Vacuum testing of high efficiency AMTEC cells

The Phillips Laboratory Power and Thermal Management Division (PL/VTP), in cooperation with JPL, AMPS, Creare, and ORION, is performing vacuum testing of high performance Alkali Metal Thermal to Electric Conversion (AMTEC) cells, including the Micro-Machined Evaporator (MME) and PL-9A cells. The MME cell was designed to test an improved evaporator, which should allow long term operation at evaporator temperatures as high as 1100 K. The higher operating temperature is a key step towards obtaining 30% conversion efficiency from AMTEC systems. The PL-9A cell was designed and built to test an improved heat shield assembly. Long term data at high temperatures can only be obtained by testing in vacuum, because the current feedthroughs will oxidize at high temperature in air over extended periods. The testing at Phillips Lab is done in a vacuum test stand which simulates the environment of an AMTEC cell operating as part of a spacecraft power system. The test configuration consists of the MIME cell (later replaced by the PL-9A cell) in the center of an array of six other AMTEC cells. The seven cells are encased in multifoil insulation. Testing shows that there is little difference between cell current/voltage performance when measured in vacuum tests compared to guard heater tests. We are also examining the differences between “fast” I-V curve sweeps, recorded manually, with the cell operating at constant heat input, over a period of five minutes or less, and “equilibrium” I-V curve sweeps, in which the cell reaches thermal equilibrium at each data point