1,860 research outputs found
A Simple Quantum Model of Ultracold Polar Molecule Collisions
We present a unified formalism for describing chemical reaction rates of
trapped, ultracold molecules. This formalism reduces the scattering to its
essential features, namely, a propagation of the reactant molecules through a
gauntlet of long-range forces before they ultimately encounter one another,
followed by a probability for the reaction to occur once they do. In this way,
the electric-field dependence should be readily parametrized in terms of a pair
of fitting parameters (along with a coefficient) for each asymptotic
value of partial wave quantum numbers . From this, the electric
field dependence of the collision rates follows automatically. We present
examples for reactive species such as KRb, and non-reactive species, such as
RbCs
Total angular momentum representation for atom-molecule collisions in electric fields
It is shown that the atom-molecule collision problem in the presence of an
external electric field can be solved using the total angular momentum
representation in the body-fixed coordinated frame, leading to a
computationally efficient method for ab initio modeling of low-temperature
scattering phenomena. Our calculations demonstrate rapid convergence of the
cross sections for vibrational and Stark relaxation in He-CaD collisions with
the number of total angular momentum states in the basis set, leading to a
5-100 fold increase in computational efficiency over the previously used
methods based on the fully uncoupled space-fixed representation. These results
open up the possibility of carrying out numerically converged quantum
scattering calculations on a wide array of atom-molecule collisions and
chemical reactions in the presence of electric fields.Comment: 19 pages, 3 figures, 1 tabl
Pair Wave Functions in Atomic Fermi Condensates
Recent experiments have observed condensation behavior in a strongly
interacting system of fermionic atoms. We interpret these observations in terms
of a mean-field version of resonance superfluidity theory. We find that the
objects condensed are not bosonic molecules composed of bound fermion pairs,
but are rather spatially correlated Cooper pairs whose coherence length is
comparable to the mean spacing between atoms. We propose experiments that will
help to further probe these novel pairs
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Radiofrequency superconductivity applied to free-electron lasers
Low wall losses and low wakefields inherent in superconducting radiofrequency (srf) cavities make them attractive candidates for accelerators that operate efficiently at high continuous-wave (cw) gradients. Such accelerators are desirable for free-electron lasers (FELs) that extract high-power cw light from a high-average-current electron beam, or that produce ultrashort-wavelength light from a high-energy electron beam. Efficiency is a prime consideration in the former case, while high electron-beam quality is a prime consideration in the latter case. This paper summarizes the status of FEL projects involving srf accelerators. It also introduces Jefferson Lab`s srf FEL and surveys its design because it is a new machine, with commissioning having commenced in October 1997. Once commissioning is complete, this FEL should produce tunable, cw, kW-level light at 3-6 {mu}m wavelength
Chaotic Orbits in Thermal-Equilibrium Beams: Existence and Dynamical Implications
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.Comment: 39 pages, including 14 figure
Linking Ultracold Polar Molecules
We predict that pairs of polar molecules can be weakly bound together in an
ultracold environment, provided that a dc electric field is present. The field
that links the molecules together also strongly influences the basic properties
of the resulting dimer, such as its binding energy and predissociation
lifetime. Because of their long-range character these dimers will be useful in
disentangling cold collision dynamics of polar molecules. As an example, we
estimate the microwave photoassociation yield for OH-OH cold collisions.Comment: 4 pages 2 figure
Fluctuations Do Matter: Large Noise-Enhanced Halos in Charged-Particle Beams
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.Comment: 13 pages total, including 5 figure
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