14

Introduction

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.

When the ultra-high-field NMR magnet project was started in

1985, the highest routinely available spectrometer frequency for1H 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 spec-

trometer frequencies. Table 1

compares the typical charac-

teristics of a standard-bore

500 MHz magnet and the new

750 MHz magnet.

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 techno-

logy. Furthermore, a completely

new cryostat technology had to

be developed to achieve the

Gerhard Roth

Bruker BioSpin GmbH

76189 Karlsruhe, Germany

Ultra-High-Field NMR Magnet Design

Table 1. Characteristics of 500 and 750 MHz cryomagnet systems withpersistent field.

500 MHz 750 MHz

Room-Temperature Bore (mm) 52 541H Frequency (MHz) a 500.13 750.13

Magnetic Field Strength (T) 11.747 17.618

Total Mass of System (kg) 442 3400

Helium Cryostat Pressure ambient ambient

Helium Cryostat Cooling none Joule-Thomson

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 < 2a 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. 1: The world’s first 17.6 T cryomagnetfor 750 MHz 1H frequency.

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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. 2: Sandard-bore magnet size comparison: 11.7 T (500 MHz 1H)vs. 17.6 T (750 MHz 1H) standard-bore (52 mm).

Fig. 3: Design issues forultra-high-field magnetsystems.

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.

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

16

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. 4: Cross sections (lightmicroscope) ofsuperconductors used inhigh-field NMR magnets:a) 0.85-mm NbTi wire with54 filaments; b) 0.85-mmNb3Sn wire with ca. 10000filaments.

Fig. 5: Comparison of rectangular wires used in ultra-high-field magnets:NbTi (66 filaments) and (NbTaTi)3Sn (ca. 50000 filaments).

Fig. 6: Behavior of the critical current vs. field strength and temperaturefor the superconductors shown in Fig. 4: (A) critical current for 0.85-mm diam. NbTi superconductor with 54 filaments and (B) Kramer plotof jc

1/2 • B01/4 vs. B0 for Nb3Sn conductors. The superconductive state

is maintained only for conditions below and to the left of the plottedlines.

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 • B0

1/4 vs. B0 is

required (jc = critical current density) in order to obtain the

17

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:

� 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 ultra-

high-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. 7: Cross sectionof a new-generationHTS-basedsuperconductorcontaining313 filaments of aBiCaSrCuOcompound embeddedin a silver matrix.

Fig. 8: Voltage vs. current for the transition from the superconducting tothe resistive state. The voltage drop U across a short piece ofsuperconductor as a function of current I can be described by the powerlaw shown with exponent (index) n. The critical current Ic is defined asthe value of I where U = U0 = 0.1 µV/cm. However, a low-driftNMR magnet requires U < 0.01 µV over the entire magnet coil.

18

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

Fig. 9: Schematic ofBruker’sUltraStabilized™cryostatfor subcooled ultra-high-field magnet systems.

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, long-

term operation of the magnet.

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 magnetsystems (as of Aug. 2003).a

1H (MHz) Europe Asia America

750 SB 1 1 2

750 WB 4 0 2

800 SB 15 11 12

800 US2 1 3 5

900 SB 4 0 1

total 25 15 22

a SB = standard-bore (54 mm); WB = wide-bore (89 mm);

US2 = standard-bore, UltraShield, UltraStabilized

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Fig. 11:Bruker’s first900 MHzmagnet installedat the ScrippsResearch Center,San Diego,USA.

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 wide-

bore 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 long-

term). 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 high-

resolution 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. 10:Bruker’s first800 US2 magnetsystem installedat the RikenNMR park inJapan.