Download Ultra-High-Field NMR Magnet Design

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