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Dive into the research topics where T. M. O’Neil is active.

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Featured researches published by T. M. O’Neil.


Physics of Fluids | 2000

Inviscid damping of asymmetries on a two-dimensional vortex

David Anton Schecter; Daniel H. E. Dubin; A. C. Cass; C. F. Driscoll; I. M. Lansky; T. M. O’Neil

The inviscid damping of an asymmetric perturbation on a two-dimensional circular vortex is examined theoretically, and with an electron plasma experiment. In the experiment, an elliptical perturbation is created by an external impulse. After the impulse, the ellipticity (quadrupole moment) of the vortex exhibits an early stage of exponential decay. The measured decay rate is in good agreement with theory, in which the perturbation is governed by the linearized Euler equations. Often, the exponential decay of ellipticity is slow compared to a vortex rotation period, due to the excitation of a quasimode. A quasimode is a vorticity perturbation that behaves like a single azimuthally propagating wave, which is weakly damped by a resonant interaction with corotating fluid. Analytically, the quasimode appears as a wave packet of undamped continuum modes, with a sharply peaked frequency spectrum, and it decays through interference as the modes disperse. When the exponential decay rate of ellipticity is comparabl...


Physics of fluids. B, Plasma physics | 1991

Guiding center atoms: Three-body recombination in a strongly magnetized plasma

Michael E. Glinsky; T. M. O’Neil

The three‐body recombination rate is calculated for an ion introduced into a magnetically confined, weakly correlated, and cryogenic pure electron plasma. The plasma is strongly magnetized in the sense that the cyclotron radius for an electron rce=(kBTe/me)1/2/Ωce is small compared to the classical distance of closest approach b=e2/kBTe, where Te is the electron temperature and Ωce=eB/mec is the electron cyclotron frequency. Since the recombination rate is controlled by a kinetic bottleneck a few kBTe below ionization, the rate may be determined by considering only the initial cascade through states of electron‐ion pairs with separation of order b. These pairs may be described as guiding center atoms since the dynamics is classical and treatable with the guiding center drift approximation. In this paper, an ensemble of plasmas characterized by guiding center electrons and stationary ions is described with the BBGKY hierarchy. Under the assumption of weak electron correlation, the hierarchy is reduced to a...


Physics of fluids. B, Plasma physics | 1990

NONAXISYMMETRIC THERMAL EQUILIBRIA OF A CYLINDRICALLY BOUNDED GUIDING-CENTER PLASMA OR DISCRETE VORTEX SYSTEM

Ralph A. Smith; T. M. O’Neil

The thermal equilibria of a two‐dimensional guiding‐center model for a single‐species plasma bounded by a cylindrical conductor are considered in the microcanonical ensemble. The same description applies to identical point vortices in a two‐dimensional, ideal fluid surrounded by a circular streamline. The statistically dominant configurations are displaced asymmetrically from the axis, for sufficiently large energies at specified canonical angular momentum. The transition between symmetric and asymmetric states resembles a second‐order phase transition, and occurs at negative temperatures. It is related to a bifurcation in the mean‐field (Vlasov) description. The theory is compared with Monte Carlo simulations of microcanonical ensembles of guiding centers.


Physics of Plasmas | 2006

Excitation of nonlinear electron acoustic waves

Francesco Valentini; T. M. O’Neil; Daniel H. E. Dubin

A particle in cell (PIC) simulation is used to investigate the excitation of electron acoustic waves (EAWs) and the stability of the EAWs against decay. An EAW is a nonlinear wave with a carefully tailored trapped particle population, and the excitation process must create the trapped particle population. For a collisionless plasma, successful excitation occurs when a relatively low amplitude driver that is spatially and temporally resonant with the EAW is applied for a sufficiently long time (many trapping periods). The excited EAW rings at a nearly constant amplitude long after the driver is turned off, provided the EAW has the largest wavelength that fits in the simulation domain. Otherwise, the excited EAW decays to a longer wavelength EAW. In phase space, this decay to longer wavelength appears as a tendency of the vortex-like trapped particle populations to merge. In a collisional plasma, successful excitation of an EAW requires the driver amplitude to exceed a threshold value. The period for a trap...


Physics of Plasmas | 1998

Thermal equilibria and thermodynamics of trapped plasmas with a single sign of charge

T. M. O’Neil; Daniel H. E. Dubin

Plasmas consisting exclusively of particles with a single sign of charge (e.g., pure electron plasmas and pure ion plasmas) can be confined by static electric and magnetic fields (e.g., in a Penning trap) and also be in a state of global thermal equilibrium. This important property distinguishes these totally un-neutralized plasmas from neutral and quasineutral plasmas. This paper reviews the conditions for and structure of the thermal equilibrium states and then develops a thermodynamic theory of the trapped plasmas. Thermodynamics provides hundreds of general relations (Maxwell relations) between partial derivatives of thermodynamic variables with respect to one another. Thermodynamic inequalities place general and useful bounds on various quantities. General and relatively simple expressions are provided for fluctuations of the thermodynamic variables. In practice, trapped plasmas are often made to evolve through a sequence of thermal equilibrium states through the slow addition (or subtraction) of ene...


Physics of Plasmas | 2009

Electron acoustic waves in pure ion plasmas

F. Anderegg; C. F. Driscoll; Daniel H. E. Dubin; T. M. O’Neil; Francesco Valentini

Standing electron acoustic waves (EAWs) are observed in a pure ion plasma. EAWs are slow nonlinear plasma waves; at small amplitude their phase velocities (vph≃1.4v¯ for small kλD) and their frequencies are in agreement with theory. At moderate amplitude, EAW-type plasma waves can be excited over a broad range of frequencies. This frequency variability comes from the plasma adjusting its velocity distribution so as to make the plasma mode resonant with the drive frequency. Wave-coherent laser-induced fluorescence shows the intimate nature of the wave-particle interaction, and how the particle distribution function is modified by the wave driver until the plasma mode is resonant with the driver.


Physics of Plasmas | 1999

Theory of asymmetry-induced transport in a non-neutral plasma

D. L. Eggleston; T. M. O’Neil

Radial transport produced by static nonaxisymmetric fields is thought to limit the confinement of non-neutral plasmas and experiments with applied asymmetries have verified that such fields do produce transport. A theoretical model of such transport is presented which is appropriate for long, thin plasmas. The theory allows for asymmetries with nonzero frequency and includes the linear collective response to applied wall voltages. For the regime where the effective collision frequency is large, the asymmetry-induced radial particle flux is derived from the drift kinetic/Poisson equations including collisions. For low collision frequencies a heuristic derivation is given. In both regimes the resulting transport is dominated by particles that move in resonance with the asymmetry. Possible applications of the theory to several experiments are discussed.


Physics of fluids. B, Plasma physics | 1992

Stability theorem for off‐axis states of a non‐neutral plasma column

T. M. O’Neil; Ralph A. Smith

A sufficient condition is given for the stability of a long non‐neutral plasma column that obeys two‐dimensional E×B dynamics. The column is confined by a uniform magnetic field and bounded by a conducting cylinder aligned with the field. The variational approach used here generalizes the well‐known stability of a centered, axisymmetric column, whose density is a monotonically decreasing function of radius. Displacement of such a column away from the axis by excitation of an l=1 diocotron mode yields a dynamical equilibrium stationary in a frame rotating with the mode. This new equilibrium is shown to be stable if the column is not too large. The analysis may explain, in part, the remarkable longevity observed for l=1 diocotron modes in experiments.


NON-NEUTRAL PLASMA PHYSICS IV: Workshop on Non-Neutral Plasmas | 2002

Numerical Simulation of Ultracold Plasmas

Stanislav G. Kuzmin; T. M. O’Neil

In recent experiments, ultracold neutral plasmas were produced by photoionizing small clouds of laser-cooled atoms. It has been suggested that the low initial temperature of these novel plasmas leads directly to strong correlation and order. In contrast, we argue that rapid intrinsic heating raises the electron temperature to the point where strong correlation cannot develop. The argument is corroborated by a molecular dynamics simulation of the early time plasma evolution.


Physics of Plasmas | 1994

Stability theorem for a single species plasma in a toroidal magnetic configuration

T. M. O’Neil; R. A. Smith

A stability theorem is developed for a single species plasma that is confined by a purely toroidal magnetic field. A toroidal conductor is assumed to bound the confinement region, and frequencies are ordered so that the cyclotron action and the toroidal action for each particle are good adiabatic invariants. The cross‐field motion is described by toroidal‐average drift dynamics. In this situation, it is possible to find plasma equilibria for which the energy is a maximum, as compared to all neighboring states that are accessible under general constraints on the dynamics. Since the energy is conserved, such states must be stable to small‐amplitude perturbations. This theorem is developed formally using Arnold’s method, and examples of stable equilibria are obtained.

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C. F. Driscoll

University of California

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F. Anderegg

University of California

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Ralph A. Smith

University of California

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A. C. Cass

University of California

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E.M. Hollmann

University of California

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