Past exam of the short options paper shared by Prelims and Parts A and B of the physics course of the University of Oxford / 2017 / S19 / 3 / b / Solution

The supplied density is $4\times 10^{24}$ m${}^{-3}$. ThereforeThe cold relativistic field scale is $E_0=m_{e}c{\omega }_p/e\simeq 1.92\times 10^{11}$ V m${}^{-1}$, corresponding to an energy gradient $\boxed{e{E}_0\simeq 192\mathrm{GeV}{\mathrm{m}}^{-1}}$. It is an upper scale for the stipulated large-amplitude wave, not proof that the particular weak driver produces that field.