Langmuir wave standing wave resonance in DC/RF plasma

The electron energy distribution function (EED$f)$ of parallel-plate plasma with a high-negative DC electrode and an opposing RF electrode is far from Maxwellian [L. Chen \textit{et} \textit{al}., Proc. on 31$^{st}$ Dry Processing Symposium, 1--3 (2009) 7]. It has four distinct energy-groups, in descending order: DC-cathode injected ballistic electrons (BE), continuum, middle-energy peak, Maxwellian. The middle-energy peak has energy ($E_{M})$ in the range of $\sim $ 40eV$