Criteria for electron runaway in a gas diode with a needle cathode

The conditions of electron runaway in a gas diode with a cathode in the form of a needle are studied theoretically. It is shown that the runaway conditions are qualitatively different for needles with relatively large and small tip radii, i.e., in fact, for different degrees of electric field inhomogeneity. In a weakly inhomogeneous field, the transition of electrons to the runaway regime is determined by the local distribution of the field near the place of their start – the tip of the needle. In a strongly inhomogeneous field, the runaway condition has a nonlocal character: it is determined by the behavior of electrons in the near-anode region. This difference leads to a nonmonotonic dependence of the threshold runaway voltage on the tip radius.