Engineering optimization of stellarator coils lead to improvements in device maintenance

As part of an early PPPL pilot plant study an engineering exercise was undertaken to straighten the stellarator modular coil (MC) back legs to provide greater access to plasma components. Saddle coils were located within separate enclosures and configured to reconstitute changes in the magnetic fields caused by straightening the MC back legs. A follow on lab directed research study looked to further improve stellarator maintenance by considering higher aspect ratio plasmas along with improving the MC defining metrics by adding engineering maintenance requirements to the physics code used in defining the modular coil winding configuration. The physics design code (COILOPT) was upgraded to provide coil solutions with straightened MC back legs and to receive inputs of engineering constraints on the MC surface geometry and winding centers. Several improvements and extensions of COILOPT++ were made which include: the ability to target coil penalties and freeze coil geometry for individual coils; adding torsion constraints on the geometry of the space curve defining the MC winding; allowing freedom to straighten MC back legs over asymmetric distances above and below the outboard midplane; the inclusion of nested saddles with enforced minimum coil-to-coil separation distances and adding the ability to accept coil winding surface geometry from Pro- E. These changes along with a number of other features have substantially improved our ability to achieve better self-consistency between engineering maintenance requirements and plasma surface reconstructions with targeted physics. Using the new COILOPT++ code, a first pass design of an improved maintenance stellarator device has been developed. The code details and the ensuing device design will be presented.