In the area of particle accelerators, superconducting windings are used in magnets for Bending, Focussing and confining particle beams as well as for conducting experiments and detecting the results. The main simulation requirements are accurate calculation of the magnitude and gradients in the fields produced in both DC and ramping operation, and protecting the system against faults due to quenching. Simply put a quench is the transition of the materials in the magnet from superconducting to resistive state during operation. If no care is taken to manage this transition in a controlled way systems can be damaged or even destroyed.
In system modelling of superconducting magnets we typically use a bulk/homogenised approximation for the coil as a whole, based on the relative fractions of the component materials in the coil. We can use expressions from theory, or tabulated data taken from measurement. With very anisotropic thermal properties, the bulk approximation needs to accurately represent the winding, not just as a single bulk value but typically in both the direction of current flow and perpendicular directions. This means that some method of describing the current direction is required so that the anisotropic material properties can be aligned with this local orientation, rather than within the global system, or some simple transformation of the global coordinate system.
One important characteristic when considering a potential fault in the system is that of the critical current. This highly non-linear property varies with both temperature and the flux density the winding is exposed to, and it defines the point at which the transition between normal and superconducting state occurs. Clearly in a homogenised model where the current flowing in the superconducting filaments is distributed equally in the volume fraction of the superconductor and matrix, the critical current density must also be diluted using this volume fraction to enable the bulk to transition correctly.
Care must be taken when ramping the magnet to operational levels so as to not quench the Niobium titanium coils with heat generated by rate dependant losses in the coils themselves or eddy currents produced by the changing fields in support structures.
Here we have a demonstration model of an MRI magnet which includes the coils, formers, spacers and coolant pipes being ramped to operational fields over a period of nearly 3 hours showing the current as it is ramped and the resultant maximum temperature rise during this process.
In this case the ramping is slow enough for the whole coil to remain below the critical temperature of 9.2 kelvin above which the Niobium titanium would become resistive (quenching to potentially disastrous effect).
Further evaluation of this system during a quench can be made giving the transient field as the current drops and the temperature distribution in the coil.
This in turn can be used to evaluate the stress in the coils at any time which can include contributions from and pre-stress or winding tension, thermal expansion of the coils, Lorentz and other electromagnetic forces acting on the coils.
The use of superconducting materials in electrical machines is not a new idea. Concepts have been put forward since the late 1970s and test machines have been developed over the last 30 years with reasonable success. However it has only really been in the last 10 to 15 years and the discovery/development of HTS wires, coupled with the drive to use renewable energy that this field has really gained momentum. Aside from the benefits of being able to use cheaper cryogenic systems one of the main benefits of HTS materials is that although in electrical machines it is possible to use superconductors to generate a DC field – with the rotation of the device causing the field modulation – they still must be able to withstand some variation in the field they are exposed to. One example of a real device constructed using a DC superconducting field winding is the prototype developed under the European Union FP6 project named Hydrogenie.
This is a 1.7 MW hydro generator that was developed in industrial partnership. In this case simulation was able to predict the performance of the device with high accuracy including estimating efficiency to within 0.1 %. The generator has demonstrated that is possible to reduce the size and weight of a generator for this purpose by up to 70 % with significant improvements to efficiencies. Recent effort within a large number of groups has been toward the more widespread use of HTS materials to improve efficiencies even further though the most successful projects to date have only used superconducting materials in the production of DC fields rather than in the AC windings.
- Particle Accelerator Magnets
- Beam Therapy Devices
- Electrical Machines
- Fault Current Limiters