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When the SPS is operated with the LHC test beam (80 bunches with 25 ns spacing), we observe a strong pressure increase for bunch intensities above about 5e10, in most sections of the ring. The picture (courtesy of J.M. Jimenez) shows the relative pressure increase at three different vacuum gauges as a function of the total beam intensity. |
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A fast instability is observed at injection and after beam excitation with about the same current threshold as the pressure increase. While the head of the LHC batch is damped, the other parts of the train show oscillations with rapidly increasing amplitudes, often resulting in significant beam loss from the trailing bunches. The instability rise time at the end of the LHC bunch train is only about 20 turns. The picture, courtesy of G. Arduini, shows growth rates measured at 6 consecutive slices along the batch, each 400 ns long. |
The multipacting was first observed as a charge build up on the feedback pick up, by W. Hoefle in 1998. At the pick up the current threshold of multipacting could be increased a factor of two using a weak solenoidal field.
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