Photoelectric Feedback Mechanism for Acceleration of Runaway Electrons in Gas Discharges at High Overvoltages

Energy conservation dictates that an electron with elementary charge e traversing a vacuum gap formed by electrodes maintained at potential difference U volts acquires maximum energy of eU. In many experiments electrons with energies as high as 3eU have been observed. The experimental discovery of this effect was made over 50 years ago and is still a subject of significant controversy in applications related to x-ray generation from high voltage discharges. Here we explain these observations by a sequential increase of energy of runaway electrons released from the cathode due to the photoelectric absorption of bremsstrahlung radiation generated by the previous cycle of runaway electrons bombarding the anode.