Examples of unexplained losses

One example of unexplained loss has been reported already a year ago [1]. The difference in the loss pattern at overinjection between IR2, where beam 1 is injected, and IR8 where beam 2 injection takes place is illustrated in Figure 1. It has been explained only partly by a presence of a chicane in IP8 which is responsible for significant decrease of the signal in monitors installed on Q1 magnet (first from left) in IP8. Another feature, the signal in the monitor on MBX magnet which is about 3-5 times higher in case of IP8, remains unexplained. Some observations considering the difference in location of the monitors have been done [2], but a simulation has to be performed to evaluate if these differences explain the difference in signal. The beam-abort thresholds for this monitor in IP8 have been temporarily increased in order to allow overinjection without dumping the beam.

Figure 1: Asymmetry in the BLM signals at injection of beam 1 and beam 2. The MBX magnet is the first dark box from the right. The irregular signal on one of the monitors installed on this magnet is visible.

Another example has a different nature. On the 3rd of October 2010 at 20:28 a sudden increase of BLM signal on the triplet magnets in IP1 led to a beam dump issued by BLMQI.02R1.B1E23_MQXB monitor. Signal averaged over long integration times 0.655 - 20.9 s exceeded the beam-abort threshold. Other monitors on the same tunnel card presented behaviour suggesting a very high loss event, but the monitors which are expected to be sensitive to such large loss (BLMs on collimators, ATLAS Beam Condition Monitors) have not confirmed it. The reason of this event, which has been observed only once, remains unclear.