In electrodynamic terms, when the first stage breaks down it creates a spherical electromagnetic wave whose electric field vector is opposed to the static high voltage. This moving electromagnetic field has the wrong orientation to trigger the next stage, and may even reach the load; such noise in front of the edge is undesirable in many switching applications. If the generator is inside a tube of (say) 1 m diameter, it requires around 10 wave reflections for the field to settle to static conditions, which restricts pulse leading edge width to 30 ns or more. Smaller devices are of course faster.