In doing research for a metal additive manufacturing tool, I was very surprised to discover a robust use of household level magnetrons to build laboratory plasma torches for atomic spectrometry, here are some references:
Uhm, Han S., Yong C. Hong, and Dong H. Shin. "A microwave plasma torch and its applications." Plasma Sources Science and Technology 15.2 (2006): S26.
Green, Karyn M., et al. "Electronic excitation temperature profiles in an air microwave plasma torch." Plasma Science, IEEE Transactions on 29.2 (2001): 399-406.
Stonies, Robert, et al. "A new small microwave plasma torch." Plasma Sources Science and Technology 13.4 (2004): 604.
Bilgic, A. M., et al. "Design and modelling of a modified 2.45 GHz coaxial plasma torch for atomic spectrometry." Spectrochimica Acta Part B: Atomic Spectroscopy 53.5 (1998): 773-777.