The ultra-cold neutral plasma (UNP) extends greatly the boundaries of classical neutral plasma physics and has been widely studied in recent years. The diagnostics methods of UNP are mainly developed from some well-defined technique of optic probes, such as laser induced fluorescence imaging, optical absorption imaging and even recombination fluorescence. 

Researchers at Dalian Institute of Chemical Physics (DICP) and Shanghai Institute of Optics and Fines Mechanics (SIOM) suggest a new method of using microwave radiation for the study of UNP. This method will give us the opportunity to measure the amount of electrons Ne and the recombination rate of plasma, which is extremely important in the research of UNP.[ Physics Letters A 375 (2011) 2158–2161] 

Their calculations indicate that the dipole radiations of UNP do not depend on specific density profile n_e(r) and incident frequency  when peo, but on the total electron number Ne. They suggest that the microwave radiation from UNP may offer a new way to get the information of recombination in UNP, or other expanding inhomogeneous plasma in similar.