Technology and engineering of liquid lithium for fusion applications
Many outstanding issues in tokamak fusion arise from the interaction of the fusion plasma with the first wall of the device, particularly in the divertor of the tokamak. Edge cooling, impurity generation, and first wall erosion among others all have root causes that lie in particle and energy exchanges that occur at the plasma boundary. Solid divertors, currently made mainly out of carbon, beryllium, or tungsten, can be strongly eroded by the edge plasma and appear as impurities in the plasma, leading to higher radiative losses. Solid divertors also tend to engender high edge recycling regimes, leading to edges that are much cooler than the core. Liquid lithium has been proposed, tested, and, on small scales, proven to be a potential replacement for current divertor concepts. Liquid lithium surfaces give rise to low recycling regimes at the plasma edge, increasing energy confinement times, encouraging quiescent H-Mode plasmas, and lowering radiative losses from the core. Current schemes for introducing lithium into a fusion device consist of lithium evaporators. However, as fusion devices evolve from pulsed to steady state, new methods will need to be employed such as the Lithium-Metal Infused Trenches (LIMIT) concept of the University of Illinois at Urbana-Champaign (UIUC), or thin flowing film lithium walls. Critical to the implementation of these devices is understanding the interactions of liquid lithium with various surfaces. Presented here are experiments investigating the material compatibility, wetting characteristics, and relative thermopower of liquid lithium with a variety of potential substrate candidates for the LiMIT concept. Experiments on the wetting characteristics of the lithium used the contact angle as a metric. Among those materials investigated are 316 stainless steel, molybdenum, tantalum, and tungsten. The contact angle, as well as its dependence on temperature was measured. For example, at 200 C, tungsten registers a contact angle of 130o, whereas above its wetting temperature of 350 C, the contact angle is less than 80o. At sufficiently high temperatures, lithium wet each material, and several methods were found to decrease the critical wetting temperature of various materials which are presented here. The thermopower of W, Mo, Ta, Li and Sn has been measured relative to stainless steel, and the Seebeck coefficient of each of these materials has then been calculated. For molybdenum the Seebeck coefficient has a linear rise with temperature from SMo = 3.9 μVK-1 at 30 oC to 7.5 μVK-1 at 275 oC, while tungsten has a linear rise from SW = 1.0 μVK-1 at 30 oC to 6.4 μVK-1 at 275 oC, and Tantalum has the lowest Seebeck coefficient of the solid metals studied with STa = -2.4 μVK-1 at 30 oC to -3.3 μVK-1 at 275 oC. The two liquid metals, Li and Sn have also been measured. The Seebeck coefficient for Li has been re-measured and agrees with past measurements. As seen with Li there are two distinct phases in Sn also corresponding to the solid and liquid phases of the metal. In its solid phase the SSn-solid = -1.5 μVK-1 at 30 oC and -2.5 μVK-1 near the melting temperature of 231 oC. There is a distinct increase in the Seebeck coefficient around the melting temperature as the Sn melts and stays relatively constant over the rest of the measured temperatures, SSn-melt = -1.4 μVK-1 from 235 oC to 275 oC.
Lithium, Fusion, Plasma, Lithium plasma facing components (PFCs), Contact Angle, Seebeck
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign