Opera-3d used to help pioneer fusion energy company

When Tokamak Energy Ltd. began a project to conduct pioneering work in the important field of fusion energy, in order to develop advanced, more compact fusion machines (tokamaks), they turned to Opera from Cobham. With Opera’s sophisticated electromagnetic design software they were able to resolve some critical issues as part of a fascinating engineering challenge.

The company grew out of the Culham Laboratory, home of JET, the world’s most powerful operating tokamak. The team at Culham also developed spherical tokomaks – pioneering a more compact, more efficient design. With the advent of high temperature superconductors (HTS), the founders of Tokamak Energy realized that it would be theoretically possible to produce spherical tokamaks that would perform similarly to existing machines with room size facilities, rather than the “aircraft hangar” size of designs like JET. This gave them the opportunity to start a company to exploit the new technology, and there has already been much success with small prototypes.

Of course, there were many technical challenges still to address. For one of these, Cobham’s Opera software was used to assess the electromagnetically induced forces and field profile in a new design. HTS coils need to be mechanically supported with minimum heat transfer. Ideally, this would be one that is a self-supporting structure which allows the use of minimal connections to the room-temperature world. To investigate possible designs, the new tokamak prototype, ST40, will use a single massive copper turn for each of the toroidal field (TF) coils (rather than HTS), as this is considerably less expensive. The consequence of this is, however, that it can no longer be assumed that current density is uniform across the cross-section. Hence, an accurate simulation of the fields, currents and resulting forces was required, since the Lorentz force acting on the coils is determined from the product of the current density and flux density (J x B),

The ST40 tokamak, around 3 metres high, has 24 TF coils, arranged in 8 groups of 3, as well as multi-turn poloidal field (PF) and central solenoid coils. The Opera model represented the PF and solenoid coils using Biot-Savart coils, while the TF coils were meshed elements in a circuit with a defined input current. Because of symmetry, it was sufficient to simulate only 45 degrees of the model, with positive periodicity boundary conditions implying the other 315 degrees. The plasma (not shown in the picture above) was also represented by a set of Biot-Savart coils making a cruciform cross-section.

Tokamak Energy simulated several variations on the design, each examining the fields and forces for multiple cases representing different times in the operating scenario. They also calculated the inductance of the TF coil system.

The force density distribution, especially around the complex shape where the centre limb of the TF coil meets the D-shape section, varies significantly – density values are high at internal corners but low at external. To gain an appreciation of how the force varied, Tokamak Energy required the force / unit length of current path in the upper part of each TF coil using a notional current path through the centre of the TF coil to be calculated. (The lower halves are identical magnitude with 180 degree rotation about the radial axis.)
The field within the interior volume of the torus also varies significantly. The maximum value of the azimuthal flux density is around 8 T close to the centre limb while, on the centre plane of a TF coil triplet, its minimum is around 1.5 T.  This reduces to about 0.7 T on the plane midway between two triplets, as the flux leaks into the gaps between the triplets.
Following the detailed Opera simulations and the modified designs, Tokamak Energy have been able to achieve the required mechanical and electromagnetic performance. The ST40 is now being assembled and is due to start operating early next year.  The initial target is to create a plasma that is hotter than the centre of the sun and then push on to reach 100 million degrees.

Paul Noonan, R&D Projects Director for ST40 at the company comments:

“At that point, Tokamak Energy will have assembled some profoundly exciting experimental and theoretical evidence of the viability of producing fusion power from compact, high field, spherical tokamaks”.