Total cross-section for photon-axion conversions in external electromagnetic field

We reconsider the conversion of the photon into axion in the external electromagnetic fields, namely in the static fields and in a periodic field of the wave guide. The total cross-sections for the conversion are evaluated in detail. The result shows that with strong strength of external electromagnetic fields, the cross-sections are large enough to measure the axion production. In the wave guide there exists the resonant conversion at the low energies, in which the value of cross-sections is much enhanced