Photoelectric feedback mechanism for acceleration of runaway electrons in gas discharges at high overvoltages

Pasko et al. [https://doi.org/10.1029/2022GL102710] provided evidence that X-ray production may involve generation of runaway electrons from the cathode due to the photoelectric absorption of bremsstrahlung radiation generated by runaway electrons bombarding the anode. Here we report results based on Monte Carlo simulations of electron and photon transport indicating that in the gap with applied voltage U this feedback process generates a group of high energy runaway electrons with energies significantly exceeding U (by a factor of 3, in agreement with work of Tarasova et al. [Sov. Phys. Tech. Phys., 19, 351, 1974] where this effect was discovered). An electron traversing the gap acquires maximum energy U and generates bremsstrahlung photons from the anode with a broad energy spectrum below U. A photon group with energy close to U releases electrons with the same energy from the cathode upon photoelectric absorption. These electrons possess energies up to 2U upon arrival at the anode, and the subsequent cycles can lead to further energy growth. There is a controversy in the existing literature as to possibility to accelerate electrons above U in high voltage discharges [Naidis et al., 27, 013001, 2018; Tarasenko, 29, 034001, 2020]. The discharge may create regions of enhanced electric field that move synchronously with runaway electrons allowing them to gain energy exceeding U [Askaryan, Sov. Phys. JETP Lett., 1, 97, 1965; Babich, Sov. Phys. Dokl., 27, 215, 1982]. Here we provide an alternative mechanism.