G. Mueller¹ , V. Engelko² , H. Bluhm¹ , V. An¹ , V. Kuznetsov², G. Viazmenova²

Measurements of the spatial distribution of the beam current density in the GESA-facilities shows that the current density oscillates in time although the total beam current is stable. To investigate the nature of these oscillations results from different diagnostics for current and power density measurements have been analyzed (Faraday cups, small metal plates, x-ray detectors, etc.). It was found that oscillations of the beam current density are due to spatial movement (rotation) of the whole beam. The amplitude and appearance of this beam precision strongly depends on magnitude and distribution of the applied magnetic field. It was found that:

• The amplitude of oscillations increased with increasing cathode inductance B_c. Oscillations occurred only at B_c > 2.0·10^-3 T and at a mirror coefficient (B_t/B_c) of 5.8, at B_c > 4.0·10^-3 T.
• At large mirror coefficients (>10) the amplitude of oscillations remained nearly constant.
• The onset of oscillations occurred 5-7 m s after the beginning of the pulse. The time of oscillations’ onset was reduced with increasing B_c.
• The period of the oscillation is practically independent of the magnetic field magnitude and distribution and is always equal to about 1.5 m s.

In this paper we discuss the results of current density measurements and the possible reasons for the appearance of beam precision during the pulse.