INVESTIGATION AND CURES OF SELFBUNCHING AT SLOW EXTRACTION OF BEAM FROM PROTON SYNCHROTRON U-70

Computer simulation as well as experimental study of selfbunching at frequency of ~ 5 MHz during the slow extraction of beam from the U – 70 were continued. The selfbunching takes place due to interaction of the beam with the passive accelerating cavities and leads to bad quality of the extracted beam. Its threshold is abnormally low due to the filament structure of the drifting bunches. The methods are described which have been proposed and realized in practice to cope with the selfbunching. The main instrument for this is the existing 200 MHz RF station. The filament structure of the beam is destroyed and the uniform momentum distribution of particles is formed due to the modulated voltage created by this station. The results of the computer simulation for the beam population up to 3·10 13 protons are shown. Longitudinal instability – selfbunching of the drifting beam occurs in the proton synchrotron U–70 at the upper flat top of magnetic field during the slow extraction. It happens at frequency of ~ 5 MHz and is seen at intensities almost an order lower then follow from the well-known Keil–Shnell criterion [1]. This problem was investigated with the help of the modernised computer code LONGIT using the macroparticles method [2]. Earlier [3] we studied the U–70 beam with the total intensity of N = 1×10 13 protons. The beam in the phase space coordinates “RF phase – energy” is shown in Fig. 1a, b, c at the moments of 25, 150, and 650 ms after switching off the RF voltage. Fig. 1a shows the specific striplike (filament) structure of the drifting beam. The beam would keep such a structure for an infinitely long time in the absence of perturbations. But acting of the voltage, induced in the accelerating cavities, becomes essential in about 15 ms after the drift has started.