AVANCE Beginners Guide
Chapter 4. System Description
4.6 The Magnet and Magnet Dewar
A range of magnets are available with different strengths. The strength of the magnet is graded according to the frequency of the NMR signals emitted by hydrogen atoms. The stronger the magnet the higher this hydrogen frequency. For example, with a 500 MHz magnet (11.7 T), this means that when a chemical sample is placed in the magnet for analysis, the1H atoms in the sample will emit signals with a frequency very close to 500 MHz. Bruker magnets are available in the range of 200-900 MHz.
Superconducting magnets are electromagnets, and as such make use of the fact that an electric current produces a magnetic field. The magnet core consists of a large coil of current carrying wire in the shape of a solenoid. At the center of the coil a very intense static magnetic field exists. The sample to be analyzed is placed inside this magnetic field.
At very low temperatures certain materials show the remarkable property of superconductivity. A superconducting wire carries electricity without the need for any driving energy (i.e. battery or mains supply). Once a current is started in a superconducting loop it will continue forever. Bruker magnets consist of such a superconducting loop. When the magnet is first installed, a current is started in the main coil. This is referred to as "charging" the magnet. Once charged, the magnet should continue to operate for many years as the current, once started, will never stop! The only maintenance required by the magnet is to ensure that the superconducting coil is kept at a sufficiently low temperature.
A temperature of 4K (-269°C) is the boiling point of helium. Under normal conditions, at temperatures above 4K, it is a gas. At temperatures below 4K it is a liquid. Thus, if the coil is kept immersed in liquid helium we can be sure that the temperature is at 4K or below. This is sufficiently cold to ensure that the magnet coil remains superconducting. However, should enough of the liquid helium evaporate, then the temperature of the superconducting coil will increase and at some stage become non-superconducting. The resistance in the coil would then result in a sudden breakdown of the magnetic field accompanied with the generation of heat which would very quickly lead to the evaporation of large quantities of liquid helium and nitrogen. The magnet room may suddenly become filled with evaporated gas and the magnet is said to have "quenched". This may or may not permanently damage the magnet. In any case, recharging the magnet is time and money consuming and should be avoided under any circumstance.
Much of the magnet's technology is involved with the difficult task of ensuring that as little as possible of the liquid helium that covers the magnet coil evaporates. This is achieved by minimizing the flow of heat from the NMR lab (room temperature) to the magnet core (4K). The magnet consists of several sections. The outer casing of the magnet is evacuated and inner surfaces are silvered. (This is the same principle as the Thermos flask except we are trying to prevent heat getting in instead of out.) Next comes a bath of nitrogen which reduces the temperature to 77.35K (-195.8°C) and finally a tank of helium within which the superconducting coil is immersed (see the figure "Superconducting Magnet")
4.6.1 Room Temperature Bore
introduced into the magnet via the top of the bore. Probes, which hold the sample and carry signals to and from the sample, are inserted from the bottom.
The helium and nitrogen tanks are wrapped around a central column known as the magnet bore. A metal plug normally closes off the top of the bore. Magnets are available with either standard bore or wide bore. Samples to be analyzed are
4.6.2 Helium Tank
It is important that the helium ports are not left open for extended periods of time (e.g. >30 seconds).
In a standard magnet the helium tank is suspended from two necks which extend high above the magnet. Access to the helium tank can be made via two ports. One of these ports permits refilling of the liquid helium and is also the entry for a helium level sensor. The other port is used only when the magnet is being charged or discharged. The helium necks may support several valves which control the release of the small quantities of helium that will inevitably evaporate. The system manager should check that these valves are working properly, i.e. are not blocked by ice.
The helium level can be checked either manually or electronically. Manual checking involves inserting a long dipstick into the helium tank via one of the access ports (experienced users only!). Electronic monitoring of the helium level is carried out using the BSMS keyboard and will be discussed in the section "BSMS Functions".
Figure 4.4. Superconducting Magnet
4.6.3 Nitrogen Tank
The three shorter necks extending above the magnet allow access to the nitrogen tank. Any one of the three ports can be used to check the nitrogen level with a suitable dipstick (experienced users only!!!) and top-up the level if required. Again the system manager should regularly check that any valves inserted onto the nitrogen ports are not blocked with ice.
As already mentioned, the magnets are designed to minimize liquid evaporation. During normal operation a small amount of nitrogen will evaporate per day. This is perfectly normal and indeed the absence of nitrogen boil-off indicates that the nitrogen ports have become blocked. How often the levels of nitrogen and helium need to be topped-up depends on the magnet size and design. It is good practice to check and top-up the nitrogen level every week. It is also good practice to check the helium level at the same time and keep a record of it. Although the helium boil-off is much less than that of nitrogen, which results in much longer intervals before one has to refill the helium (3-6 months), the regular check will ensure that the helium boil-off has not changed.