= Plasma frequency
{title2=$\omega_{ps}=\sqrt{n_s q_s^2/(\epsilon_0m_s)}$}
The plasma frequency is the characteristic electrostatic oscillation frequency of a charged species displaced against a neutralizing background. For number density $n_s$, charge $q_s$ and mass $m_s$, the displayed formula uses SI units; in <Gaussian units> it is $\omega_{ps}=\sqrt{4\pi n_sq_s^2/m_s}$. The <ion skin depth> is $c/\omega_{pi}$ for the ion species. Different species have different plasma frequencies.
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