L-H transition in tokamaks and stellarators

Shaing, K. C.; Hsu, Chiou-Ting; Christenson, P. J.

doi:10.1088/0741-3335/36/7a/007

Cited by 9 publications

(9 citation statements)

References 20 publications

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“…In this Letter we report the first experimental observations of L-H transition in a high rotational transform, lowshear stellarator (the H-1 heliac), whose critical conditions depend strongly on the magnetic configuration. We show that the onset appears to be consistent with the theory of Shaing et al [8] for stellarators and should thus allow the results to be extrapolated to larger stellarators.…”

supporting

confidence: 89%

“…Observation of the critical magnetic field as a necessary condition for the transition as well as dependence of B cr on the rotational transform has not been previously reported. The existence of the critical magnetic field and B cr ͑i͒ dependence seems to be consistent with the recent theory [8] in which the bifurcation theory of L-H transition has been extended to stellarators. Ion collisionality parameter q ‫ء‬ i n i L k ͞V ti , (where n i is the ion collision frequency and L k ഠ 0.3 m is a parallel connection length and V ti is an ion thermal velocity) is about 2 in our experiment (taking n e 1.5 3 10 18 m 23 and T i 0.5T e ), so that the comparison with the theory [8] is valid.…”

supporting

confidence: 88%

“…The existence of the critical magnetic field and B cr ͑i͒ dependence seems to be consistent with the recent theory [8] in which the bifurcation theory of L-H transition has been extended to stellarators. Ion collisionality parameter q ‫ء‬ i n i L k ͞V ti , (where n i is the ion collision frequency and L k ഠ 0.3 m is a parallel connection length and V ti is an ion thermal velocity) is about 2 in our experiment (taking n e 1.5 3 10 18 m 23 and T i 0.5T e ), so that the comparison with the theory [8] is valid. According to [8], the bifurcation occurs when the poloidal Mach number…”

supporting

confidence: 88%

“…Ion collisionality parameter q ‫ء‬ i n i L k ͞V ti , (where n i is the ion collision frequency and L k ഠ 0.3 m is a parallel connection length and V ti is an ion thermal velocity) is about 2 in our experiment (taking n e 1.5 3 10 18 m 23 and T i 0.5T e ), so that the comparison with the theory [8] is valid. According to [8], the bifurcation occurs when the poloidal Mach number…”

mentioning

confidence: 73%

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Improved Particle Confinement Mode in the H-1 Heliac Plasma

et al. 1996

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The first experimental observation of the sudden transition to the improved particle confinement mode in the H-1 heliac is reported and shows a clear dependence on magnetic configuration. In a lowtemperature plasma a transition to improved confinement followed by a twofold increase in the electron density is observed when the magnetic field exceeds a critical value B cr. At B ഠ B cr the transition occurs spontaneously (within 1 ms). This B cr strongly decreases with increasing rotational transform. The improvement in confinement correlates with the suppression of the fluctuations and fluctuationinduced particle flux and with the increase in the radial electric field. [S0031-9007(96)01636-5]

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supporting

confidence: 89%

supporting

confidence: 88%

supporting

confidence: 88%

mentioning

confidence: 73%

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Improved Particle Confinement Mode in the H-1 Heliac Plasma

et al. 1996

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“…In helical systems, neoclassical non-ambipolarity due to HF ripples provides a basic ingredient for generating the bifurcated states of E r inside the plasma and the resultant formation of E r -shear at the interface between the electron and the ion roots (neoclassical ITB) [34,45,46]. Based on the ion orbit loss, the nonlinearity of neoclassical transport coefficients on E r can also cause the bifurcation of poloidal flows [47,48]. However, in the edge region of the plasma in the collisional regime, these ion-orbit loss theories encounter some difficulties in explaining the H-mode transition for the ECH plasmas in helical systems [46].…”

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confidence: 99%

H-mode confinement of Heliotron J

et al. 2005

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The L–H transition in a helical-axis heliotron, Heliotron J, is investigated. For electron cyclotron heating (ECH), neutral beam injection (NBI) heating and ECH + NBI combination heating plasmas, the confinement quality of the H-mode is examined with an emphasis on its magnetic configuration dependence. The vacuum edge rotational transform, ι(a)/2π, is chosen as a label for the magnetic configuration where ι/2π is the rotational transform and a is the average plasma minor radius in metres. The experimental ι(a)/2π dependence of the enhancement factor over the L-mode confinement reveals that specific configurations exist where high-quality H-modes (1.3 < HISS95 < 1.8) are attained. is the experimental global energy confinement time and is the confinement time scaling from the international stellarator database given as . R is the plasma major radius in metres, is the line-averaged plasma density in 1019 m−3, PL is the power loss in megawatts that accounts for the time derivative of the total plasma energy content and Bt is the toroidal magnetic field strength in tesla (Stroth U. et al 1996 Nucl. Fusion 36 1063). The ι (a)/2π ranges for these configurations are near values that are slightly less than those of the major natural resonances of Heliotron J, i.e. n/m = 4/8, 4/7 and 12/22. To better understand this configuration dependence, the geometrical poloidal viscous damping rate coefficient, Cp, is calculated for different values of ι(a)/2π and compared with the experimental results. The threshold line-averaged density of the H-mode, which depends on the configuration, is in the region of 0.7–2.0 × 1019 m−3 in ECH (0.29 MW) + NBI (0.57 MW) operation. As for the edge plasma characteristics, Langmuir probe measurements have shown a reduced fluctuation-induced transport in the region that begins inside the last closed flux surface (LCFS) and extends into the scrape-off layer. In addition, a negative radial electric field Er (or Er-shear) is simultaneously formed near the LCFS at the transition.

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Effects of orbit squeezing on poloidal mass flow and bootstrap current in tokamak plasmas

1994

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It is shown, by solving the drift kinetic equation, that the asymptotic values of the poloidal mass flow and the bootstrap current in the banana regime of large-aspect-ratio tokamak plasmas are not affected by orbit squeezing. However, because the definition of ion collisionality u.+~ is reduced by a factor Se3'*, with S= 1 -(ppid In E,/dr) (cE,/Bpvti), the values of both the poloidal mass flow and the bootstrap current at the banana-plateau transition are different from those of the conventional neoclassical theory. Here, ppi is the ion poloidal gyroradius, E, is the radial electric field, c is the speed of light, B, is the poloidal magnetic strength, v,~ is the ion thermal speed, and r is the local radius. The implications for the experimental observations of poloidal rotation speeds in high mode (H mode) are discussed.

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L-H transition in tokamaks and stellarators

Cited by 9 publications

References 20 publications

Improved Particle Confinement Mode in the H-1 Heliac Plasma

Improved Particle Confinement Mode in the H-1 Heliac Plasma

H-mode confinement of Heliotron J

Effects of orbit squeezing on poloidal mass flow and bootstrap current in tokamak plasmas

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