Matthew W. Hooker

Industrialization of Radiation-Resistant Cyanate Ester Magnet Insulation

Future magnet systems require electrical insulation that can withstand high levels of incident radiation, while also providing the necessary mechanical robustness and dielectric strength to operate these devices. Moreover, the insulation must also be compatible with industrial fabrication processes to enable their efficient, large-scale manufacture. Cyanate ester-based insulations provide the necessary electro-mechanical performance and radiation resistance for these applications, but more information is needed to demonstrate application-specific issues related to magnet production. To accomplish this, a series of tests were performed to validate the long-term processing behavior of cyanate ester resins, their adhesion to Kapton, and the fabrication of small-scale coils. The results of this work demonstrated a working time of greater than 85 hours, good adhesion to Kapton, and the successful fabrication of test coils. Larger-scale industrial trials are ongoing at various sites to further demonstrate the use of cyanate ester insulation for the ITER TF coils, as well as commercial applications.

Radiation Resistant Electrical Insulation Qualified for ITER TF Coils

The Toroidal Field (TF) coils for the ITER device will represent the largest superconducting magnet system assembled to date, and thus creates several challenges related to the manufacture of these magnets. Most notably, the electrical insulation for the TF coils must address four simultaneous constraints: high radiation, large mechanical stresses, high voltage operation, and operation in a vacuum. In addition, these materials must meet all shipping and local environmental regulations for use in large quantities in both Europe and Japan. The TF coil insulation will undergo fast neutron fluences up to 3.2 × 1021 n/m2, which is equivalent to 10 MGy, and still be able to withstand the estimated operation in-plane shear stress in the range of 45 MPa. To address this need, CTD has developed and qualified two epoxy/cyanate ester resin systems, CTD-425 and CTD-435. These materials meet the processing requirements, mechanical strength after 30 000 load cycles, and radiation exposure specifications established by ITER IO. Both materials are qualified for use in constructing the ITER TF coils, and are supplied to European and Japanese customers by Lord Corporation. This paper summarizes the performance characterization, qualification tests, and supply chain for these materials.

Moisture Degradation of Cyanate Ester/S2 Glass Composite Insulation Systems

The fusion devices currently being developed present several challenges for magnet designers. One challenge lies within the electrical insulation, which must be able to withstand extreme temperatures, large shear and compressive stresses, high operating voltages, and high levels of incident radiation. To address the need for better performing insulation systems, Composite Technology Development (CTD), Inc. has developed CTD-403, a cyanate ester resin with increased radiation resistance, ease of processing and fabrication, low moisture absorption characteristics, and high mechanical and electrical strength at cryogenic and elevated temperatures. The moisture absorption trends of CTD-403/S2 glass composite insulation were studied. The effects of humidity exposure on interlaminar shear strength (ILSS), compressive strength, dielectric strength, and glass transition temperature were also studied. The saturation level of the insulation was seen to increase with the relative humidity of the aging environment. Fickian behavior was seen at room temperatures below 97% relative humidity exposure. Non-Fickian behavior was seen at elevated temperatures. Saturation levels after 6 months exposure were seen to be below typical epoxy-based insulation systems, averaging 0.5% weight gain. Degradation of mechanical and electrical properties was seen with increased humidity exposure and moisture absorption. ILSS showed an average retention rate of 75% after 6 months exposure. The compressive strength showed no decrease after 6 months exposure at room temperature, and show retention rates greater than 90% at 75°C/79% RH. An average dielectric strength of 98.6 kV/mm was seen for all specimens at room temperature (above 90% retention) after 6 months exposure.

Characterization and Qualification of Cyanate Ester/Epoxy Insulation for NSTX-U Fusion Magnets

In recent years, CTD has pioneered the development of radiation-resistant, cyanate ester-based resins suitable for insulating superconducting and normal magnets for fusion energy applications. These materials have been shown to exhibit very good radiation resistance, excellent mechanical properties, and good processing characteristics. More recently, a cyanate ester/epoxy formulation was qualified for use in constructing the ITER Toroidal Field coils. This product, designated CTD-425, has also been selected for use in upgrading the center stack for the National Spherical Torus Experiment (NSTX), in building the correction magnets for the Wendelstein 7-X stellarator in Germany, and is under consideration for use in the upgrade to MAST being constructed in the UK. CTD has worked closely with the Princeton Plasma Physics Laboratory and others to qualify the CTD-425 for these new applications. This paper will discuss the test methods and test philosophy used to qualify CTD-425 for use in NSTX and how this qualification procedure can be useful for future fusion and other magnet systems.

Recent Advances in the Development of Low-Cost Ceramic-Based Magnet Insulation

The wind-and-react production of complex Nb3Sn magnets requires that the coils be wound using un-reacted conductor materials, which are then reacted at 600 to 700degC to produce the superconducting phase. Application of the electrical insulation prior to the heat treatment enables more efficient construction of these magnets and minimizes handling of the strain-sensitive superconductor. Ceramic-based composite insulation materials have been successfully used to produce Nb3Sn magnets via a wind-and-react process. In this work, the mechanical and thermal properties of lower-cost ceramic insulation materials are reported. This insulation provides the compression and shear strengths needed for magnet applications, and offers increased stiffness and thermal conductivity as compared to S2-glass-reinforced polymers.

Design and Testing of ITER TF Coil Insulations

The design of the ITER Toroidal Field (TF) coils requires an insulation system that is amenable to the very large scale vacuum impregnation processes planned for the construction of these devices, and that will provide reliable electro-mechanical performance after radiation exposure. To address this need, CTD has developed an epoxy/cyanate ester resin system designated CTD-425. This material meets the processing requirements for use in the TF coils, and cyclic mechanical testing of conductor assemblies has demonstrated its electro-mechanical strength after 60,000 mechanical cycles. In addition, this product recently passed radiation exposure tests coordinated by the ITER International Organization and is now qualified for use in constructing the TF coils. This paper summarizes the performance characterization and qualification test results for this insulation.

Design and testing of the MIDAS spaceflight instrument

Several applications of high temperature superconductor technology have been identified for the National Aeronautics and Space Administrations (NASA) aerospace systems. However, validation of critical superconductive properties in the space environment is necessary before this technology can be inserted into satellite systems. Researchers at NASAs Langley Research Center have designed the Materials In Devices As Superconductors (MIDAS) experiment to evaluate the electrical characteristics of high temperature superconductive materials during extended spaceflight. The MIDAS experiment will evaluate four superconductive test circuits over a temperature range of 300 to 75 K. The MIDAS test circuit is produced by thick film printing and combines both superconductive and conventional electronics into a single, active microelectronics package designed to operate at cryogenic temperatures. All electrical measurements are performed directly on the test circuit, eliminating the need for intricate wiring and reducing thermal losses. This paper describes the design, fabrication, and testing of the primary subsystems of the MIDAS instrument.<<ETX>>

High-Tc thermal bridges for space-borne cryogenic infrared detectors

Abstract Several space-borne infrared detectors require cryogenic temperatures for successful operation. As a result, mission durations are substantially limited due to cryogen evaporation. The electrical leads connecting the detectors to the amplification electronics comprise a significant portion of the heat load on the dewar (i.e., 20% for some systems). Currently, manganin wires are used for these connections, due to the alloys low thermal conductivity at cryogenic temperatures. However, replacement of these leads within high Tc materials would result in a substantial reduction in thermal loss, translating into approximately 10–15% enhancement in mission lifetime. The potential for using the high-Tc materials as thermal bridges to replace the manganin connections is currently under investigation at NASA-LaRC.

Nonaqueous slip casting of YBa2Cu3O7−x ceramics

Abstract A ceramic casting process based on the fundamentals of slip casting has been developed to form high Tc superconductive ceramics. In this process, YBa 2 Cu 3 O 7−x powders ( μ m) dispersed in acetone are cast into foundry molds prepared by a lost wax process. After casting, the mold is peeled away from the superconductor, yielding a ceramic monolith in the shape of the mold. This work describes the casting process, as well as the result of a preliminary investigation into the use of magnetic fields to orient the YBa 2 Cu 3 O 7−x grains during the forming process.

Optimization of YBa/sub 2/Cu/sub 3/O/sub 7-x/ thick films on yttria stabilized zirconia substrates

This report describes the optimization of the firing process used in the production of YBa/sub 2/Cu/sub 3/O/sub 7-x/ thick films screen printed on yttria-stabilized zirconia substrates. The highest critical current density (J/sub c/) values were obtained by employing a double layer printing technique in which a single superconductive layer was printed onto a zirconia substrate and fired, followed by the subsequent deposition and firing of second superconductive layer. Using this procedure, thick film superconductors with a superconductive transition temperature (T/sub c/) of 85 K and a J/sub c/ of 130 A/cm/sup 2/ were obtained by sintering the printed films at 950/spl deg/C for 90 minutes, followed by a six hour oxygen annealing treatment at 600/spl deg/C. Specimens sintered for comparable periods of time at 940 and 960/spl deg/C did not exhibit superconductive behavior above 77 K due to either incomplete microstructural development or thermal decomposition of the superconductive phase respectively.<<ETX>>