The way forward: The path to monolithic additive manufacture of lower hybrid current drive launchers

Abstract

Additive Manufacturing (AM) is a key enabling technology for the rapid production of complex radio-frequency (RF) structures used in lower hybrid current drive (LHCD) launchers. Glenn Research Copper 84 (GRCop-84), a Niobium Chromide (Cr2Nb) 8 at. % Cr, 4 at. % Nb precipitation hardened alloy, is suitable for AM with Laser Powder Bed Fusion (L-PBF), achieving 99.5% density, Ra=3-4 µm surface roughness, yield strength of 470 MPa and an ultimate tensile strength (UTS) of 710 MPa in as-printed condition. AM of a high field side (HFS) lower LHCD launcher from GRCop-84 alloy demonstrated several critical advancements in AM of RF launchers. Waveguides with a pentagonal cross-section were designed to support the top internal waveguide surface with 45-degree chamfers from the sidewall, eliminating collapse of the ceiling, while maintaining RF properties near identical to a rectangular cross section. Hot Isostatic Pressing (HIPing) consolidated residual voids within the material, increasing density from 99.5% to 100%. Chemical-Mechanical Polishing (CMP) reduced residual surface roughness from the L-PBF process to Ra=0.1 µm / Rq=0.4 µm to lower RF losses. Advancements in L-PBF for the AM of copper alloys have increased the maximum build volume from 250x250x300mm on the Concept Laser M2 printer to 400x400x400mm on the EOS M400 printer. This increased build volume now enables monolithic AM of complete LHCD launchers with integrated cooling channels that eliminate the time-consuming laser welding assembly of launcher segments previously required by the smaller build volume.