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Cryogen-Free Systems
Magnets wound from NbTi or Nb
3
Sn may be conduction-cooled and operate in vacuum or helium gas at 4.2K. Current leads
constructed with High Temperature Superconductor allow high operating currents with very
low heat losses enabling a 1 Watt cryocooler to handle the entire system losses. Cryogen
free systems can replace most conventional liquid refrigerant shielded magnet systems if
a compact system is required, or if refrigerants are undesirable or unavailable. AMI
provides both solenoid and split coil based systems.
Systems shown below have optical radial access (split coil magnets) and are sealed with beryllium windows.
The system on the left is a Nb
3
Sn cryogen free system designed to provide a high gradient field at a point external to
the cryostat face.
The magnet on the lower left is a 5 Tesla, 3.5 inch (89mm) radial access cryogen-free split coil.
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Systems with Variable Temperature Insert (VTI)
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Many experimental systems require the ability to vary the sample temperature over a wide range
within the background magnetic field. AMI provides a wide variety of variable temperature inserts
for this purpose. The most common inserts accommodate sample sizes of 0.5 to 1.75 inches and
include the ability to vary the sample temperature by either flowing helium gas over the sample
or indirectly by thermal coupling of the sample chamber through an exchange gas. Temperatures
down to 1.5 K can be achieved by flooding the sample chamber with liquid helium and subsequently
pumping on the chamber to reduce the pressure. Standard features include sample mounts, helium
flow valve, helium vaporizer, rotating sample positioner, top loading sample mount, pumping port,
pressure relief and signal wire connections to the sample. The unit shown here is 6 Tesla system
with a 2 inch sample space.
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Projected Magnetic Field Superconducting Magnets
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Certain applications in Magnetic Resonance require a homogeneous field to be projected outside the cryostat
for all-round access, or for penetration inside a large object under investigation. Although some NMR
measurements have been performed in the fringe field of superconducting magnets, specially designed Field
Casting magnets have found use in mineral exploration NMR and for non invasive MRI evaluation of organic
materials in industrial settings. The example shown projects a 0.1 Tesla homogeneous field 20cm above the
top plate of the cryostat. Although conventional cryostats with liquid helium and nitrogen are most common,
it is also possible to conduction cool with a refrigerator as shown in the example.
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High Homogeneity Room Temp Bore Systems
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High homogeneity systems are characterized by the need for very high field
uniformity over the region of interest and very high temporal stability. These
high homogeneities, in the parts per million range or better, are required for
any of the resonance measurements including Nuclear Magnetic Resonance (NMR), Magnetic
Resonance Imaging (MRI), Ion Cyclotron Resonance (ICR) , and Electron Spin Resonance
(ESR). Other desirable features of these systems include self shielding to reduce the
fringing field at the extremities of the cryostat and low losses allowing for
long refrigerant hold times (typically 3 months for liquid helium and 4 weeks
for liquid nitrogen). This magnet also happens to be actively shielded as well.
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High Magnetic Field Systems
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Many solid state physics experiments require the highest possible magnetic
fields for high H/T or materials characterization measurements, particularly of
superconductors. Using a combination of Nb3Sn and NbTi, fields up to 20 Tesla
are now possible. For the highest possible field strength, a Joule Thompson
refrigerator is used to reduce the temperature of the helium bath to 1.8 K.
Alternatively, a simpler lambda point refrigerator can reduce the temperature to
2.2 K, while allowing refilling at atmospheric pressure. AMI manufactures
both high field solenoid and split coil systems. The system shown here is
integrated with a VTI and lambda plate refrigerator unit.
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Actively Shielded Magnet Systems
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Shielding refers to one of several methods to reduce the stray fields which
normally emanate from a magnet through the air. Such fields can interfere with
other equipment such as computer screens or other electronics. In close
proximity to a large magnet the forces can be large enough to attract loose
magnetic object such as carts, chairs or hand tools. Understanding your
facilities requirements with regards to magnetic fields is an important step in
installing a new magnet system. One very nice way to achieve excellent shielding
is by surrounding the main magnet coil with a properly designed and precisely
placed set of coils which generate a field in the opposite direction. These
superconducting coils are contained inside the cryostat and many times can
effectively reduce the external fields to a 5 gauss level at the surface of the
cryostat. These coils are normally wound in series with the main coil and
therefore the user see no additional operation complications. The actively
shielded magnet shown here is an 8 Tesla, 3 inch bore with an upper compensated
region for operation with a dilution refrigerator.
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Passively Shielded Magnet Systems
In instances where the control of stray magnetic fields are important one option is to surround
the magnet with ductile iron or other metal (Mu metal) with a high degree of magnetic permeability
or high field saturation point. This type of system can be effective and often is more economical
than active shielding, however, drawbacks include field reduction limitations and added weight.
The coil shown here is directly enclosed in an iron shield and designed for use in a linear
accelerator. More commonly the outside of the cryostat surface is encased with the shielding
material. Shielding should always be a consideration in high traffic areas or in areas where
restricted access administrative controls is not practical.
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Horizontal Room Temperature Bore Axial Field Systems
Room temperature bore systems give the user access to the magnetic
field in an ambient condition environment. Such systems are useful when
beams, temperature sensitive samples vacuum chambers or furnaces must be
placed in the bore. Systems containing smaller sized solenoids (to left)
may have a rectangular tail section, horizontally mounted magnet bore
and larger liquid helium reservoir above the magnet. Systems with larger
magnets and horizontal fields will often be provides like the units
shown (below).
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Perpendicular Field Room Temperature Bore Systems
These magnet systems, also referred to as Radial Access
room temperature bore systems, provide the customer with unrestricted
access perpendicular to the magnetic field direction. Most of these have
been sold to OEM vendors providing the cryostats themselves. The magnet
pictured here was a rather large 7 Tesla helmholtz coil with 2 inch
radial access over
+
32
o
from the magnet center. It was also design suitable for baking in a UHV system without damage to the coils.
3D Model of Type CH Split Coil System
3D Model of Type VH Split Coil System
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Cold Bore Radial Access Systems
These system are used when a cryogenic insert of some type (VTI, Dilution
Fridge, etc.) is used in the bore perpendicular to the magnetic field. AMI
employs proven techniques to ensure quality and minimize the chances of
expensive training quenches so often seen in split coils made by other
manufacturers.
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Bottom Loading Magnet Systems
As the name implies, the internal liquid helium chamber can be accessed from the bottom.
This is done by using a series of precision sealed flanges. The dewar also provides
conventional top opening access to the helium bath for insertion of the experimental
insert. A bottom loading dewar is a good selection in cases where a cryogenic insert
of some type is used and the magnet size is physically large in relation to the cold
bore access needed.
3D Model of Typical Bottom Loading System
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Extended Field (Pumped) Systems
In certain instances the best solution to obtain higher magnetic fields is to lower the pressure
of the helium reservoir surrounding the magnet and thus lower the Ic of the conductor. Pulling
down the pressure with a vacuum pump can easily lower the bath temperature from 4.2K to 2.5K or
lower. A Lambda Point refrigerator or heat exchanger is incorporated onto the magnet support system
just above the magnet to reduce the temperature. A physical plate loosely separates the lower 2K
bath from the upper 4K reservoir. At the reduced temperature most superconducting magnets will
achieve an additional 2 Tesla. It is important to communicate your desire to run a magnet at reduced
temperatures so that the designer can ensure the added stresses of the higher fields have been
accounted for. The system shown (on left) is an example of a 9/11 Tesla magnet with a Variable
Temperature Insert (VTI) and integral Lambda Point refrigerator.
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Optical Access Systems
AMI designs, builds and supplies many types of magnets for optical research systems. Most of these
have been sold to OEM vendors providing the optical cryostats themselves. AMI now offers an optical
multi-axis magnet system called the OptiMAxes
TM
The system details are shown in the drawing (below) which produces a 1 Tesla rotating vector and 4
Tesla single axis field.
The magnet shown (right) was a unique radial access magnet that was incorporated into an optical
2-axis system which also included internal dipole racetrack coils for an extended region of
homogeneity.
3D Model of OptiMAxes
TM
- 411 System
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Two-Axis Superconducting Vector Field MAxes
TM
-2 Systems
Two axis systems provide variable magnetic field on any two principal axes. They can be
very useful for ensuring perfect sample alignment with the magnetic field by simply tilting
the main field using the second orthogonal magnet. The system is comprised of a 2-axis
superconducting magnet, cryostat with 2 sets of helium efficient vapor cooled current leads
and other associated electronics. Specifications for the system shown here for STM work
include a high field 9T solenoid; for the principal axis, a 2.5 inch vertical clear bore
and 1T rotating vector in the x, z plane. It is possible to use such magnet systems with
existing sample inserts or AMI can provide a VTI which operates from 1.5K to 300K.
The 2-axis magnet system provides a unique way to rotate the magnetic field vector electronically without relying on
mechnical positioners. The magnet system
software provides automatic sequencing of power supply currents and thus magnetic
fields that allow the user to specify and control the magnetic field
vector from a single computer screen. The interface allows the user to
enter the desired field vector of a 2 or 3-axis magnet in Cartesian,
cylindrical, or spherical coordinates. Cryogen Free systems and Optical
Access systems are available as are many custom configurations.
3D Model of MAxes
TM
-2
Standard MAxes
TM
Systems
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Three-Axis Superconducting Vector Field MAxes
TM
-3 Systems
Three axis systems provide variable magnetic field on the three principal axes
and are particularly useful for orientation studies on a variety of samples.
The system is comprised of a 3-axis superconducting magnet, customized
support structure having 3 sets of helium efficient vapor cooled current
leads, magnet dewar and other associated electronics. Typical
specifications include high field up to 9T for the principal axis, 2.0/3.0
inch vertical clear bore and 1T rotating vector using any combination of
x, y and z-axis magnets. It is possible to use the magnet system with
existing sample inserts or AMI can provide a VTI which operates from 1.5K
to 300K (right). Our users have also used such systems with 3rd party He3 systems
and dilution refrigerators. The magnet shown (lower right) has a compensated low field
region above the vector magnet.
The 3-axis magnet system provides a unique way to rotate the magnetic field vector on
the three principal axes and this has proved useful in performing anisotropic studies
on a variety of materials. These magnet systems have also been very useful in advancing
research in the areas of spin based physics. The magnet system software provides
automatic sequencing of power supply currents and thus magnetic
fields that allow the user to specify and control the magnetic field
vector from a single computer screen (below). The interface allows the user to
enter the desired field vector of a 3-axis magnet in Cartesian,
cylindrical, or spherical coordinates. Cryogen Free systems and Optical
Access systems are available.
Customers are finding the combination of a Dilution Refrigerator
with a 3-axis vector field magnet useful in the study of Quantum Mechanics and
other nanoscale studies. The attached link is to one such site:
Del Barco Group Research
3D Model of OptiMAxes
TM
- 411 System
Standard MAxes
TM
Systems
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Level Instrumentation
