Search Results (32 total)

First direct measurements of nonambipolar magnetic fluctuation-induced charge transport in the interior of a high-temperature plasma are reported. Global resistive tearing modes drive the charge transport which is measured in the vicinity of the resonant surface for the dominant core resonant mode. Finite charge transport has two important consequences. First, it generates a potential well along with locally strong electric field and electric field shear at the resonant surface. Second, this electric field induces a spontaneous E×B driven zonal flow.

The impurity ion temperature evolution has been measured during three types of impulsive reconnection events in the Madison Symmetric Torus reversed field pinch. During an edge reconnection event, the drop in stored magnetic energy is small and ion heating is observed to be limited to the outer half of the plasma. Conversely, during a global reconnection event the drop in stored magnetic energy is large, and significant heating is observed at all radii. For both kinds of events, the drop in magnetic energy is sufficient to explain the increase in ion thermal energy. However, not all types of reconnection lead to ion heating. During a core reconnection event, both the stored magnetic energy and impurity ion temperature remain constant. The results suggest that a drop in magnetic energy is required for ions to be heated during reconnection, and that when this occurs heating is localized near the reconnection layer.

The cause for sudden reconnection in reversed field pinch plasmas is determined experimentally for two cases: large reconnection events (the sawtooth crash) and small reconnection events during improved confinement. We measure the term in the MHD equations which represents the driving (or damping) of edge tearing modes due to the axisymmetric magnetic field. The term is negative for large reconnection events (the modes are stable, implying that reconnection may be driven by nonlinear coupling to other modes) and positive for small reconnection events (modes are unstable, reconnection is spontaneous).

The first experimental study of the MHD dynamo in a quasi-single-helicity (QSH) reversed-field pinch toroidal plasma is presented. In QSH plasmas, a dominant wave number appears in the velocity fluctuation spectrum. This velocity component extends throughout the plasma volume and couples with magnetic fluctuations to produce a significant MHD dynamo electric field. The narrowing of the velocity fluctuation spectrum and the single-mode character of the dynamo are features predicted by theory and computation, but only now are observed in experiment.

The fluctuation-induced Hall electromotive force, 〈δJ→×δB→〉/n_ee, is experimentally measured in the high-temperature interior of a reversed-field pinch plasma by a fast Faraday rotation diagnostic. It is found that the Hall dynamo effect is significant, redistributing (flattening) the equilibrium core current near the resonant surface during a reconnection event. These results imply that effects beyond single-fluid MHD are important for the dynamo and magnetic reconnection.

A sum-over-histories generalized quantum theory is developed for homogeneous minisuperspace type A Bianchi cosmological models, focusing on the particular example of the classically recollapsing Bianchi type-IX universe. The decoherence functional for such universes is exhibited. We show how the probabilities of decoherent sets of alternative, coarse-grained histories of these model universes can be calculated. We consider in particular the probabilities for classical evolution defined by a suitable coarse graining. For a restricted class of initial conditions and coarse grainings we exhibit the approximate decoherence of alternative histories in which the universe behaves classically and those in which it does not. For these situations we show that the probability is near unity for the universe to recontract classically if it expands classically. We also determine the relative probabilities of quasiclassical trajectories for initial states of WKB form, recovering for such states a precise form of the familiar heuristic “J⋅dΣ” rule of quantum cosmology, as well as a generalization of this rule to generic initial states.

Confinement of runaway electrons has been observed for the first time in a reversed field pinch during improved-confinement plasmas in the Madison Symmetric Torus. Energy-resolved hard-x-ray flux measurements have been used to determine the velocity dependence of the electron diffusion coefficient, utilizing computational solutions of the Fokker-Planck transport equation. With improved-confinement, the fast electron diffusivity drops by 2 orders of magnitude and is independent of velocity. This suggests a change in the transport mechanism away from stochastic magnetic field diffusion.

We report the magnetotransport properties of thin polycrystalline films of the recently discovered nonoxide perovskite superconductor MgCNi₃. CNi₃ precursor films were deposited onto sapphire substrates and subsequently exposed to Mg vapor at 700 °C. We report transition temperatures (T_c) and critical field values (H_c2) of MgCNi₃ films ranging in thickness from 7.5 nm to 100 nm. Films thicker than ≈40 nm have a T_c∼8 K, and an upper critical field H_c2(T=0)=14 T, which are both comparable to that of polycrystalline powders. Hall measurements in the normal state give a carrier density, n=-4.2×10²² cm^-3, that is approximately 4 times that reported for bulk samples.

Magnetic field fluctuations (and the associated current perturbation) have been measured in the core of a high-temperature reversed-field pinch using a newly developed fast-polarimetry system. Radial magnetic field fluctuation levels of ∼1% are measured in standard-reversed-field pinch discharges which increase to ∼4% during the sawtooth crash (enhanced dynamo). The fluctuation level is reduced fourfold for high-confinement plasmas where the core-resonant tearing modes are suppressed.

First measurements of the current-density profile in the core of a high-temperature reversed-field pinch are presented. The current-density profile is observed to peak during the sawtooth cycle and broaden promptly at the crash. This change in profile can be linked to magnetic relaxation and the dynamo which is predicted to drive antiparallel current in the plasma core. For high-confinement discharges, the dynamo is suppressed and the current-density profile is observed to strongly peak.

Improved confinement has been achieved in the MST through control of the poloidal electric field, but it is now known that the improvement has been limited by bursts of an edge-resonant instability. Through refined poloidal electric field control, plus control of the toroidal electric field, we have suppressed these bursts. This has led to a total beta of 15% and a reversed-field-pinch-record estimated energy confinement time of 10 ms, a tenfold increase over the standard value which for the first time substantially exceeds the confinement scaling that has characterized most reversed-field-pinch plasmas.

A cause of observed anomalous plasma momentum transport in a reversed-field pinch is determined experimentally. Magnetohydrodynamic theory predicts that nonlinear interactions involving triplets of tearing modes produce internal torques that redistribute momentum. Evidence for the nonlinear torque is acquired by detecting the correlation of momentum redistribution with the mode triplets, with the elimination of one of the modes in the triplet, and with the external driving of one of the modes.

A recent study conducted on the Madison Symmetric Torus reversed-field pinch has shown that control of density fluctuations can be achieved through modification of the current density profile. Most of the power in the density fluctuations is directly associated with core-resonant resistive tearing modes. We report that, during auxiliary current drive experiments, these density fluctuations are reduced about an order of magnitude over the entire plasma cross section and the resulting electron confinement is increased eightfold.

We report an increase in particle confinement with plasma biasing in a reversed field pinch. Miniature plasma sources are used as electrodes to negatively bias the plasma at the edge (r/a∼0.9). Particle content increases and H_α radiation decreases upon application of bias and global particle confinement roughly doubles as a result. Energy confinement is not significantly affected by biasing the edge. Measurements of plasma potential, impurity flow, and floating potential fluctuations indicate that strong flows are produced and that electrostatic fluctuations are reduced.

Small-angle p-p, p-d, d-α, and α-α correlation functions were measured following the reaction ¹⁶O+²⁷Al at 40 MeV/nucleon ¹⁶O. These light charged particles (LCP’s) were measured with a closely packed hexagonal array of CsI detectors, located at 35°, with a center to center opening angle of 2.35° for adjacent detectors. Coincident particles were simultaneously detected in the NSCL 4π detector. This measurement was intended to be a complement to earlier results from the same system. Based on studies of this system at lower energies and other published correlation measurements, it was expected that at 40 MeV/nucleon there would be significant positive correlations from the nuclear force and deep anticorrelations from Coulomb repulsion. However, correlation functions from this higher energy are remarkably similar to those previously measured at ≈15 MeV/nucleon. Correlation functions formed from events with a high multiplicity or high total detected energy (central collisions) are not significantly different from the inclusive measurements. As a possible explanation we suggest that significant correlations are most readily seen in experiments sensitive to LCP’s from fast preequilibrium processes and that measurements at more backward angles are primarily sensitive to LCP’s from a longer-lived source formed after preequilibrium processes are done. This idea is supported by trends of p-p correlation functions from a wide range of systems. A schematic calculation based on a Boltzmann-Ueling-Uhlenbeck (BUU) model and statistical emission qualitatively reproduces the results from this work.

Collective radial flow of light fragments from ⁴⁰Ar+⁴⁵Sc reactions at beam energies between 35 and 115 MeV/nucleon has been investigated using the Michigan State University 4π Array. The mean transverse kinetic energy 〈E_t〉 of the different fragment types increases with event centrality and increases as a function of the incident beam energy. Comparison of our measured values of 〈E_t〉 shows agreement with predictions of Boltzmann-Uehling-Uhlenbeck model and WIX multifragmentation model calculations. The radial flow extracted from 〈E_t〉 accounts for approximately half of the emitted particle’s energy for the heavier fragments (Z≥4) at the highest beam energy studied. © 1996 The American Physical Society.

The impact parameter dependence of the disappearance of directed transverse flow has been investigated for ⁴⁰Ar+⁴⁵Sc reactions using the Michigan State University 4π Array upgraded with the High Rate Array (HRA). The energy at which collective transverse flow in the reaction plane disappears, the balance energy (E_bal), is found to increase approximately linearly as a function of impact parameter. Comparison of our measured values of E_bal(b) shows agreement with predictions of Quantum Molecular Dynamics (QMD) model calculations.

The extraction of source radii from heavy-ion collisions using coalescence models is explored. A new prescription is presented which considers nucleosynthesis via the coalescence of fragments which is, in particular, more appropriate for intermediate-energy collisions than the previous nucleon coalescence prescriptions. This fragment coalescence model provides an avenue for viewing the breakup stage of heavy-ion reactions that yields valuable complimentary information to two-nucleon and two-fragment correlation measurements. This model was applied to recent experimental data on central 55 and 115 MeV/nucleon ⁴⁰Ar+⁴⁵Sc collisions studied at midrapidity. Source radii are presented versus the transverse velocity for six different coalescence channels leading to charge one, two, and three fragments. Particular attention is paid to the temperature input in the model, as the extracted radii depend significantly on this parameter. A means of extracting the temperature from experimental data using the present model is described. Comparisons to the two temperature-dependent nucleon coalescence models will also be discussed.

We have experimentally studied small impact parameter heavy-ion collisions in the (nearly) symmetry entrance channels ¹²C+¹²C, ²⁰Ne+²⁷Al, ⁴⁰Ar+⁴⁵Sc, ⁸⁴Kr+⁹³Nb, and ¹²⁹Xe+¹³⁹La, each at many intermediate beam energies. The results from a number of analyses based on a projection of the ‘‘shapes’’ of the experimental events called the sphericity are presented. Comparisons of the relative efficiencies of various experimental methods for the selection of central events are made. The importance of autocorrelations between the sphericity and the various impact-parameter–dependent variables is evaluated. Searches for beam energy-dependent transitions from sequential binary disassembly to multifragmentation in the central events are described. Comparisons to dynamic and hybrid model code calculations will be discussed. The average sphericities of the intermediate mass fragments (IMF’s, for which 3≤Z≲20), are presented. The possibility that the IMF emission occurs following the formation of transient toroidal or disk-like geometries in the central events is explored. Increases in the average sphericities of the central events for increasing beam energies are observed which is attributed to transitions from sequential binary disassembly to multifragmentation. The transitional beam energies for the central ⁴⁰Ar+⁴⁵Sc, ⁸⁴Kr+⁹³Nb, and ¹²⁹Xe+¹³⁹La reactions are near ∼50, ∼40, and ∼40 MeV/nucleon, respectively.

The average multiplicities of intermediate mass fragments (IMFs) for central heavy-ion collisions in the (nearly) symmetric entrance channels ²⁰Ne+²⁷Al, ⁴⁰Ar+⁴⁵Sc, ⁸⁴Kr+⁹³Nb, and ¹²⁹Xe+¹³⁹La, are systematically studied over a wide range of intermediate beam energies. Cuts on experimental variables commonly assumed to be correlated with the impact parameter are used to select the most central collisions. The results for six different centrality variables are compared, and the extent to which measurements of the multiplicities of IMFs in small impact parameter collisions are affected by the variable used to select the central events is discussed. General methods for locating such ‘‘autocorrelations’’ are described. The two centrality observables that are the least autocorrelated with the number of intermediate mass fragments are identified, and these variables are used to select the most central collisions. The entrance channel mass and beam energy dependence of the experimental IMF multiplicities are presented and compared to a variety of model predictions. The models picturing the disassembly as a sequential binary process always underpredict the experimental IMF multiplicities. A generally more accurate reproduction of these multiplicities is provided by several similar chemical equilibrium models commonly assumed to be the theoretical description of multifragmentation.

The Z distributions of fragments emitted from central collisions of ⁴⁰Ar+⁴⁵Sc at beam energies from 15 to 115 MeV/nucleon have been fitted to power laws σ(Z)∝Z^-λ. The λ parameter reaches a minimum at a beam energy of 23.9±0.7 MeV/nucleon. A percolation model calculation reproduces the observed Z distributions for all beam energies, using the mean excitation energy as extracted from proton kinetic energy spectra. We extract the critical value of the deposited excitation energy for our system and make predictions for the dependence of this quantity on the size of the fragmenting system.

The disappearance of collective flow in nucleus-nucleus collisions occurs at an incident energy (E_bal) where the attractive scattering dominant at low energies balances the repulsive scattering dominant at high energies. We have performed the first systematic study of the entrance-channel mass dependence of the disappearance of flow and hence E_bal. The new data presented for the C+C, Ne+Al, Ar+Sc, and Kr+Nb systems show that E_bal scales as A^-1/3 where A is the mass of the combined system. Boltzmann-Uehling-Uehlenbeck model calculations show trends which are in qualitative agreement with these new results.

We have measured two-fragment reduced-velocity correlation functions of the intermediate mass fragments (IMF: 3≤Z≤7) produced in multifragment final states for the Kr+Nb system (E/A=35, 45, 55, 65, and 75 MeV). From the measured correlation functions we extract mean IMF emission lifetimes (τ) which are observed to decrease from τ≊400 fm/c at E/A=35 MeV to τ≊125 fm/c at E/A=55 MeV. For beam energies in excess of E/A=55 MeV, no further decrease in τ is observed, indicating a possible saturation of the mean emission lifetime for IMF produced in multifragment exit channels.

We present Z distributions for fragments with 1≤Z≤12 from central collisions of ⁴⁰ Ar+⁴⁵Sc at incident energies ranging from 35 to 115 MeV/nucleon. We find that the Z distributions can be described by a power law or an exponential and steepen with increasing incident energy. Over the range of incident energies studied, the average number of intermediate mass fragments decreases while the average number of particles increases. When combined with previous results for the charge distributions, a minimum is observed in the extracted power-law parameter.

Multifragment azimuthal correlation functions have been measured as a function of beam energy and impact parameter for the Ar+Sc system (E/A=35 to 115 MeV). The observed azimuthal correlation functions—which do not require corrections for dispersion of the reaction plane—exhibit strong asymmetries which are dependent on impact parameter and beam energy. Rotational collective motion and flow seem to dominate the correlation functions at low beam energies. It is proposed that multifragment azimuthal correlation functions can provide a useful probe for intermediate energy heavy ion reaction dynamics.