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Dynamo and anomalous transport in the reversed field pinch

Description: The reversed field pinch is an effective tool to study the macroscopic consequences of magnetic fluctuations, such as the dynamo effect and anomalous transport. Several explanations exist for the dynamo (the self-generation of plasma current)--the MHD dynamo, the kinetic dynamo, and the diamagnetic dynamo. There is some experimental evidence for each, particularly from measurements of ion velocity and electron pressure fluctuations. Magnetic fluctuations are known to produce energy and particle flux in the RFP core. Current profile control is able to decrease fluctuation-induced transport by a factor of five. Improved confinement regimes are also obtained at deep reversal and, possibly, with flow shear.
Date: August 1, 1998
Creator: Prager, S. C.
Partner: UNT Libraries Government Documents Department

Shear alfven resonances in Tokapole II

Description: It has been suggested that efficient heating can occur by coupling energy through the shear Alfven resonance that is predicted to occur at particular locations within inhomogeneous plasmas. This heating mechanism should be applicable to tokamaks; however, the existence of the resonances in tokamaks has not yet been established experimentally. We present here direct observations of localized enhancement in the driven wave magnetic field which are compatible with theoretical predictions for the shear Alfven resonance in tokamak geometry. These resonance studies at low absorbed power will be followed by higher power heating (approx. 1 MW) presently being developed.
Date: March 1, 1982
Creator: Witherspoon, F.D.; Prager, S.C. & Sprott, J.C.
Partner: UNT Libraries Government Documents Department

Current density fluctuations and ambipolarity of transport

Description: The fluctuation in the plasma current density is measured in the MIST reversed field pinch experiment. Such fluctuations, and the measured radial profile of the k spectrum of magnetic fluctuations, supports the view and that low frequency fluctuations (f < 30 kHz) are tearing modes and high frequency fluctuations (30 kHz < f < 250 kHz) are localized turbulence in resonance with the local equilibrium magnetic field (i.e., k {center dot} B = 0). Correlation of current density and magnetic fluctuations (< {tilde j}{parallel}{tilde B}{sub r} >) demonstrates that radial particle transport from particle motion parallel to a fluctuating magnetic field is ambipolar over the full frequency range.
Date: October 1, 1991
Creator: Shen, W.; Dexter, R.N. & Prager, S.C.
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

Plasma rotation, dynamo, and nonlinear coupling in the reversed field pinch

Description: Two important effects of MHD fluctuations in the RFP and tokamak are current generation (the dynamo effect) and mode locking. In the T1 and MST RFP experiments new results reveal the mode dynamics underlying these phenomena. In T1 the effect of specific magnetic Fourier modes on the current density profile is evident. In MST, the MHD dynamo term ({delta}v x {delta}B) is measured in the plasma edge, and found to account for the time dependence of the edge current throughout a sawtooth cycle. As edge resistivity is increased in T1 the fluctuation amplitude increases to maintain the dynamo-driven current, as expected from MHD computation. The modes responsible for the dynamo often lock to the local magnetic field error at the vertical cut in MST. The plasma rotation velocity has been measured with a fast Doppler spectrometer to a time resolution of 1 {mu}s. The plasma rotation and mode phase velocity are remarkably well-correlated, with both slowing, in the presence of an impulsive field error, in a 100 {mu}s timescale.
Date: July 1, 1995
Creator: Prager, S.C.; Almagri, A.F. & Cekic, M.
Partner: UNT Libraries Government Documents Department

Axisymmetric instability in a noncircular tokamak: experiment and theory

Description: The stability of dee, inverse-dee and square cross section plasmas to axisymmetric modes has been investigated experimentally in Tokapole II, a tokamak with a four-null poloidal divertor. Experimental results are closely compared with predictions of two numerical stability codes -- the PEST code (ideal MHD, linear stability) adapted to tokapole geometry and a code which follows the nonlinear evolution of shapes similar to tokapole equilibria. Experimentally, the square is vertically stable and both dee's unstable to a vertical nonrigid axisymmetric shift. The central magnetic axis displacement grows exponentially with a growth time approximately 10/sup 3/ poloidal Alfven times plasma time. Proper initial positioning of the plasma on the midplane allows passive feedback to nonlinearly restore vertical motion to a small stable oscillation. Experimental poloidal flux plots are produced directly from internal magnetic probe measurements.
Date: September 1, 1979
Creator: Lipschultz, B.; Prager, S.C.; Todd, A.M.M. & Delucia, J.
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

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

The perpendicular electron energy flux driven by magnetic fluctuations in the edge of TEXT-U

Description: A fast bolometer was used for direct measurements of parallel electron energy flux in the edge of TEXT-U. The fluctuating component of the parallel electron energy flux, combined with a measurement of magnetic fluctuations, provides an upper limit to the perpendicular electron flux. This magnetically driven energy flux cannot account for the observed energy flux.
Date: June 12, 1995
Creator: Fiksel, G.; Prager, S.C.; Bengtson, R.D. & Wootton, A.J.
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

High-power ion-cyclotron heating on the levitated octupole

Description: Experiments are underway in the Wisconsin Levitated Toroidal Octupole to create hot, dense plasmas to facilitate the study of ..beta.. limits and related phenomena such as Pfirsch-Schlueter and bootstrap currents. The question of ballooning mode instability limits on ..beta.. = 8 ..pi..nk(T/sub i/+T/sub e/)/B/sup 2/ is of general importance for all toroidal systems, and ICRF heating should permit study of high ..beta.. plasmas with lower collisionality and smaller gyroradii than those of the high ..beta.. gun-injected plasmas currently under study in the Octupole. To these ends we are developing sources capable of delivering 4 MW to the plasma (1.5 MW coupled to the plasma to date, the rest under development).
Date: March 1, 1982
Creator: Dexter, R.N.; Fortgang, C.M.; Prager, S.C.; Sprott, J.C.; Strait, E.J. & Twichell, J.C.
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

B sub 4 C solid target boronization of the MST reversed-field pinch

Description: A solid rod of hot-pressed boron carbide is being used as the source of boron during boronization of MST. The most striking result of this procedure is the reduction in oxygen contamination of the plasma (O III radiation, characteristic of oxygen at the edge, falls by about a factor of 3 after boronization.). The radiated power fraction drops to about half its initial value. Particle reflux from the wall is also lowered, making density control simpler. The rod (12.7 mm diameter) is inserted into the edge plasma of normal high-power RFP discharges. B{sub 4}C is ablated from the surface of the rod and deposited in a thin film (a-B/C:H) on the walls and limiters. The energy flux carried by superthermal'' (not runaway'') electrons at the edge of MST appears to enhance the efficient, non-destructive ablation of the boron carbide rod.
Date: October 1, 1992
Creator: Den Hartog, D.J.; Cekic, M.; Fiksel, G.; Hokin, S.A.; Kendrick, R.D.; Prager, S.C. et al.
Partner: UNT Libraries Government Documents Department

Mission and Readiness Assessment for Fusion Nuclear Facilities

Description: Magnetic fusion development toward DEMO will most likely require a number of fusion nuclear facilities (FNF), intermediate between ITER and DEMO, to test and validate plasma and nuclear technologies and to advance the level of system integration. The FNF mission space is wide, ranging from basic materials research to net electricity demonstration, so there is correspondingly a choice among machine options, scope, and risk in planning such a step. Readiness requirements to proceed with a DEMO are examined, and two FNF options are assessed in terms of the contributions they would make to closing DEMO readiness gaps, and their readiness to themselves proceed with engineering design about ten years from now. An advanced tokamak (AT) pilot plant with superconducting coils and a mission to demonstrate net electricity generation would go a long way toward DEMO. As a next step, however, a pilot plant would entail greater risk than a copper-coil FNSF-AT with its more focussed mission and technology requirements. The stellarator path to DEMO is briefly discussed. Regardless of the choice of FNF option, an accompanying science and technology development program, also aimed at DEMO readiness, is absolutely essential.
Date: December 12, 2012
Creator: Neilson, G. H.; Brown, T. G.; Gates, D. A.; Kessel, C. E.; Menard, J. E.; Prager, S. C. et al.
Partner: UNT Libraries Government Documents Department

MHD computation of feedback of resistive-shell instabilities in the reversed field pinch

Description: MHD computation demonstrates that feedback can sustain reversal and reduce loop voltage in resistive-shell reversed field pinch (RFP) plasmas. Edge feedback on {approximately}2R/a tearing modes resonant near axis is found to restore plasma parameters to nearly their levels with a close-fitting conducting shell. When original dynamo modes are stabilized, neighboring tearing modes grow to maintain the RFP dynamo more efficiently. This suggests that experimentally observed limits on RFP pulselengths to the order of the shell time can be overcome by applying feedback to a few helical modes.
Date: May 1, 1992
Creator: Zita, E.J.; Prager, S.C. (Wisconsin Univ., Madison, WI (United States). Plasma Physics Research); Ho, Y.L. & Schnack, D.D (Science Applications International Corp., San Diego, CA (United States))
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

Radio frequency wave experiments on the MST reversed field pinch

Description: Experiments, simulations, and theory all indicate that the magnetic fluctuations responsible for the poor confinement in the reversed field pinch (RFP) can be controlled by altering the radial profile of the current density. The magnetic fluctuations in the RFP are due to resistive MHD instabilities caused by current profile peaking; thus confinement in the RFP is ultimately the result of a misalignment between inductively driven current profiles and the stable current profiles characteristic of the Taylor state. If a technique such as rf current drive can be developed to non-inductively sustain a Taylor state (a current profile linearly stable to all tearing modes), the confinement of the RFP and its potential as a reactor concept are likely to increase. Whether there is a self-consistent path from poor confinement to greatly improved confinement through current profile modification is an issue for future experiments to address if and only if near term experiments can demonstrate: (1) coupling to and the propagation of rf waves in RFP plasmas, (2) efficient current drive, and (3) control of the power deposition which will make it possible to control the current profile. In this paper, modeling results and experimental plans are presented for two rf experiments which have the potential of satisfying these three goals: high-n{sub {parallel}} lower hybrid (LH) waves and electron Bernstein waves (EBWs).
Date: April 1, 1999
Creator: Forest, C.B.; Chattopadhyay, P.K.; Nornberg, M.D.; Prager, S.C.; Thomas, M.A.; Uchimoto, E. 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