As expected, this week was much more hands-on than last week. While I continued my extensive readings about the scientific theories behind NMR, I have also acquired some more practical knowledge of NMR. On Wednesday, I helped out on my first helium fill, and on Friday I watched my first nitrogen fill.
As discussed in my last post, large magnetic fields are needed in order to perform NMR. Most readings are taken using an extremely powerful, 17.6 Tesla magnet. For reference, that’s about 352,000 times more powerful than the Earth’s magnetic field. In order to generate such a powerful magnetic field, an enormously large current is needed. To obtain this current, high-power magnets are made of superconducting materials, which have very low resistance to electrical flow at low temperatures. In order to produce these temperatures, liquid helium and liquid nitrogen are used to cool the magnet. Like all liquids with low boiling points, however, these gases evaporate quite readily. As such, periodic helium and nitrogen fills are needed to ensure the magnet stays sufficiently cold.
To be blunt, liquid helium is cold. Really cold. To be precise, it boils at 4.2 Kelvin. (For reference, that’s about -452 degrees Fahrenheit .) It takes about 200 liters of liquid helium to fill the magnet. However, as mentioned previously, gases with low boiling points have a tendency to evaporate quite readily. Therefore, the magnet must be refilled with helium once every six weeks. The fill procedure, however, is quite straightforward. A high-capacity dewar of liquid helium is connected to an inlet value at the top of the magnet. A tank of pressured gaseous helium is then used to push the liquid helium out of the dewar and into the magnet. (A release value at the top of the magnet allows the helium gas to escape: if this value were not present, the pressure in the magnet would increase to dangerous levels.) Somewhat unexpectedly, as helium is added to the magnet, the magnet actually warms up. In order to achieve the desired cooling effect, pumps are used to pull helium out of the magnet – the process is akin to blowing on a hot cup of coffee to cool it down. This sets up a temperature gradient, with a lower temperature at the bottom of the magnet. The filling process itself takes an hour.
Liquid helium, though extremely cold, is also extremely expensive (about $9/liter). As such, liquid nitrogen is used in addition to helium. Though cold, liquid nitrogen is nowhere near as chilly as its noble gas counterpart. It boils at -196 degrees Celsius (or -321 degrees Fahrenheit). The magnet’s nitrogen supply must be replenished once a week. These weekly nitrogen fills are much more straightforward than the helium fills: after the transfer line is cooled, the magnet is connected to a nitrogen tank outside of Small Hall. (If the line is not cooled first, the liquid nitrogen would evaporate in the transfer line, and nitrogen gas would be pumped into the magnet, which would evaporate the remaining liquid nitrogen. Liquid nitrogen is then pumped into the magnet until it is full, a process taking about 1.5 hours.
Besides this “filler” material, I have begun experimenting on scandium oxide Sc2O3 using the low field (7.4 Telsa) magnet. More on this endeavor, however, will be posted once more experimentation takes place.