From my understanding of a Xenon flash circuit, a storage capacitor and a trigger capacitor are charged up to about 240V (both with similar RC values, but less capacitance is needed for the trigger capacitor), and then a switch allows the trigger capacitor to discharge through a transformer, producing a very high, approximately 4KV pulse which ionises the xenon gas allowing the storage capacitor (charged to 240V) to discharge through the bulb. This keeps the gas ionised until the voltage drops too low.
That last sentence is what confuses me. Why do you need a very high voltage ≈4KV for the original ionisation, but then less than 240V for it to stay ionised? From my understanding of ionisation, an electric field applied over a certain distance (a voltage) will cause the electrons to be separated from the positively charged ions when the dielectric strength of the gas is exceeded. I imagine this is something to do with the potential energy of a point charge in an electric field, which I cant find an equation for, and the ionisation energy required to move the electrons from the valence to the conduction band.
So if a certain energy is required to ionise the gas, why does it stay ionised when this energy level is decreased?
Here is a simple diagram and description of a flash circuit: