Additive Manufacture of GRCop Waveguides for Fusion

The high electrical and thermal conductivities of copper make it ideal for plasma facing components such as heat sinks and waveguides. To improve plasma performance and steady state tokamak operation, one path is off-axis current drive for current profile control. Radiofrequency (RF) power is among the leading contenders, but the harsh environment poses significant challenges. An innovative solution to this complex problem is to launch the lower hybrid waves from the high-field-side instead of the low-field-side of the tokamak. This relocation is predicted to dramatically improve wave penetration, current drive efficiency, and launcher robustness in a reactor environment. For efficient current drive, transmission losses must be minimized. Presently, first wall temperatures may be as high as 800°C, where copper is a poor structural material. Recent development of a copper alloy, GRCop, provides a potential solution with near copper like electrical and thermal conductivities with significantly improved strength at elevated temperatures. However, components produced from GRCop must be made using powder metallurgy techniques. Additive Manufacturing (AM) techniques based on laser powder bed fusion (L-PBF) have recently been used to produce GRCop components, but component size is limited. The development of AM techniques that can produce meter long GRCop waveguides would be extremely beneficial. Therefore, blown powder techniques based on High Pressure Cold Spray (HPCS), which can produce components 2m in length and greater, are being developed to produce meter size GRCop waveguides. During Phase I, HPCS processing parameters have been developed that enable the deposition of dense GRCop with properties equivalent or exceeding GRCop produced with other manufacturing techniques. For example, testing has shown HPCS GRCop has conductivity values ~80% of pure copper, which is ~15% greater than the conductivity of L-PBF GRCop. In addition, the ability to produce complex shaped components such as hexagonal and rectangular cross-sectional tubing, which is needed for advanced waveguides has been demonstrated. Profilometer measurements have shown the internal surface finish of HPCS-AM GRCop is ~3μm Ra on unpolished mandrels, which is a 3-4 times improvement in the surface finish as compared to L-PBF GRCop. To demonstrate proof-of-concept, subscale GRCop waveguide cavities were produced using HPCSAM and delivered to MIT for testing. Preliminary results have shown HPCS GRCop cavities in the as-deposited condition have equivalent Q factor to L-PBF GRCop cavities that have had their interior surface abrasively polished. During Phase II, the techniques will be optimized to produce full-size prototype waveguides, i.e., ~1m length, with as-fabricated surface finishes less than 1μm Ra to improve performance. Tests of these advanced waveguides will then be performed at MIT to yield critical RF data. The additive manufacturing techniques developed during this effort will be applicable to copper and copper alloys as well as other ductile materials for producing large free form components with improved surface finishes for government and commercial applications. These include aerospace, defense, propulsion, power generation, medical, electronic, and corrosion protection coatings.