In my previous LHC report I wrote about how the LHC was then running with a beam configuration of 13 bunches each holding 20 billion protons squeezed to 2m, providing a luminosity of 2 x 1029 Hz/cm2. This took them up to the limit of what is regarded as safe levels for the LHC. As they move towards more total energy stored in the beam, the risk of damaging sensitive parts of the collider if the beam goes astray gets higher. As it happens, it is possible to get higher luminosity without increasing the stored energy by putting more protons in each bunch but having less bunches. A couple of weeks ago the plan was to carry out tests to get these high intensity bunches stabilised. These tests would be interspersed with physics runs using the 13 bunch setup.
As this got underway the LHC was hit by a power cut that knocked loads of its systems out. If you have ever had trouble getting your PC back up after a power failure you can imagine what it must have been like for the LHC engineers with multiple components to get back into working order. The hardest of all were the cryogenic systems and in the end it took over a week before beams were again circulating.
Last week there was a conference in Hamburg to report “Physics at the LHC” (pLHC). Of course there is nothing revolutionary to report yet but it was a chance for the detector collaborations to report on the data they had taken so far and look forward to what they may see soon as luminosity goes up. A few of the physicists were not too shy to use the opportunity to plead with Steve Meyers for more physics runs before the next big conference so that they have more exciting material to present. That conference will be ICHEP 2010 which kicks off on 22 July so there is not long to prepare.
This makes quite a dilemma for Meyers and his team who control the running of the LHC. They could spend a month running with the working 13 bunch configuration, but that would be time wasted in the long run because once the LHC is up to full luminosities the same data will be provided in a run of a few minutes. It is much better to concentrate on getting to higher luminosities sooner but currently the higher intensity beams are not sufficiently stable. As well as The Hump which I wrote about last time, the beams are suffering from instabilities of a type previously seen at the HERA accelerator in Hamburg and dubbed “batman Instabilities”. Luckily, experience from HERA was taken into account when building the LHC and it has better systems including octupole magnets that can be used to cure these problems.
On Wednesday a new plan was formed to meet the challenge. There it was decided to abandon the 13 bunch config for physics and concentrate on commissioning the higher intensity bunches. This is because the process of switching between the two operation modes was causing too many delays. With some optimism they hope to have the higher intensity bunches in good shape within two weeks so that physics runs can restart at higher luminosities. If they succeed the lost time will be quickly made up and there will be plenty of physics runs in time for ICHEP 2010. If they fall behind schedule the physicists may turn up at ICHEP with little more than they had at pLHC. In truth there was no contest, the long-term goals take priority. If they miss out at ICHEP there are other conferences to follow. Patience will eventually pay-off.
So according to this new plan they will be able to restart physics runs by the end of June with nominal bunch intensities of about 110 billion protons, thus providing plenty of collisions just in time for ICHEP. The lower intensity bunches were being squeezed to beta=2m, but this process is harder at higher intensities and they had planned to aim for just 5m. After much scribbling on white boards (no doubt) they have now decided that they can get to beta=3.5m, which is better. From that point they aim to step up the number of bunches again and are hoping to be circulating 20 to 40 bunches per beam by August. They will then stick at that configuration to provide a month of physics runs. Obviously this plan allows for the fact that the French all go on holiday during the whole month of August so everything will have to run on autopilot during that time anyway. When everyone gets back they can continue stepping up the luminosity with more bunches.
What news then of the mysterious and malignant Hump? They did some further tests last week to see if The Hump was being caused by the cryogenic systems. The result was: no it isn’t. I think the hope now is that as they bring better stability systems into play (such as the transverse damper), The Hump will then be less of a problem even if the source for it is never found. We shall see.