Investigation of the role of X-ray photons in the pre-breakdown current in vacuum interrupter gaps

It is well known, that vacuum interrupters produce x-ray radiation. This is due to the high energy of the electrons accelerated in the vacuum gap before hitting the anode. Part of this energy is released in the form of high energy photons, that is x-ray radiation. Whereas this is mostly seen as a health and safety issue, it was discussed recently that x-ray radiation could play a role in the dielectric breakdown of vacuum gaps. The assumed mechanism is, that photons produced in the anode cross the gap and release secondary electrons from the cathode, amplifying the total current flowing in the gap. In addition backscattered electrons enhance the total photon yield as well. With the help of the MC simulation code PENELOPE we analyze the production of both x-rays and secondary electrons for gap voltages between 12 and 42kV. The electric field in the gap and therefore the returning of the electrons is included in the simulation. We find that these backscattered electrons enhance the x-ray photon yield. But the total number of photons produced and also the number of secondary electrons released from the cathode remains small. Their total yield in comparison with the number of primary electrons is rather small. We therefore conclude that x-ray photons play only a minor role in explaining the pre-breakdown current flowing through the gap in vacuum interrupters.