Low margin locations

The threshold setting is based on two threshold tables called master and applied [17]. The master table is the one which assures protection of the accelerator components against damage. The thresholds which are actually used in the electronics (applied thresholds) must be below the master table. The following convention is used: in the normal situation the applied thresholds are 10 times below or equal the master thresholds. This allows the operators to raise, fast and safely, the thresholds up to factor 10, when needed. This method has been found very useful in 2010 run, when UFO events became an important thread to the beam availability.

The weakness of this approach is seen for the short signal integration times. The upper bound of the dynamic range of the BLM channels (with standard ionisation chamber) is about $\rm 23~Gy/s$. According to the convention the applied threshold values must be set to maximum $\rm 2.3~Gy/s$. In case of short signal integration times this value is often much lower than the estimation of the physical threshold. Therefore the BLM system is often overprotective for short losses.

The above arguments should be kept in mind when looking at the results of the systematic search for channels where signals are close to the thresholds. Figure 11 illustrates the results of such search. Every point depicts a maximum registered ratio of signal to applied threshold for every monitor. The red line represents the dump level, and the green line 10% of the dump level. Higher beam intensities and higher luminosity expected in 2011 run will increase the losses, while higher beam energy will decrease the thresholds, so factor 10 between loss and threshold as observed last year might not be enough during 2011 run. More details on this analysis can be found in [3], where also a lists of monitors with low margin to thresholds (above green line) are published.

Figure 11: Ratio of maximum signal to the applied threshold registered during the stable beam period, as a function of monitor location, for 5 high luminosity proton fills.