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Cobham Technical Services has revealed how its Opera-3D electromagnetic field simulator is playing a key role in the design of a hybrid magnet being developed in the US.

The magnet is being developed by the National High Magnetic Field Laboratory (NHMFL) in Tallahassee, Florida.

Headquartered at Florida State University, the ‘Maglab’ facility provides scientists and engineers with a unique environment in which to conduct experiments involving very high-energy magnetic fields.

Many of the magnets used at the Maglab are designed and built on-site by the facility’s magnet science and technology group.

One of the latest additions, currently early in its construction, is a series-connected hybrid (SCH) magnet designed to provide high power efficiency and field homogeneity.

Funded by a grant from the US National Science Foundation, 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 technology developed by the MSandT 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 high-intensity central magnetic field of 36tesla.

The SCH is connected to a 650V supply and has an operating current of 20,000A; this 13MW power consumption is 66 per cent 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, 100mm-thick iron plates that 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.

Iain Dixon, research associate at the magnet science and technology division, said: ‘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,’ he added.

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 is claimed to set a new standard in powered magnet performance – its efficiency means that it will have a lower operating cost than other magnets in its class, and its configuration should provide a high level of field homogeneity and stability.

Cobham Technical Services

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