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  • 1
    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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  • 2
    Electronic Resource
    Electronic Resource
    Electron plasmas in a "partial" torus (2002)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 9 (2002), S. 766-771 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Electron plasmas are trapped in a novel "partially" toroidal (or "C"-shaped) trap designed to study issues of equilibrium, stability, and confinement of toroidal nonneutral plasmas. Plasmas with densities as high as 3.3×106 cm−3 are trapped and decay on a 100 μs timescale in a 196 G magnetic field. Successful trapping of dense electron plasmas requires the application of a strong horizontal electric field (5–10 V/cm). The confinement time scales as the 〈fraction SHAPE="CASE"〉32 power of the magnetic field. Oscillations in the image charge current to a grounded probe have a frequency that is proportional to 1/B and roughly proportional to the vacuum horizontal electric field. However, the frequency is independent of the amount of charge in the trap. Some evidence points toward the ion resonance instability as the driving mechanism for this new toroidal electron mode. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1445425
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  • 3
    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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  • 4
    Electronic Resource
    Electronic Resource
    Ambipolar magnetic fluctuation-induced heat transport in toroidal devices (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1999-2005 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The total magnetic fluctuation-induced electron thermal flux has been determined in the Madison Symmetric Torus (MST) reversed-field pinch [Fusion Technol. 19, 131 (1991)] from the measured correlation of the heat flux along perturbed fields with the radial component of the perturbed field. In the edge region the total flux is convective and intrinsically ambipolar constrained, as evidenced by the magnitude of the thermal diffusivity, which is well approximated by the product of ion thermal velocity and the magnetic diffusivity. A self-consistent theory is formulated and shown to reproduce the experimental results, provided nonlinear charge aggregation in streaming electrons is accounted for in the theory. For general toroidal configurations, it is shown that ambipolar constrained transport applies when remote magnetic fluctuations (i.e., global modes resonant at distant rational surfaces) dominate the flux. Near locations where the dominant modes are resonant, the transport is nonambipolar. This agrees with the radial variation of diffusivity in MST. Expectations for the tokamak are also discussed. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871996
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  • 5
    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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  • 6
    Electronic Resource
    Electronic Resource
    Maximum-likelihood fitting of data dominated by Poisson statistical uncertainties : Proceedings of the eleventh topical conference on high temperature plasma diagnostics (1997)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 68 (1997), S. 914-917 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The fitting of data by χ2 minimization is valid only when the uncertainties in the data are normally distributed. When analyzing spectroscopic or particle counting data at very low signal level (e.g., a Thomson scattering diagnostic), the uncertainties are distributed with a Poisson distribution. We have developed a maximum-likelihood method for fitting data that correctly treats the Poisson statistical character of the uncertainties. This method maximizes the total probability that the observed data are drawn from the assumed fit function using the Poisson probability function to determine the probability for each data point. The algorithm also returns uncertainty estimates for the fit parameters. We compare this method with a χ2-minimization routine applied to both simulated and real Thomson scattering data. Differences in the returned fits are greater at low signal level (less than ∼10 counts per measurement). The maximum-likelihood method is found to be more accurate and robust, returning a narrower distribution of values for the fit parameters with fewer outliers. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1147768
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  • 7
    Electronic Resource
    Electronic Resource
    Magnetic fluctuation induced transport in MST (abstract)a) : Proceedings of the tenth topical conference on high temperature plasma diagnostics (1995)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 66 (1995), S. 812-812 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: We made local measurements of the magnetic fluctuation induced transport of particles and energy in the outer region (r/a(approximately-greater-than)0.75) of the MST reversed field pinch plasma. These measurements allow comparison with the Rochester–Rosenbluth stochastic diffusion coefficients. An electrostatic electron energy analyzer and fast pyrobolometer were employed in combination with magnetic pickup coils to directly measure the correlated products 〈J˜θB˜r〉 and 〈q˜(parallel)B˜r〉, respectively. Magnetic fluctuation induced transport of both particles and energy is negligible at the extreme edge of MST (r/a(approximately-greater-than)0.9), but increases to a level consistent with estimates of the entire particle and energy flux as the core is approached (5×1021 m−2 s−1 and 105 W m−2). Stochastic particle transport is limited by an apparent ambipolar electric field to the ion stochastic diffusion rate (D=50 m2 s−1), the electron transport being dominated by the energetic component of the distribution (E(approximately-greater-than)50 eV). The measured heat transport is consistent with the convective energy transport carried by the fast electron component (Teffective=100 eV). © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1146229
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  • 8
    Electronic Resource
    Electronic Resource
    Global confinement and discrete dynamo activity in the MST reversed-field pinch (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 2241-2246 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Results obtained on the Madison Symmetric Torus (MST) reversed-field pinch [Fusion Technol. 19, 131 (1991)] after installation of the design poloidal field winding are presented. Values of βθe0≡2μ0ne0Te0/B2θ(a)∼12% are achieved in low-current (I=220 kA) operation; here, ne0 and Te0 are central electron density and temperature, and Bθ(a) is the poloidal magnetic field at the plasma edge. An observed decrease in βθe0 with increasing plasma current may be due to inadequate fueling, enhanced wall interaction, and the growth of a radial field error at the vertical cut in the shell at high current. Energy confinement time varies little with plasma current, lying in the range of 0.5–1.0 msec. Strong discrete dynamo activity is present, characterized by the coupling of m=1, n=5–7 modes leading to an m=0, n=0 crash (m and n are poloidal and toroidal mode numbers). The m=0 crash generates toroidal flux and produces a small (2.5%) increase in plasma current.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859642
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  • 9
    Electronic Resource
    Electronic Resource
    Nonlinear coupling of tearing fluctuations in the Madison Symmetric Torusf| (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 2540-2545 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Three-wave, nonlinear, tearing mode coupling has been measured in the Madison Symmetric Torus (MST) reversed-field pinch (RFP) [Fusion Technol. 19, 131 (1991)] using bispectral analysis of edge magnetic fluctuations resolved in "k-space.'' The strength of nonlinear three-wave interactions satisfying the sum rules m1+m2=m3 and n1+n2=n3 is measured by the bicoherency. In the RFP, m=1, n∼2R/a (6 for MST) internally resonant modes are linearly unstable and grow to large amplitude. Large values of bicoherency occur for two m=1 modes coupled to an m=2 mode and the coupling of intermediate toroidal modes, e.g., n=6 and 7 coupled to n=13. These experimental bispectral features agree with predicted bispectral features derived from magnetohydrodynamic (MHD) computation. However, in the experiment, enhanced coupling occurs in the "crash'' phase of a sawtooth oscillation concomitant with a broadened mode spectrum suggesting the onset of a nonlinear cascade.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860741
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  • 10
    Electronic Resource
    Electronic Resource
    Turbulent transport in the Madison Symmetric Torus reversed-field pinch (1992)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 2136-2141 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Measurements of edge turbulence and the associated transport are ongoing in the Madison Symmetric Torus (MST) reversed-field pinch [Fusion Technol. 19, 131 (1991)] using magnetic and electrostatic probes. Magnetic fluctuations are dominated by m=1 and n ∼2R/a, tearing modes. Particle losses induced by magnetic field fluctuations have been found to be ambipolar (〈J˜(parallel) B˜r〉/B0=0). Electrostatic fluctuations are broadband and turbulent, with mode widths Δm∼3–7 and Δn∼70–150. Particle, parallel current, and energy transport arising from coherent motion with the fluctuating E˜×B drift have been measured. Particle transport via this channel is comparable to the total particle loss from MST. Energy transport (from 〈P˜E˜φ 〉/B0) due to electrostatic fluctuations is relatively small, and parallel current transport (from 〈J˜(parallel) E˜φ〉/B0) may be small as well.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860020
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