Standard Construction of Modern High Field Magnets Used in Modern Nuclear Magnetic Resonance Devices
Nuclear magnetic resonance devices are playing an increasingly important role in healthcare and research today. As the term implies, magnets, specifically high field magnets, are an essential part of these sophisticated devices with important implications for a wide range of valuable healthcare and research applications. To gain additional insights into how these devices operate, this paper provides a discussion concerning the standard construction of modern high field magnets used in nuclear magnetic resonance devices, including a detailed graphic illustrated the different components of a representative magnet. An examination of the effects of transitions to higher magnet strengths on cooling systems is followed by an analysis of the superconducting materials used and a brief description of magnet construction. A discussion concerning the differences between shielded magnets and non-shielded magnets and innovations in technology that may allow room temperature magnet applications that avoid evaporation in the future is followed by a summary of the research and important findings and trends are presented in the conclusion.
Review and Discussion
Standard construction of modern high field magnets used in nuclear magnetic resonance devices
The history of the construction of modern high field magnets can be traced to the late 20th century when magnetic resonance applications were being routinely used for medical diagnosis (Carlisle 2004, Leroy 2003). According to Jacoby and Youngson (2005), "When high field magnets were introduced in the 1980s, scan became quicker to do and improved computer technology made the images much clearer" (p. 190). Initially termed "nuclear magnetic resonance" (NMR) when applied to human patients, the medical community determined that the use of the word "nuclear" was counterproductive and the process was renamed magnetic resonance imaging for all intents and purposes (Goldberg 2007).
The construction of the high field magnets used in these diagnostic devices improved over time as the materials and manufacturing processes continue to introduce innovations in high-field magnet performance (High field magnets 2011). At present, typical high-field magnets use multifilamentary Nb3Sn windings to generate magnetic fields ranging between 10 Tesla to more than 15 Tesla at 4.2K (High field magnets 2011). All such high-field magnets are comprised of Nb3Sn inner coil (high field region) that is surrounded by an 8 to 9 Tesla NbTi outer coil (background field) (High field magnets 2011). According to this vendor, "Coil windings are permeated with epoxy to ensure the absence of voids and prevent wire movement and its subsequent 'training' effects (High field magnets 2011, p. 3). These recent innovations have meant that nuclear magnetic resonance have become increasingly valuable research tools in the laboratory (Wanjek 2003). In fact, a high field magnet research facility at Oxford University has been in the vanguard of developing world-class superconducting materials (High field magnet facility 2011).
Discussion concerning the effects of transitions to higher magnet strengths on cooling systems
Phase transitions in higher magnet strengths represent a particularly challenging phenomenon because of the "dome-shape" it describes. Furthermore, the operation of the various types of cooling systems used in different types of superconductors are affected by a wide range of performance metrics, including design, materials composition, and the type of superconductor that is involved (Johnston 2009). Likewise, an effective energy gap in superconductors has been demonstrated in microwave absorption experiments that clearly illustrate the effects of such transitions to higher magnet strengths on the type of cooling levels that are required for optimal operation (Wang, Ono, Onose, Gu, Adno, Tokura, Uchida & Ong 2002).
According to a recent patent application from Bruker Biospin GmbH (Rheinstetten, Germany, 2011), "High temperature superconductors (HTS) of oxidic ceramic material have been known since 1986. They are particularly characterized by very high transition temperatures of up to 120K as well as very high critical magnetic field upper limits (BC2)" (Superconducting magnet coil for very high field 2011, p. 2). The functionality of these ceramic materials, though, is constrained by their fragility and the complex nature of the processes steps involved in maintaining temperatures appropriate for optimal superconduction (Superconducting magnet coil for very high field 2011).
For instance, the patent application for a superconducting magnet coil for a very high field points out that, "In a processing step, thermal treatment is carried out in an oxidizing atmosphere at temperatures in the range of 800° C. To maintain the optimum superconducting properties, the oxygen content of the atmosphere must be controlled with high precision and must be continuously provided to the superconductor in the required concentrations in accordance with a desired processing procedure" (2011, p. 2). The patent application adds that optimal operating temperatures are typically within a narrow range of tolerances, but improvements in material composition and design and reducing this constraint (Superconducting magnet coil for very high field 2011, p. 2).
Superconducting magnetic materials
According to JEOL's promotional literature, "Most modern NMR spectrometers utilize a magnet fabricated from superconducting materials and the magnet winding is cooled with liquid helium" (JEOL Nuclear Magnetic Resonance Spectrometers 2011). As to the materials needed for optimal volume of interlayer cooling channels for superconduction, Ivanov, Balashov and Shchegolev (2003) report that, "The optimal volume of interlayer cooling channels increases the conductor cryostability, and creation of additional gaps of ~200 ?m in the winding spacers is able to intensify the heat transfer" (p. 1011).
Description of magnet construction
Currently, the bore diameters of most standard high field magnets are between 1 inch and 3 inches, with corresponding homogeneities of 0.1% in a 1 cm diameter spherical volume (DSV) (High field magnets (2011). The schematic for construction of a typical high field magnet is provided in Figure 2 below.
Figure 2. Schematic of high field magnet construction
Source: High field magnets (2011) at http://www.americanmagnetics.com/images/highsol.gif
Typically, modern NMR instruments are constructed as solenoids with wiring comprised of a Niobium-titanium alloy (< 750 MHz) that superconducts at 4K (-269C). Niobium tin (above 750 MHz) that is superconducting at 4K (-271C) (SMA007 NMR course summary, 2011). The solenoid is immersed in a bath of liquid helium which is placed in a flask surrounded by a jacket of liquid N2. The He and N2 evaporate and must be replaced (N2 capacity 20 days, He 200 days). The superconducting magnet creates a powerful magnetic field and the solenoid is aimed at the targeted field with the optimal field strength being equal at all points in the targeted sample (see Figures 3 and 4 below).
Differences between shielded magnets and non-shielded magnets
In sharp contrast to unshielded magnets which do not offer any protection from the magnetic field being generated, shield magnets provide some level of protection, depending on the material composition of the shielding used. At present, although there are no materials known that completely block magnetic fields without being attracted to the magnetic force, high-permeability shielding alloys can redirect the magnetic field in useful ways (Magnetic fields and shields 2011). Although the precise composition of most shielding materials remain propriety and confidential, Agilent reports that the main magnet in its Premium Shielded Narrow Bore Magnets array (pictured in Figure 5 below) is "housed within a low-loss cryostat; liquid helium transfer siphon and extension tube; braided liquid nitrogen transfer line" (2011, p. 2).
One industry vendor also cites that proprietary nature of the various ferromagnetic shielding materials used, including the observation that, "Their alloy composition is a highly guarded secret, based on years of extensive research and application. CO-NETIC-AA, NETIC S3-6 and MuMetal are three unique shielding materials provided by Magnetic Shield Corp." (Magnetic fields and shields 2011, p. 2). Another general observation from this vendor provides some useful insights into the differences between shielded and unshielded magnets. According to Magnetic Shield Corporation (2011), "All commercially available magnetic shielding materials are ferromagnetic. This means they are attracted by a magnet just like iron or steel. Ferromagnetic materials are necessary because shields work by pulling the magnetic field towards them and away from what needs to be shielded" (Magnetic fields and shields 2011, p. 2).
Innovations in Supporting Technology
Currently, very high-field magnets must maintained at cold temperatures using liquid hydrogen and liquid nitrogen (Depalma 2003, p. 45). Seminal research in the early 1970s by Osheroff serendipitously identified the superfluidity of helium 3 at very low temperatures while investigating the nuclear antiferromagnetism of helium 3 (Leroy 2003). According to Leroy, "For his experiments he needed to use a large magnet to produce solid helium [and] identified a phase transition in an experiment using nuclear magnetic resonance [and reported a] BCS type transition in liquid helium [that] described the transition of a material to the superconductive state by the coupling of fermions to give bosons" (2003, p. 218). The promotional literature from JEOL emphasizes that today, samples needed for NMR analysis remain at room temperature avoiding evaporation (JEOL Nuclear Magnetic Resonance Spectrometers 2011). Likewise, Agilent (2011) notes that the features of its Premium Shielded…