Showing papers by "Shoji Yamamoto published in 2016"

Transition from chirping to steady NBI-driven Alfvén modes caused by magnetic configuration variations in the TJ-II stellarator

Abstract: Beam-driven Alfven eigenmodes (AEs) have been studied in the TJ-II low-magnetic-shear flexible heliac (B 0 = 0.95 T, 〈R〉 = 1.5 m, 〈a〉 = 0.22 m), in L-mode hydrogen plasmas with hydrogen NBI and ECR heating (P NBI ≤ 1.0 MW, E NBI = 32 keV, P ECRH ≤ 0.6 MW). In low-density plasmas in the range = (0.3–1.5) × 1019 m−3, a large variety of AEs have been observed in the plasma core with the heavy ion beam probe diagnostic and Mirnov coils, in the frequency range 50 kHz < f AE < 380 kHz. In experiments in which the vacuum rotational transform is varied during the shot (dynamic configuration scan), some AEs exhibit changes in their nonlinear evolution from bursting-amplitude AEs with chirping frequency to steady-frequency AEs, and back. The range of intervals within which the AEs studied are chirping or steady-state is determined.

Influence of magnetic well on electromagnetic turbulence in the TJ-II stellarator

Abstract: A magnetic well scan has been performed in the TJ-II stellarator to investigate the confinement properties with different values of the well, or even of the hill, and to explore the properties of electromagnetic turbulence. Stable plasmas have been obtained in theoretically Mercier-unstable configurations, and the electrostatic turbulence levels in the edge are increased. Three families of modes appear during the experiments: (1) a family of modes of Alfvenic nature with high frequencies; (2) a second set of modes of middle frequencies (tens of kHz) and (3) an oscillation at f ≈ 10–20 kHz happens in several cases. In spite of the fact that the vacuum rotational transform is very similar in all of the cases, the Alfvenic mode family changes drastically when decreasing the magnetic well, showing a non-monotonic behaviour of the amplitude, and a decrease of the typical frequencies. This behaviour cannot be explained only by current or density variations, so the effect of the modification of the configuration is playing a key role. Regarding the intermediate frequencies, a coherent mode appears with decreasing frequency as the magnetic well decreases. This mode is a candidate for a GAM, which can survive in these TJ-II plasmas, despite of the strong damping these modes should suffer in this device.

Antiferromagnetic order in single crystals of the S = 2 quasi-one-dimensional chain MnCl 3 (bpy)

Abstract: The prevailing theme of ``form follows function'' requires structural coherence over sufficient distances to allow subtle magnetic coupling to be revealed in high magnetic field studies at low temperatures. Single-crystal studies of the $S$=2 quasi-one dimensional chain material MnCl${}_{3}$(2,2'-bipridine) allows the long-range order to be identified, while also revealing the presence of an energy gap at the spin-flop transition field, whose nature remains intriguing.

Injection barrel with a tapered structure for a low speed and small size cryogenic hydrogen pellet in medium-sized plasma fusion devices.

Abstract: An injection barrel was designed and fabricated for a small size 0.8 mm cryogenic pellet with a low speed of 200-300 m/s in medium-sized plasma fusion devices. Pellet injection with pneumatic acceleration was examined using a conventional in situ technique. A tapered structure was applied in the downstream side of the injection barrel to satisfy the requirement of pellet speed reduction by expansion of the propellant gas. Shadowgraph and light gate measurements show that the intact pellets have speeds of 260 ± 30 m/s and a typical size of 1.1-1.2 mm. The pellet ablation code based on a neutral gas shielding model shows that the penetration depth of the measured pellet parameters does not cross the plasma center, even in medium-sized plasma devices such as the Heliotron J helical device. The injection barrel with a tapered structure developed in this study is feasible for low speed pellet injection.

Nuclear spin-lattice relaxation at field-induced level crossings in a Cr 8 F 8 pivalate single crystal

Abstract: We construct a microscopic theory for the proton spin-lattice relaxation-rate 1 / T 1 measurements around field-induced level crossings in a single crystal of the trivalent chromium ion wheel complex [Cr 8 F 8 (OOC t Bu) 16 ] at sufficiently low temperatures [E. Micotti et al., Phys. Rev. B 72 (2005) 020405(R)]. Exactly diagonalizing a well-equipped spin Hamiltonian for the individual clusters and giving further consideration to their possible interactions, we reveal the mechanism of 1 / T 1 being single-peaked normally at the first level crossing but double-peaked intriguingly around the second level crossing. We wipe out the doubt about poor crystallization and find out a solution—intramolecular alternating Dzyaloshinsky–Moriya interaction combined with intermolecular coupling of antiferromagnetic character, each of which is so weak as several tens of mK in magnitude.

Gas puff modulation experiment measured by interferometers in Heliotron J

Abstract: An HCN laser (λ = 337 μm) interferometer with a high time resolution has been developed in a helical device, Heliotron J, for the study of plasma confinement and transport. Using the new interferometer in combination with a microwave interferometer, a gas puff modulation experiment has been performed to clarify the particle transport in ECH and ECH + NBI heated plasmas. Based on the particle balance equation, the diffusion coefficient D and the convection velocity V are evaluated on the assumption of profile shapes for D, V and particle source. The result indicates that ECH plasma has better particle transport characteristics, smaller value on D and V, than the case of NBI heated plasmas. The influence of the source profile shape on this analysis is considered, because there is ambiguity on the edge plasma parameters around LCFS, which determines the source profile shape. Although evaluated values of D and V can depend on the source profiles, the difference still remains within the error bars at the present accuracy in this experimental condition, suggesting that more careful treatment of the assumption on particle source is required for the particle transport study with higher accuracy.

Development of beam emission spectroscopy for turbulence transport study in Heliotron J

Abstract: This paper describes the development study of the beam emission spectroscopy (BES) for the turbulent transport study in Heliotron J. Modification of the sightlines (10 × 4 for edge and 10 × 2 for edge) enables us to obtain 2-dimensional BES imaging. The cooling effect on the reduction in the electrical noise of avalanche photodiode (APD) assembly has been investigated using a refrigerant cooling system. When the temperature of the APD element has set to be -20 °C, the electrical noise can be reduced more than 50%. The measurement error of the phase difference in the case of low signal level has been tested by two light-emitting diode lamps. The APD cooling has an effect to improve the measurement error at the low signal level of APD.

Faraday-cup-type lost fast ion detector on Heliotron J.

Abstract: A Faraday-cup type lost-fast ion probe (FLIP) has been designed and installed in Heliotron J for the purpose of the studies of interaction between fast ions and MHD instabilities. The FLIP can measure the co-going fast ions whose energy is in the range of 1.7–42.5 keV (proton) and pitch angle of 90∘–140∘, especially for fast ions having the injection energy of neutral beam injection (NBI). The FLIP successfully measured the re-entering passing ions and trapped lost-fast ions caused by fast-ion-driven energetic particle modes in NBI heated plasmas.