History

The 270 MHz (6.3 T) NMR magnet was originally manufactured by Oxford Instruments, Oxford, UK, in 1978-79 and installed by JEOL USA on a JEOL FX-270 NMR Spectrometer in 1979. The spectrometer was upgraded to a Delta-GSX 270 in 1995 before being retired in 1999 to make space for a JEOL Eclipse+ 500 NMR Spectrometer system.

Because the 270 MHz NMR frequency is now very uncommon, and the magnet could not sustain sufficient current to make 300 MHz, we thought that cutting open the magnet would be a useful teaching aid for our NMR customers and the NMR community.

Although NMR magnets can be properly disassembled for repair by the manufacturer, in order to show relative positions of the various parts, the method of disassembly used in this demonstration is destructive. Do not attempt to repeat this demonstration unless you intend to destroy your NMR magnet. The magnet was carefully sectioned by sequentially cutting and removing each layer of material.

What follows is a view of a superconducting NMR magnet that very few people have ever seen.

	Outside Vacuum Chamber The outside shell of the magnet is made from stainless steel and is about 1/8" (3 mm) thick. The shell took about 8 hours to cut open with an air-powered cutoff grinding tool. The picture right shows the outside of the liquid nitrogen vessel with a portion of the outer can and aluminized Mylar super-insulation removed. The Mylar insulation reflects the infra-red heat radiation from the inside of the room temperature surface. There are about 165 layers of the aluminized Mylar insulation. During normal operation of the magnet this 'space' is evacuated under high vacuum. The high vacuum is maintained by the cryo-pumping action of the 4K liquid helium can.
	Liquid Nitrogen Vessel The liquid nitrogen vessel is made of approximately 3/16" (4.8 mm) aluminum. The vessel required about 30 minutes to cut open with a panel saw. The picture at the right shows the interior of the liquid nitrogen vessel. During normal operation this space is filled with liquid nitrogen (77K). The liquid nitrogen level sensor and the passageway for the magnet charging lead can be seen inside of the liquid nitrogen vessel. Note that the liquid nitrogen reservoir space is mostly above the magnet. The purpose of the liquid nitrogen is to act as a less expensive refrigerant to block infra-red radiation from reaching the liquid helium vessel.
	20 Kelvin Radiation Shield The 20 K (Kelvin) radiation shield is made of aluminum and was wrapped with alternating layers of aluminum foil and open weave gauze. The purpose of the 20 K shield is to block infra-red radiation coming from the 77 K liquid nitrogen vessel. The elimination of infra-red radiation lowers the liquid helium boil-off rate. The 20 K radiation shield is thermally isolated from the liquid helium (4.2 K) and liquid nitrogen (77 K) and reaches an intermediate temperature near 20 K. The picture at the left shows the 20K radiation shield after the inside of the liquid nitrogen vessel is removed. During normal operation the 20 K shield is surrounded by high vacuum.
	Liquid Helium Vessel The liquid Helium vessel is made of stainless steel and is wrapped with a single layer of aluminum foil which acts a radiation shield to help lower radiant heating. The liquid helium can is about 1/16" ( 1.6 mm) thick and took about 1 hour to cut with the air-powered cutoff grinder. In the picture at the left, the 20 Kelvin shield has been removed, showing the outside of the 4.2 K liquid helium vessel. Note that the copper bore tubes are clearly visible in the picture.
	Liquid Helium Baffle Inside the liquid helium vessel around the magnet is an aluminum baffle. This baffle acts both as an infra-red radiation shield and protects the superconducting magnet from any fluctuations in the liquid helium reservoir, particularly during a liquid helium refill. This is a critical feature because superconducting magnets at low fields, such as a 54 mm bore 270 MHz, are not fully submerged in liquid helium. Higher field superconducting magnets, such as 500 MHz, must maintain the superconducting solenoid fully immersed in liquid helium. The helium vapor above the liquid is actually sufficient to maintain superconductivity of the 270 MHz magnet. When the magnet is near empty the top of the magnet is at about 6 K, the magnet will stay superconducting to about 10K before it quenches. However, this also means that any disturbance of the vapor temperature could quench the magnet. The only path for the liquid helium to reach the magnet surface is a small opening (1/4", 6 mm) at the very base. During a liquid helium refill, the magnet is protected from an accidental discharge of helium gas that might otherwise cause a quench. The picture at the left shows the liquid helium baffle with the outside stainless steel liquid helium vessel removed. The black section is cloth tape wound around the baffle to secure the liquid helium syphon tubing.
	Superconducting Magnet The picture at the left shows the internal liquid helium baffle cut away to expose the superconducting solenoid wrapped in black tape. A section on the black tape was cut away to expose a clear tape through which the superconducting shim coils are visible. The superconducting shim coils are wound on the outside of the solenoid and are used to adjust the magnetic field gradients at the probe in much the same way as room temperature shim coils. The superconducting magnet wire is made of a copper-clad niobium-titanium alloy. This magnet contains approximately 12 miles (19 km) of superconducting wire.
	Quench Resistor & Charging Plug The picture at the left shows a close-up of one of the quench resistors and the plug for magnet charging partly inserted into the magnet. The quench resistors protect the magnet during a quench by dissipating the heat generated from the 85.5 kJ of energy that is stored in the energized magnet. A quench is what happens when a superconducting magnet stops superconducting and has a finite resistance.
	Superconducting Shims The superconducting shims can be seen in the picture at the right. The superconducting shims are wound on the outside of the magnet. This allows the amount of wire required for the shim to be adjusted to meet the shim strength needs for the magnet. The vertical wires are from one of the radial shims, X, Y, ZX, ZY, XY, or X2-Y2.

Notices: All images and text are Copyrighted 2000 by JEOL USA, Inc. All rights reserved. JEOL USA, Inc. will allow non-commercial use of the text and images for educational purposes provided that JEOL USA, Inc. is acknowledged as the source of the images and text and that JEOL USA, Inc. is notified of your usage with an e-mail to nmr@jeol.com. JEOL would appreciate that you also add a link to our site, www.jeolusa.com, from any derivative web sites based on our magnet web site.

Acknowledgments: We would like to thank Oxford Instruments for discussions relating to the characteristics of the magnet.