Threshold changes

The changes of thresholds which took place in 2010 were due to hardware modifications (for instance filter installations) as well as due to debugging of the threshold generation code. These changes are described in [3,18,19].

The most important change, affecting almost all monitors on superconducting magnets, follows the better understanding of the quench limits due to lack of quench induced by UFO event (millisecond scale, described before) and as a conclusion of quench test for long losses (1-5 second scale).

The quench tests performed before Autumn 2010 were testing only MB magnets at injection energy and for very short losses, where the quench limit can be easily calculated as an enthalpy limit of a dry superconducting cable [13]. A series of tests performed in September and October 2010 used orbital bump technique to provoke slow losses on arc MQ magnet for injection energy and at 3.5 TeV. An example of the loss signals observed during the quench test at 3.5 TeV are shown in Figure 12. The small blue and red squares mark the positions of the BLM monitors observing beam 1 and beam 2 respectively. The beam direction was from right to left (beam 2). The green line shows the signals expected in the second and the third monitor in the moment of quench. The red line connects the signal actually registered by the BLMs during the quench. The existing threshold was overestimated by factor 2 to 3 [20].

Figure 12: Signals registered in the BLM system during the quench test of MQ magnet at 3.5 TeV.

Corrections to BLM thresholds on cold magnets have been applied before the start of 2011 run. They are depicted in Figure 13, where the solid line is an example of old 3.5 TeV quench thresholds, and dashed line shows the new thresholds.

Figure 13: Example of BLM thresholds for beam energy 3.5 TeV.