Ph.D. Thesis - Physics

250 psi, which increases to around 310 psi in the high-pressure line when the compressor
is running. It is water-cooled with a chilled water supply with a flow rate of about 1.5
gal/min.
The DMX-20 anti-vibration interface thermally connects the cold head to the experiment
without mechanical contact. A reservoir is filled with ultra-high-purity (99.9999%) helium
gas at a pressure of about 1 psi. The reservoir is closed with a rubber bellows that does not
transmit vibration from the expander to the experiment. Heat is exchanged by a pair of
copper coils, one attached to the cold head and the other to the DMX-20 interface, between
which helium gas is present.
When being operated, the expander is anchored to the ceiling with an 80/20 aluminum
framework, while the experiment remains fixed to the table. If alignment of the cold head
is not correct, then vibrations can be felt by hand on the experiment and the cold head
position readjusted.
7.4.2 Vacuum chamber and optical access
The experimental assembly was mounted on an optical breadboard using an 80/20 aluminum
framework. The breadboard was converted into a table using four 3 in. wide 80/20 pieces,
with rubber feet on the bottom of each and some supports across the legs for stability.
When the cryostat is not running, standoffs hold the expander in place within the DMX-20.
The vacuum chamber is mounted on a pivot so that it can be rotated up 90 ◦ and opened.
The vacuum chamber is made of con-flat parts for maximum isolation from atmosphere.
The DMX-20 unit is encased in a 8 in. flanged nipple. On the bottom, a 4 1/2 in. CF
viewport, AR coated for 422 nm and 1092 nm, is mounted on an 8 in. flange. When the
cryostat is opened, this flange is removed. A photograph of the exterior setup is presented
in Fig. 7-17.
Cryogenic vacuum setups differ in two main ways from room-temperature ones. On one
hand, the choice of materials is broader than in a room-temperature setup. Materials that
outgas too much at room temperature, such as plastic and lead solder, are acceptable at
4 K, making the experimental design somewhat simpler. On the other hand, obtaining this
low outgassing, as well as minimizing the electrode temperatures, requires good thermal
isolation from 300 K. Thermal “shorts” can be caused by conduction or radiation.
A radiation shield is constructed and anchored to the 40 K stage. This is composed of
a 4 1/2 in. CF spherical octagon from Kimball physics, on top of which is a copper plate
that holds the imaging lens assembly. Viewports are installed on two sides of the octagon
to permit the lasers to access the trap, while other ports of the octagon are covered with
copper baffles. Wiring from the external feedthroughs goes around these baffles. Fig. 7-18
shows the 4K baseplate with electrical connections for dc and rf. The 40K shield is also
visible around this.
Preventing thermal shorts due to conduction is achieved by a combination of good wire
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