Room Temperature Access Systems

If experiments are to be performed at room temperature, room temperature access systems can be provided. The room temperature bore tube is surrounded by one or two radiation shield bore tubes, which are thermally tied to the liquid nitrogen shield. The liquid helium bore tube surrounds the radiation shield tubes and the cold bore of the magnet surrounds the entire room temperature bore assembly. Room temperature access can be provided in either the vertical position illustrated in Figure B or the horizontal position shown in Figure C. Horizontal room temperature access systems are frequently enclosed in the dewar by welding and are not readily accessible after assembly.

Variable Temperature Access Systems

Frequently experiments must be performed over a wide range of temperatures. By pumping on the helium bath the temperature can be lowered from the normal boiling point of liquid helium (4.2K) to the lambda point (2.17K) where superfluidity occurs. Because a substantial amount of helium is consumed in reducing the temperature of the magnet, it is generally more practical to insert a second liquid helium dewar into the bore of the magnet so the temperature of the sample can be reduced without cooling the magnet unnecessarily.

A insert dewar of this type can be used for experiments in the temperature range from 2K to 5K by filling with liquid helium and pumping or pressurizing as required, or operate at 77K by inserting liquid nitrogen. Experiments at room temperature can also be performed with a insert dewar.

Where continuous variation of the temperature is required, a variable temperature insert is used. Cooling is achieved by withdrawing a controlled amount of liquid helium from the reservoir through a valved capillary tube. The helium is fed into the bottom of the sample tube, flows over the sample and is extracted at the top of the dewar. By heating the helium with the electrical heater on the bottom of the sample tube, the temperature of the sample can be varied. This arrangement can be used to achieve temperatures from 1.5K to 300K and can be used with a separate temperature controller to regulate the temperature of the cryostat.

Each of the options mentioned above requires that the bore of the magnet be enlarged to provide room for the extra vacuum jacket required to isolate the sample from the liquid helium.

Most experiments are performed with the sample mounted parallel to the magnetic field. However, experiments requiring that the sample be transverse to the magnetic field can be performed in two ways. First, the sample can be mounted in a solenoid with the sample transverse to the field; or second, the magnet can be constructed so access is provided in a radial direction through the midplane of the magnet. Variable temperature insert dewars can also be provided for this magnet configuration.

Optical Access Systems

Optical access to the sample can be provided either along the axis of the magnet or through its midplane. When the optical path is parallel to the field, windows can be provided in the bottom of the dewar and light can be passed along the vertical axis. If optical access is required both along the axis and transverse to it, a split coil configuration is used. The magnet can be mounted with the magnetic axis either horizontal or vertical.

If the magnetic axis is vertical, four or more optical ports can be provided in the horizontal plane, each providing a view transverse to the magnetic field, and an additional window can be mounted in the bottom to permit light to be transmitted along the axis. If the magnetic axis is horizontal, four ports can be provided in the horizontal plane to provide two optical paths; one parallel to and one transverse to the magnetic axis.

If helium can be tolerated in the light path, windows are placed on the outside of the helium reservoir and on the outer jacket of the dewar. The magnet can be simply inserted into the dewar. Where the light path must be evacuated, the magnet is constructed on a bobbin that is vacuum tight, and the magnet is welded into the dewar and becomes an integral part of it. Windows are then placed only on the room temperature outer jacket; or optionally on the radiation shield of the dewar, unless temperature control of the sample is required.

Temperatures can be varied in optical dewars by placing the samples in a separate variable temperature insert having optical windows. The temperature is varied by vaporizing and heating helium as discussed earlier.

Once the configuration and temperature variation requirements for your experiments are specified, the size of your samples should be considered carefully. The minimum size sample that can be used without compromising the results of your present or future experiments should be ascertained. This factor ultimately affects the size and cost of the magnet and dewar in your system.