Superconducting
magnets are usually operated in liquid helium at 4.2 K. Their
performance can
often be enhanced by cooling the magnet to lower temperatures as
described in
section
6.3 on page 41. The simplest way to achieve temperatures below
4.2 K is to
pump the whole liquid helium reservoir with a rotary pump, to reduce
the
vapour pressure
above the liquid. If the bath is cooled to 2.2 K in this way, about 35%
of
the helium is
evaporated to cool the remaining liquid. Temperatures below 2.2 K can
be
achieved, but
if the bath is cooled below the lambda point, the liquid helium
consumption
increases
significantly (both to reach the low temperature and to maintain
it).
This simple
approach has several disadvantages. A large amount of liquid is used to
cool
the magnet
down, and since the reservoir is then below atmospheric pressure, access
to
the reservoir
is difficult and all the fittings on the top plate have to be reliably leak
tight.
The liquid
helium can only be re-filled by de-energising the magnet to its 4.2 K field
and
filling the
reservoir to atmospheric pressure with helium gas, which interrupts
the
experiment.
Lambda point
refrigerators (also known as 'lambda plates' or 'pumped plates') are
used
to cool
superconducting magnets to about 2.2 K and maintain this
temperature
continuously.
See Figure
7. They consist of a needle valve (to control the flow of
liquid
helium into the
refrigerator) and a tube or chamber with a pumping line. They are
normally built
into the 'magnet support system'. The refrigerator is in good
thermal
contact with
the liquid helium just above the magnet.
Liquid is
continuously fed into the refrigerator and pumped to a low pressure so that
it
cools. The
cooling power is determined by the liquid flow rate and the size of the
pump,
and it can be
adjusted using the needle valve. High flow rates are typically used at
high
temperatures to
cool the system quickly or to obtain high cooling power, but when
base
temperature is
reached, the flow can be reduced to make operation as economical as
possible.
The density of
liquid helium changes rapidly with temperature, so strong
convection
currents are
set up, around the magnet. The cold liquid from the refrigerator sinks to
the
bottom of the
reservoir, cooling the magnet and keeping it at about 2.2 K.
Meanwhile
the warmer
liquid above the refrigerator is affected very little. The thermal
conductivity
of the liquid
is so low that the region immediately above the plate has a steep
temperature
gradient, and the liquid surface remains at 4.2 K and at atmospheric
pressure.
It is important
to make sure that this thermal gradient is maintained, and not
short
circuited by
high conductivity components.