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  • 1
    Electronic Resource
    Electronic Resource
    Modifications to the edge current profile with auxiliary edge current drive and improved confinement in a reversed-field pinch (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 3491-3494 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Auxiliary edge current drive is routinely applied in the Madison Symmetric Torus [R. N. Dexter, D. W. Kerst, T. W. Lovell et al., Fusion Technol. 19, 131 (1991)] with the goal of modifying the parallel current profile to reduce current-driven magnetic fluctuations and the associated particle and energy transport. Provided by an inductive electric field, the current drive successfully reduces fluctuations and transport. First-time measurements of the modified edge current profile reveal that, relative to discharges without auxiliary current drive, the edge current density decreases. This decrease is explicable in terms of newly measured reductions in the dynamo (fluctuation-based) electric field and the electrical conductivity. Induced by the current drive, these two changes to the edge plasma play as much of a role in determining the resultant edge current profile as does the current drive itself. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1287913
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  • 2
    Electronic Resource
    Electronic Resource
    E×B flow shear and enhanced confinement in the Madison Symmetric Torus reversed-field pinch : The 39th annual meeting of division of plasma physics of APS (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 1848-1854 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Strong E×B flow shear occurs in the edge of three types of enhanced confinement discharge in the Madison Symmetric Torus [Dexter et al., Fusion Technol. 19, 131 (1991)] reversed-field pinch. Measurements in standard (low confinement) discharges indicate that global magnetic fluctuations drive particle and energy transport in the plasma core, while electrostatic fluctuations drive particle transport in the plasma edge. This paper explores possible contributions of E×B flow shear to the reduction of both the magnetic and electrostatic fluctuations and, thus, the improved confinement. In one case, shear in the E×B flow occurs when the edge plasma is biased. Biased discharges exhibit changes in the edge electrostatic fluctuations and improved particle confinement. In two other cases, the flow shear emerges (1) when auxiliary current is driven in the edge and (2) spontaneously, following sawtooth crashes. Both edge electrostatic and global magnetic fluctuations are reduced in these discharges, and both particle and energy confinement improve. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872855
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  • 3
    Electronic Resource
    Electronic Resource
    Experimental scaling of fluctuations and confinement with Lundquist number in the reversed-field pinch (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 1004-1014 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The scaling of the magnetic and velocity fluctuations with Lundquist number (S) is examined experimentally over a range of values from 7×104 to 106 in a reversed-field pinch (RFP) plasma. Magnetic fluctuations do not scale uniquely with the Lundquist number. At high (relative) density, fluctuations scale as b˜∝S−0.18, in agreement with recent numerical results. Fluctuations are almost independent of S at low (relative) density, b˜∝S−0.07. The range of measured exponents is narrow and is in clear disagreement with theories predicting b˜∝S−1/2. At high relative density, the scaling of the energy confinement time follows expectations for transport in a stochastic magnetic field. A confinement scaling law (nτE∝β4/5⋅T−7/10⋅a−3/5⋅Iφ2) is derived, assuming the persistent dominance of stochastic magnetic diffusion in the RFP and employing the measured scaling of magnetic fluctuations. The peak velocity fluctuations during a sawtooth cycle scale marginally stronger than magnetic fluctuations but weaker than a simple Ohm's law prediction. The sawtooth period is determined by a resistive-Alfvénic hybrid time (Tsaw∝τRτA) rather than a purely resistive time. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872670
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  • 4
    Electronic Resource
    Electronic Resource
    Measurement of the dynamo effect in a plasma (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1935-1942 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: 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 α effect) 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: (1) The α effect accounts for the dynamo current generation, even in the time dependence through a "sawtooth'' cycle; (2) at low collisionality the dynamo is explained primarily by the widely studied pressureless magnetohydrodynamic (MHD) model, i.e., the fluctuating velocity is dominated by the E×B drift; (3) 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 (1974); Rev. Mod. Phys. 58, 741 (1986)] conjecture. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871989
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  • 5
    Electronic Resource
    Electronic Resource
    Measurement of core velocity fluctuations and the dynamo in a reversed-field pinch : The 40th annual meeting of the division of plasma physics of the american physical society (1999)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 6 (1999), S. 1813-1821 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Plasma flow velocity fluctuations have been directly measured in the high-temperature magnetically confined plasma in the Madison Symmetric Torus (MST) Reversed-Field Pinch (RFP) [R. N. Dexter et al., Fusion Technol. 19, 131 (1991)]. These measurements show that the flow velocity fluctuations are correlated with magnetic field fluctuations such that the electromotive force 〈v˜×B˜〉 approximately balances parallel Ohm's law, E(parallel)+〈v˜×B˜〉(parallel)=ηJ(parallel). This initial measurement is subject to limitations of spatial localization and other uncertainties, but is evidence for sustainment of the RFP magnetic field configuration by the magnetohydrodynamic (MHD) dynamo, 〈v˜×B˜〉. Both the flow velocity and magnetic field fluctuations are the result of global resistive MHD modes of helicity m=1, n=5–10 in the core of MST. Chord-averaged flow velocity fluctuations are measured in the core of MST by recording the Doppler shift of impurity line emission with a specialized high resolution and throughput grating spectrometer. Magnetic field fluctuations are recorded with a large array of small edge pickup coils, which allows spectral decomposition into discrete modes and subsequent correlation with the velocity fluctuation data. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873439
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  • 6
    Electronic Resource
    Electronic Resource
    Locking of multiple resonant mode structures in the reversed-field pinch (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 2942-2946 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Locking of a rotating mode by applying a resonant magnetic perturbation having the same helicity has been observed on various devices. Experiments have been carried out on the Madison Symmetric Torus reversed-field pinch (RFP) [Dexter et al., Fusion Technol. 19, 131 (1991)] which show that an externally applied magnetic perturbation can cause locking of the dominant magnetic modes (poloidal mode number m=1, toroidal mode numbers n=5–10) when the perturbation is resonant with them. A perturbation which is not resonant (m=0 or 2) produces no such effect. Thus, resonant torques may lock a stochastic magnetic structure arising from several modes, as likely exists in the RFP, as well as a distinct island as exists in tokamaks, although the details of the interaction are likely to be different. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873017
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  • 7
    Electronic Resource
    Electronic Resource
    Fivefold confinement time increase in the Madison Symmetric Torus using inductive poloidal current drive : The 38th annual meeting of the Division of Plasma Physics (DPP) of the American Physical Society (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 1632-1637 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Current profile control is employed in the Madison Symmetric Torus [R. N. Dexter et al., Fusion Technol. 19, 131 (1991)] reversed field pinch to reduce the magnetic fluctuations responsible for anomalous transport. An inductive poloidal electric-field pulse is applied in the sense to flatten the parallel current profile, reducing the dynamo fluctuation amplitude required to sustain the equilibrium. This technique demonstrates a substantial reduction in fluctuation amplitude (as much as 50%), and improvement in energy confinement (from 1 to 5 ms); a record low fluctuation (0.8%) and record high temperature (615 eV) for this device were observed simultaneously during current drive experiments. Plasma beta increases by 50% and the Ohmic input power is three times lower. Particle confinement improves and plasma impurity contamination is reduced. The results of the transient current drive experiments provide motivation for continuing development of steady-state current profile control strategies for the reversed field pinch. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872324
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  • 8
    Electronic Resource
    Electronic Resource
    Sawteeth and energy confinement in the Madison Symmetric Torus reversed-field pinch (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 709-711 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Most Madison Symmetric Torus (MST) [Fusion Technol. 19, 131 (1991)] reversed-field pinch discharges exhibit sawtooth oscillations with a period of 2–5 ms, corresponding to magnetohydrodynamic (MHD) 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871775
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  • 9
    Electronic Resource
    Electronic Resource
    High confinement plasmas in the Madison Symmetric Torus reversed-field pinch : Review,Tutorial and Invited Papers from the 43rd Annual Meeting of the APS Division of Plasma Physics (2002)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 9 (2002), S. 2061-2068 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Reduction of core-resonant m=1 magnetic fluctuations and improved confinement in the Madison Symmetric Torus [Dexter et al., Fusion Technol. 19, 131 (1991)] 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 m=0 magnetic fluctuations. Now, through refined poloidal electric field control, plus control of the toroidal electric field, it is possible to reduce simultaneously the m=0 and m=1 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 kA). At low current (200 kA), 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. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1456930
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  • 10
    Electronic Resource
    Electronic Resource
    Control of magnetic fluctuations in the reversed field pinch with edge current drive (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 1463-1466 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The generation of auxiliary current in the extreme edge of the reversed field pinch is shown to affect edge and core resonant magnetic fluctuations, the recurrence time of relaxation oscillations (sawteeth), and the energy and particle confinement. Current is driven in the edge by electrostatic current sources. Although the injected current is expected to primarily affect edge resonant fluctuations, the coupling of edge and core modes enables changes in the extreme edge to have global consequences. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1365103
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