Monte-Carlo simulations of the capture and cooling of alkali-metal atoms by a supersonic helium jet
arxiv(2024)
Abstract
We present three-dimensional Monte-Carlo simulations of the capture of 1000 K
^7Li or 500 K ^87Rb atoms by a continuous supersonic ^4He jet and show
that intense alkali-metal beams form with narrow transverse and longitudinal
velocity distributions. The nozzle creating the ^4He jet is held at
approximately 4 K. These conditions are similar to those in the cold ^7Li
source developed by some of us as described in [Phy. Rev. A 107, 013302
(2023)]. The simulations use differential cross-sections obtained from quantum
scattering calculations of ^7Li or ^87Rb atoms with ^4He atoms for
relative collision energies between k× 1 mK to k× 3000 K, where
k is the Boltzmann constant. For collision energies larger than ≈
k× 4 K the collisions favor forward scattering, deflecting the ^7Li or
^87Rb atoms by no more than a few degrees. From the simulations, we find
that about 1% of the lithium atoms are captured into the ^4He jet,
resulting in a lithium beam with a most probable velocity of about 210 m/s
and number densities on the order of 10^8 cm^-3. Simulations predict
narrow yet asymmetric velocity distributions which are verified by comparing to
fluorescence measurements of the seeded ^7Li atoms. We find agreement between
simulated and experimentally measured seeded ^7Li densities to be better than
50% across a range of ^4He flow rates. We make predictions for capture
efficiency and cooling of ^87Rb by a supersonic ^4He jet. The capture
efficiency for ^87Rb is expected to be similar to ^7Li.
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