If UV photon with 3.9 eV energy strikes the Caesium,the electron is ejected.It starts to move in the direction of the positive electret,once its reached,it will gain 390 eV worth of kinetic energy.I ask you,where does this energy come from? Its not ioniastion energy thats for sure,since that is only 3.9 eV,two orders of magnitude bellow our final kinetic energy.
Like I said before:
The work constructing the electric field was consumed during mechanical arrangement of the components.
In turn, in this situation, that work is done on the electron-ion pair. As a result, energy is consumed: the electron and ion become trapped in the electret surfaces, neutralizing some charge. The field weakens.
Suppose you continue ionizing atoms in the gap. With each additional charge, the field drops: 390V at first, then 389.9999..., and so on until the field is consumed. Once the field is at very low levels (probably more than 3.9V, because of statistics and imperfect vacuum in practice), the current decreases significantly, as some atoms recombine rather than neutralize the charge.
If slightly more energy is applied, so that ionization occurs with some excess energy (as kinetic energy), then the ion and electron will take random paths in the near-zero field, with the result that the field remains balanced: if a field is applied, slightly more of each kind will be attracted to one side or the other.
Ionization, in bulk, is simply free space made slightly conductive. So it should be no surprise that the system evolves towards a state identical to that of two electrets slapped on either side of a metallic block: the field in the block is zero, current can flow through the block, and layers of charge are concentrated on its surface.
It may be easy to ignore the mechanical work done arranging little hunks of ceramic -- a bumbling human can't sense forces that small. That does not excuse the fact that work is, and must be, performed, in the process.
Tim