X-Ray Diffraction

Conduction-cooled (i.e. cryogen-free) magnets specially designed for X-Ray Diffraction measurements have the advantage of compact size, compared to a conventional liquid refrigerant system, permitting the assembly to be fitted to existing goniometers. A variable temperature insert, typically 2 to 300 K allows for sample rotation in the magnetic field. Thin Beryllium windows are used throughout to minimize secondary scattering. The system pictured (on left) is of a 5T cryogen-free s X-ray diffraction system. An exampled of magnets used in such systems is also shown (below).

Neutron Diffraction

Atom/Plasma Traps

Magnetic trapping has developed over the past few years as a tool for the study of atomic and condensed matter. The number of particles that can be loaded into a magnetic trap increases as the size and confining potential of the trap is increased. A common magnetic trap is one constructed of four racetrack coils for radial confinement and two solenoids at each end for axial confinement. Many other configurations including three axis systems are also used for magnetic trapping.

Scanning Tunneling Microscope (STM)

The scanning tunneling microscope (STM) is widely used in both industrial and fundamental research to obtain atomic-scale images of metal surfaces. It provides a three-dimensional profile of the surface which is very useful for characterizing surface roughness, observing surface defects, and determining the size and conformation of molecules and aggregates on the surface. The system shown (left) is a 9/11 Tesla, 3.5" bore system. An additional consideration is to keep the cryostat top plate as uncluttered as possible (right). This allows the magnet system to be easily supported from the underside of a vibration isolation table and provides maximum working area for the STM insert itself.

Magnetic Levitation

		For levitation of diamagnetic materials a high magnetic field and field gradient product B(dB/dz) is required. Diamagnetic materials are subject to repulsive force in magnetic fields. When such magnetic force counterbalances the gravitational force, the materials will levitate in the gradient magnetic field. By changing the magnetic field direction an increased force is applied. The magnetic levitation produced by this technique provides an ideal equivalent condition to microgravity. This technique is also very useful in growth of protein crystals in microgravity conditions. The system shown has a 16T 2 /cm product in a 1 inch room temperature bore. The important design feature for stable levitation is the field profile in the radial direction that must increase with increasing distance from the magnetic axis. A superconducting magnet offers superior performance to a high field resistive magnet because it has high temporal stability and no vibrations. Additional research to use this artificial gravity capability on chemical processes shows promise by achieving improved results with smaller sample volumes and smaller processing equipment sizes. Increased liquid holdup time through a catalytic trickle-bed reactor is one example. The magnet below was used for this purpose. The system pictured (left) is a 9.5 Tesla Levitation System producing 16.6 T 2 /cm within a 1" room temperature bore. The system (below) has a 2" inch room temperature bore and produces 9 Tesla with > 5.75 T 2 /cm.

Particle Trapping Systems

Medical and Pharmaceutical Systems

AMI has designed, manufactured and delivered a number of different types of custom magnet systems for commercial or research applications in the medical field. Most projects of this type are sponsored by corporate concerns requiring non-disclosure agreements to protect proprietary information within this highly competitive arena. AMI treats the trust of our customers very seriously and welcomes the opportunity to explore mutually beneficial long term relationships.

The medical MRI unit below 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 mechanical refrigerator as shown.

Ultra-Low Temperature Systems

		There are two major components to most ultra-low temperature systems operating in the mK range: a Dilution Refrigerator Insert and a Nuclear Demagnetization Magnet System. AMI specializes in working closely with customers who wish to purchase the dilution refrigerator and magnet system from separate vendors. Our engineering staff carefully reviews customer provided drawings of the insert and designs the magnet system around this. Approval drawings are provided prior to production to ensure there is no mismatch when the final unit is integrated. By purchasing the magnet system separately many customers realized significant cost savings and get a custom system exactly tailored to their experiment. The photo shown here is of a 14T/9T double demagnetization system. Magnets for nuclear demagnetization are required to have a large volume at high fields into which the sample to be demagnetized (frequently a bundle of copper wires) is placed. A He 3 -He 4 dilution refrigerator is generally used to cool the sample while it is in the magnetic field and before it is demagnetized. During the demagnetization process, the refrigerator is thermally decoupled from the sample by means of a superconductive heat switch. To accomplish these operations, the high field magnet used to magnetize the copper wire bundle must be compensated so it will not affect the operation of the heat switch or switches and so it will not generate appreciable eddy currents in the mixing chamber of the dilution refrigerator. This field free region is also used for the ultra low temperature experiment itself. The magnet system for this application is a high field magnet, a nulling coil, and a series of compensating coils mounted on an integral cylinder extending above the main magnet. The dilution refrigerator and the experiment itself are located inside the compensation coils where the field is typically reduced to less than 3 mT when the main field is at 8 T. In some cases, an additional highly homogeneous magnet is placed in the compensated region to permit the temperature to be measured using nuclear magnetic resonance techniques.
When the ultimate in low temperatures is required, two stages of demagnetization are employed. In this case, PrNi 5 is sometimes used in the first stage to achieve higher cooling capacities at higher temperatures. After the first stage is thermally decoupled from the dilution refrigerator and demagnetized, the second stage is decoupled and demagnetized to achieve the ultimate low temperatures. The dilution refrigerator and magnet designs must be closely coordinated to assure that the proper low magnetic fields are achieved at the superconducting heat switches and that adequate space is available inside the magnet for the various refrigerator components. 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 customer's site: UCF Del Barco Group Research

Adiabatic Demagnetization Refrigeration (ADR) Systems

The refrigeration cycle of an ADR uses the interaction between the atomic magnetic moments in a paramagnetic material (often a salt) and an externally applied magnetic field. A salt pill is thermally connected to a heat sink through a thermal switch; the entropy is reduced by increasing the magnetic field, thus aligning the spins. When the field is maximized the thermal contact to the heat sink is broken. As the field is reduced the temperature drops as the spins relax towards their normal state. In a cryogenic ADR system 4K or 2K helium is the base temperature and the salt pill can reach temperatures in the 300 mK range or less. Taking advantage of this unique cooling cycle means maintaining the mK temperature stable over a useful period of time to collect data from either a sample or a cooled detector of some sort. This is achieved by slowly and precisely decreasing the magnetic field once the desired operating temperature is reached to offset the unavoidable heat leak into the system. It is therefore very important that the bipolar power supply system used be very stable and posses the ability to ramp very slowly so that it supplies just the right amount of current to reduce the field over time and thus maintain the desired temperature with good stability. AMI provides low current bipolar power supplies with ramp rates as low as 10 μ A/minute. Magnets for this application are typically of a low current design such as the one shown here. Magnetic shielding is also an important system consideration.

Particle Accelerator Beam Line Systems

Superconducting magnets in particle accelerators contain, steer or focus the particle beam. They act as focusing devices in key locations and are often used in experimental systems at the target end of the tunnel. Racetrack dipole and quadrupole magnets are normally used to guide the beam because they produce a long region of uniform field axially along the flight path of the particles. AMI has the capability to produce precision wound single conductor Racetrack dipole and quadrupole magnets (see below). A dipole version has two elongated coils opposite each other. A quadropole or quadrupole magnet has four coils symmetrically spaced around the radial bore. Quadrupoles produce a higher degree of field homogeneity and strength as compared to dipoles. The achievable field in magnets of this type is lower than that of comparably sized solenoids wound with the same material. AMI's custom production capabilities are best suited for low volume custom magnets for beam lines. Two such identical systems used in an experimental target area are shown below.

Custom Resonance Magnet Systems

The common feature of all resonance magnets are high homogeneity and high temporal stability. Magnetic field strength varies widely between 0.05 and 21 Tesla depending upon the specific application. AMI has built many magnets for Magnetic Resonance Imaging, Nuclear Magnetic Resonance, Electron Spin Resonance and Ion cyclotron Resonance.

In addition to whole body MRI systems now in widespread use, AMI has designed and built specialized extremity MRI magnet systems. AMI has also designed and built high resolution and solid state NMR magnets together with ESR and ICR magnets. In addressing these market segments AMI has the experience required to make correct decisions concerning the appropriate superconductors, winding and jointing techniques to use for specific applications. The systems shown here range from 0.35, 0.5 and 1 Tesla whole body MRI units equipped with cryo-coolers to minimize loss rates of both nitrogen and helium. AMI produced these complete systems including magnet cryostat and control electronics.

High Gradient Magnetic Separation (HGMS) Systems

HGMS uses a superconducting magnet that provides the high magnetic field background required to produce intense magnetic field gradients at the surface of the separation medium, typically a ferromagnetic mesh. HGMS has been used for such diverse applications as the removal of harmful impurities from drinking water, purification of minerals such as Kaolin, and the separation of red blood cells from plasma. A room temperature bore magnet system provides the housing for the HGMS canister that is periodically removed from the magnetic field for backflushing the separated impurities. Industrial or high production systems utilize reciprocating magnetic filter canisters so to one unit is backflushing while the other is processing material. This type of arrangement can provide nearly continuous uptime for long periods. Computer based power supply systems with safeguards, interlocks and limited operator interfaces can be provided for industrial settings such as this. Cryogen free systems are highly recommended to avoid the safety, logistics and special training issued involved with liquid helium.

Superconducting Magnetic Energy Storage (SMES) Systems

SMES systems are used primarily to provide momentary power at key locations in an electrical grid during voltage sags or power outages. The magnet is yoked to a specialized set of electronics which senses the impending outage and dumps the energy from the magnet back into the power grid in a rapid but controlled fashion. The system then re-energized (re-charges) the magnet to get ready for the next occurrence. AMI can work with SMES system research teams or corporate engineering groups to provide magnets and other components for this application. The magnet can be cooled by liquid helium and incorporate a mechanical cryocooler to re-condense the gas and keep helium costs low. This also reduces maintenance effort needed top off the system. AMI can design and manufacture magnets of significant size to be useful in such applications by storing and releasing Mega Joules of energy during a short time period. Stored energy, physical size, charge and discharge rates are all considerations when designing such systems.

Nuclear Magnetic Resonance (NMR) Systems

Nuclear Magnetic Resonance magnet systems are in such widespread use that their features have been perfected to a greater extent than any other superconducting magnet system. They are characterized by very high homogeneity, temporal stability and long cryogen hold times to the extent they may be treated almost like permanent magnets. The sensitivity of the NMR measurement increases with field strength and that has inexorably increased field strength over the past 35 years from 5 Tesla to 21.5 Tesla as higher performance superconductors have been developed. American Magnetics specialize in custom NMR magnet systems that fall outside the range of the magnets supplied with standard model NMR spectrometers. Examples are those magnets used for specialized experiments requiring modulation coils or unusual homogeneous volumes. In addition those magnets used for solid state NMR or a combination of MRI and MRS are within our area of expertise. Typical specifications are up to 0.1ppm homogeneity from the superconducting magnet alone and field stability of up to 0.1ppm/Hr. 3D Model of NMR System

Semiconductor Crystal Growth

High magnetic fields have been shown to be beneficial in the growth of high purity and high quality crystals. Such systems are often used to improve yield of high cost materials such as Gallium Arsenide or Germanium.

Semiconductor Crystal Annealing

The application of a uniform magnetic field across (perpendicular) to a stack of wafers provides better yield, quality and operating parameter flexibility to the wafer manufacture. The magnetic field is applied during the annealing annealing process with the wafers residing inside a high temperature vacuum furnace. Fields in the 2 Tesla range are normally used. The limiting factor to field strength is often the size of the oven which must reside inside the bore of the magnet. Such systems operate in a production environment and therefore cryogen free technology is preferred to avoid the need for routine cryogen handling by production personnel. System reliability, safety and simplified user interface are all key considerations when planning for the design or installation of such systems. Other considerations include ease of access to the oven for loading/unloading, stray field implications or controls. AMI has the experience to work with semicon furnace manufacturers to provide magnet systems with large uniform fields, fast/reliable cryogen free cooling, robust digitally controlled power supplies and the engineering resources to make your project a success.

Mössbauer Magnets

The Mössbauer effect (or nuclear gamma resonance) is based on the principle that sometimes a nucleus in a solid matrix can emit and absorb gamma rays without recoil; because when it is in a solid matrix the nucleus is no longer isolated, but is fixed within the lattice. In this situation the recoil energy may be less than the lowest quantised lattice vibrational energy and consequently the gamma ray be emitted without any loss of energy due to the recoil of the nucleus. Since the probability of such a recoil-free event depends on the energy of the nuclear gamma ray the Mössbauer effect is restricted to certain isotopes with low-lying excited states. In general, the Mössbauer effect is optimized for low-energy

gamma rays associated with nuclei strongly bound in a crystal lattice at low temperatures. A Mössbauer experiment consists of a radioactive source, which is moved rapidly back and forth by a precision velocity transducer (drive unit). Gamma-rays emitted from this source pass through a sample into a detector which collects the data for the spectrometer. It is sometimes necessary to place the sample in a high magnetic field to observe certain effects. In Mössbauer Spectroscopy it is important that the source and absorber be as close together as is feasible. The source, usually the isotope Fe 57, will emit broad signals in the presence of a magnetic field and a narrow line in zero field. Therefore AMI designs such coils with a bucking coil to quickly reduce the field after the sample region and thereby enable more sensitive measurements.

Magnetic Modulation Field Sweep Systems

Some experiments require field modulation of the main field. In such cases AMI has designed many zero inductively coupled modulated magnet systems. The highest field produced and the highest modulation (sweep) frequency are directly related and will depend on issues such as background field and the balance of frequency and magnitude of the modulation field. Typically modulation coils are in the tens of Gauss and up to the KHz range. Similar designs are also useful for field sweeping a small field range in a large background field. Many magnets for Electron Spin Resonance (ESR) use this technique.

Low Cost Magnet Systems

Even a small superconducting solenoid based magnet system normally must include a helium cryostat, power supplies, support structure and sample insert. These costs can add up in a hurry. To address this problem AMI has developed a low cost solution that allows a liquid helium transport canister to serve as the system cryostat. This system is typically available with solenoids of 6 to 8 Tesla. The actual sample space and field strength with vary depending on the size and type of transport container available. A top loading sample rod is provided with electrical connections and is easily accessed without removing the magnet. The magnet support structure includes low loss vapor cooled power leads, safety pressure relief valve, and a quick flange for coupling to the storage canister.

It is always a good idea to prescreen a sample before running it on comprehensive systems like PPMS/MPMS or high field dilution refrigerator based systems. Pre-screening of the samples can be easily performed using the low cost storage dewar system which can save precious time & resources for multi-user facilities. The low cost system can also serve as ideal educational tool to demonstrate Quantum Hall Effect or train students on basic magnet system operation before entrusting them to operate more expensive and complex systems. A complete system includes helium supply dewar, magnet insert and associated electronics. These systems are ideal when very limited funds are available.

SQUID Magnetometer Systems

The SQUID is the most sensitive detector of magnetic fields with an energy resolution under optimized research laboratory conditions, approaching the fundamental quantum limit. The high sensitivity of a SQUID enables measurements of very weak magnetic moments.

A typical measurement consists of moving a sample through accurately balanced superconducting detection coils (gradiometers). The sample sits in an applied magnetic field of 0-9 Tesla produced by a superconducting magnet. The superconducting magnet is placed in persistent mode during each measurement so that no noise from electronic power supply can induce noise into the measurement. AMI has provided low noise superconducting magnets to individual scientists and companies who build complete turnkey SQUID Magnetometers.

Vibrating Sample Magnetometer (VSM) Systems

The VSM uses an induction technique, and is widely used for characterizing materials. This technique allows measuring the magnetic moment in sweeping magnetic fields and is very effective in performing dynamic magnetic measurements. The sample under investigation is mounted at the end of a carbon fiber rod and attached to a mechanical resonator that oscillates the sample (usually in a vertical direction) at a fixed frequency in presence of external magnetizing field which is usually provided by a superconducting magnet. The superconducting magnet used for such a measurement is designed for fast sweeping rates since magnetization measurements are performed in sweeping fields. The technique involves two pickup coils, placed above and below the sample, and the coils experience a change of the magnetic flux due to the motion of the sample. EMF voltage, proportional to the rate of change of flux, is induced in the two pickup coils. This signal is proportional to the moment, amplitude, and frequency of vibration. The measurements are performed at cryogenic temperatures using low temperature cryostats or variable temperature inserts (VTI) and measurements above room temperature are possible using high temperature oven assemblies. AMI is one of the leading suppliers of superconducting magnets for companies that manufacture turnkey VSM systems.

Faraday Balance Susceptometer Systems

Unlike all the above induction based methods, the Faraday balance technique is based on measurement of force. The technique uses a primary magnetizing field and a superimposed field gradient in the vertical direction. The gradient field based on sample magnetization creates a magnetic force on the sample that adds or subtracts the sample weight. The magnetic force acting on the sample is detected by a micro balance and is directly proportional to the product of the field and gradient field. AMI has designed various gradient field systems for customer designed Faraday balance systems.

Magneto Hydro Dynamic (MHD) and Magneto Plasma Dynamic (MPD) Systems

A fluid moving perpendicular to the direction of the magnetic field generates an electromotive force perpendicular to both. The electromotive force can be used to produce electric power using the dynamo effect or converted to mechanical thrust using the inverse of the dynamo effect. Magnet systems have been designed and produced by AMI for a variety of experimental systems that use these principles. AMI has provided magnets for both military and commercial MHD research programs. The large split coil magnet on the right is a magnet used in an MHD propulsion research system.

Plasma Physics Systems

AC Susceptometer Systems

AC susceptibility measurements is a very useful experimental technique in studying magnetic properties of materials. They are especially important to detect magnetic phase transitions and these measurements give insight into the critical state model of type II superconductors. AC Susceptometers are fairly easy to build and are very widely used. The method is based on the fact that mutual inductance of two coils changes if the magnetic sample is placed in one of the coils and the resulting voltage is proportional to the susceptibility of the sample. These measurements are typically performed in presence of homogeneous DC magnetic fields produced by superconducting magnet. The variation in sample temperature is achieved by using a VTI. AMI has provided many superconducting magnet systems for use with home made AC Susceptometers.

Gyrotron Systems

Superconducting gyrotron magnets are used in gyrotron amplifiers that generate millimeter-wave electromagnetic radiation. The electromagnetic radiation produced has become increasingly useful for certain radar applications and for heating plasma in controlled fusion. More recently they have found use in industrial heating applications. Most early superconducting gyrotron magnets were cooled by liquid helium in conventional cryostats, however, now that they are gaining wider acceptance in the industrial market there has been a shift towards conduction-cooled gyrotron magnets (i.e. cryogen-free). Conduction-cooled superconducting gyrotron magnet systems combine ease of operation and low maintenance, which are two essential features for a cost-effective industrial application.

As each application requires a unique magnetic solution, American Magnetics Inc. welcomes inquiries for specific applications that use gyrotron magnets.