Study on the charge exchange of heavy ions during their acceleration in the U-200, U-300, U-400 cyclotrons

Experimental data on ion losses during charge exchange are analyzed. The data were obtained in experiments in the JINR U-200, U-300, U-400 cyclotrons. The pressure distribution and residual gas composition under different conditions of the U-400 cyclotron operation were studied. The measurements has shown that calculation of ion losses must be conducted with allowance made for the real pressure distribution in the cyclotron gap. A special attention must be paid to the cyclotron central region near the ion source where losses can be as high as 30-70% due to high pressures (up to 10-3 torr) and charge exchange cross sections (up to 10-14 cm2). A considerable part of the gas flow in the U-400 cyclotron is caused by desorption, stimulated by ions subjected to charge exchange as well as by a wide spectrum of ions, of various charges and masses, extracted from the ion source. As follows from the radial pressure distributions in the accelerating gap, an increase in the pumping rate of peripherally located pumps of the U-400 cyclotron would not produce a marked effect on the improvement of the pressure pattern in the accelerating gap, that is, it would not cause a reduction in the ion losses. A pressure reduction in the accelerating gap can be attained by a reduction in the gas flow from a PIG type source employing a pulse gas supply, by creation of additional means for pumping out (cryogenic panels, titanium magnetic discharge pumps, etc.) in the central region and by decreasing desorbing effect of accompanying ions via the installation of controlling gaps on the first turns in the cyclotron central region