Some Notes from the LHC Commissioning Report
Mike Lamont delivered a report on the commissioning of the LHC. The slides are here. Below I offer some notes from his talk.
Problem: as discovered at the final stages of the hardware commissioning, the quench protection system (QPS) can be triggered erroneously due to a converter switch being turned off at the same time as a fast discharge. The workaround involves new thresholds, and the implication is that ramping must be slower than planned – it will take 3/4 of an hour. A real solution will come in a few weeks.
Success: the availability of the technical systems was good. For week 10 of 2010, the LHC was available 66% of the time, with 17% planned downtime (technical stop) and only 17% unavailable for unplanned reasons.
Success: lifetimes of 450 GeV beams were quite high, on the order of 100 hours.
Success: the aperture was measured by kicking the beam and seeing at what transverse distance and where along the beam losses occur. The optics show apertures that are more than ten times the size (σ) of the beam.
Success: the magnet model turns out to be remarkably accurate. The largest deviations are at the couple 10-4 level.
Success: the beam dump system, which is highly nontrivial and may be crucial to the survival of the experiments, works beautifully. See this illustration of 10 bunches dumped into the target precisely where they are meant to go (red line).
Critical Path: the machine protection system will dump the beam if “anything out there decides it’s had enough.” There are many inputs to the decision to dump the beam, and subtle interplay among them. Careful testing has proceeded well so far.
Mystery: “the hump” drives beam excitations which are bad because the emittance blows up leading to lower luminosities and higher backgrounds. The experts are open to suggestions…
Success: the ramp Friday morning with two pilot beams was a complete success – on the first try. The orbit looks stable and reproducible.
Puzzle/Problem: apparently the bunch length is not as short as expected after the ramp is completed, and it grows over time. This is not understood and may be a concern if fills last for many hours – see the plot below.
The ramp ends just before 60 minutes. The lower green cross shows what the bunch length should be, while the upper cross shows that it is significantly larger (don’t miss the suppressed zero!). The purple crosses show a small increase from 60 to 130 minutes.
Success: beta beating relates to the stability of the orbit and is a measure of success of the commissioning of the machine. For the LHC, this is already at the 20% level – it took weeks/months for earlier machines to achieve this.
In short, the LHC is in good shape, and the main area of concern is the machine and quench protection systems. According to Lamont, there are no show stoppers, and all of the hard work and thorough preparations over the last months and years puts the LHC in an excellent position for physics this year.
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