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Impurities, temperature, and density in a miniature electrostatic plasma and current source

Description: We have spectroscopically investigated the Sterling Scientific miniature electrostatic plasma source-a plasma gun. This gun is a clean source of high density (10{sup 19} - 10{sup 20} m{sup -3}), low temperature (5 - 15 eV) plasma. A key result of our investigation is that molybdenum from the gun electrodes is largely trapped in the internal gun discharge; only a small amount escapes in the plasma flowing out of the gun. In addition, the gun plasma parameters actually improve (even lower impurity contamination and higher ion temperature) when up to 1 kA of electron current is extracted from the gun via the application of an external bias. This improvement occurs because the internal gun anode no longer acts as the current return for the internal gun discharge. The gun plasma is a virtual plasma electrode capable of sourcing an electron emission current density of 1 kA/cm{sup 2}. The high emission current, small size (3 - 4 cm diameter), and low impurity generation make this gun attractive for a variety of fusion and plasma technology applications.
Date: October 1, 1996
Creator: Den Hartog, D. J.; Craig, D. J.; Fiksel, G. & Sarff, J. S.
Partner: UNT Libraries Government Documents Department

Locked modes and magnetic field errors in MST

Description: In the MST reversed field pinch magnetic oscillations become stationary (locked) in the lab frame as a result of a process involving interactions between the modes, sawteeth, and field errors. Several helical modes become phase locked to each other to form a rotating localized disturbance, the disturbance locks to an impulsive field error generated at a sawtooth crash, the error fields grow monotonically after locking (perhaps due to an unstable interaction between the modes and field error), and over the tens of milliseconds of growth confinement degrades and the discharge eventually terminates. Field error control has been partially successful in eliminating locking.
Date: June 1, 1992
Creator: Almagri, A. F.; Assadi, S.; Prager, S. C.; Sarff, J. S. & Kerst, D. W.
Partner: UNT Libraries Government Documents Department

Strong radial electric field shear and reduced fluctuations in a reversed-field pinch

Description: A strongly sheared radial electric field is observed in enhanced confinement discharges in the MST reversed-field pinch. The strong shear develops in a narrow region in the plasma edge. Electrostatic fluctuations are reduced over the entire plasma edge with an extra reduction in the shear region. Magnetic fluctuations, resonant in the plasma core but global in extent, are also reduced. The reduction of fluctuations in the shear region is presumably due to the strong shear, but the causes of the reductions outside this region have not been established.
Date: May 1, 1997
Creator: Chapman, B. E.; Chiang, C. S.; Prager, S. C. & Sarff, J. S.
Partner: UNT Libraries Government Documents Department

Measurement of the dynamo effect in a plasma

Description: A series of the detailed experiments has been conducted in three laboratory plasma devices to measure the dynamo electric field along the equilibrium field line (the {alpha} 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 x B drift and electron diamagnetic drift. The three major findings are (1) the {alpha} 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 x B drift; (3) at high collisionality, a new ``electron diamagnetic dynamo`` is observed, in which the fluctuating velocity is dominated by the 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 J.B. Taylor`s conjecture.
Date: November 1, 1995
Creator: Ji, H.; Prager, S.C.; Almagri, A.F.; Sarff, J.S.; Hirano, Y. & Toyama, H.
Partner: UNT Libraries Government Documents Department

Increased confinement and beta by inductive poloidal current drive in the RFP

Description: Progress in understanding magnetic-fluctuation-induced transport in the reversed field pinch (RFP) has led to the idea of current profile control to reduce fluctuations and transport. With the addition of inductive poloidal current drive in the Madison Symmetric Torus (MST), the magnetic fluctuation amplitude is halved, leading to a four- to five-fold increase in the energy confinement time to {tau}{sub E}{approximately}5 ms as a result of both decreased plasma resistance and increased stored thermal energy. The record low fluctuation amplitude coincides with a record high electron temperature of {approximately}600 eV (for MST), and beta {beta} = 2{mu}{sub 0}<p> / B(a){sup 2} increases from 6% to 8% compared with conventional MST RFP plasmas. Other improvements include increased particle confinement and impurity reduction. 19 refs., 4 figs., 1 tab.
Date: October 1, 1996
Creator: Sarff, J.S.; Lanier, N.E.; Prager, S.C. & Stoneking, M.R.
Partner: UNT Libraries Government Documents Department

Fivefold confinement time increase in the Madison Symmetric Torus using inductive poloidal current drive

Description: Current profile control is employed in the Madison Symmetric Torus 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 ms 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.
Date: December 1, 1996
Creator: Stoneking, M.R.; Lanier, N.E.; Prager, S.C.; Sarff, J.S. & Sinitsyn, D.
Partner: UNT Libraries Government Documents Department

Measurement of core velocity fluctuations and the dynamo in a reversed-field pinch

Description: 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). These measurements show that the flow velocity fluctuations are correlated with magnetic field fluctuations. 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. 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.
Date: December 31, 1998
Creator: Den Hartog, D. J.; Craig, D.; Fiksel, G.; Fontana, P. W.; Prager, S. C.; Sarff, J. S. et al.
Partner: UNT Libraries Government Documents Department

Integrated simulation and modeling capability for alternate magnetic fusion concepts

Description: This document summarizes a strategic study addressing the development of a comprehensive modeling and simulation capability for magnetic fusion experiments with particular emphasis on devices that are alternatives to the mainline tokamak device. A code development project in this area supports two defined strategic thrust areas in the Magnetic Fusion Energy Program: (1) comprehensive simulation and modeling of magnetic fusion experiments and (2) development, operation, and modeling of magnetic fusion alternate- concept experiment
Date: November 3, 1998
Creator: Cohen, B. I.; Hooper, E.B.; Jarboe, T. R.; LoDestro, L. L.; Pearlstein, L. D.; Prager, S. C. et al.
Partner: UNT Libraries Government Documents Department

The effect of collisionality and diamagnetism on the plasma dynamo

Description: Fluctuation-induced dynamo forces are measured over a wide range of electron collisionality in the edge of TPE-1RM20 Reversed-Field Pinch (RFP). In the collisionless region the Magnetohydrodynamic (MHD) dynamo alone can sustain the parallel current, while in the collisional region a new dynamo mechanism resulting from the fluctuations in the electron diamagnetic drift becomes dominant. A comprehensive picture of the RFP dynamo emerges by combining with earlier results from MST and REPUTE RFPs.
Date: April 28, 1995
Creator: Ji, H.; Yagi, Y.; Hattori, K.; Hirano, Y.; Shimada, T.; Maejima, Y. et al.
Partner: UNT Libraries Government Documents Department

Turbulent transport in the MST reversed-field pinch

Description: Measurements of edge turbulence and the associated transport are ongoing in the Madison Symmetric Torus (R = 1.5 m, a = 0.52 m) reversed-field pinch using magnetic and electrostatic probes. Magnetic fluctuations are dominated by m = 1 and n {approximately} 2R/a tearing modes. Particle losses induced by magnetic field fluctuations have been found to be ambipolar (<{tilde J}{sub parallel}{tilde B}{sub r}> = O). Electrostatic fluctuations are broadband and turbulent, with mode widths {delta}m {approximately} 3--7 and {delta}n {approximately}70--150. Particle, parallel current, and energy transport arising from coherent motion with the fluctuating {tilde E}xB drift has been measured. Particle transport via this channel is comparable to the total particle loss from MST. Energy transport (from <{tilde P}{tilde E}{sub phi}>/B{sub o}) due to electrostatic fluctuations is relatively small, and parallel current transport (from <{tilde J}{sub parallel}{tilde E}{sub chi}>/B{sub o}) may be small as well.
Date: November 1, 1991
Creator: Rempel, T.D.; Almagri, A.F.; Assadi, S.; Den Hartog, D.J.; Hokin, S.A.; Prager, S.C. et al.
Partner: UNT Libraries Government Documents Department

Nonlinear coupling of tearing fluctuations in the Madison Symmetric Torus

Description: 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 m[sub 1] + m[sub 2] = m[sub 3] and n[sub 1] + n[sub 2] = n[sub 3] is measured by the bicoherency. In the RFP, m=l, n[approximately]2R/a (6 for MST) internally resonant modes are linearly unstable and grow to large amplitude. Large values of bicoherency occur for two m=l 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 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.
Date: November 1, 1992
Creator: Sarff, J.S.; Almagri, A.F.; Cekic, M.; Den Hartog, D.J.; Fiksel, G.; Hokin, S.A. et al.
Partner: UNT Libraries Government Documents Department