Ph.D. Thesis - Physics

Figure 7-16: Photograph of the expander. This unit had been removed due to a helium
leak. Some corrosion is visible, which may be due to the presence of liquid water if the
bellows had not been adequately flushed with helium.
a low-pressure (20 psi) return line, in a process called the Gifford-McMahon refrigeration
cycle. Two stages are present on the cold head, one that cools to 40 K with a cooling power
of 35 W, and one that cools to 4.2 K, with a cooling power at that temperature of 0.8 W.
The three key components of the cryostat presented here are the expander, the compressor,
and the vibration isolation mechanism. In this section, we discuss these components, then
present the vacuum chamber housing and the mounting of the entire system.
7.4.1 The cryostat
The expander is the part of the cryostat in which high-pressure helium is allowed to expand
into a low-pressure return line and thereby fall in temperature. The low-pressure gas is then
returned to the compressor to be pressurized again. The expander contains a valve that
is operated by electrical power sent from the compressor, which opens to allow the high-
pressure helium to expand at a rate of about 2 Hz. Inside the lower portion of the assembly
is a displacer assembly, with first and second stage displacer units, the second being of a
smaller diameter. The function of the displacer units is to capture a small amount of the
helium that has been cooled by expansion, and then bring the cold head to that temperature
by the use of heat exchanging elements. The displacers open and close according to the
pressure above them. A photograph of the expander is shown in Fig. 7-16.
The compressor is the part of the cryostat that provides the necessary high- and low-
pressure helium gas flow to and from the expander. It holds a static helium pressure of
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