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Electromagnetic field simulator helps speed characterisation of innovative hybrid magnet

Magnet Lab's new Series-Connected Hybrid promises 36 T at 13 MW.

Cobham Technical Services' Opera 3D electromagnetic field simulator is playing a vital role in the design of an innovative hybrid magnet being developed by the USA's National High Magnetic Field Laboratory (NHMFL) in Tallahassee, Florida.

The NHMFL - popularly known as the MagLab - is home to some of the largest and most powerful laboratory magnets in the world. Headquartered in a 370,000-square-foot complex at Florida State University, the facility provides scientists and engineers with a unique environment in which to conduct experiments involving very high energy magnetic fields.

Many of the innovative magnets used at the MagLab are designed and built on-site, by the facility's Magnet Science & Technology group. One of the latest additions, currently early in its construction, is a Series-Connected Hybrid (SCH) magnet designed to provide unprecedented power efficiency and field homogeneity. Development of this resistive-superconducting hybrid magnet is being funded by an $11.7 million grant from the US National Science Foundation, and the magnet will be used for research purposes in high field nuclear magnetic resonance (NMR), condensed matter physics, biology and chemistry.

The SCH uses a water-cooled Florida-Bitter resistive magnet - a highly innovative technology developed by the MS&T group, capable of generating high magnetic field strengths more efficiently than alternative means - nested within a superconducting magnet that is cooled by liquid helium. The two magnets are connected in series and together create a very high intensity central magnetic field of 36 tesla. The SCH is connected to a 650 V supply and has an operating current of 20,000 A; this 13 MW power consumption is 66 percent less than what would be required for an all-resistive magnet providing the same field and bore.

The Opera-3D electromagnetic simulator from Cobham Technical Services was used to aid the design of the SCH magnet and its shielding.

Electromagnetic shielding of the SCH is provided by a set of eight magnetically soft, 100 mm thick iron plates which form an octagonal wall around the entire magnet system. The magnetic properties of these shields were characterised by a B-H curve, obtained by using Opera 3D.

According to Iain Dixon, Research Associate at the Magnet Science & Technology division, "We chose to use Opera v12.0 to evaluate the 3D magnetic field uniformity and fringe fields of the SCH because the simulator is accurate and fast. The inner coils of the SCH are of Florida-Bitter type with essentially non-uniform current densities. To emulate this, we subdivided each coil into a large number of radially thin coils with uniform current densities, and the whole coil system was generated by use of an external program called by Opera from command line."

Opera 3D has also been used for a variety of other performance-related studies on the SCH, including evaluating the magnetic fields around the magnet's HTS (high temperature superconductor) vapour-cooled leads, and calculating eddy currents in the thermal radiation shields surrounding the superconducting coil. In the latter case, these calculations are of critical importance; during fast discharge of the magnet - such as in the case of a quench - eddy currents can exert large mechanical forces on the shields, and they therefore need to be accurately simulated for safety.

The SCH promises to set a new standard in powered magnet performance. Its efficiency means that it will have a lower operating cost than any other magnet in its class, and its innovative magnet configuration should provide an unprecedented level of field homogeneity and stability. Such is the design expectation that the MagLab is already involved in two further variations of the SCH; one other under construction is destined for the Helmholtz Zentrum Berlin where it will be used for neutron-scattering experiments, the other in design is for the Spallation Neutron Source at the Oak Ridge National Laboratory in Tennessee.

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