An Ultra-Short Dense Paraxial Bunch of Sub-Relativistic Runaway Electrons

This letter reports new results on the formation of a paraxial flow of runaway electrons emitted in an elongated air gap near the tip of a conical steel cathode. In the axial magnetic field > 4 T, the peak current density of ≈ 0.6 kA/cm
²
has been achieved in the bunch core with a 0.7 mm diameter. Similarly to the vacuum case, dependence of the bunch parameters on the magnetic field was obtained and interpreted. Analysis of recorded current waveforms and oscilloscope transient response shows that the original current pulse rise time and width did not exceed 5 and 10 ps, respectively. Using the time-of-flight method, we have estimated the particle kinetic energy (≈200 keV) at the peak of the distribution function.