Direct observation of potential profiles with a 200 keV heavy ion beam probe on the Compact Helical System

Fujisawa, A.; Iguchi, H.; Lee, S.; Crowley, T. P.; Hamada, Y.; Kubo, S.; Idei, H.; Sanuki, H.; Itoh, K.; Minami, Takashi; Tanaka, K.; Nishimura, Satoshi; Ida, K.; Hidekuma, S.; Kojima, M.; Takahashi, C.; Okamura, S.; Matsuoka, K.

doi:10.1063/1.872310

Cited by 18 publications

(13 citation statements)

References 19 publications

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“…There, a large gradient in electron temperature is observed around the core at the location of the connection layer of Ӎ0. 25. The E r -shear plays an important role on the barrier formation by reducing turbulence, as is similar to the barrier formation of tokamaks.…”

Section: Discussionmentioning

confidence: 89%

“…23 After the charge exchange recombination spectroscopy ͑CXRS͒ was used to measure the internal radial electric field, 24 advanced measurements were performed using a heavy ion beam probe ͑HIBP͒. 25,26 The high spatio-temporal resolution of the HIBP was sufficient to reveal the bifurcation nature of the radial electric field. 27 The stationary potential profile showed characteristic patterns to be classified into five categories.…”

Section: Introductionmentioning

confidence: 99%

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Experimental study of the bifurcation nature of the electrostatic potential of a toroidal helical plasma

et al. 2000

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The bifurcation nature of the electrostatic structure is studied in the toroidal helical plasma of the Compact Helical System ͑CHS͒ ͓K. Matsuoka et al., Proceedings of the 12th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Nice, 1988 ͑International Atomic Energy Agency, Vienna, 1989͒, Vol. 2, p. 411͔. Observation of bifurcation-related phenomena is introduced, such as characteristic patterns of discrete potential profiles, and various patterns of self-sustained oscillations termed electric pulsation. Some patterns of the electrostatic structure are found to be quite important for fusion application owing to their association with transport barrier formation. It is confirmed, as is shown in several tokamak experiments, that the thermal transport barrier is linked with electrostatic structure through the radial electric field shear that can reduce the fluctuation resulting in anomalous transport. This article describes in detail spatio-temporal evolution during self-sustained oscillation, together with correlation between the radial electric field and other plasma parameters. An experimental survey to find dependence of the temporal and spatial patterns on plasma parameters is performed in order to understand systematically the bifurcation property of the toroidal helical plasma. The experimental results are compared with the neoclassical bifurcation property that is believed to explain the observed bifurcation property of the CHS plasmas. The present results show that the electrostatic property plays an essential role in the structural formation of toroidal helical plasmas, and demonstrate that toroidal plasma is an open system with a strong nonlinearity to provide a new attractive problem to be studied.

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Section: Discussionmentioning

confidence: 89%

Section: Introductionmentioning

confidence: 99%

Experimental study of the bifurcation nature of the electrostatic potential of a toroidal helical plasma

et al. 2000

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“…This diagnostics worked well for the middle and high density plasmas heated by NBI. The HIBP measured full profiles of electric potential for both ECH and NBI plasmas with a relatively low density [23]. Figure 8 shows the electric field profiles of NBI plasmas measured by TVCXS for three different densities.…”

Section: Electric Field In the Standard Configurationmentioning

confidence: 99%

Confinement physics study in a small low aspect ratio helical device: CHS

et al. 1999

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The configuration parameter of the plasma position relative to the center of the helical coil winding is very effective one for controlling the MHD stability and the trapped particle confinement in Heliotron/Torsatron systems. But these two characteristics are contradictory to each other in this parameter. The inward shifted configuration is favorable for the drift-orbit-optimization but it is predicted unstable with the Mercier criterion. Various physics problems, such as electric field structure, plasma rotation and MHD phenomena, have been studied in CHS with a compromising intermediate position. With this standard configuration, CHS has supplied experimental results for understanding general toroidal confinement physics and low-aspect-ratio helical systems. In the recent experiments, it was found that the wide range of inward shifted configurations gives stable plasma discharges without any restriction to the special pressure profile. Such enhanced range of operation made it possible to study experimentally the drift-orbit-optimized configuration in the Heliotron/Torsatron systems. The effect of configuration improvement was studied with plasmas in a low collisionality regime.

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“…The poloidal rotation of impurity ions is also large in this peripheral region, since it is mainly driven by the radial electric field determined by the ambipolar condition of ripple diffusion fluxes. [4][5][6] Therefore, the flow of impurity ions may not be incompressible as assumed in the neoclassical theory.…”

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confidence: 99%

“…The standard neoclassical theory 1,2 predicts an inboard/outboard asymmetric parallel flow ͑a ''Pfirsch-Schlueter-like'' toroidal flow͒ to compensate for the unbalance of the perpendicular fluxes on the inboard and the outboard regions and to make the flow incompressible ("•vϭ0) in a steady state. In heliotron/ torsatron, [3][4][5][6][7] however, the strong toroidal viscosity due to helical ripples 3 damps this toroidal flow, as well as the net toroidal flows due to the radial electric field, the ion pressure gradient, and the toroidal momentum injection by neutral beams. This damping is strong in the peripheral region where the helical ripple is large.…”

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confidence: 99%

Inboard/outboard asymmetry of poloidal flow observed in the Compact Helical System

Nishimura¹,

Ida²,

Osakabe³

et al. 2000

Physics of Plasmas

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The poloidal flow velocity and the density of fully ionized carbon were measured in the Compact Helical System ͓K. Ida et al., Phys. Rev. Lett. 67, 58 ͑1991͔͒ using the charge exchange spectroscopy with the bidirectional viewing. The poloidal asymmetry of the poloidal flow velocity observed in the outward shifted plasma is due to the electrostatic potential being constant on the magnetic flux surfaces. The poloidal asymmetry of the ion density can be explained by the conservation of the poloidal ion flux under the strong toroidal viscosity. © 2000 American Institute of Physics. ͓S1070-664X͑00͒04002-7͔The spectroscopic measurements of plasma rotations are widely used to evaluate radial electric fields in magnetically confined toroidal plasmas. The rotation velocity is connected to the radial electric field and the ion pressure gradient by the radial force balance. In general, radial electric fields, ion pressure gradients, and rotation velocities are not constant on magnetic flux surfaces, since electrostatic potentials and ion pressures are considered to be constant on the magnetic flux surfaces. When the plasmas have toroidal curvatures ͑toroi-dal effects͒, the conservation of the poloidal plasma flux causes the difference of the poloidal rotation velocities between the inboard region of the magnetic axis (RϽR ax , where R ax is the major radius of the magnetic axis͒ and the outboard region (RϾR ax ). The standard neoclassical theory 1,2 predicts an inboard/outboard asymmetric parallel flow ͑a ''Pfirsch-Schlueter-like'' toroidal flow͒ to compensate for the unbalance of the perpendicular fluxes on the inboard and the outboard regions and to make the flow incompressible ("•vϭ0) in a steady state. In heliotron/ torsatron, 3-7 however, the strong toroidal viscosity due to helical ripples 3 damps this toroidal flow, as well as the net toroidal flows due to the radial electric field, the ion pressure gradient, and the toroidal momentum injection by neutral beams. This damping is strong in the peripheral region where the helical ripple is large. The poloidal rotation of impurity ions is also large in this peripheral region, since it is mainly driven by the radial electric field determined by the ambipolar condition of ripple diffusion fluxes.4-6 Therefore, the flow of impurity ions may not be incompressible as assumed in the neoclassical theory.The difference of poloidal rotation velocities and densities between the inboard and the outboard regions has not been studied in detail in either tokamaks or helical systems. In tokamaks, the poloidal rotation measurements are restricted in the outboard regions [8][9][10][11] because of the difficulty of installing chords viewing vertically the inboard regions where there are only very narrow spaces between toroidal and poloidal coils. In order to study the inboard/outboard asymmetry of poloidal flow, very precise measurements of Doppler shifts of impurity lines are required. The inboard/ outboard asymmetry has never been reported for lack of the accuracy of the absolute Doppl...

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Direct observation of potential profiles with a 200 keV heavy ion beam probe on the Compact Helical System

Cited by 18 publications

References 19 publications

Experimental study of the bifurcation nature of the electrostatic potential of a toroidal helical plasma

Experimental study of the bifurcation nature of the electrostatic potential of a toroidal helical plasma

Confinement physics study in a small low aspect ratio helical device: CHS

Inboard/outboard asymmetry of poloidal flow observed in the Compact Helical System

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