BLM THRESHOLD STRATEGY ( VISA-VIS UFOS AND QUENCHES )

The interaction of the LHC’s proton beam with falling macroparticles (dust) in the beam tube causes beam losses with durations ranging from tens of microseconds to several milliseconds. After the long shutdown, the beam energy will be increased from 4 towards 6.5 TeV, as a consequence of which some of these beam-particle interactions, colloquially called “UFOs”, are predicted to cause quenches in superconducting magnets. In-depth experimental and numerical studies have been performed to make the most efficient use possible of the LHCs beam-loss monitoring (BLM) system to minimize the number of quenches, while keeping the number of avoidable beam dumps due to the BLM system to a minimum. The results of these studies are presented here, as well as preliminary strategies for the setting of BLM thresholds for the protection of warm magnets and collimators. ARC UFOS PRE AND POST LS 1 Predictions for UFOs in the Arcs As the beam energy in the LHC will be increased from 4 to 6.5 TeV, the energy-deposition in superconducting coils due to collisions of the proton-beam with falling macroparticles (UFOs) will increase by a factor 2.4 . At the same time, the minimum quench-energy in the superconducting coils will decrease by a factor 2-3, depending on the duration of the UFO losses [1]. The combination of these two effects means that some UFO losses are expected to be sufficiently important to quench superconducting magnets in the LHC; compare [2]. The most likely functional region around the LHC ring for UFO-induced quenches and/or beam dumps is expected to be the arc region[3]. This assumption is supported by the observation that the UFO hotspots in the injection-kicker regions, which exhibited a high rate of activity prior to the LHC long shutdown (LS 1), have been overhauled with measures that have proven their efficiency in selected locations already during Run 1 [5]. Another relevant observation from Run 1 is the (de)conditioning seen during 2011/12 and illustrated in Fig. 1. After every winter stop, the rate of UFOs in the arcs increased (deconditioning), slowly approaching a lower asymptotic value over the subsequent weeks (conditioning). Another marked increase in UFO rate was observed during opertion with 25-ns bunch spacing. For the early weeks of Run 2 we have to expect an increased UFO activity with a subsequent conditioning, both, during the initial ∗bernhard.auchmann@cern.ch, CERN TE/MPE Figure 1: Number of arc UFOs per hour during stable beams in 2011 and 2012. Courtesy T. Baer, [2]. 50-ns operation, and after the switch to 25-ns bunch spacing. Based on the semi-analytical model of beammacroparticle interactions of [6, 7], the loss-duration of UFO events decreases linearly with beam size. The reduced beam size at 6.5 TeV will therefore lead to ∼20% shorter losses than at 4 TeV. The significance of this lies in the comparison of the maximum design-response time of the LHC machine protection system (MPS) with the rise time of UFO-induced losses. The maximum MPS response time is 3 turns or ∼270μs, with typical response times ranging between 80 and 170μs [8]. UFO-loss rise-times in ∼6000 events recorded during 2011-2012 (3.5 and 4 TeV beam energy, respectively) were found in the range between 50 and 300μs [9]. Even though the semi-analytical model predicts that UFO events with higher losses also have longer durations, it cannot be excluded that some UFOs can cause a magnet to quench before the MPS can dump the beam due to a BLM trigger. Measures Taken during LS 1 A mitigation of the origin of UFOs in the arcs, similar to the actions taken in the injection kickers, was not possible during LS 1. Certainly, the risk attached to quenches in the main circuits of the LHC are a lesser after LS 1. This is due to the refurbishment and control of all interconnections, and the qualification by CSCM tests [10] of the current bypass in the RB circuits. Nonetheless, quenches in main magnets at currents equivalent to 6.5 TeV beam energy are expected to lead to more than eight hours of down time – considerably more than a beam dump due to a BLM trigger. The avoidance of quenches, as well as the avoidance of unnecessary beam dumps, are, therefore, decisive factors for the availability of the machine at 6.5 TeV in the Figure 2: Relocation of BLMs in the arcs and DS from horizontal positions on MQ magnets to vertical positions above the MB-MB interconnects; courtesy A. Lechner [1]. Figure 3: BLM signals in the in the new BLM locations in an arc cell. The signals are plotted as a function of UFO location. Courtesy A. Lechner [1].