A Simple Solution to the Multiple Cooling Requirements for Vertical
Cryostats used for 1.8K - 4.2K Superconducting Magnet Testing

A. F. Lietzke

Abstract

The motivation for an improved heat exchanger for vertical magnet test
cryostats is presented. The basic system design constraints, challenges
and conflicts are described, including issues around dual usage for 80K
and 1.8K cool-downs. A conceptual design is presented that meets the
challenges in a novel way by using some of the annular space surrounding
the top of the magnet for a heat exchanger. Practical implementation
problems are discussed and possible solutions are presented.

I\) Introduction

In recent times there has been an increased interest in operating some
accelerator magnets at temperatures below 4.2K. This not only allows the
magnet to train more quickly, run more reliably and/or operate at a
higher field strength, but the presence of the superfluid (T \< 2.17K @
1 Atm) improves the ability to remove heat from the magnet. The improved
thermal conductivity is especially desirable for magnets subjected to
volumetric heating (as from synchrotron radiation), permitting operation
under conditions that would otherwise preclude superconducting magnets,
or require the construction of relatively expensive cooling schemes.
Hence there is a desire for a simple, inexpensive, and reliable system
to remove heat at sub atmospheric pressures.

One solution is to simply pump on the entire system. This however,
entails some risk of solid air plugging (in the presence of a leak), and
usually provides inadequate cooling for the magnet current-leads at the
lowest temperatures. The other solution is to provide a small,
sub-atmospheric heat exchanger that allows the magnet chamber to operate
above 1 Atm.

Operationally, it is desirable to test such magnets in a vertical
cryostat in order to simplify magnet insertion and removal operations,
by permitting the use of warm pressure and vacuum seals. The minimum
thermal leak-path required with this design, coupled with lifting device
height limitations and minimum desired magnet lengths, often places a
premium on vertical real-estate. Hence, there is often some desire to
place the evaporator off to one side of the magnet, usually at some
penalty in complexity, cost, thermal leakage, and/or operational
reliability. A second reason to place the evaporator off to one side
involves one's desire to protect its relatively thin walls from damage
during the insertion and removal of rather heavy (1 - 10 ton) magnets.

There is usually a desire to cool these magnets from 300K to 80K in a
rather slow and benign fashion (to reduce the effects of differential
thermal contraction), within a helium atmosphere (to reduce the effects
of solid nitrogen within the magnet structure). This requires some kind
of heat exchanger to cool the helium gas. Hence, one either installs a
second heat exchanger for 80K cooling, or finds a way to use one heat
exchanger for both operations.

The purpose of this paper is to describe the conceptual design of a heat
exchanger that is simple, rugged, takes up very little expensive real
estate, and can be used for 80K cool-down operations within vertical
cryostats.

II\) General Design Considerations

Super-fluid operation of a cryostat places some severe constraints on
the cryostat design. Due to the decreasing cooling capability as the
temperature is lowered, a premium exists for low thermal leakage. This
not only requires some attention to insulating blanket details, but it
requires considerable attention to insuring super-fluid-tight seals at
all penetrations into the super-fluid volume. In particular, for magnets
which are inserted with 300K seals, some kind of low conductivity plate
("lambda plate") is used to separate the superfluid/magnet system from
the normal stratified helium above it. This kind of geometry requires
that the lambda-plate diameter exceed the diameter of the largest magnet
to be tested. It also means that the magnet must be inserted (and
removed) past some kind of sealing ring with enough sealing length to
meet the super-fluid sealing requirements (2-4 cm is usually considered
to be adequate). The axial location of this ring depends upon the
maximum allowed thermal gradient within the cryostat wall material
(controlling the heat load to the 4.5K system), plus the desired local
4.5K liquid helium reservoir height (above the Lambda-plate). Typical
lambda-plate locations range between 0.5 - 1.2 m below the top of the
cryostat. Several penetrations are required and need to be sealed
adequately: 1) supports for the magnet's weight, 2) current-supply leads
for the magnet, 3) pressure relief ports, 4) non-vapor-lock liquid
helium supply ports, 5) diagnostic access. Care must be taken to avoid
over-constraining the penetrations as this may interfere with the
quality of the superfluid seal.

One also needs the refrigeration capability to handle the heat-load at
the desired operating conditions. Cooling a 1 Atm helium system below
4.2K requires an evaporator from which heat is evaporatively removed
after conduction across some portion of the evaporator's outer surface.
This not only requires some minimum pumping capability, but the Kapitza
thermal impedance at each surface of the heat exchanger, plus the
thermal impedance within the heat exchanger wall material require a
minimum heat exchanger area for satisfactory operation. A heat exchanger
system at the Lawrence Berkeley Laboratory, having 1 m^2^ of 100 mm
long, 0.4 mm wall electrolytic copper tubes effectively removed 35 watts
at 1.8K.

To reduce the heat load to the evaporator, the in-coming liquid helium
should be single phase, so as to be precooled as much as practical by
heat exchanging with the low pressure evaporant. This inlet should be
metered by a valve that is controlled by the liquid level within the
evaporator. Redundant valves will increase the operational reliability.

To keep the 1.8K cool-down time reasonable, the pumping conductance
should be adequate for 35 watts of cooling at 1.7K.

Using the evaporator to cool the system from 300K to 80K without a
second valve would require injecting LN into the chamber above the
Lambda-plate. This is not recommended as one could not assure that LN
would stay out of the lower chamber. Hence, a second valve is required
to inlet liquid nitrogen into the evaporator chamber. As one also
desires to remove excess LN without excessive pumping and have
absolutely no chance of plugging the evaporator during 1.8K operation,
the LN entrance into the evaporator needs to be at the very bottom of
the chamber (to allow pressurized LN removal) and LN control valve needs
to be reasonably super-fluid tight, with the volume above the valve
connected to the evaporator pumping (for insurance against plugging the
system with solid air).

Laboratory pressure safety requirements place a lower limit on the
strength and number of reinforcement rings that are required for any
proposed heat-exchanger wall thickness. The thickness of the copper wall
is influenced by the need for an adequately small thermal impedance, yet
a reasonable number of reinforcing rings and a reasonable freedom from
accidental puncture during magnet insertion or removal.

Differential thermal expansion must be considered at all superfluid
joints. Namely, a tight fit at 300K does not necessarily insure a tight
fit when cold. Furthermore, a tight fit when cold may mean a
shrunk-stuck joint when warmed up.

III\) Specific Design Features:

A rugged, beveled, 3 cm wide, stainless-steel Lambda-plate sealing-ring
is used approximately 1 meter below the top of the cryostat. The bevel
extends all the way to the wall and is fairly close (15 degrees) to
vertical for four reasons: 1) to facilitate blind centering of the
magnet during its insertion, 2) to facilitate blind centering of the
lambda plate, 3) to provide a tight seal when cold (as the stainless
shrinks more than the lambda-plate during cool-down), and 4) to reduce
the chance of debris lying on the sealing surface and degrading the
super-fluid seal.

The lambda-plate is a 40 mm thick Nema-Glass disk and has a matching
outer bevel. There are eight penetrations of this plate which have to be
superfluid-sealable:

1.  Three stiff (25 mm), nearly vertical stainless-steel rods support
    the magnet's weight. Their exact position depends upon their
    attachment to the magnet, the magnet temperature and the natural
    hanging location of the magnet (which may vary from magnet to
    magnet). Hence, each rod has a spring-loaded seal that can adjust
    itself to its circumstances. Each rod is polished in the
    neighborhood of the sliding seal, which is made from teflon that is
    sized to be tight when cold and loose when warm. The teflon is
    backed by a stiff plate that helps center the spring and distribute
    its force more uniformly. Stop-rings are present to help pop the
    lambda-plate loose during magnet removal and are positioned to keep
    the springs in their seats when relaxed.

2.  Two high-current magnet supply leads are made flexible enough to
    permit permanent seals (swaged Teflon)

3.  The diagnostic umbilical (300 small, stranded, copper wires) is also
    flexible enough to permit a permanent seal (swaged Teflon).

4.  A spring-loaded pressure relief valve serves as a cold-gas or liquid
    cryogen supply valve when forced open against its spring (during
    gaseous cool-down and liquid accumulation). It lies near the outside
    of the lambda plate in order to minimize cold-gas/hot-gas mixing
    during gaseous cool-down.

5.  A second spring-loaded pressure relief valve, lies diametrically
    opposite and as far as practical from the above-mentioned
    cold-supply valve. When forced open against its spring, it serves as
    a return path for warm gas during a gaseous cool-down and
    circumvents vapor-lock during liquid accumulation.

Some of the annular volume immediately beneath the lambda-plate
sealing-ring is used for the annular evaporator. with the inner (copper)
wall of the reservoir serving as a heat-exchanger for the magnet volume.
In order to provide dual temperature (80K or 1.8K) operation, plumbing
is supplied which allows the injection of either liquid nitrogen or
precooled liquid helium into the reservoir. Plumbing is also supplied to
provide a pumping capability that is adequate for 35 watts of cooling at
1.7K.

Conclusions