249 Matching Results

Search Results

Advanced search parameters have been applied.

Helicity, Reconnection, and Dynamo Effects

Description: The inter-relationships between magnetic helicity, magnetic reconnection, and dynamo effects are discussed. In laboratory experiments, where two plasmas are driven to merge, the helicity content of each plasma strongly affects the reconnection rate, as well as the shape of the diffusion region. Conversely, magnetic reconnection events also strongly affect the global helicity, resulting in efficient helicity cancellation (but not dissipation) during counter-helicity reconnection and a finite helicity increase or decrease (but less efficiently than dissipation of magnetic energy) during co-helicity reconnection. Close relationships also exist between magnetic helicity and dynamo effects. The turbulent electromotive force along the mean magnetic field (alpha-effect), due to either electrostatic turbulence or the electron diamagnetic effect, transports mean-field helicity across space without dissipation. This has been supported by direct measurements of helicity flux in a laboratory plasma. When the dynamo effect is driven by electromagnetic turbulence, helicity in the turbulent field is converted to mean-field helicity. In all cases, however, dynamo processes conserve total helicity except for a small battery effect, consistent with the observation that the helicity is approximately conserved during magnetic relaxation.
Date: November 1, 1998
Creator: Ji, Hantao
Partner: UNT Libraries Government Documents Department

Design, Installation and Performance of the New insulator for NSTX CHI Experiments

Description: Coaxial Helicity Injection (CHI), a non-inductive method to initiate plasma and generate toroidal plasma current, is being investigated in the National Spherical Torus Experiment (NSTX). The center stack and outer vacuum vessel are separated by insulating gaps at the top and bottom of the slim central column so that a high voltage (up to 2 kV) can be applied between them from a pulsed power supply or a capacitor bank to initiate an arc discharge. In the presence of a suitable poloidal magnetic field, the discharge is initiated at the lower gap (the injector gap) and because of the strong toroidal field develops a helical structure resulting in substantial toroidal plasma current being driven. In NSTX, up to 390 kA of toroidal current has been generated for an injected current of 25 kA. The early investigations of CHI however frequently developed arcs across the insulator at the top of the machine (the absorber gap), which terminated the desired discharge. This arcing greatly restricted the operational space available for CHI studies. During 2002, the absorber region was modified to suppress these arcs. The new design includes a new ceramic insulator on the high field side of the absorber region with a much longer tracking distance between conducting elements at the different potentials. Furthermore, two new coils were installed near the absorber to provide the ability to minimize the poloidal field connecting the center stack and outer vacuum vessel. During the subsequent experimental campaign, CHI operation was less prone to arcing in the absorber and those arcs that did occur did not terminate the main discharge.)
Date: March 5, 2008
Creator: Mueller, D; Chrzanowski, J; Gates, D; Menard, J; Raman, R; Jarboe, T R et al.
Partner: UNT Libraries Government Documents Department

Ramp-up of CHI Initiated Plasmas on NSTX

Description: Experiments on the National Spherical Torus (NSTX) have now demonstrated flux savings using transient coaxial helicity injection (CHI). In these discharges, the discharges initiated by CHI are ramped up with an inductive transformer and exhibit higher plasma current than discharges without the benefit of CHI initiation.
Date: October 29, 2009
Creator: Mueller, D; Bell, R E; LeBlanc, B; Roquemore, A L; Raman, R; Jarboe, T R et al.
Partner: UNT Libraries Government Documents Department

Molecular Structure Analyses of Asymmetric Hydrocarbon Liquid Compounds in the Gas Phase Using Chirped-pulse Fourier Transform Microwave Spectroscopy: Acyl Chlorides and Perfluorinated Acyl Chlorides

Description: Examinations of the effects of (a.) alkyl carbon chain length and (b.) perfluorination of acyl chlorides; propionyl chloride, butyryl chloride, valeroyl chloride, and perfluorinated acyl chlorides; perfluoropropionyl chloride and perfluorobutyryl chloride, are reported and compared using CP-FTMW spectroscopy. All of these molecules are already published in various journals except for valeroyl chloride. The chapters are organized by molecule alkyl chain length and include some background theory. Conformational stability, internal rotation, helicity, and ionic character of the C-Cl bond via the nuclear electric quadrupole coupling constant (χzz) are analyzed. Results show syn, syn-anti/syn-gauche, and syn-anti-anti/syn-gauche-anti stable conformations. Internal rotation was only seen in propionyl chloride. Helicity was not observed. (χzz) was observed to be inert to alkyl chain length, ~ 60 MHz and ~ 65 MHz for the nonfluorinated and fluorinated acyl chlorides. Partial fluorination and varying functional groups are recommended.
Date: August 2011
Creator: Powoski, Robert A.
Partner: UNT Libraries

Coupled spheromak-helicity injector in the sustained spheromak physics experiment, SSPX

Description: The Sustained Spheromak Physics Experiment, now under construction at LLNL, will be sustained by helicity injected from a coaxial plasma source. The ideal MHD equilibrium of the coupled spheromak and source plasmas is calculated in this report, with the plasma in the injector treated in the force free approximation. The two regions are linked by a current-carrying edge plasma region, with field lines which end on the injector electrodes and a width determined by the ratio of the poloidal flux applied to the injector by external coils and that generated in the spheromak. The safety factor, q, on the magnetic axis and its profile within the spheromak separatrix are determined primarily by the ratio of the external and internal values of {lambda} =j{sub {vert_bar}{vert_bar}}/B and by the internal profile of {lambda}. The q-profile is expected to have significant effect on the operation of the experiment, particularly on the stability of the equilibrium and the associated magnetic dynamo.
Date: April 1, 1998
Creator: Hooper, E.B.
Partner: UNT Libraries Government Documents Department

Turbulent Dynamos and Magnetic Helicity

Description: It is shown that the turbulent dynamo alpha-effect converts magnetic helicity from the turbulent field to the mean field when the turbulence is electromagnetic while the magnetic helicity of the mean-field is transported across space when the turbulence is elcetrostatic or due to the elcetron diamagnetic effect. In all cases, however, the dynamo effect strictly conserves the total helicity expect for a battery effect which vanishes in the limit of magnetohydrodynamics. Implications for astrophysical situations, especially for the solar dynamo, are discussed.
Date: April 1, 1999
Creator: Ji, Hantao
Partner: UNT Libraries Government Documents Department

A theoretical description of inhomogeneous turbulence

Description: This is the final report of a three-year, Laboratory-Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). In this LDRD, we have developed a highly compact and descriptive formalism that allows us to broach the theoretically formidable morass of inhomogeneous turbulence. Our formalism has two novel aspects: (a) an adaptation of helicity basis functions to represent an arbitrary incompressible channel flow and (b) the invocation of a hypothesis of random phase. A result of this compact formalism is that the mathematical description of inhomogeneous turbulence looks much like that of homogeneous turbulence--at the moment, the most rigorously explored terrain in turbulence research. As a result, we can explore the effect of boundaries on such important quantities as the gradients of mean flow, mean pressure, triple-velocity correlations and pressure velocity correlations, all of which vanish under the conventional, but artificial, assumption that the turbulence is statistically spatially uniform. Under suitable conditions, we have predicted that a mean flow gradient can develop even when none is initially present.
Date: March 1, 2000
Creator: Turner, L.
Partner: UNT Libraries Government Documents Department

Demonstration of Tokamak Ohmic Flux Saving by Transient Coaxial Helicity Injection on NSTX

Description: Transient Coaxial Helicity Injection (CHI) started discharges in NSTX have attained peak currents up to 300 kA and when these discharges are coupled to induction, it has produced up to 200 kA additional current over inductive-only operation. CHI in NSTX has shown to be energetically quite efficient, producing a plasma current of about 10 A/Joule of capacitor bank energy. In addition, for the first time, the CHI produced toroidal current that couples to induction continues to increase with the energy supplied by the CHI power supply at otherwise similar values of the injector flux, indicating the potential for substantial current generation capability by CHI in NSTX and in future toroidal devices. __________________________________________________
Date: April 23, 2010
Creator: Raman, R.; Mueller, D.; Nelson, B. A.; Jarboe, T. R.; Gerhardt, S.; Kugel, H. W. et al.
Partner: UNT Libraries Government Documents Department

Determining How Magnetic Helicity Injection Really Works

Description: OAK-B135 The goal of the Caltech program is to determine how helicity injection works by investigating the actual dynamics and topological evolution associated with magnetic relaxation. A new coaxial helicity injection source has been constructed and brought into operation. The key feature of this source is that it has maximum geometric simplicity. Besides being important for fusion research, this work also has astrophysical implications. Photos obtained using high-speed cameras show a clear sequence of events in the formation process. In particular, they show initial merging/reconnection processes, jet-like expansion, kinking, and separation of the plasma from the source. Various diagnostics have been developed, including laser induced fluorescence and soft x-ray detection using high speed diodes. Gas valves have been improved and a patent disclosure relating to puffed gas valves has been filed. Presentations on this work have been given in the form of invited talks at several university physics departments that were previously unfamiliar with laboratory plasma experiments.
Date: October 9, 2001
Creator: Bellan, Paul M.
Partner: UNT Libraries Government Documents Department

Observation of Persistent Edge Current Driven by Coaxial Helicity Injection (CHI)

Description: Coaxial Helicity Injection, CHI, has been used on the National Spherical Torus Experiment (NSTX) [M. Ono et al., Nucl. Fusion 40 (2000) 557], the Helicity Injected Torus (HIT) [B.A. Nelson et al., Phys. Rev. Lett. 72 (1994) 3666] and HIT-II [T.R. Jarboe et al., Phys. Plasmas 5 (1998) 1807] to initiate plasma and to drive up to 400 kA of toroidal current. On HIT-II, CHI initiated discharges have been successfully coupled to ohmic sustainment [R. Raman et al., Phys. Plasmas 11 (2004) 2565]. This paper presents the first results on the use of CHI to demonstrate the persistence of edge current drive in a preestablished single-null diverted inductive discharge. Edge current drive has the potential to improve plasma stability limits [J.E. Menard et al., Nucl. Fusion 37 (1997) 595]. While most current drive methods drive current in the interior of the plasma, CHI is well suited for driving current in the edge plasma.
Date: April 21, 2005
Creator: Mueller, D.; Nelson, B.A.; Hamp, W.T.; Redd, A.J.; Jarboe, T.R.; O'Neill, R.G. et al.
Partner: UNT Libraries Government Documents Department

Nucleon Resonance Electrocouplings from the CLAS Data on Exclusive Meson Electroproduction off Protons

Description: {gamma}{sub v}NN* transition helicity amplitudes (electrocouplings) of several prominent excited proton states are determined for the first time in independent analyses of {pi}{sup +}n, {pi}{sup 0}p, and {pi}{sup +}{pi}{sup -}p electroproduction off protons. Consistent results on resonance electrocouplings obtained from major meson electroproduction channels offer an evidence for reliable extraction of these fundamental quantities. Analysis of {pi}{sup +}{pi}{sup -}p electroproduction has extended considerably information on electrocouplings of high lying N* states, which decay preferentially to the N{pi}{pi} final states.
Date: December 1, 2011
Creator: Victor I. Mokeev, Inna G. Aznauryan, Volker D. Burkert
Partner: UNT Libraries Government Documents Department

A simple relation between the gamma N -> N(1535) helicity amplitudes

Description: It is shown that the helicity amplitudes A{sub 1/2} and S{sub 1/2} in the {gamma}N {yields} N(1535) reaction, can be well related by S{sub 1/2} = {radical}1+{tau}/{radical}2 M{sub S}{sup 2}-M{sup 2}/2M{sub S}Q A{sub 1/2} in the region Q{sup 2} > 2 GeV{sup 2}, where M and M{sub S} are the nucleon and N(1535) masses, q{sup 2} = -Q{sup 2} the four-momentum transfer squared, and {tau} = Q{sup 2}/(M{sub S} + M){sup 2}. This follows from the fact that the Pauli-type transition form factor F*{sub 2} extracted from the experimental data, turns up to show F*{sub 2} {approx_equal} 0 for Q{sup 2} > 1.5 GeV{sup 2}. The observed relation is tested by the experimental data and the MAID parametrization. A direct consequence of the relation is that the assumption,|A{sub 1/2}| >> |S{sub 1/2}|, is not valid for high Q{sup 2}. Instead, both amplitudes A{sub 1/2} and S{sub 1/2} have the same Q{sup 2} dependence in the high Q{sup 2} region, aside from that S{sub 1/2} has an extra factor, - 1/{radical} M{sub S}-M/2M{sub s}. The origin of this relation is interpreted in a perspective of a quark model.
Date: September 1, 2011
Creator: Gilberto Ramalho, Kazuo Tsushima
Partner: UNT Libraries Government Documents Department

Electroproduction of the Roper Resonance as a Hybrid State

Description: The Q^2 dependence of the helicity amplitudes for the Roper resonance as a hybrid state is presented.Our study shows that the magnitude of the transverse helicity amplitude decreases rapidly as Q^2 increases, while the longitudinal helicity amplitude vanishes for the hybrid state.This feature is quite different from the prediction of the q^3 potential quark model, in which the Roper resonance is assumed to be an orbitally excited state.The comparison with data shows that the hybrid interpretation of the Roper is favoured.Future experiments at CEBAF can provide the information needed to definitely determine the spin-flavour content of this resonance.
Date: August 1, 1991
Creator: Burkert, Volker; Li, Zhenping & Li, Zhujun
Partner: UNT Libraries Government Documents Department

Measurement of the N --> Delta^+ (1232) Transition at High-Momentum Transfer by pi^0 Electroproduction

Description: We report a new measurement of the exclusive electroproduction reaction gamma*_p --> pi0_p to explore the evolution from soft nonperturbative physics to hard processes via the Q2 dependence of the magnetic (M1+), electric (E1+), and scalar (S1+) multipoles in the N --> Delta transition. 9000 differential cross section data points cover W from threshold to 1.4 GeV/c2, 4pi center-of-mass solid angle, and Q2 from 3 to 6 GeV2/c2, the highest yet achieved. It is found that the magnetic form factor G^*M decreases with Q2 more steeply than the proton magnetic form factor, the ratio E1+/M1+ is small and negative, indicating strong helicity nonconservation, and the ratio S1+/M1+ is negative, while its magnitude increases with Q2.
Date: September 1, 2006
Creator: Ungaro, M.; Stoler, P.; Aznauryan, I.; Burkert, V. D.; Joo, K.; Smith, L. C. et al.
Partner: UNT Libraries Government Documents Department

The Search for Reconnection and Helicity During Formation of a Bounded Spheromak

Description: Recent results from investigations using insertable magnetic probes at the Sustained Spheromak Physics Experiment (SSPX) [E. B. Hooper et al., Nucl. Fusion 39, 863 (1999)] are presented. Experiments were carried out during pre-programmed, constant amplitude coaxial gun current pulses, where magnetic field increases stepwise with every pulse, but eventually saturates. Magnetic traces from the probe, which is electrically isolated from the plasma and spans the flux conserver radius, indicate there is a time lag at every pulse between the response to the current rise in the open flux surfaces (intercepting the electrodes) and the closed flux surfaces (linked around the open ones). This is interpreted as the time to buildup enough helicity in the open flux surfaces before reconnecting and merging with the closed ones. Future experimental and diagnostic plans to directly estimate the helicity in the open flux surfaces and measure reconnection are briefly discussed.
Date: April 30, 2007
Creator: Romero-Talamas, C A; McLean, H S; Hooper, E B; Wood, R D; LoDestro, L L & Moller, J M
Partner: UNT Libraries Government Documents Department

Magnetic Reconnection in the Spheromak: Physics and Consequences

Description: Magnetic reconnection in the spheromak changes magnetic topology by conversion of injected toroidal flux into poloidal flux and by magnetic surface closure (or opening) in a slowly decaying spheromak. Results from the Sustained Spheromak Physics Experiment, SSPX, are compared with resistive MHD simulations using the NIMROD code. Voltage spikes on the SSPX gun during spheromak formation are interpreted as reconnection across a negative-current layer close to the mean-field x-point. Field lines are chaotic during these events, resulting in rapid electron energy loss to the walls and the low T{sub e} < 50 eV seen in experiment and simulation during strong helicity injection. Closure of flux surfaces (and high T{sub e}) can occur between voltage spikes if they are sufficiently far apart in time; these topology changes are not reflected in the impedance of the axisymmetric gun. Possible future experimental scenarios in SSPX are examined in the presence of the constraints imposed by reconnection physics.
Date: February 28, 2006
Creator: Hooper, E B; Cohen, B I; Hill, D N; LoDestro, L L; McLean, H S; Romero-Talamas, C A et al.
Partner: UNT Libraries Government Documents Department

Initial Results from Coaxial Helicity Injection Experiments in NSTX

Description: Coaxial Helicity Injection (CHI) has been investigated on the National Spherical Torus Experiment (NSTX). Initial experiments produced 130 kA of toroidal current without the use of the central solenoid. The corresponding injector current was 20 kA. Discharges with pulse lengths up to 130 ms have been produced.
Date: May 10, 2001
Creator: Raman, R.; Jarboe, T.R.; Mueller, D.; Schaffer, M.J.; Maqueda, R.; Nelson, B.A. et al.
Partner: UNT Libraries Government Documents Department

Noninductive Current Generation in NSTX using Coaxial Helicity Injection

Description: Coaxial Helicity Injection (CHI) on the National Spherical Torus Experiment (NSTX) has produced 240 kA of toroidal current without the use of the central solenoid. Values of the current multiplication ratio (CHI produced toroidal current/injector current) up to 10 were obtained, in agreement with predictions. The discharges which lasted for up to 200 ms, limited only by the programmed waveform, are more than an order of magnitude longer in duration that any CHI discharges previously produced in a Spheromak or a Spherical Torus (ST).
Date: May 10, 2001
Creator: Raman, R.; Jarboe, T.R.; Mueller, D.; Schaffer, M.J.; Maqueda, R.; Nelson, B.A. et al.
Partner: UNT Libraries Government Documents Department

Initial Physics Results From the National Spherical Torus Experiment

Description: The mission of the National Spherical Torus Experiment (NSTX) is to extend the understanding of toroidal physics to low aspect ratio (R/a approximately equal to 1.25) in low collisionality regimes. NSTX is designed to operate with up to 6 MW of High Harmonic Fast Wave (HHFW) heating and current drive, 5 MW of Neutral Beam Injection (NBI) and Co-Axial Helicity Injection (CHI) for non-inductive startup. Initial experiments focused on establishing conditions that will allow NSTX to achieve its aims of simultaneous high-bt and high-bootstrap current fraction, and to develop methods for non-inductive operation, which will be necessary for Spherical Torus power plants. Ohmic discharges with plasma currents up to 1 MA and with a range of shapes and configurations were produced. Density limits in deuterium and helium reached 80% and 120% of the Greenwald limit respectively. Significant electron heating was observed with up to 2.3 MW of HHFW. Up to 270 kA of toroidal current for up to 200 msec was produced noninductively using CHI. Initial NBI experiments were carried out with up to two beam sources (3.2 MW). Plasmas with stored energies of up to 140 kJ and bt =21% were produced.
Date: January 3, 2001
Creator: Kaye, S.M.; Bell, M.G.; Bell, R.E. & Bialek, J.
Partner: UNT Libraries Government Documents Department

Probing the Structure of Nucleons in the Resonance Region with CLAS at Jefferson Lab

Description: The physics of electromagnetic excitation of nucleon resonances and of their relevance in nucleon structure studies are discussed. Preliminary data from the CLAS detector on the N-Delta(1232) transition multipoles, the helicity amplitudes of the N*(1535), and the search for so-called ''missing resonances'' at Jefferson Lab are presented.
Date: March 1, 2000
Creator: Burkert, Volker D.
Partner: UNT Libraries Government Documents Department

Gun requirements to achieve high field spheromaks

Description: It is shown that a gun similar to that in the SSPX could demonstrate the high fields required for Pulsed Spheromak reactors merely by prolonging the pulse. Important considerations are choosing the voltage to exceed ohmic losses; designing the gun to avoid wasteful short-circuiting of current within the gun; and the injection efficiency factor, f, determined by the ''sag'' in the profile of {lambda} = {mu}{sub o}j/B. Typically f = 0.75 in experiments, giving an overall efficiency > 50 % if short-circuiting is avoided. Theoretical transport models agree qualitatively with the need for a finite gradient in h to pump in helicity by current-driven tearing modes and suggest that pressure-driven resistive modes would not compete with current-driven modes during a buildup to ohmic ignition.
Date: March 4, 1999
Creator: Fowler, T K
Partner: UNT Libraries Government Documents Department