Technical FAQ

Charged Particles

## What emitter models does the Opera Charged Particle module offer?

Opera’s Charged Particle module includes:

• Thermionic - Thermal saturation - Space-charge limited emission - Langmuir/Fry

• Field effect

• Plasma free surface

• Surface secondary emission (backscattered and true secondary)

• Volume interactions (backscatter, ionisation)

• Magnetized plasma

• User defined emitter

## What does Opera display in terms of ion beam statistics?

When it comes to evaluating the statistics of an ion beam, the starting point is usually a patch placed across the beam at some user-defined location. Standard tools in the Post-Processor can then extract intersection data, such as the current and velocity components of each intersecting trajectory and the number and current densities. From these, simple additional computations can generate beam metrics, including the moments, emittance and phase space.

## Why are charged particle solutions iterative?

The beamlet space charge affects the electric field distribution and may also affect the current in the beamlets. A consistent voltage, current and space charge distribution is required and this is calculated by an iteration that updates the space charge distribution, recalculates the electric fields and then the beamlet trajectories.

## Can Opera model Magnetron Sputtering?

Opera combines accurate finite element analysis with detailed models for plasma, sputtering, and film deposition to provide the first practical tools for magnetron design and optimization. Opera can be used to predict target erosion and to optimize utilization. It can accurately characterize the design of magnet systems, including multi-target coaters, and it can predict deposited film profiles and the deposition dynamics.

## What is meant by Secondary Emission?

Secondary emission properties can be applied to labelled surfaces of the model. Collisions of the particle beamlets with these labelled surfaces are detected and secondary particles are introduced. These secondaries may also collide to produce further new secondary particles; the maximum number of generations of secondary particles can be limited. The space charge effects created by secondary particles can be excluded from the calculation.

## Can Opera calculate the temperature rise due to the impact of kinetic charged particles onto a surface?

Opera is multiphysics software, so the heat generated due to the particle beam can be passed seamlessly to Opera's thermal module to calculate the resultant temperature rise.

## For analysis of a conventional space charge limited, tungsten cathode, triode gun, which do you suggest is most appropriate emission model?

Comparison with measurement has shown that the Langmuir-Fry Thermionic emission model with a virtual cathode (Type 1) is the most accurate. This model includes a realistic initial velocity for the electrons leaving the cathode and is therefore able to correctly represent the potential minimum that will exist if the emission is space charge limited. The Child's law model is less accurate because it does not use a Maxwell distribution of initial velocities. The results were compared in the range 0.05 to 0.5 of the thermal saturation limit for helical tungsten filaments and dispenser cathodes. The Child's law results become less accurate at smaller currents, because the assumed initial velocity has more effect.

Magnets

## In an accelerator magnet, magnetic fields are used to steer and focus charged particle beams. What tools does Opera have to rapidly assess field quality? And can I see how the particle beams behave?

Opera’s post-processor includes tools for characterizing field homogeneity or gradients in free-space volumes. For dipole, quadrupole and higher order multi-pole magnets, characteristics are usually calculated as a set of Fourier coefficients on a circle or part circle. Similar functions are also available to display field homogeneity on spherical surfaces. Field values can be calculated at any point and Opera can directly track individual charged particles or systems of particles through or beyond the magnetic aperture. Results can be displayed in various ways, including:

• 3D track lines through the geometry

• projections onto the major coordinate planes

• intersection points on any 2d plane

• current or power density maps

## Can Opera model pulsed magnets, especially the redistribution of currents in the conductors? And how does it deal with laminations?

Opera can perform transient simulations with a user-defined pulse for either the current in, or the voltage across the winding. The simulation is able to capture skin-effects and proximity effects. For magnets built using laminations, Opera has a material model that can treat a bulk volume – such as the magnet yoke - as a “packed” structure, thereby removing the need to model the individual laminations. The designer only needs to specify the packing factor of the lamination stack and the orientation of the plane of the laminations.

## What physics can Opera capture in the superconducting windings of my magnet?

In most cases superconducting magnets can be accurately represented using stand¬ard coil models. However, Opera includes comprehensive facilities for modelling the detailed physics of superconductors when it is necessary to do so, for exam¬ple shielding produced by super-currents under Meissner effect conditions, or the hysteretic behaviour as a magnet current is ramped up and down. Opera’s most sophisticated model for superconducting windings is to simulate a quench. The Opera-3d quench simulator solves the time-transient coupled thermal and electromagnetic equations. The tool is able to model complete coils connected to supply/protection circuits and evaluate the effectiveness of the protection. Simulations can include heating from eddy currents in formers and support structures. Whilst Opera can simulate quench in a complete coil or system of coils, it can also model individual superconducting wires of both LT and HT superconductors.

## How accurate is Opera for simulating NMR/MRI magnets?

Superconducting NMR/MRI magnets are usually represented by a proprietary coil model that is capable of producing highly accurate results for field homogeneity in the imaging region, measured in parts per million. This level of accuracy is essential for this application which relies on magnetically sensitive high-Q NMR resonances. Opera’s track-record for achieving such high accuracy has made it the premier simulation software for designing superconducting NMR/MRI magnets worldwide.

## I’m designing superconducting solenoid MRI magnets and need to assess the small effect of the shielded room and the steel reinforcing on the magnet’s homogeneity?

The formulation used in Opera-3d’s magnetostatic solver (previously known as TOSCA) allows the field from this type of perturbation problem to be computed very accurately. The field from unshielded solenoids can be calculated using the Biot-Savart expression to one part in 100 million. The shield and the reinforcing structure will make a small perturbation to the central field, usually in the order of one part in one thousand which is well within the accuracy of Opera.

## Can I assess how effective the shielding is for my proposed MRI facility?

Yes, Opera computes relatively small residual fields in shielded space very accurately, even if fields elsewhere in the model are much higher. Isosurfaces can be easily displayed in the Post-processor, allowing a 3D visualisation of the 5 Gauss limit.

Machines

## Can I calculate a Torque-Speed curve for my Synchronous Reluctance Machine?

Yes. You can use either Opera’s general-purpose user interface, or the Machines Environment, which automates analysis processes for electrical machines. If you use the Opera-3d Machines Environment you can select a Torque-Speed analysis, which uses a flux-linkage technique to generate the torque-speed curve from a series of static runs. This is much more efficient than running a large number of motional analyses.

## Can you display a Gorges Diagram for my electrical machine?

The Opera Winding Tool allows several key parameters for electrical windings, including the Gorges Diagram to be calculated and displayed.

## Can Opera analyse my electrical machine under fault conditions?

Opera is general-purpose finite element simulation software that is available for 2d and 3d analyses. Opera solves the fundamental equations that describe the intrinsic electromagnetic behaviour of any electrical machine. This formulation means that the software applies equally well to fault conditions and normal operation, and predicts accurately the real-life performance in either state.

## How do I account for demagnetization of my permanent magnets in my electrical machine?

Opera was designed with advanced material modelling in mind. It is able to treat a wide range of material properties, from the simplest linear material to full hysteresis models of soft magnetic materials, and the demagnetization of hard permanent magnets. In a demagnetization analysis, Opera records the progress of the material magnetization along the virgin characteristic, until the magnetizing field starts to reduce. Secondary ‘demagnetization’ characteristics are then used to determine the remnant magnetization vector when the magnetization process is complete. In both the magnetization and demagnetization processes, the effect of eddy currents and circuit transients are captured. During demagnetization, the values of the pre-stored values determine which demagnetization (second quadrant) curve each element follows and its direction of magnetization. Again the flux density in each element is monitored and the minimum values are stored in variables. The values can then be transferred to the standard Opera transient solvers. In such a simulation where the applied field from current sources etc are opposing the magnet’s field, the variables will show the operating point of the magnet. In a transient simulation, they will show the lowest operating point that was reached during the transient event. Demagnetization in service can therefore be modelled. The minimum field will be tracked and updated during subsequent simulations, and the appropriate demagnetization curve or recoil permeability will be used.

## Can I look at magnetization of my permanent magnets before they are assembled into my electrical machine?

Opera was designed with advanced material modelling in mind. It is able to treat a wide range of material properties, from the simplest linear material to full hysteresis models of soft magnetic materials, and the demagnetization of hard permanent magnets. In a magnetization analysis, Opera records the progress of the material magnetization along the virgin characteristic, until the magnetizing field starts to reduce. Secondary ‘demagnetization’ characteristics are then used to determine the remnant magnetization vector when the magnetization process is complete. In both the magnetization and demagnetization processes, the effect of eddy currents and circuit transients are captured. The result is a magnetized sample, where the magnetization distribution is correctly defined. This can then be used in other simulations to model the performance of the magnetized sample in its designated application (eg. an electrical machine).

## Can Opera calculate the losses in my machine?

Iron losses (including eddy current, hysteresis and excess/rotational components) can be evaluated using one of the relevant solvers for any type of machine, using Fourier methods with losses described by Steinmetz based formulations or directly from manufacturers curves.

Copper losses can be calculated simply from the current flowing in simulated windings.

Hysteresis losses including rotational component losses can be explicitly obtained using the hysteresis solver and eddy current losses by explicitly defining the materials’ conductivities.

Any loss quantity can be used as a heat source in thermal analyses.

Copper losses can be calculated simply from the current flowing in simulated windings.

Hysteresis losses including rotational component losses can be explicitly obtained using the hysteresis solver and eddy current losses by explicitly defining the materials’ conductivities.

Any loss quantity can be used as a heat source in thermal analyses.

## Can you consider both Electromagnetic and Structural performance of a Synchronous Reluctance (SynchRel) Machine at the same time, or do I not need to?

Opera can solve both electromagnetic and structural problems, and couple them. The design of a SynchRel machine is a multiphysics challenge. Magnetic Flux flows freely parallel to the barriers in the rotor, and hence there is a low reluctance Ld. Flux on the quadrature axis is impeded by the barriers, and hence there is a high reluctance Lq. Torque is increased by improving the Ld/Lq ratio. Hence, we want to increase the width of the magnetic barriers, but this affects the mechanical integrity of the rotor. To improve structural strength, bridges can be introduced into the barriers. This has the effect of locally strengthening the component, without overly affecting the magnetic circuit. Opera can pass forces and deformations back-and-forth to solve both problems in a coupled fashion on a single mesh.

## Can Opera model Superconducting Generators; do you have any experience in this field?

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 it is possible to reduce the size and weight of a generator for this purpose by up to 70 % with significant improvements to efficiencies.

Transformers

## Normal operation is one thing, how about performance under fault conditions?

Opera is general-purpose finite element simulation software that is available for 2d and 3d analyses. Opera solves the fundamental equations that describe the intrinsic multi-physics behaviour of any power systems device. This formulation means that the software applies equally well to fault conditions and normal operation, and predicts accurately the real-life performance in either state.

## Can Opera analyse a transformer under open-circuit, short-circuit and inrush conditions?

Yes, these are standard pre-configured analyses in the Transformers Environment. If you are not using the Environment then they can still be defined using the Modeller with its Circuit Editor.

## Can Opera use my full hysteresis curve for my soft magnetic material?

In the Opera suite a semi-empirical method for modelling hysteresis has been developed alongside industrial partners. The magnetic behaviour is considered as a trajectory B(H). The trajectory is based on a measured major symmetric loop that is supplied by the user. This data may be easily obtained from measurements or published data-sheets, and imported into Opera as a magnetic characteristic table. The Opera hysteresis model includes the issues of nested minor loops and ‘wiping out’ of minor loops, which occurs when the trajectory goes through an earlier turning point. Moreover, the model recognises oscillating fields and minimises the storage of turning points. Assuming you have a licence for the applicable module, you need to supply data for only the major hysteresis loop. The algorithm uses a reconstruction technique to determine minor loops and turning points of the trajectory and to erase turning points when the magnetization of a material exceeds the previous excursion. The algorithm also correctly transfers to the saturated material curve beyond the end of the user data, in the same way as for anhysteretic materials in Opera.

## Which variables can I use in an Opera Optimization process?

The optimization variables can be chosen from any of the user defined variables in Opera, in effect allowing geometry data, drives and even material data to be varied in order to obtain the desired objectives. Inequality and equality constraints can be defined in order to restrict the range of the design space. Different levels of constraints can be defined, depending on whether they can be evaluated as a feasible design during the model creation phase or they are obtained as an output of the finite element analysis. The Optimizer is capable of solving both single objective and multi-objective problems. The optimization objectives are defined as the output of the simulation and are evaluated and stored by the Optimizer. The output is an informative set of results which can be interrogated to choose the optimal design

Cathodic Protection

## Is it possible to perform Cathodic Protection calculations using Finite Element Analysis such as Opera?

You can use Opera’s static solver to perform current flow calculations, which are suitable for cathodic protection (CP) modelling on ships / submarines. CP systems inject current into the conducting sea-water / sea-bed (which do need to be included in the model in this case) to modify the electrochemical potential distribution that occurs because the ship is made from different metals and causes corrosion. The modified potential distribution then prevents (or at least reduces) corrosion.

Marine Signatures

## Is it possible to analyse Ferromagnetic & Eddy Current Signatures in a Marine Environment using Opera? How will the modelling of the surrounding sea-water be taken into account?

With regard to ferromagnetic and eddy current signatures from ships and submarines, you need the static and dynamic electromagnetic modules of Opera-3d (with Modeller / Post) to do these calculations. A high frequency solver is not needed - in these type of calculations, the sea-water does not have any effect and can be treated as free-space. In defence applications, the signatures have to be minimized without reducing the effectiveness of any cathodic protection system. Opera’s static solver can help in this by modelling the ship and CP system to determine if (a) the CP system is effective (from the potential solution) and (b) the signatures are sufficiently low.