Experiments indicate that ionization‐diffusion instabilities (striations) of the convective type, present in low‐pressure mercury/noble‐gas lamps, are generated by current fluctuations in the inhomogeneous part at the cathode end of the discharge. Self‐excitation of striations is made possible by feedback via the electrical supply circuit of the lamp. An experimental boundary condition is found at the point of excitation. A simple plasma model, with an extension that takes feedback into account, fully describes the laser‐like phenomenon of mode selection and the influence of the discharge length.