Global confinement and discrete dynamo activity in the MST reversed-field pinch

Hokin, S.; Almagri, A. F.; Assadi, S.; Beckstead, Jeffrey Andrew; Chartas, G.; Crocker, N. A.; Cudzinovic, M.; Hartog, Daniel Den; Dexter, R. N.; Holly, D. J.; Prager, S. C.; Rempel, T. D.; Sarff, J. S.; Scime, Earl; Shen, Weimin; Spragins, C.W.; Sprott, C.; Starr, Gregory P.; Stoneking, M. R.; Watts, Christopher; Nebel, Richard

doi:10.1063/1.859642

Cited by 77 publications

(70 citation statements)

References 18 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…[1][2][3] Similar to the case in tokamaks, the RFP sawtooth crash coincides with a sudden transfer of energy from the core plasma to the edge and is interpreted as magnetohydrodynamic ͑MHD͒ relaxation. During the slow sawtooth rise phase in the RFP, the profile, where (r)ϭ 0 J-B/B 2 , peaks as a result of resistive diffusion and the peaked parallel equilibrium inductive electric field.…”

mentioning

confidence: 90%

Sawteeth and energy confinement in the Madison Symmetric Torus reversed-field pinch

et al. 1996

View full text Add to dashboard Cite

Most Madison Symmetric Torus ͑MST͒ ͓Fusion Technol. 19, 131 ͑1991͔͒ reversed-field pinch discharges exhibit sawtooth oscillations with a period of 2-5 ms, corresponding to magnetohydrodynamic ͑MHD͒ instability and increased transport. However, in discharges where the plasma-facing wall has been boronized, the plasma resistivity is reduced, and sawteeth are often suppressed for periods up to 20 ms. The energy confinement time during these sawtooth-free periods is triple the normal value, corresponding to a higher plasma temperature and lower Ohmic input power. In addition, the steady growth of the dominant magnetic fluctuations normally observed between sawtooth crashes is absent. © 1996 American Institute of Physics. ͓S1070-664X͑96͒02703-5͔Sawtooth oscillations have been observed in several reversed-field pinch ͑RFP͒ experiments. 1-3 Similar to the case in tokamaks, the RFP sawtooth crash coincides with a sudden transfer of energy from the core plasma to the edge and is interpreted as magnetohydrodynamic ͑MHD͒ relaxation. During the slow sawtooth rise phase in the RFP, the profile, where (r)ϭ 0 J-B/B 2 , peaks as a result of resistive diffusion and the peaked parallel equilibrium inductive electric field. The amplitudes of the dominant poloidal mode number mϭ1 magnetic fluctuations ͑or modes͒, resonant in the plasma core, gradually increase during this phase. The faster sawtooth crash phase results in the redistribution of parallel current, flattening (r). The crash is a discrete dynamo event, during which edge current is driven, and toroidal flux is generated. Sawteeth degrade confinement in two ways. First, the growth of magnetic fluctuations increases transport. Second, plasma-wall interaction, most prominent in the crash phase, injects radiative impurities into the plasma. Hence, control of sawtooth oscillations and understanding of their origin is essential.In the Madison Symmetric Torus ͑MST͒ RFP, 4 sawteeth with periods of several ms have, until recently, accompanied nearly all discharges. 5 In this paper we report the production of discharges, following boronization 6 of the plasma-facing wall, in which sawteeth are absent for extended periods ͑up to 20 ms͒. We believe that these sawtooth-free periods ͑SFP͒ are due primarily to decreased current diffusion ͓and slowed peaking of (r)͔ arising from a reduced plasma resistivity. During SFP we observe that ͑1͒ the energy confinement time triples relative to normal sawtoothing discharges, and ͑2͒ the steady growth of the dominant mϭ1 modes normally observed between sawtooth crashes is absent. Also observed in SFP ͑and in some boronized discharges without SFP͒ are discrete bursts of mϭ0, toroidal mode number nϭ1 mode activity, each corresponding to a small increase in the toroidal flux.Shown in Fig. 1 are data from a typical MST discharge in which sawteeth occur throughout. Each sawtooth crash causes a rapid increase in the overall toroidal magnetic flux ͓Fig. 1͑a͔͒, while enhancing the reversal of the toroidal magnetic field at the edge ͓Fig. 1͑b͔͒. Increased ...

show abstract

mentioning

confidence: 90%

Sawteeth and energy confinement in the Madison Symmetric Torus reversed-field pinch

et al. 1996

View full text Add to dashboard Cite

show abstract

“…Studies of the scrape-off layer (SOL) plasma have found that temperature scale lengths and floating potential scale with n/I P [34,35]. In RFPs, I/N provides a good scaling parameter for Z EFF and β [36]. Finally, note that the critical N β for the onset of tearing modes can be scaled with n/ n G .…”

Section: Physics Correlated With the Density Limit Scalingmentioning

confidence: 99%

“…Pellet fueling in RFX allowed the limit to be overcome, but only transiently. The I/N ratio has proved to be a good scaling quantity for other phenomena in the RFP, including the plasma β, Z EFF , energy confinement [185,36] and the low density limit [174,175]. Poor performance at low density (high I/N) has been attributed to instabilities driven by large values of the streaming parameter V DRIFT /V THERMAL [174].…”

Section: Reversed Field Pinchmentioning

confidence: 99%

Density limits in toroidal plasmas

Greenwald

2002

Plasma Phys. Control. Fusion

467

477

View full text Add to dashboard Cite

In addition to the operational limits imposed by MHD stability on plasma current and pressure, an independent limit on plasma density is observed in confined toroidal plasmas. This review attempts to summarize recent work on the phenomenology and physics of the density limit. Perhaps the most surprising result is that all of the toroidal confinement devices considered operate in similar ranges of (suitably normalized) densities. The empirical scalings derived independently for tokamaks and reversed field pinches (RFP) are essentially identical, while stellarators appear to operate at somewhat higher densities with a different scaling. Dedicated density limit experiments have not been carried out for spheromaks and field-reversed configurations (FRC), however "optimized" discharges in these devices are also well characterized by the same empirical law. In tokamaks, where the most extensive studies have been conducted, there is strong evidence linking the limit to physics near the plasma boundary thus it is possible to extend the operational range for line-averaged density by operating with peaked density profiles. Additional particles in the plasma core apparently have no effect on density limit physics. While there is no widely accepted, first principles model for the density limit, research in this area has focussed on mechanisms which lead to strong edge cooling. Theoretical work has concentrated on the consequences of increased impurity radiation which may dominate power balance at high densities and low temperatures. These theories are not entirely satisfactory as they require assumptions about edge transport and make predictions for power and impurity scaling that may not be consistent with experimental results. A separate thread of research looks for the cause in collisionality enhanced turbulent transport. While there is experimental and theoretical support for this approach, understanding of the underlying mechanisms is only at a rudimentary stage and no predictive capability is yet available..

show abstract

“…Measurements were performed in the relatively low plasma current discharges to avoid heat damage to the inserted probes. All measurements were taken around the current flattop period, in which MST discharges typically show the discrete sawtooth oscillations with several millisecond repetition period 18 while REPUTE and TPE discharges contain rather irregular oscillations.…”

Section: Experimental Apparatusmentioning

confidence: 99%

Measurement of the dynamo effect in a plasma

Prager

Almagri

et al. 1996

Physics of Plasmas

View full text Add to dashboard Cite

A series of detailed experiments has been conducted in three laboratory plasma devices to measure the dynamo electric field along the equilibrium field line ͑the ␣ effect͒ arising from the correlation between the fluctuating flow velocity and magnetic field. The fluctuating flow velocity is obtained from probe measurement of the fluctuating E؋B drift and electron diamagnetic drift. The three major findings are the following: ͑1͒ The ␣ effect accounts for the dynamo current generation, even in the time dependence through a ''sawtooth'' cycle; ͑2͒ at low collisionality the dynamo is explained primarily by the widely studied pressureless magnetohydrodynamic ͑MHD͒ model, i.e., the fluctuating velocity is dominated by the E؋B drift; ͑3͒ at high collisionality, a new ''diamagnetic dynamo'' is observed, in which the fluctuating velocity is dominated by the electron diamagnetic drift. In addition, direct measurements of the helicity flux indicate that the dynamo activity transports magnetic helicity from one part of the plasma to another, but the total helicity is roughly conserved, verifying Taylor's ͓Phys. Rev. Lett. 33, 1139 ͑1974͒; Rev. Mod. Phys. 58, 741 ͑1986͔͒ conjecture.

show abstract

Global confinement and discrete dynamo activity in the MST reversed-field pinch

Cited by 77 publications

References 18 publications

Sawteeth and energy confinement in the Madison Symmetric Torus reversed-field pinch

Sawteeth and energy confinement in the Madison Symmetric Torus reversed-field pinch

Density limits in toroidal plasmas

Measurement of the dynamo effect in a plasma

Contact Info

Product

Resources

About