Capillary wicking of liquid lithium on laser textured surfaces for plasma facing components

Liquid lithium has been proposed as a coating for plasma facing components (PFC) in fusion reactors in order to reduce recycling and protect the PFCs from high heat loads. Lithium can be introduced onto the surface of the PFCs through a passive capillary wicking mechanism. Surfaces capable of wicking molten lithium to support passive cooling are traditionally produced using a porous material which is added to the surface of a structural material. Laser micro-machining or texturing processes, on the other hand, allow functional surfaces to be applied directly onto existing structural materials. Both 316L stainless steel and titanium-zirconium-molybdenum (TZM) alloys were textured using a Nd:VO₄ laser to produce a pattern of open channels with depths between 40 μm and 260 μm. Screening tests using an isopropanol solution were used to evaluate the ease of wicking for different texturing geometries, and molten lithium wicking experiments were then performed with the samples placed at 45° and 90°orientations. In order to quantitatively measure the progress of the wicking front, a time of flight (TOF) technique, which measures the progress of a temperature gradient using pre-placed thermocouples, was developed to track the progress of the lithium wicking front. The laser textured surfaces on both the 316L and TZM alloys displayed the ability to fully wick both isopropanol and liquid lithium and outperformed commercially available porous materials in the wicking of an isopropanol solution. A comparison of the different texturing geometries on 316L stainless steel samples shows that deeper grooves in the wicking direction combined with shallow grooves, of approximately half the depth, in the cross-channel direction improve the wicking rate and uniformity, and achieve full wetting of the sample surface.