Experiments to Control Atom Number and Phase-Space Density in ...

the shorter the bake can be. However, multiple elements in the chamber limit both
the maximum temperature and the maximum temperature gradients during the baking
process. A multiple step bake-out is therefore used.
Even before the assembly of the vacuum chamber, an air-bake of the differential
pumping tube is done to remove any residual oil deposited by the machining process.
The differential pumping tube, after thorough cleaning with acetone and methanol, is
thus heated to about 200 ◦ C for about 24 hours.
For the first bake-out the ZnSe viewports are not attached to the vacuum cham-
ber. The maximum baking temperature of the ZnSe viewports is low (120 ◦ C) due to the
AR coating, and would limit the overall temperature of the vacuum chamber during the
bake. Two blank flanges seal the ports typically occupied by the ZnSe viewport. The
chamber is wrapped in flexible heater-tapes and covered in aluminum foil before being
heated to 200 ◦ C, the maximum temperature specified for the rotary feedthrough and
the highest recommended temperature for the reentrant viewports.
After the initial bake, the ZnSe viewports are attached to the spherical octagon.
To minimize the contact of any chamber walls with air, the whole chamber is vented with
high-purity argon gas and the viewports are attached as quickly as possible. The thick
AR coating on the ZnSe viewports is extremely sensitive to any spatial and temporal
temperature gradients. To minimize the risk of damaging the coating, a brick oven
surrounds the spherical octagon. The bricks are made from an insulating ceramic and
wrapped in aluminum foil to minimize the release of dust. The large air volume inside
the brick oven mitigates temperature gradients. The final baking temperature with the
ZnSe viewports attached is about 100 ◦ C.
The lithium reservoir is run at a temperature of 350 ◦ C during the experiment. It
is therefore necessary to outgas the lithium to at least this temperature. The lithium used
in the experiment is 95% enriched 6 Li from Sigma-Aldrich and Medical Isotopes Inc. The
enriched lithium is stored in oil to avoid oxidation. Even after multiple cleaning cycles,
involving removal of the outermost layer of the chunk of lithium and chemical cleaning
of the metal, oil contamination cannot be completely ruled out. Before outgassing the
lithium at a temperature of up to 400 ◦ C, the gate valve to the science chamber is closed.
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