resolution and reproducible controlled laboratory experiments to deepen the understanding of space weather. Here, we present the spontaneous excitation of chirping whistler waves in hot-electron high-ß plasma (ß is the ratio of the plasma pressure to the magnetic pressure) in an "artificial magnetosphere", a levitated dipole experiment. These experiments suggest that the generation and nonlinear growth of coherent chorus emissions are ubiquitous in dipole magnetic configuration. We anticipate that these experiments will accelerate the laboratory investigation of space weather phenomena.

Valley filters using graphene blister defects from first principles.

Valleytronics, which makes use of the two valleys in graphenes, attracts considerable attention and a valley filter is expected to be the central component in valleytronics. We propose the application of the graphene valley filter using blister defects to the investigation of the valley-dependent transport properties of the Stone-Wales and blister defects of graphenes by density functional theory calculations. It is found that the intervalley transition from theKvalley to theK'valleys is completely suppressed in some defects. Using a large bipartite honeycomb cell (BHC) including several carbon atoms in a cell and replacing atomic orbitals with molecular orbitals in the tight-binding model, we demonstrate analytically and numerically that the symmetry between the A and B sites of the BHC contributes to the suppression of the intervalley transition. In addition, the universal rule for the atomic structures of the blisters suppressing the intervalley transition is derived. Furthermore, by introducing additional carbon atoms to graphenes to form blister defects, we can split the energies of the states at which resonant scattering occurs on theKandK'channel electrons. Because of this split, the fully valley-polarized current will be achieved by the local application of a gate voltage.

Collective Thomson scattering diagnostic with in situ calibration system for velocity space analysis in large helical device.

A collective Thomson scattering (CTS) diagnostic with a ±3 GHz band around a 77 GHz gyrotron probe beam was developed to measure the velocity distribution of bulk and fast ions in high-temperature plasmas. We propose a new in situ calibration method for a CTS diagnostic system combined with a raytracing code. The method is applied in two situations for electron cyclotron emission in plasmas and in a CTS diagnostic with a modulated probe beam. Experimental results highlight the importance of refraction correction in probe and receive beams. The CTS spectrum is measured with the in situ calibrated CTS receiver and responds to fast ions originating from a tangential neutral beam with an energy of 170 keV and from a perpendicular beam with an energy of 60 keV, both in the large helical device. From a velocity space analysis model, the results elucidate the measured anisotropic CTS spectrum caused by fast ions. The calibration methods and analyses demonstrated here are essential for CTS, millimeter-wave diagnostics, and electron cyclotron heating required under fusion reactor conditions.

Calibration of coherence imaging spectroscopy using spectral line sources.

Coherence imaging spectroscopy (CIS) measures the two-dimensional profiles of both ion temperature and ion velocity in plasmas. The interferometric technique is realized by a certain relation between the phase and the wavelength of light emerging from a birefringent crystal. The calibration for the CIS system requires monochromatic and tunable light sources near the He II line (468.6 nm) or C III line (465 nm) where the CIS measures. In this research, the CIS system has been upgraded by implementing an electron multiplier CCD and a CIS cell. A monochromator validates the linearity of the phase relation on the wavelength near the He II line. As an in situ calibration at the Ring Trap 1 plasma device, two spectral lines of Ti and Zn lamps obtain the accurate dispersion function of phase. It is found that a simple method with two spectral lines is reliable and sufficient for the calibration.

Fast ion measurement using a hybrid directional probe in the large helical device.

A hybrid directional probe was newly installed in the large helical device for fast ion measurement. The collector of the probe mounts a thermocouple to estimate local power flux and can be also utilized as a collector of a conventional Langmuir probe; therefore, the hybrid directional probe can simultaneously measure both local power density flux and current flux at the same collector surface. The concept and design of the hybrid directional probe, the calibration of the power density measurement, and preliminary result of the fast ion measurement are presented.

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.

Measurements of spatial structure of plasma potential and density fluctuations by multichannel heavy ion beam probe on large helical device.

Heavy ion beam probe (HIBP) on large helical device is currently equipped with three channel detectors, which can observe three spatial points simultaneously inside the plasma with resolution of approximately 10 mm. The beam trajectories and observation point location are calculated numerically and optimized allowing for the identification of the mode structure in multichannel (up to 9) HIBP measurements. The calculations show that the radial and poloidal wavenumbers can be identified by proper changing and choosing of the beam energy and trajectory.