DESIGN, MATERIALS AND R-AND-D ISSUES OF INNOVATIVE THERMAL CONTACT JOINTS FOR HIGH HEAT-FLUX APPLICATIONS

Plasma facing components in fusion machines are designed with a layer of sacrificial armour material facing the plasma and a high-conductivi ty material in contact with the coolant. One of the most critical issu es associated with making the proposed design concept work, from a pow er handling point of view, is achieving the necessary contact conducta nce between the armour and the heat sink. This paper presents a novel idea for the interface joint between the sacrificial armour and the ac tively cooled permanent heat sink. It consists of a thermal bond layer of a binary or more complex alloy, treated in the semi-solid region i n such a way as to lead to a fine dispersion of a globular solid phase into a liquid matrix (rheocast process). The alloy in this ''mushy st ate'' exhibits a time-dependent, shear rate-dependent viscosity, which is maintained reversibly when the material is solidified and heated a gain in the semi-solid state. The function of the thermal bond layer i s to facilitate heat transfer between the replaceable armour and the p ermanent heat sink without building up excessive thermal stresses, as in conventional brazed joints, and allow an easy replacement whenever needed without disturbing the coolant system. No contact pressure is r equired in this case to provide the desired heat transfer conductance, and the reversible thixotropic properties of the rheocast material sh ould guarantee the stability of the layer in the semi-solid conditions . Key design, material and testing issues are identified and discussed in this paper with emphasis on specific needs for future research and development work. Examples of suitable material options which are bei ng considered are reported together with some initial heat transfer an alysis results.