336 Matching Results

Search Results

Advanced search parameters have been applied.

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

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

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

Nonlinear tearing mode interactions and mode locking in reversed field pinches

Description: The nonlinear interaction of a set of tearing instabilities and plasma flow is studied in a cylindrical plasma. An analytic theory of mode locking is developed which includes the effects of the localized electromagnetic torques, plasma inertia and cross-field viscosity. The calculation is specialized for the case of mode locking on the Madison Symmetric Torus (MST) reversed field pinch. In MST plasmas, a set of m = 1 tearing instabilities become phase locked and form a toroidally localized, rotating magnetic disturbance. An evolution equation for the phase velocity of this magnetic disturbance is derived which accounts for two types of electromagnetic torques. The external torques describe the interaction of the tearing modes with static magnetic perturbations located outside the plasma region. The interior torques describe the nonlinear interaction of three tearing modes which satisfy a wave number resonance condition. For conditions typical of MST, the internal torques dominate the external torques, which suggest the nonlinear interaction of tearing instabilities play a prominent role in the momentum degradation and mode locking.
Date: June 1, 1996
Creator: Hegna, C.C.
Partner: UNT Libraries Government Documents Department

Adiabatic compression of a closed-field-line configuration by a centimeter-size liner

Description: The paper considers adiabatic compression of a pre-formed closed field line configuration by an imploding liner. Three configurations are discussed: the field-reversed configuration, the spheromak and the Z-pinch. It is shown that, by employing a 2D-compression, one can reach a break-even condition with an energy input as low as 100 kJ. It is emphasized that the possibility of performing crucial experiments on small-to-medium-scale experimental devices may considerably shorten the development path for the system under consideration.
Date: September 1, 1995
Creator: Drake, R.P.; Hammer, J.H.; Hartman, C.W.; Perkins, L.J. & Ryutov, D.D.
Partner: UNT Libraries Government Documents Department

Time-resolved observation of discrete and continuous MHD dynamo in the reversed-field pinch edge

Description: We report the first experimental verification of the MHD dynamo in the RFP. A burst of magnetohydrodynamic (MHD) dynamo electric field is observed during the sawtooth crash, followed by an increase in the local parallel current in the MST RFP edge. By measuring each term, the parallel MHD mean-field Ohm`s law is observed to hold within experimental error bars both between and during sawtooth crashes.
Date: January 6, 1994
Creator: Ji, H.; Almagri, A. F.; Prager, S. C. & Sarff, J. S.
Partner: UNT Libraries Government Documents Department

Lower hybrid accessibility in a large, hot reversed field pinch

Description: Accessibility and damping of the slow wave in a reversed field pinch (RFP) plasma is investigated theoretically, using projected Reversed Field Experiment (RFX) plasma parameters. By numerically solving the hot plasma dispersion relation, regions of propagation are found and the possibility of mode conversion is analyzed. If the parallel index of refraction of the wave is chosen judiciously at the edge of the plasma, the slow wave is accessible to a target region located just inside the reversal surface without mode conversion. Landau damping is also optimized in this region. A representative fast electron population is then added in order to determine its effect on accessibility and damping. The presence of these electrons, whose parameters were estimated by extrapolation of Madison Symmetric Torus (MST) data, does not affect the accessibility of the wave. However, the initial phase velocity of the wave needs to be increased somewhat in order to maintain optimal damping in the target zone.
Date: November 1, 1995
Creator: Dziubek, R. A.; Harvey, R. W.; Hokin, S. A. & Uchimoto, E.
Partner: UNT Libraries Government Documents Department

Microstability theory for the field reversed configuration. Final report

Description: This report summarizes the work done in the last contract period. Previous work has been described in Annual Performance Reports. The work carried on under this Research Grant and not included in previous progress and annual reports includes two distinct items. One work is a study of the nonlocal high beta microstability of the FRC (Field Reversed Configuration), which they began sometime ago. This study identified the limiting beta (=4{pi}nT/B{sup 2}) for the mode to remain unstable. The study found that as beta increases, the wavenumbers (k{sub y}, K{sub z}) for maximum growth changes, so that the limiting beta is not the one found by fixing (k{sub y}, K{sub z}) and increasing beta. It also appears that the criterion for nonlocal terms to influence the result, as beta increases, is substantially weaker than might have been thought. The authors identify the parameter that determines this effect. This study is presented as Appendix 1 of this report. The second study is of the effect of collisions on the lower hybrid drift instability. The result is that the effect of collisions is substantially more important than might have been expected. These two studies are in different stages of completion. The second is in fact complete, and could be published virtually as is, although it would benefit from a small amount of numerical analysis. The first study is far richer than the second, in that it includes a variety of regimes and effects. The formulation presented in it could e used as the basis for a series of papers, although in its present stage it is not ready for publication. It is unfortunate, but the level of the research Grant, and its untimely end, did not permit further progress on that study.
Date: November 5, 1997
Creator: Krall, N.A.
Partner: UNT Libraries Government Documents Department

Reversed field pinch experiments. Final report, November 24, 1991--November 23, 1994

Description: The Reversatron reversed field pinch (RFP) is an experiment to investigate the effect of boundary conditions on plasma confinement. Earlier RFP experiments, particularly ZT-40 and Los Alamos and Eta Beta II at Padua, had found an improvement in confinement when the vacuum chamber wall was surrounded by a conducting shell. The data showed a longer discharge duration and a reduction in the applied loop voltage. The latter determines the energy input to the plasma, thus a lower loop voltage (with relatively constant plasma parameters) indicated improved energy confinement. The Reversatron RFP was designed to allow the conducting wall to be easily changed so that differing electrical boundary conditions could be compared in the same device. Specifically, it was proposed to measure the growth rates and wavenumbers of magnetohydrodynamic (MHD) instabilities as a function of the boundary conditions and to determine their effect on plasma confinement. These measurements were made and reported in a series of publications (Appendices) in Physics of Fluids B and in Plasma Physics and Controlled Fusion.
Date: March 1, 1995
Creator: Robertson, S.
Partner: UNT Libraries Government Documents Department

Measurement of magnetic fluctuation-induced heat transport in tokamaks and RFP

Description: It has long been recognized that fluctuations in the magnetic field are a potent mechanism for the anomalous transport of energy in confined plasmas. The energy transport process originates from particle motion along magnetic fields, which have a fluctuating component in the radial direction (perpendicular to the confining equilibrium magnetic surfaces). A key feature is that the transport can be large even if the fluctuation amplitude is small. If the fluctuations are resonant with the equilibrium magnetic field (i.e., the fluctuation amplitude is constant along an equilibrium field line) then a small fluctuation can introduce stochasticity to the field line trajectories. Particles following the chaotically wandering field lines can rapidly carry energy across the plasma.
Date: August 1, 1996
Creator: Fiksel, G.; Hartog, D.D.; Cekic, M. & Prager, S.C.
Partner: UNT Libraries Government Documents Department

Toroidal pinch experiments. Annual progress report, November 24, 1992--November 23, 1993

Description: The effect of boundary conditions on plasma confinement in the reversed field pinch (RFP) was studied. Early theoretical studies of the magnetohydrodynamic stability of the reversed field pinch had shown that the pinch was unstable unless it was surrounded by a conducting shell which prevented the penetration of the magnetic field during the lifetime of the pinch. The presence of the sheer, however, complicated construction and prevented control of the plasma equilibrium by feedback systems. This research project was undertaken to determine the effect on confinement of varying the magnetic penetration time of the shell. A reversed field pinch, the Reversatron, was constructed so that shells of varying thickness could be installed. Shells were made with magnetic penetration times both longer and shorter than the duration of the plasma discharge. These shells allowed the performance to be determined as a function of the magnetic penetration time of the shell. This research showed that a shell with a short penetration time degraded the discharges. Diagnostic measurements showed reductions in plasma current, discharge duration, and the Doppler temperature of impurity ions and showed increases in the plasma resistivity and loop voltage. The greater power input and lower temperature implied a reduced energy confinement time. Magnetic diagnostics showed that two classes of MHD instabilities with poloidal mode number m = 1 were responsible for the degraded confinement. These were the dynamo modes resonant on the magentic axis (with toroidal mode numbers -9, -10, -11, -12) and the external kink modes (with toroidal mode numbers 3, 4, 5, and 6).
Date: January 24, 1994
Creator: Robertson, S.
Partner: UNT Libraries Government Documents Department

Particle transport due to magnetic fluctuations

Description: Electron current fluctuations are measured with an electrostatic energy analyzer at the edge of the MST reversed-field pinch plasma. The radial flux of fast electrons (E>T{sub e}) due to parallel streaming along a fluctuating magnetic field is determined locally by measuring the correlated product <{tilde J}{sub e}{tilde B}{sub r}>. Particle transport is small just inside the last closed flux surface ({Gamma}{sub e,mag} < 0.1 {Gamma}{sub e,total}), but can account for all observed particle losses inside r/a=0.8. Electron diffusion is found to increase with parallel velocity, as expected for diffusion in a region of field stochasticity.
Date: January 1, 1994
Creator: Stoneking, M. R.; Hokin, S. A.; Prager, S. C.; Fiksel, G.; Ji, H. & Den Hartog, D. J.
Partner: UNT Libraries Government Documents Department

Generation of fast electrons in reversed field pinches by the equilibrium grad {vert_bar}B{vert_bar} force

Description: It is shown that a decreasing magnetic field profile in reversed-field pinch plasmas leads to formation of an anisotropic electron distribution function at the plasma edge. The mechanism is the conservation of the magnetic moment and the energy of electrons that collisionlessly travel outward in a stochastic magnetic field. As a result, the electrons have high parallel energies and low perpendicular energies at the edge. The details of the distribution function correspond well to experimental results.
Date: October 1, 1994
Creator: Fiksel, G.
Partner: UNT Libraries Government Documents Department

Measurement of magnetic turbulence structure and nonlinear mode coupling of tearing fluctuations in the Madison Symmetric Torus reversed field pinch edge

Description: Linear and nonlinear magnetohydrodynamic (MHD) stability of current-driven modes are studied in the MST reversed field pinch. Measured low frequency (f < 35 kHz) magnetic fluctuations are consistent with the global resistive tearing instabilities predicted by 3-D MHD simulations. At frequencies above 35 kHz, the magnetic fluctuations were detected to be localized and externally resonant. Discrete dynamo events, ``sawtooth oscillations,`` have been observed in the experimental RFP plasmas. This phenomenon causes the plasma to become unstable to m = 1 tearing modes. The modes that may be important in different phases of these oscillations are identified. These results then assist in nonlinear studies and also help to interpret the spectral broadening of the measured data during a discrete dynamo event. Three-wave nonlinear coupling of spectral Fourier modes is measured in the MST by applying bispectral analysis to magnetic fluctuations measured at the plasma edge at 64 toroidal locations and 16 poloidal locations, permitting observation of coupling over 8 poloidal and 32 toroidal modes. Comparison to bispectra predicted by resistive MHD computation indicates reasonably good agreement. However, during the crash phase of the sawtooth oscillation the nonlinear coupling is strongly enhanced, concomitant with a broadened k-spectrum. During the sawtooth formation the plasma is undergoing a pure diffusive process. The dynamo only occurs during the sawtooth crash. High frequency activity prior to a sawtooth crash is caused by nonlinear frequency (small-scale) mode coupling. Growth rate and coupling coefficients of toroidal mode spectra are calculated by statistical modeling. Temporal evolution of edge toroidal mode spectra has been predicted by transfer function analysis. The driving sources of electrostatic fields are different than for the magnetic fields. The characteristics of tearing modes can be altered by external field errors and addition of impurities to the plasma.
Date: January 1, 1994
Creator: Assadi, S.
Partner: UNT Libraries Government Documents Department

The fluctuation induced Hall effect

Description: The fluctuation induced Hall term, {le}{approximately}{ovr J} {times} {approximately}{ovr B}{ge}, has been measured in the MST reversed field pinch. The term is of interest as a possible source of current self-generation (dynamo). It is found to be non-negligible, but small in that it can account for less than 25% of the dynamo driven current.
Date: February 1, 1993
Creator: Shen, W. & Prager, S. C.
Partner: UNT Libraries Government Documents Department

Coaxial Slow Source. Final technical report, November 16, 1991--November 15, 1993

Description: This report describes work performed by the University of Washington Aerospace and Energetics Research Program involving the Field Reversed Configuration (FRC) approach to magnetic confinement fusion. The group`s effort involved continued research on the Coaxial Slow Source annular FRC experimental device located on campus, as well as support for the Large s Experiment (LSX) operated by Spectra Technology Incorporated, Bellevue, Washington.
Date: December 31, 1993
Creator: Brooks, R. D. & Jarboe, T. R.
Partner: UNT Libraries Government Documents Department

A simple, high performance Thomson scattering diagnostic for high temperature plasma research

Description: This Thomson scattering diagnostic is used to measure the electron temperature and density of the plasma in the MST reversed-field pinch, a magnetic confinement fusion research device. This diagnostic system is unique for its type in that it combines high performance with simple design and low cost components. In the design of this instrument, careful attention was given to the suppression of stray laser line light with simple and effective beam dumps, viewing dumps, aperatures, and a holographic edge filter. This allows the use of a single grating monochromator for dispersion of the Thomson scattered spectrum onto the microchannel plate detector. Alignment and calibration procedures for the laser beam delivery system, the scattered light collection system, and the spectrometer and detector are described. A sample Thomson scattered spectrum illustrates typical data.
Date: February 1, 1994
Creator: Hartog, D. J. D. & Cekic, M.
Partner: UNT Libraries Government Documents Department

Confinement improvement with rf poloidal current drive in the reversed-field pinch

Description: External control of the current profile in a reversed-field pinch (RFP), by means such as rf poloidal current drive, may have beneficial effects well beyond the direct reduction of Ohmic input power due to auxiliary heating. Reduction of magnetic turbulence associated with the dynamo, which drives poloidal current in a conventional RFP, may allow operation at lower density and higher electron temperature, for which rf current drive becomes efficient and the RFP operates in a more favorable regime on the n{tau} vs T diagram. Projected parameters for RFX at 2 MA axe studied as a concrete example. If rf current drive allows RFX to operate with {beta} = 10% (plasma energy/magnetic energy) at low density (3 {times} 10{sup 19} m{sup {minus}3}) with classical resistivity (i.e. without dynamo-enhanced power input), 40 ms energy confinement times and 3 keV temperatures will result, matching the performance of tokamaks of similar size.
Date: March 8, 1994
Creator: Hokin, S.; Sarff, J.; Sovinec, C. & Uchimoto, E.
Partner: UNT Libraries Government Documents Department

Field-reversed configuration (FRC) experiments

Description: FRCs with equilibrium separatrix radii up to 0.18 m have been formed and studied in FRX-C/LSM. For best formation conditions at low fill pressure, the particle confinement exceeds the predictions of LHD transport calculations by up to a factor of two; however, the inferred flux confinement is more anomalous than in smaller FRCs. Higher bias field produces axial shocks and degradation in confinement, while higher fill pressure results in gross fluting during formation. FRCs have been formed in TRX with s from 2 to 6. These relatively collisional FRCs exhibit flux lifetimes of 10 {yields} 20 kinetic growth times for the internal tilt mode. The coaxial slow source has produced annular FRCs in a coaxial coil geometry on slow time scales using low voltages. 16 refs., 4 figs., 1 tab.
Date: January 1, 1988
Creator: Siemon, R.E.; Chrien, R.E.; Hugrass, W.N.; Okada, S.; Rej, D.J.; Taggart, D.P. et al.
Partner: UNT Libraries Government Documents Department

Magnetohydrodynamic simulations of noninductive helicity injection in the reversed-field pinch and tokamak

Description: Numerical computation is used to investigate resistive magnetohydrodynamic (MHD) fluctuations in the reversed-field pinch (RFP) and in tokamak-like configurations driven solely by direct current (DC) helicity injection. A Lundquist number (S) scan of RFP turbulence without plasma pressure produces the weak scaling of S{sup -0.18} for the root-mean-square magnetic fluctuation level for 2.5x10{sup 3}{le}S{le}4x10{sup 4}. The temporal behavior of fluctuations and the reversal parameter becomes more regular as S is increased, acquiring a {open_quotes}sawtooth{close_quotes} shape at the largest value of S. Simulations with plasma pressure and anisotropic thermal conduction demonstrate energy transport resulting from parallel heat fluctuations. To investigate means of improving RFP energy confinement, three forms of current profile modification are tested. Radio frequency (RF) current drive is modeled with an auxiliary electron force, and linear stability calculations are used.
Date: December 31, 1995
Creator: Sovinec, C.R.
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

Radial electric fields from larmor radius effects in the field-reversed theta pinch

Description: The generation of radial electric field from the different Larmor radii between the diffusing ions and electrons in Field-Reversed Theta Pinch (FR theta P) has been evaluated by developing a new computer code, FLR. The code treats the background electrons at point particles, while for large-orbit ions the effects of finite Larmor radius are incorporated into an accurate density formulation. This is necessary since in a small device whose size is comparable to an ion gyroradius (e.g., FR theta P) most ions will contribute to the density over a fairly large region of the plasma. The actual ion density at any point in the plasma represents a time-averaged contribution from all particles whose gyro-orbits pass through that point. The FLR code has predicted the electric field strengths between 10/sup 3/ and 10/sup 4/ volts/cm, which is in good agreement with the experimental values of FRX-A measurements which correspond roughly to the order of 10/sup 3/ volts/cm.
Date: January 1, 1980
Creator: Hu, Y.
Partner: UNT Libraries Government Documents Department

Dynamo theory: can amplification of magnetic-field profiles arise from a cross-field alpha effect

Description: The answer to the title question is here provided in cylindrical geometry for an essentially arbitrary radial dependence of this alpha effect, except that it is subject to a simple, physically required constraint. The interest in this type of alpha effect derives from its connection with compressible turbulence, which is usually not considered in kinematic dynamo theory.
Date: April 1, 1982
Creator: Gerwin, R. & Keinigs, R.
Partner: UNT Libraries Government Documents Department

Detection of plasma equilibrium shifts with fiber optic sensing of image currents

Description: The radial equilibrium position of Reverse Field Pinch experiments is determined by the j x B force on the plasma. The current density is that of the toroidal plasma current and the B field is the vertical magnetic field which is present in the plasma. This magnetic field is the result of several components. The main field, generated by the toroidal current windings, is corrected by adjustable trim windings to achieve a desired equilibrium position. There is an additional component to the field due to induced image currents in the close fitting conducting shell which encircles the plasma. These currents vary in time due to the finite L/R time of the conducting shell. It is the object of this paper to investigate the possibility of measuring these shell currents accurately using fiber optics so as to provide an analog signal to the equilibrium feedback circuit. 7 refs., 7 figs.
Date: January 1, 1988
Creator: Forman, P.R.; Jahoda, F.C. & Miller, G.
Partner: UNT Libraries Government Documents Department