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The molecular dynamics simulations of Yukawa~i.e., screened-Coulomb! systems that were applied to the regime of weak screening in an earlier study @S. Hamaguchi, R. T. Farouki, and D. H. E. Dubin, J. Chem. Phys. 105, 7641~1996!# are extended to the strong screening regime. Transition temperatures at the fluid-solid phase boundary and the solid-solid phase boundary are obtained as functions of the screening parameter k5a/l D i.e., the ratio of the Wigner-Seitz radius a to the Debye length l D !. The resulting phase diagram also covers the triple point-the intersection of the fluid-solid and solid-solid phase boundaries-at k54.28 and G55.6 310 3 , where G is the ratio of the Coulomb potential energy to the kinetic energy per particle~i.e., G 5Q 2 /4pe 0 akT, where Q is the charge of each Yukawa particle and T is the system temperature!. Yukawa systems serve as models for plasmas and colloidal suspensions of charged particulates.

JWri.ffMNonlinear gyrokinetic equations are derived from a systematic Hamiltonian theory. The derivation employs Lie transforms and a noncanonical perturbation theory flrst used by Littlejohn for the simpler problem of asymptotically small gyroradius. For deflniteness, we emphasize the limit of electrostatic fluctuations in slab zeometry; however, there is a straight forward generalization to arbitrary field geometry and electromagnetic per turbations. An energy invariant for the nonlinear system is derived, and various of its limits are considered. The weak-turbulence theory of the equations is examined. In particular, the wave-kinetic equation of Galeev and Sagdeev is derived from an asystematic truncation of the equations, implying that this equation fails to consider all gyrokinetic effects. The equations are simplified for the case of small but finite gyroradius and put in a form suitable for efficient computer simulation. Although it is possi ble to derive the Terry-Horton and Hasegawa-Mima equations as limiting cases of our theory, several new nonlinear terms absent from conventional theories appear and are discussed. The resulting theory is very similar in content to the recent work of Lee. However, the systematic nature of our derivation provides considerable insight into the structure and interpreta tion of the equations. DISCLAIMERThii report WH prepared as u account of work sponsored by an agency of the United States Gorerament Neither lb« United Slates Government nor any agency thereof, nor any of their employees, makes any warranty, exprcu o r implied, or asunKS uy legal liability or responsibility for the eccancy, compkteacsi, or usefulness of Bay information, apparatus product, or proem disclosed, « represent* that iu aic would not infringe privately owned rights. Refereoce bereiii to any specific comntcrcisJ product, process, or service by traic lame, tradeaurk, manufacturer, or otherwise don ant acccaurily coutitate or issply iu endurremeat, noonmendatkie, or favoring by the United Slatm Government or any agency thereof. The views and opinioni of authon expressed herein do not aeceisarily aute or reflect those of the United States Government or any agency thereof. -1-E&rZW.X C-TS' S ;CJ';rC:iT IS UiiLIMTO PPPL-1969 DEB3 010846 -t-

In recent years, there has been a wealth of new science involving low-energy antimatter (i.e., positrons and antiprotons) at energies ranging from 10 2 to less than 10 −3 eV. Much of this progress has been driven by the development of new plasma-based techniques to accumulate, manipulate and deliver antiparticles for specific applications. This article focuses on the advances made in this area using positrons. However many of the resulting techniques are relevant to antiprotons as well. An overview is presented of relevant theory of single-component plasmas in electromagnetic traps. Methods are described to produce intense sources of positrons and to efficiently slow the typically energetic particles thus produced. Techniques are described to trap positrons efficiently and to cool and compress the resulting positron gases and plasmas. Finally, the procedures developed to deliver tailored pulses and beams (e.g., in intense, short bursts, or as quasi-monoenergetic continuous beams) for specific applications are reviewed. The status of development in specific application areas is also reviewed. One example is the formation of antihydrogen atoms for fundamental physics [e.g., tests of invariance under charge conjugation, parity inversion and time reversal (the CPT theorem), and studies of the interaction of gravity with antimatter]. Other applications discussed include atomic and materials physics studies and study of the electron-positron many-body system, including both classical electron-positron plasmas and the complementary quantum system in the form of Bose-condensed gases of positronium atoms. Areas of future promise are also discussed. The review concludes with a brief summary and a list of outstanding challenges.

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 moment͒ 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 comparable to the vortex rotation frequency, the vorticity perturbation does not resemble a quasimode; rather, it is rapidly dominated by spiral filaments. Over longer times, linear theory predicts algebraic decay of ellipticity; however, nonlinear oscillations of ellipticity emerge in the experiment before a transition to algebraic decay would occur.

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Transition inverse temperatures ͑or ⌫ values͒ at the fluid-solid phase boundary of Yukawa systems near the one-component-plasma ͑OCP͒ limit have been evaluated by molecular dynamics simulations. These values are systematically smaller than those obtained in an earlier study by Farouki and Hamaguchi ͓J. Chem. Phys. 101, 9885 ͑1994͔͒. The discrepancy is attributed to the fact that, in the earlier study, the harmonic entropy constants were approximated by that of the OCP, whereas the new results are based on more accurate harmonic entropy constants obtained from lattice-dynamics calculations. The new molecular dynamics simulations also confirm that the bccfcc phase transition curve is in good agreement with that of the quasiharmonic theory in the regime р1.4, where is the ratio of the Wigner-Seitz radius to the Debye length. Examples of Yukawa systems include dusty plasmas and colloidal suspensions.

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