E×B
           flow shear and enhanced confinement in the Madison Symmetric Torus reversed-field pinch

Chapman, B.E.; Almagri, A. F.; Anderson, J. K.; Chiang, C. S.; Craig, D.; Fiksel, G.; Lanier, N. E.; Prager, S. C.; Sarff, J. S.; Stoneking, M. R.; Terry, P. W.

doi:10.1063/1.872855

Cited by 25 publications

(33 citation statements)

References 30 publications

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“…The mechanism controlling the edge energy transport in a RFP is still a matter of debate, as there are not yet definite conclusions on whether electrostatic or magnetic turbulence is mostly responsible for it. Recent measurements [9][10][11] have shown the existence of a strongly sheared plasma flow in the edge region of a RFP discharge, suggesting a quenching of turbulent transport with a mechanism similar to the one identified in tokamaks [12].Reducing the impact of magnetic turbulence on transport has become one of the major challenges of RFP research. In particular, external poloidal current drive has been proposed as a tool for improving confinement properties of the RFP configuration by alleviating the need of the dynamo field.…”

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confidence: 98%

Core Transport Improvement during Poloidal Current Drive in the RFX Reversed Field Pinch

Bartiromo¹,

Martin

Martini³

et al. 1999

Phys. Rev. Lett.

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We describe plasma profiles evolution during pulsed poloidal current drive experiments performed with the RFX reversed field pinch device. With external drive of edge poloidal current, magnetic fluctuations are reduced suggesting a concomitant reduction of the spontaneous dynamo action. The electron temperature profile is seen to peak in the plasma core, consistently with a reduction of the heat conductivity due to a substantial decrease of MHD dynamo fluctuations. Our results also indicate that the magnetic turbulence due to these fluctuations, which dominates heat transport in the core of the reverse field pinch configuration, does not drive an appreciable heat flux at the edge.[S0031-9007(98)08371-9] PACS numbers: 52.55. Hc, 52.25.Fi, 52.25.Gj, 52.35.Py A growing body of experimental and theoretical work is indicating that in the reverse field pinch (RFP) configuration [1] for magnetic confinement of a thermonuclear plasma a large fraction of the energy losses is due to transport mechanisms linked to MHD turbulence. In fact, magnetic fluctuations are intrinsic to the RFP, since the configuration is usually maintained by a strong dynamo mechanism, which converts part of the energy externally supplied by the toroidal transformer to the poloidal magnetic field component B u into energy associated with the toroidal component B f . This process regenerates toroidal magnetic flux lost by resistive diffusion.Many of the mechanisms (see [2], and references therein, and [3,4]) proposed to explain the dynamo are based on the nonlinear interaction of global, low-m resistive MHD modes resonant inside the toroidal field reversal surface. According to the most developed theory, the MHD dynamo (see [2] for a review), they produce coherent magnetic fieldb and velocityỹ fluctuations which combine to generate an electromotive electric field, E d ͗ỹ 3b͘. This dynamo electric field is found in three-dimensional numerical simulations [5-7] and has been directly measured [8]. Magnetic fluctuations in the RFP are then intimately connected to the dynamo, and in standard conditions they are somewhat unavoidable. This leads to the stochasticization of the magnetic field lines over a large part of the core plasma and, as a consequence, thermal isolation is only provided by a thin layer located at the plasma edge. The mechanism controlling the edge energy transport in a RFP is still a matter of debate, as there are not yet definite conclusions on whether electrostatic or magnetic turbulence is mostly responsible for it. Recent measurements [9][10][11] have shown the existence of a strongly sheared plasma flow in the edge region of a RFP discharge, suggesting a quenching of turbulent transport with a mechanism similar to the one identified in tokamaks [12].Reducing the impact of magnetic turbulence on transport has become one of the major challenges of RFP research. In particular, external poloidal current drive has been proposed as a tool for improving confinement properties of the RFP configuration by alleviating the need of the dyna...

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confidence: 98%

Core Transport Improvement during Poloidal Current Drive in the RFX Reversed Field Pinch

Bartiromo¹,

Martin

Martini³

et al. 1999

Phys. Rev. Lett.

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“…Periods of spontaneous improved confinement, which can last up to 20 ms, are also characterized by mϭ0 bursts and a reduction of magnetic fluctuations between bursts. 7,[13][14][15] One burst occurs just as the current drive begins in Fig. 4.…”

Section: Fluctuation Reductionmentioning

confidence: 99%

“…16 There is also a reduction of electrostatic ͑po-tential͒ fluctuations in the plasma edge. 7,15 The relative contribution to improved energy confinement of magnetic and electrostatic fluctuation reduction remains to be determined.…”

Section: Fluctuation Reductionmentioning

confidence: 99%

High confinement plasmas in the Madison Symmetric Torus reversed-field pinch

et al. 2002

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Reduction of core-resonant mϭ1 magnetic fluctuations and improved confinement in the Madison Symmetric Torus ͓Dexter et al., Fusion Technol. 19, 131 ͑1991͔͒ reversed-field pinch have been routinely achieved through control of the surface poloidal electric field, but it is now known that the achieved confinement has been limited in part by edge-resonant mϭ0 magnetic fluctuations. Now, through refined poloidal electric field control, plus control of the toroidal electric field, it is possible to reduce simultaneously the mϭ0 and mϭ1 fluctuations. This has allowed confinement of high-energy runaway electrons, possibly indicative of flux-surface restoration in the usually stochastic plasma core. The electron temperature profile steepens in the outer region of the plasma, and the central electron temperature increases substantially, reaching nearly 1.3 keV at high toroidal plasma current ͑500 kA͒. At low current ͑200 kA͒, the total beta reaches 15% with an estimated energy confinement time of 10 ms, a tenfold increase over the standard value which for the first time substantially exceeds the constant-beta confinement scaling that has characterized most reversed-field-pinch plasmas.

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“…2 In the Madison Symmetric Torus ͑MST͒, 3 discharges where the m = 1 and also the edge-resonant m = 0 modes are controlled exhibit a tenfold improvement in energy confinement and an approximate doubling of beta, the ratio of plasma pressure to the confining magnetic field pressure. 4,5 This technique has also been successfully applied to other RFP plasmas. [6][7][8] The largest improvements in energy confinement and beta have been limited to relatively low density, n e ഛ 10 19 m −3 .…”

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High-β, improved confinement reversed-field pinch plasmas at high density

et al. 2008

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In Madison Symmetric Torus [Dexter et al., Fusion Technol. 19, 131 (1991)] discharges where improved confinement is brought about by modification of the current profile, pellet injection has quadrupled the density, reaching ne=4×1019m−3. Without pellet injection, the achievable density in improved confinement discharges had been limited by edge-resonant tearing instability. With pellet injection, the total beta has been increased to 26%, and the energy confinement time is comparable to that at low density. Pressure-driven local interchange and global tearing are predicted to be linearly unstable. Interchange has not yet been observed experimentally, but there is possible evidence of pressure-driven tearing, an instability usually driven by the current gradient in the reversed-field pinch.

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E×B flow shear and enhanced confinement in the Madison Symmetric Torus reversed-field pinch

Cited by 25 publications

References 30 publications

Core Transport Improvement during Poloidal Current Drive in the RFX Reversed Field Pinch

Core Transport Improvement during Poloidal Current Drive in the RFX Reversed Field Pinch

High confinement plasmas in the Madison Symmetric Torus reversed-field pinch

High-β, improved confinement reversed-field pinch plasmas at high density

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