Jefferson Laboratory's kW-level infrared free-electron laser utilizes a superconducting accelerator that recovers about 75% of the electron-beam power. In achieving first lasing, the accelerator operated "straight ahead" to deliver 38-MeV, 1.1-mA cw current for lasing near 5 &mgr;m. The waste beam was sent directly to a dump while producing stable operation at up to 311 W. Utilizing the recirculation loop to send the electron beam back to the linac for energy recovery, the machine has now recovered cw average currents up to 5 mA, and has lased cw with up to 1720 W output at 3.1 &mgr;m.

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We analyze N-body simulations of halo mergers to investigate the mechanisms responsible for driving mixing in phase space and the evolution to dynamical equilibrium. We focus on mixing in energy and angular momentum and show that mixing occurs in a steplike fashion following pericenter passages of the halos. This makes mixing during a merger unlike other well-known mixing processes such as phase mixing and chaotic mixing, whose rates scale with local dynamical time. We conclude that the mixing process that drives the system to equilibrium is primarily a response to energy and angular momentum redistribution that occurs due to impulsive tidal shocking and dynamical friction rather than a result of chaotic mixing in a changing potential. We also analyze the merger remnants to determine the degree of mixing at various radii by monitoring changes in radius, energy, and angular momentum of particles. We confirm previous findings that show that the majority of particles retain strong memory of their original kinetic energies and angular momenta, but do experience changes in their potential energies owing to the tidal shocks they experience during pericenter passages. Finally, we show that a significant fraction of mass (%40%) in the merger remnant lies outside its formal virial radius, and that this matter is ejected roughly uniformly from all radii outside the inner regions. This highlights the fact that mass, in its standard virial definition, is not additive in mergers. We discuss the implications of these results for our understanding of relaxation in collisionless dynamical systems.

The formation of beam halos has customarily been described in terms of a particle-core model in which the space-charge field of the oscillating core drives particles to large amplitudes. This model involves parametric resonance and predicts a hard upper bound to the orbital amplitude of the halo particles. We show that the presence of colored noise due to space-charge fluctuations and/or machine imperfections can eject particles to much larger amplitudes than would be inferred from parametric resonance alone.

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Phase mixing of chaotic orbits exponentially distributes these orbits through their accessible phase space. This phenomenon, commonly called ''chaotic mixing,'' stands in marked contrast to phase mixing of regular orbits which proceeds as a power law in time. It is operationally irreversible; hence, its associated e-folding time scale sets a condition on any process envisioned for emittance compensation. A key question is whether beams can support chaotic orbits, and if so, under what conditions? We numerically investigate the parameter space of three-dimensional thermal-equilibrium beams with space charge, confined by linear external focusing forces, to determine whether the associated potentials support chaotic orbits. We find that a large subset of the parameter space does support chaos and, in turn, chaotic mixing. Details and implications are enumerated.

Beams in which space charge forces are stronger than the force from thermal pressure are nonneutral plasmas, since particles interact mostly via the long-range collective potential. An ever-increasing number of applications demand such high-brightness beams. The University of Maryland Electron Ring ͓P. G. O'Shea et al., Nucl. Instrum Methods Phys. Res. A 464, 646 ͑2001͔͒, currently under construction, is designed for studying the physics of space-charge-dominated beams. Indirect ways of measuring beam emittance near the UMER source produced conflicting results, which were resolved only when a direct measurement of phase space indicated a hollow velocity distribution. Comparison to self-consistent simulation using the particle-in-cell code WARP ͓D. P. Grote et al., 193 ͑1996͔͒ revealed sensitivity to the initial velocity distribution. Since the beam is born with nonuniformities and granularity, dissipation mechanisms and rates are of interest. Simulations found that phase mixing by means of chaotic particle orbits is possible in certain situations, and proceeds much faster than Landau damping. The implications for using beams to model other N-body systems are discussed.

This paper focuses on the dynamical implications of close supermassive black hole binaries both as an example of resonant phase mixing and as a potential explanation of inversions and other anomalous features observed in the luminosity profiles of some elliptical galaxies. The presence of a binary comprised of black holes executing nearly periodic orbits leads to the possibility of a broad resonant coupling between the black holes and various stars in the galaxy. This can result in efficient chaotic phase mixing and, in many cases, systematic increases in the energies of stars and their consequent transport towards larger radii. Allowing for the presence of a supermassive black hole binary with plausible parameter values near the center of a spherical, or nearly spherical, galaxy characterised initially by a Nuker density profile enables one to reproduce in considerable detail the central surface brightness distributions of such galaxies as NGC 3706.Comment: 14 pages plus 15 figures, revised and condensed version to appear in Astrophysical Journa