A certain substance, with a dielectric constant $$k = 2.5$$ and the dielectric strength $$E = 1.8\times10^7\,\text{N C}^{-1}$$, completely fills the space between the plates of a parallel plate capacitor, with circular plates, of capacitance $$C = 72.0\,\text{nF}$$. The minimum diameter of the circular plates, to ensure that the capacitor can withstand a potential difference of $$V = 4.0\,\text{kV}$$, is