A Simultaneous Spectroscopic Measurement of the Global and Edge Local Structures in the Ion Temperature and Plasma Rotation Profiles in the Compact Helical System

Nishimura, Satoshi; Nagaoka, K.; Yoshimura, Y.; Nakamura, Katsumi; Iguchi, H.; Akiyama, Tsuyoshi; Minami, Takashi; Oishi, T.; Ida, K.; Toi, K.; Shimizu, A.; Isobe, M.; Suzuki, C.; Takahashi, C.; Okamura, S.; Matsuoka, K.

doi:10.1585/pfr.2.037

Cited by 3 publications

(2 citation statements)

References 16 publications

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“…Note that we use the expression "pth harmonic" focusing on the shape of the frequency spectra of the HO (core) and HO (edge) having frequencies which are p times larger than those of the fundamental oscillation, even though the dispersion relationships of both the HO (core) and HO (edge) are different from that of the conventional MHD oscillation having harmonics. If the frequency of the 1st harmonic is determined by the E × B poloidal rotation, the rotation velocities at ρ = 0.95 (HO (edge), m = 1, 4 kHz) and ρ = 0.53 (HO (core), m = 2, 3.5 kHz) are estimated to be about 4800 m/s and 1200 m/s in the electron diamagnetic direction, respectively, where the averaged minor radius of the last closed flux surface is 0.2 m. Each value is almost consistent with the rotation velocity measured using charge exchange recombination spectroscopy [21]. On the other hand, the phase velocity of the harmonic components other than the 1st harmonic cannot be explained by the E × B rotation.…”

Section: Resultssupporting

confidence: 73%

Measurement of 3-D Mode Structure of the Edge Harmonic Oscillations in CHS using Beam Emission Spectroscopy

Oishi

Kado

Yoshinuma

et al. 2007

Plasma and Fusion Research

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The 3-D spatial structure-radial locality and poloidal/toroidal mode numbers-of the magnetohydrodynamic fluctuation called "edge harmonic oscillation (EHO)" in the compact helical system (CHS) was investigated using beam emission spectroscopy (BES) as the diagnostic method of the local density fluctuations and the magnetic probe array. We found two groups of harmonic oscillations in CHS, one with a frequency of 4.0 kHz and a harmonic located in the edge region of the normalized minor radius ρ = 0.95 near the rotational transform ι = 1 surface, and the other with a frequency of 3.5 kHz and a harmonic located in the core region ρ = 0.53 near the ι = 0.5 surface. The magnetic probe signals showed that the poloidal/toroidal mode numbers of the edge mode and the core mode were −1/1 and −2/1, respectively. They were consistent with the rotational transform of the magnetic field at the locations of those modes.

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Section: Resultssupporting

confidence: 73%

Measurement of 3-D Mode Structure of the Edge Harmonic Oscillations in CHS using Beam Emission Spectroscopy

Oishi

Kado

Yoshinuma

et al. 2007

Plasma and Fusion Research

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“…However, in CHS, the radial gradient of the RS is poor after the transition. This observation suggests that the RS does not play significant roles on sustaining plasma rotation [7] after the transition. Further studies are necessary to clarify the existence of the radial electric field and edge structure of H-mode plasmas in CHS.…”

Section: Observation Of Edge Reynolds Stress Increase Preceding An L-mentioning

confidence: 91%

Observation of Edge Reynolds Stress Increase Preceding an L-H Transition in Compact Helical System

Nagashima

Nagaoka

Itoh

et al. 2010

Plasma and Fusion Research

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An increase in turbulent Reynolds stress preceding an L-H transition in the Compact Helical System (CHS) was observed. A positive increase in the Reynolds stress is associated with a negative jump in the floating potential. The relationship of signs is consistent with the momentum balance equation. Therefore, this observation supports the hypothesis that the Reynolds stress plays an important role in triggering the L-H transition in CHS plasmas. Understanding of the mechanism that triggers the transition to high-confinement plasmas [1] is important for realizing fusion plasmas. In some tokamak operations, Hmode transitions are triggered by a sawtooth crash. However, such events have not been observed in H-mode transitions in helical plasmas. In this rapid communication, we present a direct observation of turbulent Reynolds stress (RS) preceding an L-H transition in Compact Helical System (CHS) [2]. Compression of RS could be a source of shear flows and is a candidate mechanism for the formation of a radial electric field, leading to H-mode plasmas [3].The CHS is a low-aspect ratio middle size stellarator with a major radius R 0 = 1 m, minor radius a = 0.2 m, number of helical windings l = 2, and toroidal period number N t = 8. In these experiments, the magnetic axis is located at R = 92.1 cm (R axis ), and the toroidal magnetic field B t = 0.9 T. Two neutral beam injection (NBI) units were used to make reproducible H-mode plasmas. Edge fluctuations were measured with the hybrid probe (HP) [4]. The HP has six electrodes by which the floating potential signals were measured. The RS is calculated from the floating potential measured with two pairs of electrodes separated radially and poloidally, respectively. The HP is movable in the poloidal plane, and the two-dimensional RS structure was obtained, neglecting temperature fluctuations.Figure 1 (a) shows the time evolution of twodimensional maps of the RS. Positive RS indicates radially outward transport of velocity in the electron diamagnetic drift direction. The maps are reconstituted from shot-byauthor's e-mail: nagashima@k.u-tokyo.ac.jp shot scan data from the HP. Timings of the H α drop are used as the standard to synchronize different shot data in the same time series in two-dimensional maps. The RS starts to increase at 78 ms, indicated by red square (1) in Fig. 1 (a), has a maximum at 86 ms (2), and vanishes at 94 ms (3). We have discovered an increase in the RS preceding the H α drop (the L-H transition). At the beginning of the increase, the RS is localized near the outermost surface of the plasma at Z = 0 m. However, around the period when the RS has a maximum (84 ms), a large RS is distributed along the plasma surface, and the radial gradient of the RS is strong. The RS can transfer poloidal momentum in the radial direction and/or radial momentum in the poloidal direction. The RS gradient is a source/sink for momentum. Therefore, the finite radial gradient observed here can drive poloidal shear flow.The momentum balance equation determines the correspond...

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Investigation of a high spatial resolution method based on polar coordinate maximum entropy method for analyzing electron density fluctuation data measured by laser phase contrast

Matsuo

Iguchi

Okamura

et al. 2012

Review of Scientific Instruments

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Laser phase contrast is a powerful diagnostic method to determine the spatial distribution of electron density fluctuations in magnetically confined plasmas, although its applicability depends on magnetic field configurations. The spatial resolution of fluctuations is linked with the resolution of the propagation direction that is derived from the two-dimensional spectral analysis of the wavenumber for the fluctuations. The method was applied to fluctuation measurements in a compact helical system. In order to improve the resolution of the propagation direction with a relatively small number of data points, the maximum entropy method with polar coordinates was employed. A spatial resolution of the order of 1 cm was obtained, which is satisfactory in a plasma with a 20 cm minor radius.

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A Simultaneous Spectroscopic Measurement of the Global and Edge Local Structures in the Ion Temperature and Plasma Rotation Profiles in the Compact Helical System

Cited by 3 publications

References 16 publications

Measurement of 3-D Mode Structure of the Edge Harmonic Oscillations in CHS using Beam Emission Spectroscopy

Measurement of 3-D Mode Structure of the Edge Harmonic Oscillations in CHS using Beam Emission Spectroscopy

Observation of Edge Reynolds Stress Increase Preceding an L-H Transition in Compact Helical System

Investigation of a high spatial resolution method based on polar coordinate maximum entropy method for analyzing electron density fluctuation data measured by laser phase contrast

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