Cryogen-Free Systems

Magnets wound from NbTi or Nb₃Sn 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 on right and below have optical radial access (split coil magnets) and are sealed with beryllium windows. The system on the left is a Nb₃Sn 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.

Systems with Variable Temperature Insert (VTI)

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.

Projected Magnetic Field Superconducting Magnets

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.

High Homogeneity Room Temp Bore Systems

High Magnetic Field Systems

Actively Shielded Magnet Systems

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.

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.  

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).

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

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.

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

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.

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

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

Three-Axis Superconducting Vector Field MAxes^TM-3 Systems

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