accelerator 3 is listed here below as condition 4 condition 4 equ4 here e m is the gain in energy in the main accelerator 3 e q is the gain in energy in source 1 f q is the operating frequency of the source 1 f b is the operating frequency of the preaccelerator 2 and of the main accelerator 3 and u03b1 3 is the preacceleration factor a not absolutely necessary but advantageous further condition 5 equ5 must be met in order to attain a minimum dependency on field errors at the input of preaccelerator 2 and at the output of the preaccelerator 2 if all these conditions are met precisely an electron beam can be obtained whose energy is increased up to 10 mev and whose energy constancy or its directional be m values respectively are extremely high fig3 shows a simple high frequency field emission source 1 which can be used in the arrangement of fig1 it includes two parts 9 and 10 which can be connected together they enclose a recess 11 which serves as a resonator energy is fed in via inlet 12 on the raised bottom 13 of part 10 a field emission cathode 15 is centrally disposed on a rod 14 the field emission cathode 15 is placed a short distance away from the iris aperture 16 the energy coupled in through inlet 12 produces in cavity 11 a standing wave at a frequency of 24 ghz the voltage maximum lies between the peaked i e pointed field emission cathode 15 and the opposite terminal.