Unmagnetized plasmas are typically modeled as Drude materials. The complex permittivity of a Drude material with N poles is given by:
:<math> \varepsilon(\omega) = \varepsilon_{\infty} - \sum_{p=1}^N \dfrac{\omega_p^2}{\omega^2 - j\omega \nu_p} </math><!--[[Image:FDTD19(1).png]]-->
where ω<sub>p</sub> and v<sub>p</sub> are the angular plasma frequency and angular collision frequency corresponding to the p''th'' pole, respectively, and both are expressed in rad/s. For an unmagnetized plasma, ε<sub>∞</sub> = 1.