Compression of a runaway electron flow in an air gap with a nonuniform magnetic field

The first results of study of the compression of a picosecond runaway electron flow in an air electrode gap by a pulsed guiding magnetic field increasing along the electron trajectory by a factor of 10–20 are reported. The main aim is to increase the density and homogeneity of the current of runaway electrons that are generated near the edge of a tubular cathode and are accelerated in an inhomogeneous electric field to a collector inside a solenoid. Restrictions on the integral current of runaway electrons caused by the reflection of particles with high transverse velocities from a region of concentration of magnetic field lines (magnetic mirror) have been analyzed. Under “optimal” conditions (magnetic field, the diameter of the cathode, and its position with respect to the solenoid are varied), a magnetized tubular runaway electron flow radially compressed by a factor of 3–4 with a current density up to 100 A/cm$^2$ is formed.