Measurement and simulations of electron beam precession in GESA 1 and GESA 2 facilities

Transportation of intense electron beams in vacuum by an applied magnetic field leads to precession of the beam around the axis of the transport channel with a frequency of about 1 MHz. The frequency, amplitude and onset time of the beam precession were measured by e.g. x-ray diagnostic showing a dependency on magnitude and distribution of the applied magnetic field and on target material. In this paper we present results of further investigations to clarify the reasons for appearance of beam precession. A systematic experimental study of beam precession parameters as function of magnetic field configuration is presented. The precession parameters were estimated by the similar x-ray diagnostic described in [1]. The analyses of the experimental results were supported by simulations using the PIC-code ESRAY. The results imply that a prerequisite for beam precession is a spatial pulsation of the beam profile, resulting from deviation of the real magnetic field from Brillouin-conditions and is supposed to be triggered or even maintained by the dynamic of the cathode plasma (movement towards the anode), leading to continuously alteration of the beam perveance.