Nonlocal Condition for Electron Runaway in a Gas Diode with a Needle Cathode

The conditions of electron runaway in an air gap with an inhomogeneous electric field caused by using a needle cathode are analyzed. Significant differences in the behavior of electrons have been revealed for different degrees of inhomogeneity of the field distribution provided by varying the radius of the needle tip. In a weakly inhomogeneous field (the radius of the tip exceeds a certain threshold), electrons are continuously accelerated throughout the gap. In a strongly inhomogeneous field (the tip radius is relatively small), electrons can reach maximum energy inside the gap and then begin to slow down in the near-anode region. The difference in the dynamics of free electrons leads to the need to use different, local or non-local, conditions for their runaway.