Well, what I could find quickly comes from the 1960s, out of Avco Laboratories and the Naval Research Labs:
"Plasma Diagnostics in Supersonic Flow Containing Cesium and Potassium" [1] Burn Cs in a rocket engine, check for absorption up to 70 GHz (or cps, in those days). Looks like fun work.
"Plasma Diagnostics in Supersonic Flow Containing Cesium and Potassium" [1] Burn Cs in a rocket engine, check for absorption up to 70 GHz (or cps, in those days). Looks like fun work.
[1] https://ieeexplore.ieee.org/document/4323385 (paywalled)