Note: The Taylor cone generates ion sources of very high brightness. This design enables the gun to provide beam diameters in the range $50\mathrm{nm}$ to $1\mathrm{\mu }\mathrm{m}$ for ion beam energies in the range $5\mathrm{keV}$ to $30\mathrm{keV}$, the smallest diameters generally being at the higher energies.