LHC Status Report

last week we celebrated 1 inverse femtobarn (1/fb) of integrated luminosity delivered to ATLAS and CMS. Of that data ATLAS has recorded about 95% and CMS about 92% so with a little more added ATLAS have now recorded over 1/fb.

The milestones have been celebrated with a CERN press release

Last week the LHC Control group held an Open meeting to report on progress of the beams and experiments. Slides and videos are available for some of the talks including  the Machine Status Report by Steve Myers who revealed that during the last Machine Development period the bunch intensity was tested up to 195 billion protons, going well beyond the 170 billion ultimate intensity limit. The intensity currently in use is about 120 billion, but there is hope that this may be increased later in the year.

Although the LHC has delivered 1/fb in record time as a result of its better than expected early performance, there is some frustration that technical problems are holding it back from achieving even better results. A string of difficulties has been making it hard to get the beams circulating while other glitches cause the beams to be dumped early. The time in stable beams has been about 36% since they started running with 1092 bunches and it should be possible to do better than that.

Unidentified Falling Objects

In his talk Myres gave some more information about UFOs. These are mysterious rapid beam loss events thought to be caused by particles falling into the beam path. They can trigger the protection mechanisms to dump the beams. Studies have shown that they most often occur at the injection points and almost always shortly after injection causing problems before they get to stable beams. Surprisingly their frequency is not increasing with further intensity advances. They were 110 of these UFO events last year and already 5000 this year, but only the strongest cases can trigger a beam dump.

An extensive report on UFOs can be found here

RF Power Couplers

Another series of problems concerns the RF components. The couplers can take 200 kW of power and currently are being loaded up to 190 kW. This figure increases with beam intensity. If one of the ceramic couplers breaks it would put the LHC out of action for five to six weeks. These and other concerns have been preventing them from raising the bunch number to the next step of 1236 bunches. There is also a special report on the RF power issues and how they have been addressed. With the situation coming back under control it is hoped that the next luminosity step can still be taken this weekend.

Mini-Chamonix

In order to decide how to proceed for the rest of the year there will be a “mini-Chamonix” meeting on the 15th July. There We may hear more about addressing these and other problems as well as prospects for any further luminosity increases e.g. by raising bunch intensity.

Status Reports of the Experiments

At the LPCC meeting there were also reports from the individual LHC experiments. CMS has produced 80 papers using LHC data while ATLAS has about 190 and there are also good initial results from LHCb and ALICE. With the luminosity increasing at faster than expected rates there has been more pileup of events in the detector than anticipated. ATLAS reports an average of 6 events for each bunch crossing. There is significant impact on the calorimeter reconstruction resulting in increased systematic uncertainties in the analysis. Low transverse momentum jet events are the worst affected, but it is a small price to pay for so much extra data. Pileup will get worse if the bunch intensity is raised further.

Summary of Physics Results

Most of the physics results published so far have used just the 40/pb of data collected in 2010 with just a handful using up to 240/pb . A selective summary of results from ATLAS is shown on this slide (click to see full-sized). Within a few weeks we will have many more results including some using the 1/fb now collected. the EPS-HEP conference at the end of July is the next major opportunity for physics presentations.


11 Responses to LHC Status Report

  1. algernon says:

    Nice summary Philip, as always. Thank you.

    BTW right now Page 1 is announcing that they’re going to try and use 1236 bunches for the next physics fill.

  2. Bill K says:

    Not if us kobolds have anything to say about it! Cryogenics at point 8 is our target..

  3. carla says:

    It’s bizarre that the LHC is designed for 1380b, yet they’re concerned about the RF couplers being pushed to their limit because a ceramic might break. Does this mean that when the intensity is pushed futher, they’re going to have to replace the present couplers?

  4. Philip Gibbs says:

    I’d like to understand it better than I do, but the crux is that the problem arises in a failure situation where a klystron trips. The klystrons are key components of the RF systems that accelerate the protons in sections of the rings. If one trips they can do one of two things:

    (1) They can allow the beams to continue and cope with the loss but the beam then induces large voltages in the RF cavities. This reflects through the circulator and the couplers are hit with a power surge. They can take up to 200 kW. With present beam intensities they will be fine. With 1236 bunches they will be really on the limit. Any more and they will break.

    (2) The alternative is to set an interlock which dumps the beams whenever a klystron fails. If they do that then the danger is averted, but it also increases the number of beam dumps and there are more than enough of those already.

    Up to now they have been fine with option (1) but to step up they need to move to option (2)

    What they have done is to take some remedial measures that reduce the number of RF beam dumps from other faults, then they have turned on the interlock (or will turn it on during technical stop). Based on previous events they expect the combined effect to be a similar number of dumps as before, but with the interlock active it is safe to increase the intensity.

    Further steps will be taken during the technical stop to improve the situation further. This might make it possible for them to go back to option (1), or more likely it will just reduce the number of trips. If bunch numbers are increased further next year they will face the same problem again.

    • carla says:

      Aha! that makes so much more sense. These guys are so talented when it comes to getting the best out of the machine, it makes me wonder at what intensity the machine will be running two months from now – 2x the current intensity? I wouldn’t put it past them. May the failures continue as evidence they’re continuing to push the machine!

    • algernon says:

      That looks like an acceptable plan right now, but what happens when they switch to 25ns bunch spacing in 2012? Aren’t they supposed to crank it up to 2808b?
      I guess the surge on the couplers isn’t simply proportional to the total number of bunches, or is it? That would put the entire 2012 run on jeopardy unless they replace all the couplers during the winter break or definitely adopt option (2), no?

  5. phil says:

    In Myers’ talk he says that the LHC design assumes an interlock with beam dump upon failure. I presume then that will definitely be the strategy once they reach design luminosity. It’s an issue now only because they want to avoid more glitches if possible, and have been able to get away safely without turning on the interlock so far.

    • Philip Gibbs says:

      Yes, that’s how I understand it. It was always intended to run with the interlock on. The induced voltages are proportional to the beam currents which will double with 2808 bunches and more if higher intensity bunches are used.

      I don’t know if they can avoid the interlock this year but next they will need it and will just have to look for ways of keeping the number of klystron trips as low as possible.

      • carla says:

        On the plus side, it looks as if they’ve got the rf problems sorted out now upon reading the Sunday meeting notes. It’s back to any one klystron trip dumping the beam and the intensity can now be increased!

  6. […] noticia antes que éste, pero quisiera destacar a Philip Gibbs, “LHC Status Report,” viXra log, June 18, 2011, quien también nos resume lo presentado en la reunión del grupo de control de todos los […]

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