High Current Beam Transport Experiments at GSI

The status of the high current ion beam transport experiment is reported. 190 keV Ar1+ ions were injected into six periods of a magnetic quadrupole channel. Since the pulse length is > 0.5 ms partial space charge neutralization occurs. In our experiments, the behavior of unneutralized and partially space charge compensated beams is compared. With an unneutralized beam, emittance growth has been measured for high intensities even in case of the zero-current phase advance σ0 < 90° . This initial emittance growth at high tune depression we attribute to the homogenization effect of the space charge density. An analytical formula based on this assumption describes the emittance growth very well. Furthermore the predicted envelope instabilities for σ0 > 90° were observed even after 6 periods. In agreement with the theory, unstable beam transport was also experimentally found if a beam with different emittances in the two transverse phase planes was injected into the transport channel. Although the space charge force is reduced for a partially neutralized beam a deterioration of the beam quality was measured in a certain range of beam parameters. Only in the range where an unneutralized beam shows the initial emittance growth, the partial neutralization reduces this effect, otherwise the partially neutralized beam is more unstable.