1,208 research outputs found
Emergence of Kinetic Behavior in Streaming Ultracold Neutral Plasmas
We create streaming ultracold neutral plasmas by tailoring the photoionizing
laser beam that creates the plasma. By varying the electron temperature, we
control the relative velocity of the streaming populations, and, in conjunction
with variation of the plasma density, this controls the ion collisionality of
the colliding streams. Laser-induced fluorescence is used to map the spatially
resolved density and velocity distribution function for the ions. We identify
the lack of local thermal equilibrium and distinct populations of
interpenetrating, counter-streaming ions as signatures of kinetic behavior.
Experimental data is compared with results from a one-dimensional, two-fluid
numerical simulation.Comment: 8 pages, 6 figure
Ultracold Neutral Plasmas
Ultracold neutral plasmas are formed by photoionizing laser-cooled atoms near
the ionization threshold. Through the application of atomic physics techniques
and diagnostics, these experiments stretch the boundaries of traditional
neutral plasma physics. The electron temperature in these plasmas ranges from
1-1000 K and the ion temperature is around 1 K. The density can approach
cm. Fundamental interest stems from the possibility of
creating strongly-coupled plasmas, but recombination, collective modes, and
thermalization in these systems have also been studied. Optical absorption
images of a strontium plasma, using the Sr
transition at 422 nm, depict the density profile of the plasma, and probe
kinetics on a 50 ns time-scale. The Doppler-broadened ion absorption spectrum
measures the ion velocity distribution, which gives an accurate measure of the
ion dynamics in the first microsecond after photoionization.Comment: 12th International Congress on Plasma Physics, 25-29 October 2004,
Nice (France
High Resolution Ionization of Ultracold Neutral Plasmas
Collective effects, such as waves and instabilities, are integral to our
understanding of most plasma phenomena. We have been able to study these in
ultracold neutral plasmas by shaping the initial density distribution through
spatial modulation of the ionizing laser intensity. We describe a relay imaging
system for the photoionization beam that allows us to create higher resolution
features and its application to extend the observation of ion acoustic waves to
shorter wavelengths. We also describe the formation of sculpted density
profiles to create fast expansion of plasma into vacuum and streaming plasmas
Electron Temperature Evolution in Expanding Ultracold Neutral Plasmas
We have used the free expansion of ultracold neutral plasmas as a
time-resolved probe of electron temperature. A combination of experimental
measurements of the ion expansion velocity and numerical simulations
characterize the crossover from an elastic-collision regime at low initial
Gamma_e, which is dominated by adiabatic cooling of the electrons, to the
regime of high Gamma_e in which inelastic processes drastically heat the
electrons. We identify the time scales and relative contributions of various
processes, and experimentally show the importance of radiative decay and
disorder-induced electron heating for the first time in ultracold neutral
plasmas
Experimental Realization of an Exact Solution to the Vlasov Equations for an Expanding Plasma
We study the expansion of ultracold neutral plasmas in the regime in which
inelastic collisions are negligible. The plasma expands due to the thermal
pressure of the electrons, and for an initial spherically symmetric Gaussian
density profle, the expansion is self-similar. Measurements of the plasma size
and ion kinetic energy using fluorescence imaging and spectroscopy show that
the expansion follows an analytic solution of the Vlasov equations for an
adiabatically expanding plasma.Comment: 4 pages, 4 figure
Theory of the cold collision frequency shift in 1S--2S spectroscopy of Bose-Einstein-condensed and non-condensed hydrogen
We show that a correct formulation of the cold collision frequency shift for
two photon spectroscopy of Bose-condensed and cold non-Bose-condensed hydrogen
is consistent with experimental data. Our treatment includes transport and
inhomogeneity into the theory of a non-condensed gas, which causes substantial
changes in the cold collision frequency shift for the ordinary thermal gas, as
a result of the very high frequency (3.9kHz) of transverse trap mode. For the
condensed gas, we find substantial corrections arise from the inclusion of
quasiparticles, whose number is very large because of the very low frequency
(10.2Hz) of the longitudinal trap mode. These two effects together account for
the apparent absence of a "factor of two" between the two possibilities.
Our treatment considers only the Doppler-free measurements, but could be
extended to Doppler-sensitive measurements. For Bose-condensed hydrogen, we
predict a characteristic "foot" extending into higher detunings than can arise
from the condensate alone, as a result of a correct treatment of the statistics
of thermal quasiparticles.Comment: 16 page J Phys B format plus 6 postscript figure
Mass scaling and non-adiabatic effects in photoassociation spectroscopy of ultracold strontium atoms
We report photoassociation spectroscopy of ultracold Sr atoms near the
intercombination line and provide theoretical models to describe the obtained
bound state energies. We show that using only the molecular states correlating
with the asymptote is insufficient to provide a mass scaled
theoretical model that would reproduce the bound state energies for all
isotopes investigated to date: Sr, Sr and Sr. We attribute
that to the recently discovered avoided crossing between the
() and () potential
curves at short range and we build a mass scaled interaction model that
quantitatively reproduces the available and bound state energies
for the three stable bosonic isotopes. We also provide isotope-specific
two-channel models that incorporate the rotational (Coriolis) mixing between
the and curves which, while not mass scaled, are capable of
quantitatively describing the vibrational splittings observed in experiment. We
find that the use of state-of-the-art ab initio potential curves significantly
improves the quantitative description of the Coriolis mixing between the two -8
GHz bound states in Sr over the previously used model potentials. We
show that one of the recently reported energy levels in Sr does not
follow the long range bound state series and theorize on the possible causes.
Finally, we give the Coriolis mixing angles and linear Zeeman coefficients for
all of the photoassociation lines. The long range van der Waals coefficients
~a.u. and ~a.u. are reported.Comment: 14 pages, 7 tables, 5 figures. Submitted to Phys. Rev.
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