Conditions for the generation of runaway electrons in an air gap with an inhomogeneous electric field: theory and experiment

Conditions for the generation of runaway electrons (RAEs) in a magnetically insulated coaxial air diode with graphite cathodes of different geometries—needle and conical with a Taylor opening half angle of $49,3^\circ $—are compared. The axial magnetic field allows the RAE beam to be focused on a current probe collector, thereby increasing the sensitivity of the recording technique in use. The threshold RAE generation voltage for the Taylor cone is found to be lower than that for the needle (i.e., a cone with a small opening angle), which indicates its nonmonotonic angular dependence with a minimum at an angle not exceeding the Taylor angle. According to our estimates, the dynamics of free electrons change qualitatively at the Taylor angle. At large angles, they accelerate throughout the entire gap; at smaller angles, they accelerate near the cathode and then slow down at the periphery.