23a: Functionally-Graded Cu-W Interlayers for Enhanced Divertor/Heatsink Bonding

At present, an international consortium is engaged in a project that involves construction of an historical large-scale, energy-positive fusion reactor. Among the numerous components undergoing development and refinement are the divertor modules situated at the bottom of the vacuum vessel, which extract heat and ash produced by the fusion reaction, minimize plasma contamination, and protect the surrounding walls from thermal and neutronic loads. In order to provide the high heat transfer capacity necessary to handle the anticipated flux from the fusion plasma, it is planned to use copper alloy heatsinks in combination with tungsten plasma-facing components. However, the dramatic mismatch in coefficients of thermal expansion between tungsten and copper represents a significant outstanding challenge for this combination of materials. In this Phase I program, fabrication of functionally-graded bonding interlayer films by pulsed electrochemical deposition will be investigated. The films to be evaluated are tungsten-included copper composite and iron/tungsten alloy, graded to afford high and low tungsten abundance near the plasma-facing component- and heatsink-facing surfaces, respectively. It is anticipated that these functionally graded interlayers will provide a smooth gradient in the coefficient of thermal expansion, facilitating strong joints between the plasma-facing component and heatsink components. The proposed Phase I activities will begin with adaptation of one or more custom electrodeposition cells for the target substrate materials and geometries. Progressive development work will then be performed with the tungsten-included copper (iron-tungsten alloy) candidate interlayer systems, beginning with deposition of homogeneous copper (iron), continuing to deposition of non-graded tungsten-in-copper (iron-tungsten) composite (alloy) films, and concluding with fabrication of functionally- graded interlayers. Selected interlayer samples with promising functional gradients will be bonded between tungsten and copper-chrome-zirconium coupons and tested for mechanical strength as well as characterized for various mechanical/materials properties. An economic and scale-up analysis of the electrodeposition- based interlayer fabrication technology will be performed.The future applications of the proposed fabrication technology for functionally-graded films in the fusion power market are uncertain and strictly long-term. Successful development of fusion power, however, would represent an incalculable boon to human well- being. In addition, numerous other industries, such as gas and steam turbines in aerospace and power generation, also are exploiting or investigating the potential of functionally-graded materials for bonding and other applications. Thus, numerous alternative commercialization pathways exist beyond the fusion power market.