Time-of-flight technique for estimation of the energy of runaway electron bunches formed in magnetized gas diodes

Time-of-flight estimates of the kinetic energy of magnetically insulated bunches of moderately relativistic runaway electrons (RAEs) were obtained with the use of an air diode equipped with a drift section. The measurements have demonstrated that the bunch energy can be controlled by changing the conditions for RAE emission in an air diode with a sharply inhomogeneous electric field, which can be done by varying the cathode geometry and material and the length of the cathode-anode gap. It has been demonstrated that if the measured electron current peaks are resolved to within 10 ps, and the bunch duration and spread in RAE emission moments are close to this resolution, the characteristic energies of the bunches can be estimated with an accuracy of ≈10%. Examples of energy measurements are given for paraxial and tubular bunches accelerated at cathode potentials of up to −250 kV. The estimated characteristic energies of the bunches, even not corrected for drift losses in the fill gas, turned out to be greater than the energies that could be achieved for bunches accelerated in a vacuum diode with due account for the variations in diode voltage.