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Ultra-High-Field NMR Magnet Design Gerhard Roth Table 1. Characteristics of 500 and 750 MHz cryomagnet systems with persistent field. Bruker BioSpin GmbH 76189 Karlsruhe, Germany Room-Temperature Bore (mm) 52 54 1 H Frequency (MHz) a 500.13 750.13 Magnetic Field Strength (T) 11.747 17.618 Introduction Total Mass of System (kg) 442 3400 It is now more than ten years since the first ultra-high-field NMR magnet was introduced to the market. On December 5, 1992, the first of a new magnet type reached its nominal field of 17.62 T for 750 MHz 1H frequency (Fig. 1), and only a few days later excellent high-resolution spectra were presented. Helium Cryostat Pressure ambient ambient Helium Cryostat Cooling none Joule-Thomson When the ultra-high-field NMR magnet project was started in 1985, the highest routinely available spectrometer frequency for 1 H NMR was 500 MHz. The project’s goal of 750 MHz required a 50% increase in the available maximum field strength, calling for new approaches with respect to a number of design features and techniques. Fig. 2 illustrates the increase in size for the first 750 MHz magnet vs. a 500 MHz magnet - an increase that is more than proportional to field strength. But the increase in size was only one of the challenges; a whole new magnet technology had to be developed, breaking ground for a new magnet generation and opening the door for even higher magnetic fields and spectrometer frequencies. Table 1 compares the typical characteristics of a standard-bore 500 MHz magnet and the new 750 MHz magnet. Fig. 1: The world’s first 17.6 T cryomagnet for 750 MHz 1H frequency. 14 500 MHz 750 MHz In addition to the more than proportional increase in magnet mass and cryostat volume for the 750 vs. 500 MHz system, it can be seen from Table 1 that the magnet current and stored energy increase by factors of 2.8 and 11, respectively. Despite the large increase in magnet current and field strength, it was possible to reduce the drift specification through the use of a completely new joint technology. Furthermore, a completely new cryostat technology had to be developed to achieve the a Operating Temperature (kelvins) 4.2 ~2 Helium Cryostat Volume (L) 72 425 Helum Consumption (mL/h) < 20 < 180 Magnet Current (A) 70 200 Stored Energy (MJ) 0.45 5.0 Field Inhomogeneity for 1H (Hz) < 0.2 < 0.2 Typical Field Drift (Hz/h) <5 <2 Larmor freq. for H2O = 42.57637888 MHz/T reduced operating temperature and the overall stability that is required for high-resolution NMR spectroscopy. Fig. 3 illustrates schematically the various design considerations and new developments that are key to this new magnet technology, which was developed in a continuing long-term cooperation with the Research Center (Forschungszentrum) Karlsruhe. In the following sections we will take a closer look at two of the many issues that influence the design of a high-field NMR magnet. Superconducting Wire High-field magnets for NMR spectroscopy place the most stringent demands on field strength, homogeneity, and stability, and their design is, therefore, critically dependent on the availability of appropriate superconducting wires. A key property of such wire is its maximum critical current Ic (A) which is a function of the temperature T and the magnetic field B experienced by the wire. Likewise, there is a critical temperature Tc which depends on I and B and a critical field Bc which depends on T and I. If any of the parameters I, B, or T exceed their critical values anywhere along the wire, there is a transition from the superconducting to the resistive state. The high current flowing in a now resistive section of the wire generates heat which, in a kind of chain reaction, rapidly propagates throughout the magnet, “quenching” the superconductive state and causing the entire energy stored in the magnetic field to be converted to heat with rapid boil-off of the surrounding liquid helium bath. Other key parameters influencing magnet design are the commercially available wire cross sections and lengths as well as the wire’s mechanical (tensile) strength. Fig. 2: Sandard-bore magnet size comparison: 11.7 T (500 MHz 1H) vs. 17.6 T (750 MHz 1H) standard-bore (52 mm). Quite frequently it is reported, that new superconducting alloys or compounds have been discovered which are superconducting even at very high fields. However, despite their promising properties, these new superconductors exist at first only as small-scale laboratory samples. For practical use in magnet construction, they must be developed into an industrial conductor which can be reliably manufactured in sufficient lengths while retaining the new, promising short-sample characteristics. Very few of the new superconductors are at all suitable for manufacturing in the quality and quantity needed for NMR magnets. Typically 10 to 20 years of development are required before a new superconducting alloy or compound becomes a successful commercial product. The latest example is the discovery of ceramic or high-temperature superconductors (HTS) in 1986, which only recently could be manufactured as a tape in lengths of a few hundred meters. Since an NMR magnet typically contains ca. 100 km of superconducting wire, the use of HTS conductors for this application is at best a prospect for the distant future. magnet operating at 4.2 K. Therefore, in 1979 a new superconductor based on Nb3Sn was developed and successfully used for the innermost section of the first 11.7 T magnet. A typical Nb3Sn conductor (Fig. 4b), which must have sufficient flexibility and tensile strength for precise winding into a solenoid form, consists of ca. 10000 pure Nb filaments in a bronze matrix (copper with 13.5% tin). The bronze matrix is surrounded by a tantalum barrier and a pure copper sheath. In its native state this wire has the desired mechanical properties for winding, but the Nb filaments are poor superconductors, quenching at field strengths above ca. 2 T. Thus, after the coil has been wound, it requires thermal treatment (baking) at ca. 700 °C for several days to weeks. At this temperature the Sn atoms in the bronze matrix diffuse into the Nb filaments, forming in situ the desired Nb3Sn compound in the so-called A15 phase with the crystal structure of a typical metallic high-field superconductor. This A15 phase has excellent superconducting properties, but it is extremely brittle and cannot be used in the original (flexible) wire. Fig. 5 compares NbTi and Nb3Sn superconducting wires that have rectangular cross sections and are used in 800 and 900 MHz ultra-high-field magnets (18.79 and 21.14 T). A magnet designer must take into account all of the properties of the special conductors mentioned above when laying out a complete magnet design for a particular field strength B0 and bore diameter. For those coil sections requiring Nb3Sn, special Fig. 4 presents cross-sectional views of two typical superconductors used in the construction of high-field NMR magnets. NbTi multifilament wires are typically used to construct solenoids for field strengths of up to 9.4 T (400 MHz) since these conductors offer the best performance: high current density, mechanical strength, ease of winding. The wire shown contains 54 NbTi filaments in a hexagonal array embedded in a copper matrix. The NbTi alloy has the desired superconducting properties (toleration of high currents and high fields) at 4.2 K, the temperature of liquid helium at atmospheric pressure, while the copper matrix serves as a mechanical support and as the main conductor at temperatures above the critical transition temperature Tc (i.e., during a quench), thus protecting the NbTi from damaging heating since it is a poor conductor in the normal state (T > Tc ). The superconducting properties of NbTi multifilament wire are not sufficient for the construction of a persistent 500 MHz Fig. 3: Design issues for ultra-high-field magnet systems. 15 Fig. 4: Cross sections (light microscope) of superconductors used in high-field NMR magnets: a) 0.85-mm NbTi wire with 54 filaments; b) 0.85-mm Nb3Sn wire with ca. 10000 filaments. fiber glass wire insulation must be used, and all other materials must be chosen to withstand the heat treatment at 700 °C. Moreover, since the finished magnet will operate near -273 °C, such coil sections must retain the desired mechanical and physical properties over a temperature range of about 1000 °C. The extremely brittle character of the final Nb3Sn conductor after heat treatment means that all connections to such coil sections have to be predefined. After heat treatment no changes or corrections to the coil section can be applied. Thus, the properties of the wire before and after baking are particularly important parameters for the mechanical design of a superconducting magnet. Of course, magnet design is strongly dependent on the wire’s critical current Ic, which increases with decreasing temperature but decreases with increasing magnetic field strength experienced by the wire. The current carrying capacity for NbTi and Nb3Sn multifilament superconductors at 4.2 K and 1.8 K are shown in Fig. 6. For NbTi wire with a diameter of only 0.85 mm, the maximum possible current at 4.2 K Fig. 5: Comparison of rectangular wires used in ultra-high-field magnets: NbTi (66 filaments) and (NbTaTi)3Sn (ca. 50000 filaments). 16 Fig. 6: Behavior of the critical current vs. field strength and temperature for the superconductors shown in Fig. 4: (A) critical current for 0.85mm diam. NbTi superconductor with 54 filaments and (B) Kramer plot of jc1/2 • B01/4 vs. B0 for Nb3Sn conductors. The superconductive state is maintained only for conditions below and to the left of the plotted lines. decreases linearly from a remarkable 600 A at ca. 4.7 T to only 100 A at 9.4 T (Fig. 6A). Obviously, at these huge currents such a thin wire would be destroyed if it suddenly became resistive as in the case of a magnet quench – unless an appropriate protection circuit is used to protect the coil. This issue will be discussed in a future article about high-field magnets. Lowering the magnet temperature to 1.8 K shifts the Ic vs. B line to the right but does not alter its slope. According to Fig. 6A, a 400 MHz NMR magnet (9.4 T) can only be operated at currents up to ca. 100 A in liquid helium at ambient pressures. With further cooling to 1.8 K, the available current capacity would increase dramatically (by a factor of 4) to about 400 A at a current density of ca. 550 A/ mm2. Such an increase in current carrying capacity of the superconductor, would allow us to reduce the wire diameter (cross section) by a corresponding factor while retaining a given total current. For NbTi superconductor with 0.50 vs. 0.85 mm diameter, for example, the cross section is reduced by a factor of 2.9, and as a critical current this conductor could carry up to ca. 140 A. Smaller diameter wire means reduced magnet volume and mass, less stored energy, and a smaller stray field, i.e., a smaller magnet with increased safety and stability. Note, however, that it is not possible to simply take a magnet designed for 400 MHz at 4.2 K, cool it to 1.8 K, and increase the magnet current by a factor of 3 (e.g. from 100 to 300 A) to achieve a factor of 3 increase in B0. Fig. 6A shows that at 1.8 K a current of ca. 130 A would result in a field of ca. 12 T, which is at the critical boundary. Any attempt to further increase the current at this field would lead to a quench. In Fig. 6B critical plots for Nb3Sn conductors are shown; however, in this case, a Kramer plot of jc1/2 • B01/4 vs. B0 is required (jc = critical current density) in order to obtain the Fig. 7: Cross section of a new-generation HTS-based superconductor containing 313 filaments of a BiCaSrCuO compound embedded in a silver matrix. approximately linear relationship shown. At 17 T the maximum available current density is 76 A/mm2 at 4.2 K and 155 A/mm2 at 1.8 K. Thus, subcooling of the superconductor to temperatures below 4.2 K has two main effects: K K an increase in current density at a given field, allowing a reduction in size for all coil sections which are not being operated at the maximum field (reduction in overall magnet size), an increase in maximum critical field, giving access to magnetic field strengths which are not accessible at 4.2 K. Therefore, superconducting magnets which generate the highest possible field for a given conductor type must be cooled below 4.2 K. There is no possibility to reach that field value at 4.2 K. A further benefit is that use of a higher current at reduced temperatures allows for a more compact and simultaneously more relaxed magnet design, which increases production yield and overall safety of the system. The practical limit for the today’s Nb3Sn superconductor is a field strength of ca. 21 T, corresponding to 900 MHz 1H frequency. For next-generation systems with field strengths of 23.5 T, corresponding to a 1000 MHz spectrometer, it will be necessary to use a new type of conductor, at least for the innermost magnet section, which must remain superconducting at the full field strength. It appears that high temperature superconductors (HTS) have the required properties to achieve fields above 22 T. However, although the basic physical properties of HTS materials have been known since 1986, it will take several more years of development before a suitable conductor for the design of a high-resolution 1 GHz NMR magnet will be available. Fig. 7 shows the first example of such a superconductor of the future, recently manufactured at laboratory scale. The critical currents or current densities shown in Fig. 6 do not represent the actual maximum currents that can be used in practise. Note that Ic is defined as the current at which a voltage drop U of 0.1 µV occurs over 1 cm of wire, corresponding to a resistance of 10-7 / Ic Ohm per cm. If an NMR magnet with ca. 100 km (107 cm) of wire were operated close to Ic, then a voltage drop of 1 V would occur across the magnet terminals, which is on the order of a typical discharge voltage. Hence, the magnet would lose field rapidly, i.e., exhibit unacceptable drift. In practise, drift voltages must be kept to < 0.01 µV over the entire magnet, i.e., about eight orders of magnitude below the critical transition voltage. Fig. 8 illustrates this behavior with a schematic representation of the transition from the superconducting to the normal conducting state as the current is increased. Note that the transition is not simply a step function but obeys a power law; the steepness of the U vs. I curve is described by the exponent n. Depending on n, which is typically in the range 20 to 40, the maximum usable or nominal current In for a low-drift NMR magnet must be substantially lower than Ic such that the voltage drop across the whole magnet will remain below the drift voltage limit of 0.01 µV. For each coil section and for each wire type that is being used in an NMR magnet, the usable In must be determined reliably in order to achieve a successful magnet design. Ic and n depend on the properties of the materials, the wire manufacturing process itself, and variations for each individual wire batch. Thus, not only the exact determination of these properties by careful measurement is important, but also the experience and know-how of the magnet manufacturer play a key role in making the choice of an appropriate magnet current. Typically, the maximum In that can be used for an NMR magnet are ca. 30% to 70% of Ic. Subcooling Technology The stable and long-term continuous operation of an ultrahigh-field NMR magnet requires that the cryostat be designed to be insensitive to possible disturbances such as changes in helium evaporation rate, room temperature, ambient pressure, and the cryogen levels inside the cryostat. Fig. 9 shows a schematic cross sectional view of the cryostat used for Bruker’s UltraStabilized™magnet systems (B0 > 17 T). From the outside the cryostat looks much like a conventional Fig. 8: Voltage vs. current for the transition from the superconducting to the resistive state. The voltage drop U across a short piece of superconductor as a function of current I can be described by the power law shown with exponent (index) n. The critical current Ic is defined as the value of I where U = U0 = 0.1 µV/cm. However, a low-drift NMR magnet requires U < 0.01 µV over the entire magnet coil. 17 helium which is continuously flowing through the needle valve. With this technique all of Bruker’s subcooled magnets achieve an extraordinarily high temperature stability with variations only in the range of 0.1 mK - hence, the term ultrastabilized. The needle valve used in Bruker’s UltraStabilized magnets can handle a wide range of flow settings, ranging from the high cooling power needed during magnet cool down and energization to the very small flow settings for continuous, longterm operation of the magnet. Fig. 9: Schematic of Bruker’s UltraStabilized™cryostat for subcooled ultra-highfield magnet systems. cryostat, consisting of an outer vacuum case, a liquid nitrogen vessel, some radiation shields and a liquid helium vessel in which the magnet solenoid is mounted. However, in contrast to conventional cryostats where a single helium bath remains at ca. 4.2 K, the liquid helium vessel in Fig. 9 is divided into two parts: an upper section at 4.2 K, and a lower section, containing the magnet, which is cooled to ca. 2 K. To ensure long-term, safe operation, the complete content of the helium vessel is kept at a slight overpressure as helium gas from the upper vessel passes through the cryostat towers and exits through a check valve. Overpressure prevents air from being drawn into the cryostat and minimizes the danger of ice formation within the towers. The lower helium vessel is connected to the upper one through narrow channels which ensure that both containers are always at the same pressure (slightly over ambient). However, the temperature of the lower helium bath is maintained at ca. 2 K, which corresponds to an equilibrium vapor pressure of 30 mbar above the liquid. To avoid such a low pressure above the helium bath, the lower temperature of the lower bath is generated using a Joule-Thomson cooling unit in which liquid helium is allowed to expand through a needle valve into a heat exchanger. Only the heat exchanger circuit is kept at a pressure of < 30 mbar via a vacuum pump and, thus, always below the operating temperature of the lower helium bath. Since the heat flow from the helium bath into the heat exchanger is equal to the heat load of the lower helium bath, the operating temperature of the lower bath remains constant. The heat flow into the heat exchanger determines the required flow of liquid helium through the needle valve and, therefore, the necessary pumping speed. With this design the helium temperature in the magnet bath no longer depends on atmospheric pressure (as in conventional cryostats) but only on the stability of the cooling power for the lower bath. The stability of the lower bath temperature depends mainly on the stabilities of the needle valve setting and the pumping speed, the two factors which define the amount of 18 The design of this new cryostat type has been patented because of the unique features which had not been employed before in subcooling technology. The cryostat is a so-called low-loss cryostat, which, for conventional 4.2 K systems, means that the enthalpy of the escaping helium gas evaporating through the towers is used to cool the intermediate radiation shields and to lower the overall helium consumption. However, for a subcooled cryostat the major helium flow is generated by the subcooling of the lower bath, resulting in only a small residual gas flow through the towers and little enthalpic capacity for cooling the intermediate radiation shields. To better utilize the available enthalpy of the helium gas leaving the cryostat, it was necessary to find a way to use the enthalpy of the helium that escapes via the needle valve - the predominant fraction of helium exiting from the cryostat. In our patented technology the helium passing through the needle valve of the cooling unit is fed back into the towers, and its remaining enthalpy is, thus, available for cooling of the intermediate radiation shields. The method is very efficient and makes full use of the available enthalpy of the helium gas. This can easily be verified by measuring the temperature of the pumped helium leaving the cryostat (touch the plumbing where it exits the cryostat) - it is nearly at room temperature. Thus, helium which flows through the needle valve exercises all of its cooling power within the cryostat structure, and subcooled systems with this cryostat design have the lowest possible helium consumption for a given magnet size. Table 2. Installations of Bruker UltraStabilized™NMR magnet systems (as of Aug. 2003).a 1 H (MHz) Asia America 750 SB 1 1 2 750 WB 4 0 2 800 SB 15 11 12 800 US 1 3 5 900 SB 4 0 1 25 15 22 2 total a Europe SB = standard-bore (54 mm); WB = wide-bore (89 mm); US2 = standard-bore, UltraShield, UltraStabilized Fig. 10: Bruker’s first 800 US2 magnet system installed at the Riken NMR park in Japan. Over the past ten years, a large number of these ultra-high-field NMR magnets have been installed world-wide (Table 2). The majority of applications are in biochemical research (e.g. proteomics); however, more recently, the number of systems being used for solids NMR, even at 900 MHz, has been increasing. The techniques originally developed for the 750 MHz systems have been expanded to a number of new magnets and higher fields. Once the technology existed, it could be scaled up to give the 800 MHz standard-bore system. Then a 750 MHz widebore magnet was developed, specifically for solids and imaging applications, followed by the 900 MHz standard-bore system in 2001. Finally, a new 800 US² standard-bore magnet with UltraShield and UltraStabilized technology was added to the product line, featuring active shielding to reduce the radius of the horizontal 0.5 mT stray field line from 6.1 to only 2.2 m. Fig. 10 shows the first 800 US2 magnet at the Riken NMR Park in Japan (installed Sept. 2001). Fig. 11 shows the first 900 MHz magnet, which was installed at Scripps Research Center in July 2001. In the initial phase of development our new technology had to face a number of objections, initiated by concerns about safety and stability during long-term operation. However, the subcooled ultra-high-field magnet systems were designed from the start for trouble-free, continuous operation (very longterm). Now we have over 60 UltraStabilized magnets in the field with a total accumulated operating time of over 200 magnet-years, and the first installed systems have been in continuous, failure-free operation for more than 9 years. Today, Bruker is working on the next step to reach even higher magnetic fields - the 1000 MHz magnet - a joint project with the Forschungszentrum Karlsruhe and Vacuumschmelze in Hanau. For this ambitious project the first and prerequisite step is the development of a new HTS-based conductor with the proper physical properties for the construction of highresolution NMR magnets. In July, 2003, Bruker acquired Vacuumschmelze’s superconductor business and founded the new company called EAS (European Advanced Superconductors). Thus, we now have more direct control over superconductor characteristics which are key to the development of our future ultra-high-field NMR spectrometers. Fig. 11: Bruker’s first 900 MHz magnet installed at the Scripps Research Center, San Diego, USA. 19