Past exam of the physics B course of the University of Cambridge / 2012 / Paper 2 / Section B / B6 / d

Use a cold, unmagnetized, collisionless electron plasma with immobile ions. For an oscillating field proportional to $e^{-i\omega t}$, the electron displacement follows $-m_e{\omega }^2\mathbf{x}=-e\mathbf{E}$. Hence $\mathbf{P}=-n_{e}e\mathbf{x}=-n_e{e}^2\mathbf{E}/(m_e{\omega }^2)$ andThe induced polarization opposes the field because electron inertia changes the phase of its displacement. Maxwell's equations give transverse electromagnetic dispersionThus transverse waves propagate above the plasma frequency, while below it the wavenumber is imaginary and a semi-infinite lossless plasma reflects an incident wave with an evanescent interior field. In the cold electrostatic model, a longitudinal density oscillation instead has frequency ${\omega }_p$.

Collisions at rate $\nu$ give ${ϵ}_r=1-{\omega }_p^2/[\omega (\omega +i\nu )]$ for this time convention, introducing absorption. A background magnetic field makes the response anisotropic, and thermal motion supplies additional dispersion and static screening. The divergent zero-frequency cold collisionless formula should not be mistaken for a complete description of static Debye screening. The simple dielectric function is a dynamical electron-inertia response under stated assumptions.