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

Okamura, S.; Matsuoka, K.; Akiyama, R.; Darrow, D.S.; Ejiri, A.; Fujisawa, A.; Fujiwara, Masami; Goto, M.; Ida, K.; Idei, H.; Iguchi, H.; Inoue, Noriyuki; Isobe, M.; Itoh, K.; Kado, S.; Khlopenkov, K.; Kondo, Takashi; Kubo, S.; Lazaros, A.; Lee, S.; Matsunaga, G.; Minami, Takashi; Morita, S.; Murakami, S.; Nakajima, N.; Nikai, N.; Nishimura, Satoshi; Nomura, I.; Ohdachi, S.; Ohkuni, K.; Osakabe, M.; Pavlichenko, R. O.; Peterson, B.J.; Sakamoto, R.; Sanuki, H.; Sasao, M.; Shimizu, A.; Shirai, Y.; Sudo, S.; Takagi, Shigeyuki; Takahashi, C.; Takayama, Shuichi; Takechi, M.; Tanaka, K.; Toi, K.; Watari, T.; Yamazaki, K.; Yokoyama, M.; Yoshimura, Y.

doi:10.1088/0029-5515/39/9y/310

Cited by 52 publications

(26 citation statements)

References 26 publications

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“…The neoclassically predicted ambipolar radial electric field is generally in agreement with observations in the core of CHS [26] and W7-AS [27,28,29,30] in the 'ion root' regime. Agreement has also been obtained in plasmas with net toroidal current and significant magnetic shear in W7-AS [31], and qualitative agreement has been reported in TJ-II [32].…”

Section: Neoclassical Transport In Stellarators: Theory and Experimentssupporting

confidence: 83%

“…In both CHS [26] and LHD [9,47] 'inward shifted' configurations with slightly smaller R o have somewhat improved energy confinement relative to the standard R o . The improvement is attributed to smaller neoclassical orbit drifts in these inward shifted configurations, but anomalous transport is significant in the core for all R o in both devices, and becomes small relative to neoclassical only for outward shifted LHD configurations with strongly de-optimized drift orbits and enhanced neoclassical ripple transport.…”

Section: -11mentioning

confidence: 95%

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Assessment of Transport in NCSX

Mikkelsen

Maaßberg

Zarnstorff

et al. 2007

Fusion Science and Technology

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We explore whether the energy confinement and planned heating in NCSX are sufficient to test MHD stability limits, and whether the configuration is sufficiently quasi-axisymmetric to reduce the neoclassical ripple transport to low levels, thereby allowing tokamak-like transport. A 0-D model with fixed profile shapes is related to global energy confinement scalings for stellarators and tokamaks; neoclassical transport properties are assessed with the DKES, NEO, and NCLASS codes; and a power balance code is used to predict temperature profiles. Reaching the NCSX goal of =4% at low collisionality will require H_ISS-95=3, which is higher than the best achieved in present stellarators. However, this level of confinement is actuallỹ 10% lower than that predicted by the ITER-97P tokamak L-mode scaling. By operating near the stellarator density limit, the required H_ISS-95 is reduced by 35%. The high degree of quasiaxisymmetry of the configuration and the self-consistent 'ambipolar' electric field reduce the neoclassical ripple transport to a small fraction of the neoclassical axisymmetric transport. A combination of neoclassical and anomalous transport models produces pressure profile shapes that are within the range of those used to study the MHD stability of NCSX. We find that =4% plasmas are 'neoclassically accessible', and are compatible with large levels of anomalous transport in the plasma periphery. 7-2

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Section: Neoclassical Transport In Stellarators: Theory and Experimentssupporting

confidence: 83%

Section: -11mentioning

confidence: 95%

Assessment of Transport in NCSX

Mikkelsen

Maaßberg

Zarnstorff

et al. 2007

Fusion Science and Technology

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“…The Compact Helical System (CHS) [23] is a Heliotron/Torsatron device (poloidal period number L 2, and toroidal period number M 8) with a major radius R 0 1 m, an average minor radius a 20 cm, and a rotational transform i ͓͗RB u ͞rB f ͔͘ is 0.3 (center) -1.0 (edge), where R and r are the major and minor radii, B u and B f are the poloidal and toroidal magnetic fields, and ͗ ͘ denotes a magnetic flux average. The plasma is produced initially by electron cyclotron heating (ECH) (53.2 GHz ECH with short pulse) with hydrogen gas and sustained with a hydrogen neutral beam tangentially injected.…”

mentioning

confidence: 99%

Observation of Toroidal Flow Antiparallel to the〈Er×Bθ〉Drift Direction in the Hot Electron Mode Plasmas in the Compact He

et al. 2001

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A toroidal flow antiparallel to the ͗E r 3 B u ͘ drift direction is observed in the hot electron mode plasmas when a large positive electric field and a sharp electron temperature gradient are sustained inside the internal transport barrier in the Compact Helical System. This toroidal flow reaches up to 5 3 10 4 m͞s at the plasma center, and it is large enough to reverse the toroidal flow driven by a tangentially injected neutral beam. These observations clearly show the plasma favors flow in the minimum =B direction at the transport barrier. DOI: 10.1103/PhysRevLett.86.3040 PACS numbers: 52.55.HcA spontaneous poloidal flow has been recognized to be important in the confinement of toroidal plasmas, since poloidal flow velocity and radial electric field shear were found to contribute an improvement of plasma confinement in H-mode plasmas [1,2]. The turbulence in the plasma is predicted to be suppressed by a E 3 B velocity shear through the mechanism of a nonlinear decorrelation [3,4]. It has been confirmed experimentally that the E 3 B shearing rate exceeds the growth rate of the turbulence at the transport barrier [5][6][7]. The toroidal flow has been considered to be determined by the radial transport of momentum driven by neutral beam injection (NBI) with anomalous shear viscosity [8,9]. Although the E 3 B velocity shear due to the toroidal flow can be important in the plasma core [10], there have been few experiments that demonstrate a spontaneous toroidal flow not driven by NBI in tokamaks [11][12][13][14]. It is generally the case in tokamaks that the plasma rotates parallel (antiparallel) to the plasma current for the coinjected (counterinjected) NBI, which corresponds to a positive (negative) radial electric field ͑E r ͒ in L-mode plasmas [15]. However, when a negative E r is produced at the transport barrier, both localized toroidal flow in the counterdirection and localized poloidal flow in the electron diamagnetic direction are observed even for plasmas with coinjected NBI [5,16,17]. These results show the need for detailed measurements of both toroidal and poloidal flow at the transport barrier, where the toroidal flow profile is not determined by the transport of momentum driven by NBI alone. Yet, despite the importance of the coupling between toroidal and poloidal flow, no studies until now have discussed the toroidal flow at the internal transport, where there is a large radial electric field.In a Heliotron/Torsatron device, there is no toroidal symmetry, and the minimum =B trace has a helical structure and helical flow along the minimum =B can be expected. The significant difference between tokamaks and Heliotron devices is the relationship between the poloidal field direction and the direction of dominant symmetry.The pitch angle of dominant symmetry is even larger than that of the averaged poloidal field in a Heliotron device, while it is zero due to the toroidal symmetry in tokamaks. This helical flow should change sign depending on the sign of E r or on the direction of the magnetic field. H...

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“…We have developed a Cotton-Mouton polarimeter using a HCN laser ͑wavelength of 337 m͒ on the Compact Helical System ͑CHS͒. 8 The method of measuring the Cotton-Mouton effect as a phase difference is adopted. It is insensitive to amplitude variations of detected signals due to oscillation instabilities of the laser and beam refraction in the plasma.…”

Section: Introductionmentioning

confidence: 99%

Cotton-Mouton polarimeter with HCN laser on CHS

Akiyama

Kawahata

Ito

et al. 2006

Review of Scientific Instruments

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Polarimeters based on the Cotton-Mouton effect hold promise for electron density measurements. We have designed and installed a Cotton-Mouton polarimeter on the Compact Helical System. The Cotton-Mouton effect is measured as the phase difference between probe and reference beams. In this system, an interferometric measurement can be performed simultaneously with the same probe chord. The light source is a HCN laser ͑wavelength of 337 m͒. Digital complex demodulation is adopted for small phase analysis. The line averaged density evaluated from the polarimeter along a plasma center chord is almost consistent with that from the interferometer.

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

Cited by 52 publications

References 26 publications

Assessment of Transport in NCSX

Assessment of Transport in NCSX

Observation of Toroidal Flow Antiparallel to the〈Er×Bθ〉Drift Direction in the Hot Electron Mode Plasmas in the Compact He

Cotton-Mouton polarimeter with HCN laser on CHS

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