The study of UFO events will continue during the 2011 run. As the frequency of these events is expected to increase, a strategy to avoid spurious beam dumps must be developed.
One possibility is so called scrubbing run foreseen during the first weeks of 2011 operation. During the scrubbing a high intensity beam at injection energy is circulating in the machine in order to enhance outgassing from the internal surfaces of the beam chamber. It is possible that the scrubbing will also enhance the release rate of the dust particles therefore suppressing the rate of UFO events in the following physics runs. The UFO activity before, during and after scrubbing run will be carefully monitored.
The second possibility relies on the lack of UFO-induced quenches during 2010 run. The BLM thresholds at the end of the run have been already increased to about 60% above the originally estimated quench level. This modification allowed to avoid spurious beam dumps which would have been generated by the BLM system if lower thresholds had still been applied.
The signals observed during the UFO events have exceeded the expected BLM signal at quench. The reason can be two-fold:
In order to investigate the impact of these two unknowns, a simulation study has started and a special quench test has been performed.
It must be stressed that, in case of the beam energy increase, the amplitude of the loss generated by UFO will increase (as in case of BLM signal during the wire scan, see Figure 9). Together with the reduced quench level, this might lead to a significant increase of the quench probability when running the LHC with energies above 3.5 TeV.
![\includegraphics[width=65mm, clip]{plots/WS_ramp.eps}](img22.png)