162 research outputs found
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Ordered one-component plasmas: Phase transitions, normal modes, large systems, and experiments in a storage ring
The property of cold one-component plasmas, confined by external forces, to form an ordered array has been known for some time both from simulations and from experiment. The purpose of this talk is to summarize some recent work on simulations and some new experimental results. The author discusses some experimental work on real storage rings, magnetic storage devices in which partials circulate with large kinetic energies and for which laser cooling is used on partially ionized ions to attain temperatures ten or more orders of magnitude lower than their kinetic energies
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The physics of crystalline beams
It seems that the time has come in the pursuit of lower and lower beam temperatures to start focusing more detailed attention to the reality of storage rings--conventional cooling techniques and measures of temperature are generally not the appropriate ones at the lowest temperatures. Finding solutions to these serious problems does not appear to be impossible, but these considerations must be kept in mind in designing new storage rings with the aim to approach the regime of ordered three-dimensional beams. In particular, such rings will have to: Use calculations of the lattice with the full effects of space charge included. (N.B. averaged over time, space charge exactly cancels the focusing fields for a cold beam and therefore must be explicitly included.) Find technical solutions and incorporate several of; cooling to introduce a longitudinal velocity gradient and favor constant angular velocity; high multiplicity in bending and focusing elements; stronger focusing (high betatron tune); and high symmetry in the ring design. Finally, simulations should try to incorporate as much realism as possible, with larger repeating cells and more detailed descriptions of the lattice
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Order in very cold confined plasmas
The study of the structure and dynamic properties of classical systems of charged particles confined by external forces, and cooled to very low internal energies, is the subject of this talk. An infinite system of identical charged particles has been known for some time to form a body-centered cubic lattice and is a simple classical prototype for condensed matter. Recent technical developments in storage rings, ion traps, and laser cooling of ions, have made it possible to produce such systems in the laboratory, though somewhat modified because of their finite size. I would like to discuss what one may expect in such systems and also show some examples of experiments. If we approximate the potential of an ion trap with an isotropic harmonic force F = {minus}Kr then the Hamiltonian for this collection of ions is the same as that for J. J. Thomson`s ``plum pudding`` model of the atom, where electrons were thought of as discrete negative charges imbedded in a larger, positive, uniformly charged sphere. The harmonic force macroscopically is canceled by the average space-charge forces of the plasma-, and this fixes the overall radius of the distribution. What remains, are the residual two-body Coulomb interactions that keep the particles within the volume as nearly equidistant as possible in order to minimize the potential energy. The configurations obtained for the minimum energy of small ionic systems [2] in isotropic confinement are shown in figure 1. Indeed this is an `Exotic Atom` and fits well into the subject of this symposium honoring the 60th birthday of Professor Toshi Yamazaki
Stroboscopic Laser Diagnostics for Detection of Ordering in One-Dimensional Ion beam
A novel diagnostic method for detecting ordering in one-dimensional ion beams
is presented. The ions are excited by a pulsed laser at two different positions
along the beam and fluorescence is observed by a group of four
photomultipliers. Correlation in fluorescence signals is firm indication that
the ion beam has an ordered structure.Comment: 7 pages, REVTEX, fig3 uuencoded, figs 1-2 available upon request from
[email protected], to appear in Phys. Rev.
Searches for Stable Strangelets in Ordinary Matter: Overview and a Recent Example
Our knowledge on the possible existence in nature of stable exotic particles
depends solely upon experimental observation. Guided by this general principle
and motivated by theoretical hypotheses on the existence of stable particles of
strange quark matter, a variety of experimental searches have been performed.
We provide an introduction to the theoretical hypotheses, an overview of the
past searches, and a more detailed description of a recent search for
helium-like strangelets in the Earth's atmosphere using a sensitive laser
spectroscopy method
Search for sharp lines in e+-e- coincidences from positrons on Th
Two mini-orange spectrometers were used to view e+-e- pairs from a positron source and a Th scatterer from opposite directions, 180° apart. No statistically significant evidence for peaks previously reported was found in the coincidence spectra
Structural phase transitions in multipole traps
A small number of laser-cooled ions trapped in a linear radiofrequency
multipole trap forms a hollow tube structure. We have studied, by means of
molecular dynamics simulations, the structural transition from a double ring to
a single ring of ions. We show that the single-ring configuration has the
advantage to inhibit the thermal transfer from the rf-excited radial components
of the motion to the axial component, allowing to reach the Doppler limit
temperature along the direction of the trap axis. Once cooled in this
particular configuration, the ions experience an angular dependency of the
confinement if the local adiabaticity parameter exceeds the empirical limit.
Bunching of the ion structures can then be observed and an analytic expression
is proposed to take into account for this behaviour
Isovector and isoscalar superfluid phases in rotating nuclei
The subtle interplay between the two nuclear superfluids, isovector T=1 and
isoscalar T=0 phases, are investigated in an exactly soluble model. It is shown
that T=1 and T=0 pair-modes decouple in the exact calculations with the T=1
pair-energy being independent of the T=0 pair-strength and vice-versa. In the
rotating-field, the isoscalar correlations remain constant in contrast to the
well known quenching of isovector pairing. An increase of the isoscalar (J=1,
T=0) pair-field results in a delay of the bandcrossing frequency. This
behaviour is shown to be present only near the N=Z line and its experimental
confirmation would imply a strong signature for isoscalar pairing collectivity.
The solutions of the exact model are also discussed in the
Hartree-Fock-Bogoliubov approximation.Comment: 5 pages, 4 figures, submitted to PR
Colossal magnetooptical conductivity in doped manganites
We show that the current carrier density collapse in doped manganites, which
results from bipolaron formation in the paramagnetic phase, leads to a colossal
change of the optical conductivity in an external magnetic field at
temperatures close to the ferromagnetic transition. As with the colossal
magnetoresistance (CMR) itself, the corresponding magnetooptical effect is
explained by the dissociation of localized bipolarons into mobile polarons
owing to the exchange interaction with the localized Mn spins in the
ferromagnetic phase. The effect is positive at low frequencies and negative in
the high-frequency region. The present results agree with available
experimental observations.Comment: 4 pages, REVTeX 3.0, two eps-figures included in the tex
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