Why is an alkali seed material added to the hot gas in an MHD generator?

In a magnetohydrodynamic (MHD) generator, an alkali seed material—such as cesium, potassium, or sodium—is added to the hot gas to increase its electrical conductivity.1

For an MHD generator to operate with high performance and reasonable dimensions, the plasma must achieve a sufficient level of electrical conductivity, typically between 10 and 50 siemens per metre. Because the gas must be highly ionized to conduct electricity effectively, this readily ionizable "seed" material is injected into the flow—usually at about 1 percent by mass—to ensure the gas becomes an effective conductor at the required operating temperatures.1