R & D of polyimide insulated JET ELM control coils for operation at 350 C

Abstract

A study has confirmed the feasibility of designing, fabricating and installing resonant magnetic field perturbation (RMP) coils in JET with the objective of controlling edge localized modes (ELM). These coils present several engineering challenges. Conditions in JET necessitate the installation of these coils via remote handling, which will impose weight, dimensional and logistical limitations. And while the encased coils are designed to be conventionally wound and bonded, they will not have the usual benefit of active cooling. Accordingly, coil temperatures are expected to reach 350C during bakeout as well as during plasma operations from resistive heating. These elevated temperatures are beyond the safe operating limits of conventional OFHC copper and the epoxies that bond and insulate the turns of typical coils. This has necessitated the use of an alternative copper alloy conductor C18150 (CuCrZr). More importantly, an alternative to epoxy had to be found. An R&D program was initiated to find the best available insulating and bonding material. The search included polyimides and ceramic polymers. Ultimately, these ELM coils must be able to withstand the elevated thermal conditions as well as the structural stresses resulting from electromagnetic loads, which include eddy current and halo current effects. Not only do these loads affect the performance of the coils and cases, but also impact the design of joints, leads, jumpers, and the mounting of the coils to the interior of the vacuum vessel wall. In order to qualify the proposed insulating and bonding materials, prototypical coil samples were built to the design specifications of the proposed JET ELM coils. These samples were impregnated with polyimide then cured. This paper will detail the R&D program, including the results of testing to determine mechanical properties of the polyimide bonded coil samples.