Ph.D. Thesis - Physics

Spherical octagon, feedthroughs, and oven
The spherical octagon is a stainless steel piece manufactured by Kimball physics (Part No.
MCF450-SO20008). It has two 4 1/2 in. con-flat (CF) sealing surfaces, and eight 1 1/3 in.
CF sealing surfaces. The top 4 1/2 in. surface is used for imaging. A anti-reflective (AR)
coated fused silica viewport is used, while on the bottom there is another viewport, although
it was not used in this work. The system used a total of three electrical feedthroughs. One
provides the high-voltage rf signal to the trap. A separate feedthrough is used for this
to limit stray capacitance that can adversely affect the Q factor of the rf resonance. The
second feedthrough is a 9-pin sub-d connection that carries any dc voltages that may be
needed for the trap. The setup in Ch. 6 is very similar to this one, and does make use of
these connections. The final feedthrough holds the strontium oven and is used to supply it
with current.
The oven used in the experiments of Ch. 5 and 6 is made of a piece of thin tantalum
foil that is folded into a tube. One end of the oven is spot-welded closed and then grains of
strontium metal are added to the open end. This end is then spot-welded closed, and both
ends of the oven are spot-welded directly to the stainless steel wires of the feedthrough. A
hole is poked in the side of the oven that faces the ion trap; it is important that this be the
only opening in the oven. A typical resistance for the finished oven, including feedthroughs,
is about 0.5 Ω, and currents of 3 A are normally sufficient to load the trap.
Vacuum pumps and pressure gauge
Initial pumpdown of the system to O(10 −6 torr) is done with a turbomolecular pump backed
by a roughing pump. After reaching this vacuum level, a valve to the turbo pump is closed
and two pumps that are an integral part of the vacuum system are used.
The ion pump is a Varian 60 l/s triode pump. The triode configuration pumps noble
gases more efficiently than the original diode configuration, which is very useful in Ch. 6,
but less essential here. A titanium sublimation pump (Ti-sub) is also used. It consists
of a long titanium filament through which ≈ 40 A of current is flowed. Titanium is then
deposited on the walls of the vacuum housing. A six-inch-diameter stainless steel nipple
serves as this surface; it is significantly wider than the filament to increase the surface area
on which titanium is deposited. As shown in Fig. 5-4, the filament is mounted at a right
angle to the aperture that leads to the ion trap, reducing the chances of depositing titanium
on the trap. The Ti-sub does not need to be fired continuously; typically, it is fired once a
week (for 1-5 minutes) as long as some improvement in pressure is observed after firing.
These two pumps are sufficient to reach pressures in the 10 −10 torr range after bakeout.
Bakeout is a process in which the entire vacuum chamber is heated to around 200 ◦ C and
pumped on. The higher temperature increases the outgassing rate of material adsorbed on
surfaces within the chamber, and essentially speeds up the pumpdown. This is a standard
procedure for ultra high vacuum (UHV) apparatus. A typical baking time is around 1-2
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