51 research outputs found
Model for the overall phase-space acceptance in a Zeeman decelerator
We present a new formalism to calculate phase-space acceptance in a Zeeman
decelerator. Using parameters closely mimicking previous Zeeman deceleration
experiments, this approach reveals a hitherto unconsidered velocity dependence
of the phase stability which we ascribe to the finite rise and fall times of
the current pulses that generate the magnetic fields inside the deceleration
coils. It is shown that changing the current switch-off times as the sequence
progresses, so as to maintain a constant mean acceleration per pulse, can lead
to a constant phase stability and hence a beam with well-defined
characteristics. We also find that the time overlap between fields of adjacent
coils has an influence on the phase-space acceptance. Previous theoretical and
experimental results suggested unfilled regions in phase space that influence
particle transmission through the decelerator. Our model provides, for the
first time, a means to directly identify the origin of these effects due to
coupling between longitudinal and transverse dynamics. Since optimum phase
stability is restricted to a rather small parameter range in terms of the
reduced position of the synchronous particle, only a limited range of final
velocities can be attained using a given number of coils. We evaluate phase
stability for different Zeeman deceleration sequences, and, by comparison with
numerical three-dimensional particle trajectory simulations, we demonstrate
that our model provides a valuable tool to find optimum parameter sets for
improved Zeeman deceleration schemes. An acceleration-deceleration scheme is
shown to be a useful approach to generating beams with well-defined properties
for variable-energy collision experiments. More generally, the model provides
significant physical insights applicable to other types of particle
decelerators with finite rise and fall time fields
Getting a Grip on the Transverse Motion in a Zeeman Decelerator
Zeeman deceleration is an experimental technique in which inhomogeneous,
time-dependent magnetic fields generated inside an array of solenoid coils are
used to manipulate the velocity of a supersonic beam. A 12-stage Zeeman
decelerator has been built and characterized using hydrogen atoms as a test
system. The instrument has several original features including the possibility
to replace each deceleration coil individually. In this article, we give a
detailed description of the experimental setup, and illustrate its performance.
We demonstrate that the overall acceptance in a Zeeman decelerator can be
significantly increased with only minor changes to the setup itself. This is
achieved by applying a rather low, anti-parallel magnetic field in one of the
solenoid coils that forms a temporally varying quadrupole field, and improves
particle confinement in the transverse direction. The results are reproduced by
three-dimensional numerical particle trajectory simulations thus allowing for a
rigorous analysis of the experimental data. The findings suggest the use of a
modified coil configuration to improve transverse focusing during the
deceleration process.Comment: accepted by J. Chem. Phy
Ionization of Rydberg atoms embedded in an ultracold plasma
We have studied the behavior of cold Rydberg atoms embedded in an ultracold
plasma. We demonstrate that even deeply bound Rydberg atoms are completely
ionized in such an environment, due to electron collisions. Using a fast pulse
extraction of the electrons from the plasma we found that the number of excess
positive charges, which is directly related to the electron temperature Te, is
not strongly affected by the ionization of the Rydberg atoms. Assuming a
Michie-King equilibrium distribution, in analogy with globular star cluster
dynamics, we estimate Te. Without concluding on heating or cooling of the
plasma by the Rydberg atoms, we discuss the range for changing the plasma
temperature by adding Rydberg atoms.Comment: To be published in P.R.
Loading Stark-decelerated molecules into electrostatic quadrupole traps
Beams of neutral polar molecules in a low-field seeking quantum state can be
slowed down using a Stark decelerator, and can subsequently be loaded and
confined in electrostatic quadrupole traps. The efficiency of the trap loading
process is determined by the ability to couple the decelerated packet of
molecules into the trap without loss of molecules and without heating. We
discuss the inherent difficulties to obtain ideal trap loading, and describe
and compare different trap loading strategies. A new "split-endcap" quadrupole
trap design is presented that enables improved trap loading efficiencies. This
is experimentally verified by comparing the trapping of OH radicals using the
conventional and the new quadrupole trap designs
Deceleration and electrostatic trapping of OH radicals
A pulsed beam of ground state OH radicals is slowed down using a Stark
decelerator and is subsequently loaded into an electrostatic trap.
Characterization of the molecular beam production, deceleration and trap
loading process is performed via laser induced fluorescence detection inside
the quadrupole trap. Depending on details of the trap loading sequence,
typically OH () radicals are trapped at a density
of around cm and at temperatures in the 50-500 mK range. The 1/e
trap lifetime is around 1.0 second.Comment: 4 pages, 3 figure
Direct measurement of the radiative lifetime of vibrationally excited OH radicals
Neutral molecules, isolated in the gas-phase, can be prepared in a long-lived
excited state and stored in a trap. The long observation time afforded by the
trap can then be exploited to measure the radiative lifetime of this state by
monitoring the temporal decay of the population in the trap. This method is
demonstrated here and used to benchmark the Einstein -coefficients in the
Meinel system of OH. A pulsed beam of vibrationally excited OH radicals is
Stark decelerated and loaded into an electrostatic quadrupole trap. The
radiative lifetime of the upper -doublet component of the level is determined as ms, in good
agreement with the calculated value of ms.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
Decelerating molecules with microwave fields
We here report on the experimental realization of a microwave decelerator for
neutral polar molecules, suitable for decelerating and focusing molecules in
high-field-seeking states. The multi-stage decelerator consists of a
cylindrical microwave cavity oscillating on the TE 11n mode, with n=12 electric
field maxima along the symmetry axis. By switching the microwave field on and
off at the appropriate times, a beam of state-selected ammonia molecules with
an incident mean velocity of 25 m/s is guided while being spatially focussed in
the transverse direction and bunched in the forward direction. Deceleration
from 20.0 m/s to 16.9 m/s and acceleration from 20.0 m/s to 22.7 m/s is
demonstrated.Comment: 4 Pages, 3 Figure
LS-CMA-ES: a Second-order algorithm for Covariance Matrix Adaptation
International audienceEvolution Strategies, Evolutionary Algorithms based on Gaussian mutation and deterministic selection, are today considered the best choice as far as parameter optimization is concerned. However, there are multiple ways to tune the covariance matrix of the Gaussian mutation. After reviewing the state of the art in covariance matrix adaptation, a new approach is proposed, in which the covariance matrix adaptation method is based on a quadratic approximation of the target function obtained by some Least-Square minimization. A dynamic criterion is designed to detect situations where the approximation is not accurate enough, and original Covariance Matrix Adaptation (CMA) should rather be directly used. The resulting algorithm is experimentally validated on benchmark functions, performing much better than CMA-ES on a large class of problems
Optical pumping of trapped neutral molecules by blackbody radiation
Optical pumping by blackbody radiation is a feature shared by all polar
molecules and fundamentally limits the time that these molecules can be kept in
a single quantum state in a trap. To demonstrate and quantify this, we have
monitored the optical pumping of electrostatically trapped OH and OD radicals
by room-temperature blackbody radiation. Transfer of these molecules to
rotationally excited states by blackbody radiation at 295 K limits the
trapping time for OH and OD in the state to
2.8 s and 7.1 s, respectively.Comment: corrected small mistakes; added journal reference
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