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High current plasma electron emitter

Description: A high current plasma electron emitter based on a miniature plasma source has been developed. The emitting plasma is created by a pulsed high current gas discharge. The electron emission current is 1 kA at 300 V at the pulse duration of 10 ms. The prototype injector described in this paper will be used for a 20 kA electrostatic current injection experiment in the Madison Symmetric Torus (MST) reversed-field pinch. The source will be replicated in order to attain this total current requirement. The source has a simple design and has proven very reliable in operation. A high emission current, small size (3.7 cm in diameter), and low impurity generation make the source suitable for a variety of fusion and technological applications.
Date: July 1, 1995
Creator: Fiksel, G.; Almagri, A.F. & Craig, D.
Partner: UNT Libraries Government Documents Department

Controlling chaos in low and high dimensional systems with periodic parametric perturbations

Description: The effect of applying a periodic perturbation to an accessible parameter of various chaotic systems is examined. Numerical results indicate that perturbation frequencies near the natural frequencies of the unstable periodic orbits of the chaotic systems can result in limit cycles for relatively small perturbations. Such perturbations can also control or significantly reduce the dimension of high-dimensional systems. Initial application to the control of fluctuations in a prototypical magnetic fusion plasma device will be reviewed.
Date: June 1, 1998
Creator: Mirus, K. A. & Sprott, J. C.
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

Calibration techniques for a fast duo-spectrometer

Description: The authors have completed the upgrade and calibration of the Ion Dynamics Spectrometer (IDS), a high-speed Doppler duo-spectrometer which measures ion flow and temperature in the MST Reversed-field Pinch. This paper describes an in situ calibration of the diagnostic`s phase and frequency response. A single clock was employed to generate both a digital test signal and a digitizer trigger thus avoiding frequency drift and providing a highly resolved measurement over the system bandwidth. Additionally, they review the measurement of the spectrometer instrument function and absolute intensity response. This calibration and subsequent performance demonstrate the IDS to be one of the fastest, highest throughput diagnostics of its kind. Typical measurements are presented.
Date: June 1, 1996
Creator: Chapman, J.T. & Den Hartog, D.J.
Partner: UNT Libraries Government Documents Department

A simple, low-cost, versatile CCD spectrometer for plasma spectroscopy

Description: The authors have constructed a simple, low-cost CCD spectrometer capable of both high resolution ({Delta}{lambda} {le} 0.015 nm) and large bandpass (110 nm with {Delta}{lambda} {approximately}0.3 nm). These two modes of operation provide two broad areas of capability for plasma spectroscopy. The first major application is measurement of emission line broadening; the second is emission line surveys from the ultraviolet to the near infrared. Measurements have been made on a low-temperature plasma produced by a miniature electrostatic plasma source and the high-temperature plasma in the MST Reversed-Field Pinch. The spectrometer is a modified Jarrell-Ash 0.5 m Ebert-Fastie monochromator. Light is coupled into the entrance slit with a fused silica fiber optic bundle. The exposure time (2 ms minimum) is controlled by a fast electromechanical shutter. The exit plane detector is a compact and robust CCD detector developed for amateur astronomy by Santa Barbara Instrument Group. The CCD detector is controlled and read out by a Macintosh{reg_sign} computer. This spectrometer is sophisticated enough to serve well in a research laboratory, yet is simple and inexpensive enough to be affordable for instructional use.
Date: June 1, 1996
Creator: Den Hartog, D.J. & Holly, D.J.
Partner: UNT Libraries Government Documents Department

Confinement in the RFP: Lundquist number scaling, plasma flow, and reduced transport

Description: Global heat and particle transport in the reversed field pinch (RFP) result primarily from large-scale, resistive MHD fluctuations which cause the magnetic field in the core of the plasma to become stochastic. Achieving a better understanding of t his turbulent transport and identifying ways to reduce it are critical RFP development issues. The authors report measurements of the Lundquist number (S-scaling) of magnetic and ion flow velocity fluctuations in the Madison Symmetric Torus (MST) RFP. The S-scaling of magnetic fluctuations in MST is weaker than previous measurements {tilde b}/B {approximately} S{sup {minus}1/2} in smaller (lower S) RFP plasmas. Impurity ion flow velocity fluctuations (measured with fast Doppler spectroscopy) have a scaling similar to the magnetic fluctuations, falling in the range {tilde V}/V{sub A} {approximately} S{sup {minus}[0.08-0.10]}. The MHD dynamo ({tilde V} x {tilde b}) up to 15 V/cm was measured in the plasma core. Interestingly, the scaling of the MHD dynamo ({tilde V} x {tilde b}) {approximately} S{sup {minus}[0.64-0.88]} is stronger than for its constituents, a result of decreased coherency between {tilde V} and {tilde b} with increasing S. A weak S-scaling of magnetic fluctuations implies fluctuation suppression measures (e.g., current profile control) may be required in higher-S RFP plasmas. Two types of current profile modifications have been examined--inductive and electrostatic. The inductive control halves the amplitude of global magnetic fluctuations and improves the confinement by a factor of 5. The electrostatic current injection, localized in the edge plasma, reduces edge resonant fluctuations and improves the energy confinement. In addition, regimes with confinement improvement associated with the plasma flow profile are attained.
Date: October 1, 1998
Creator: Fiksel, G.; Almagri, A.F. & Anderson, J.K.
Partner: UNT Libraries Government Documents Department

Spectroscopic measurement of the MHD dynamo in the MST reversed field pinch

Description: The author has directly observed the coupling of ion velocity fluctuations and magnetic field fluctuations to produce an MHD dynamo electric field in the interior of the MST reversed field pinch. Chord averaged ion velocity fluctuations were measured with a fast spectroscopic diagnostic which collects line radiation from intrinsic carbon impurities simultaneously along two lines of sight. The chords employed for the measurements resolved long wavelength velocity fluctuations of several km/s at 8--20 kHz as tiny, fast Doppler shifts in the emitted line profile. During discrete dynamo events the velocity fluctuations, like the magnetic fluctuations, increase dramatically. The toroidal and poloidal chords with impact parameters of 0.3 a and 0.6 a respectively, resolved fluctuation wavenumbers with resonance surfaces near or along the lines of sight indicating a radial velocity fluctuation width for each mode which spans only a fraction of the plasma radius. The phase between the measured toroidal velocity fluctuations and the magnetic fluctuations matches the predictions of resistive MHD while the poloidal velocity fluctuations exhibit a phase consistent with the superposition of MHD effects and the advection of a mean flow gradient past the poloidal line of sight. Radial velocity fluctuations resolved by a chord through the center of the plasma were small compared to the poloidal and toroidal fluctuations and exhibited low coherence with the magnetic fluctuations. The ensembled nonlinear product of the ion velocity fluctuations and fluctuations in the magnetic field indicates a substantial dynamo electric field which peaks during the periods of spontaneous flux generation.
Date: September 1, 1998
Creator: Chapman, J.T.
Partner: UNT Libraries Government Documents Department

Control of nonlinear systems using periodic parametric perturbations with application to a reversed field pinch

Description: In this thesis, the possibility of controlling low- and high-dimensional chaotic systems by periodically driving an accessible system parameter is examined. This method has been carried out on several numerical systems and the MST Reversed Field Pinch. The numerical systems investigated include the logistic equation, the Lorenz equations, the Roessler equations, a coupled lattice of logistic equations, a coupled lattice of Lorenz equations, the Yoshida equations, which model tearing mode fluctuations in a plasma, and a neural net model for magnetic fluctuations on MST. This method was tested on the MST by sinusoidally driving a magnetic flux through the toroidal gap of the device. Numerically, periodic drives were found to be most effective at producing limit cycle behavior or significantly reducing the dimension of the system when the perturbation frequency was near natural frequencies of unstable periodic orbits embedded in the attractor of the unperturbed system. Several different unstable periodic orbits have been stabilized in this way for the low-dimensional numerical systems, sometimes with perturbation amplitudes that were less than 5% of the nominal value of the parameter being perturbed. In high-dimensional systems, limit cycle behavior and significant decreases in the system dimension were also achieved using perturbations with frequencies near the natural unstable periodic orbit frequencies. Results for the MST were not this encouraging, most likely because of an insufficient drive amplitude, the extremely high dimension of the plasma behavior, large amounts of noise, and a lack of stationarity in the transient plasma pulses.
Date: June 1, 1998
Creator: Mirus, K.A.
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

Lower hybrid accessibility in a large, hot reversed field pinch

Description: Recent theoretical and experimental results indicate that driving a current in the outer radius of an RPF suppresses sawtooth activity and increases particle and energy confinement times. One candidate for a form of steady state current drive is the slow wave at the lower hybrid frequency. Here, the accessibility of such a wave in an RFP plasma is investigated theoretically, with focus on the RFX machine of Padua, Italy. To drive current, the slow wave with frequency between 1.0--1.5 GHz is considered where optimal Landau damping is desired at r/a {approximately} 0.7. By numerically determining the values of the wave`s perpendicular index of refraction which satisfy the hot plasma dispersion relation, regions of propagation and evanescence can be found. The path of the wave can then be traced over a contour map of these regions so that accessibility can be clearly seen. The possibility of mode conversion events can be ascertained by plotting the values of the perpendicular index of refraction for the fast and slow wave and observing convergence points. To locate regions of maximum Landau damping, a technique developed by Stix was adapted for use with the slow wave in an RFP plasma. Results show that the slow wave is accessible to the target region without mode conversion so long as the value of the parallel index of refraction is correctly chosen at the edge of the plasma. Landau damping can also be optimized with this method. In an RFP, 2--20% of the electron population consists of fast electrons. Because this species alters the total electron distribution function and raises the effective temperature in the outer regions of the plasma, its presence is expected to shift the location of ideal Landau damping.
Date: February 1, 1995
Creator: Dziubek, R.A.
Partner: UNT Libraries Government Documents Department

Controlling fluctuations and transport in the reversed field pinch with edge current drive and plasma biasing

Description: Two techniques are employed in the Madison Symmetric Torus (MST) to test and control different aspects of fluctuation induced transport in the Reversed Field Pinch (RFP). Auxiliary edge currents are driven along the magnetic field to modify magnetic fluctuations, and the particle and energy transport associated with them. In addition, strong edge flows are produced by plasma biasing. Their effect on electrostatic fluctuations and the associated particle losses is studied. Both techniques are accomplished using miniature insertable plasma sources that are biased negatively to inject electrons. This type of emissive electrode is shown to reliably produce intense, directional current without significant contamination by impurities. The two most important conclusions derived from these studies are that the collective modes resonant at the reversal surface play a role in global plasma confinement, and that these modes can be controlled by modifying the parallel current profile outside of the reversal surface. This confirms predictions based on magnetohydrodynamic (MHD) simulations that auxiliary current drive in the sense to flatten the parallel current profile can be successful in controlling magnetic fluctuations in the RFP. However, these studies expand the group of magnetic modes believed to cause transport in MST and suggest that current profile control efforts need to address both the core resonant magnetic modes and those resonant at the reversal surface. The core resonant modes are not significantly altered in these experiments; however, the distribution and/or amplitude of the injected current is probably not optimal for affecting these modes. Plasma biasing generates strong edge flows with shear and particle confinement likely improves in these discharges. These experiments resemble biased H modes in other magnetic configurations in many ways. The similarities are likely due to the common role of electrostatic fluctuations in edge transport.
Date: September 1, 1998
Creator: Craig, D.J.G.
Partner: UNT Libraries Government Documents Department


Description: The comprehensive CQL3D Fokker-Planck/Quasilinear simulation code has been benchmarked against experiment over a wide range of electron cyclotron conditions in the DIII-D tokamak (C.C. Petty et al., 14th Topical Conf. on RF Power in Plasmas, 2002). The same code, in disagreement with experiment, gives 560 kA of ECCD for a well documented, completely ECCD-driven, 100 kA TCV shot [O. Sauter et al, PRL, 2000]. Recent work (R.W. Harvey et al, Phys. Rev. Lett., 2002) has resolved the differences as due to radial transport at a level closely consistent with ITER scaling. Transport does not substantially affect DIII-D ECCD, but at similar ECH power has an overwhelming effect on the much smaller TCV. The transport is consistent with electrostatic-type diffusion (D{sub {rho}{rho}} constant in velocity-space) and not with a magnetic-type diffusion (D{sub {rho}{rho}} {proportional_to} |v{parallel}|). Fokker-Planck simulation of Ohmic reversed field pinch (RFP) discharges in the MST device reveals transport velocity dependence stronger than |v{parallel}| will give agreement with current and soft X-ray spectra in standard discharges, but in the higher confinement, current profile controlled PPCD discharges, transport is again electrostatic-like. This is consistent with the object of PPCD, which is to replace magnetic turbulence driven current with auxiliary CD to improve transport. The tokamak and high-confinement RFP results mutually reinforce the constant-in-velocity-space ''electrostatic-type turbulence'' conclusion. The steady-state energy and toroidal current are governed by the same radial transport equation.
Date: July 1, 2002
Partner: UNT Libraries Government Documents Department

Maximum-likelihood fitting of data dominated by Poisson statistical uncertainties

Description: The fitting of data by {chi}{sup 2}-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. The authors 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. They compare this method with a {chi}{sup 2}-minimization routine applied to both simulated and real data. Differences in the returned fits are greater at low signal level (less than {approximately}20 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.
Date: June 1996
Creator: Stoneking, M. R. & Den Hartog, D. J.
Partner: UNT Libraries Government Documents Department

Fast electron generation and transport in a turbulent, magnetized plasma

Description: The nature of fast electron generation and transport in the Madison Symmetric Torus (MST) reversed field pinch (RFP) is investigated using two electron energy analyzer (EEA) probes and a thermocouple calorimeter. The parallel velocity distribution of the fast electron population is well fit by a drifted Maxwellian distribution with temperature of about 100 eV and drift velocity of about 2 {times} 10{sup 6} m/s. Cross-calibration of the EEA with the calorimeter provides a measurement of the fast electron perpendicular temperature of 30 eV, much lower than the parallel temperature, and is evidence that the kinetic dynamo mechanism (KDT) is not operative in MST. The fast electron current is found to match to the parallel current at the edge, and the fast electron density is about 4 {times} 10{sup 11} cm{sup {minus}3} independent of the ratio of the applied toroidal electric field to the critical electric field for runaways. First time measurements of magnetic fluctuation induced particle transport are reported. By correlating electron current fluctuations with radial magnetic fluctuations the transported flux of electrons is found to be negligible outside r/a{approximately}0.9, but rises the level of the expected total particle losses inside r/a{approximately}0.85. A comparison of the measured diffusion coefficient is made with the ausilinear stochastic diffusion coefficient. Evidence exists that the reduction of the transport is due to the presence of a radial ambipolar electric field of magnitude 500 V/m, that acts to equilibrate the ion and electron transport rates. The convective energy transport associated with the measured particle transport is large enough to account for the observed magnetic fluctuation induced energy transport in MST.
Date: May 1, 1994
Creator: Stoneking, W. R.
Partner: UNT Libraries Government Documents Department

A fast spectroscopic diagnostic for the measurement of plasma impurity ion dynamics

Description: A high-resolution spectrometer has been developed and used to measure simultaneously impurity ion temperature and flow velocities in high temperature plasmas with 10 {mu}s temporal resolution (limited by digitization rate). This device is actually a duo-spectrometer: measurements from two different chordal views of the plasma can be made simultaneously via two separate quartz input fiber optic bundles coupled to the entrance slits which are tilted to compensate for line curvature. The dispersed spectra on the exit plane of the duo-spectrometer are coupled via quartz fiber optics to two arrays of 16 photomultiplier tubes each. Measurement made by recording the Doppler broadened and shifted 227.091 nm emission from the CV impurity ions in the MST reversed-field pinch (RFP) plasma have achieved precisions of <6 eV for temperatures of 150 Ev and <0.7 km/s for flow velocities of 6 km/s. Representative results from the MsT RFP indicate that the toroidal flow velocity drops and ion temperature increases during saw tooth events in MST.
Date: April 1, 1994
Creator: Den Hartog, D. J. & Fonck, R. J.
Partner: UNT Libraries Government Documents Department

Conservation of magnetic helicity during plasma relaxation

Description: Decay of the total magnetic helicity during the sawtooth relaxation in the MST Reversed-Field Pinch is much larger than the MHD prediction. However, the helicity decay (3--4%) is smaller than the magnetic energy decay (7--9%), modestly supportive of the helicity conservation hypothesis in Taylor`s relaxation theory. Enhanced fluctuation-induced helicity transport during the relaxation is observed.
Date: July 1, 1994
Creator: Ji, H.; Prager, S. C. & Sarff, J. S.
Partner: UNT Libraries Government Documents Department

Lower-hybrid poloidal current drive for fluctuation reduction in a reversed field pinch

Description: Current drive using the lower-hybrid slow wave is shown to be a promising candidate for improving confinement properties of a reversed field pinch (RFP). Ray-tracing calculations indicate that the wave will make a few poloidal turns while spiraling radially into a target zone inside the reversal layer. The poloidal antenna wavelength of the lower hybrid wave can be chosen so that efficient parallel current drive will occur mostly in the poloidal direction in this outer region. Three-dimensional resistive magnetohydrodynamic (MHD) computation demonstrates that an additive poloidal current in this region will reduce the magnetic fluctuations and magnetic stochasticity.
Date: June 1, 1994
Creator: Uchimoto, E.; Cekic, M.; Harvey, R. W.; Litwin, C.; Prager, S. C.; Sarff, J. S. et al.
Partner: UNT Libraries Government Documents Department

Solid target boronization of the MST reversed-field pinch during pulsed discharge cleaning

Description: A solid rod of hot-pressed boron carbide is currently being used as the source of boron during boronization of MST. In previous work the authors have demonstrated that boronization can be effectively accomplished by insertion of a low apparent density B{sub 4}C rod into the edge hydrogen plasma of normal high-power RFP discharges. The authors have now extended that technique and can boronize MST by inserting a negatively biased B{sub 4}C rod into pulsed discharge cleaning (PDC) helium plasmas. The same positive results of reduced impurity contamination and particle reflux are achieved with this new boronization method. The bias for the target is provided by the ohmic heating transformer which is pulsed to produce the PDC discharges. Current flow through the B{sub 4}C rod is limited by an inductor. The amount of hydrogen in the amorphous boron carbide films is minimal because the only hydrogen in the PDC plasma is that left over from RFP discharges.
Date: June 1, 1994
Creator: Den Hartog, D. J. & Kendrick, R. D.
Partner: UNT Libraries Government Documents Department