As reported yesterday, the Large Hadron Collider has now collided nominal intensity proton bunches for the first time. This is an important turning point in the commissioning of the collider and today Oliver Bruning gave a useful talk explaining why that is. This then, could be a good moment to review the progress so far and look at the future plans for the gradual build up of energy and luminosity.
First of all, here is a table showing how the collider has gradually worked up to this point since its restart at the end of 2009:
|30/03/2010||3.5 TeV||2||1||11m||10 Gp||0.001 MHz/b|
|24/04/2010||3.5 TeV||3||2||2m||12 Gp||0.01 MHz/b|
|14/05/2010||3.5 TeV||4||2||2m||20 Gp||0.035 MHz/b|
|15/05/2010||3.5 TeV||6||3||2m||20 Gp||0.077 MHz/b|
|24/05/2010||3.5 TeV||13||8||2m||20 Gp||0.21 MHz/b|
|25/06/2010||3.5 TeV||3||1||3.5m||90 Gp||0.25 MHz/b|
E/proton is the energy per proton. The centre of mass energy which is important for the physics is twice this number because two protons collide head-on.
The protons in the beam are concentrated in bunches and nb is the number of bunches circulating in each direction round the collider ring. The number of collisions per turn nc is what counts towards luminosity and for small numbers of bunches this can be less than nb . It depends on how the bunches are distributed in order to collide at the different intersection points where the experiments live.
β is a parameter that measures how much the beams are squeezed at the collision points. Squeezing them causes more protons to collide so the luminosity increases. Finally Ib is the number of protons in each bunch (1 Gp = 1 billion protons). The luminosity L (units are given as MegaHertz per barn which is 1030 cm-2 s-1 because this is the unit used on some of the LHC luminosity displays) determines the rate at which collision events can take place and it is roughly proportional to nc x Ib2 /β, so increasing the number of protons per bunch is the most effective way to increase luminosity.
So that is why the goal for the last few weeks has been to increase Ib to its nominal value of 110 Gp. Going beyond this number may be possible later but it is very difficult because as the protons get closer together they start to interact and form instabilities in the beam. The controllers of the LHC beams have a number of tools at their disposal to fix these problems but with the lower intensities used until now these were not needed. To bring on the higher intensities they had to test and calibrate the tools and that takes time. That is why it has now been nearly four weeks without any real physics runs in the collider, a frustrating time for the experimenters.
In his talk today Bruning explained in basic terms a little about the methods they use to control the instabilities. These are things that have been worked out in the past at other colliders so they are the results of many years of research. It is because of this experience that it is possible to get the LHC working in such a relatively short space of time. In case you are curious about the details and don’t have the time to sit through the video of the talk, I’ll give a quick summary. The primary way to keep the beam under control is by tuning it so that its chromaticity is positive but not too large. if chromaticity still goes negative it is possible to keep the beam stable using transverse dampers. The instability is measured on one side of the collider ring and then a signal is sent across the diameter of the ring to control magnets at the opposite point. The signal must go at nearly the speed of light to arrive in time before the beam gets there leaving enough time to adjust the magnets and correct the beam.
Another trick for making the bunches less prone to instabilities is to stretch them out. This lowers the density of protons without much loss of luminosity. When the beam is accelerated to higher energies the length of the bunches shrinks due to Lorentz contraction at the highly relativistic speeds. The LHC has controls to spread them back out. Finally, there is one other system that helps control the stability which is Landau damping using octopole magnets.
All these systems have now been commissioned or nearly commissioned over the last few weeks, so running the collider with nominal intensity bunches is now possible. The other thing they needed to do was set up the collimators. These are solid blocks that can be positioned near the beam to strip out any protons that move away from the centre. There are no less than 76 of these and each one has to be placed in the optimal position by trail and error. At lower intensities this is a relatively quick setup process, but at higher intensities it is a more delicate process. The energy in the bunches is higher and passes a safe limit for the collider. At lower intensities it is OK to disable some of the colliders built-in protection systems but above 30 billion protons per bunch that would be a very unwise risk. This means that the beams are likely to be dumped during the process of setting up the collimators. For example, if the number of protons drops suddenly by just 0.1% because a collimator isd moved in too far, or if it drops by 50% overall, then the whole beam is dumped and further collimator setup has to wait until they can be reinjected.
Because this process can take so long it is important to fix the beam parameters now with values that can be used for the rest of this year. Increasing the number of bunches is the one thing they can do to increase luminosity further without having to redo the collimation. As the number of bunches is increased the bunches will come closer together and at some point later this year this could lead to unwanted “parasitic” collisions between bunches away from the intersection points. The way to avoid this is to introduce a cross angle between the beams so they only meet at the desired point, but if the crossing angle is too large it becomes more difficult to squeeze the beams to lower β. One of the smaller experiments LHCf needs a crossing angle to be able to function. In fact the LHCf collaboration would like quite a large crossing angle but the loss of luminosity would not be acceptable to the other experiments. This would not matter so much if they could have different collimation setups ready for different parameters but to save time they have selected one compromise setting with β at 3.5m and a crossing angle of 100 micro-radians.
These parameters are likely to remain fixed for the rest of this year with just a gradual increase in the number of bunches. In another report at the pLHC conference a couple of weeks ago. Mike Lamont described the schedule for this in the “short to medium term”, where medium term means the next ten years! The long-term plan is to upgrade the LHC to provide higher luminosities after 2020, but even before then there may be scope to increase bunch intensities from the nominal value of 110 Gp to as high as 170 Gp. Taking this into account the tentative plan for the next ten years is shown in this table:
|25/06/2010||3.5 TeV||3||1||3.5m||90 Gp||0.25 MHz/b|
|01/07/2010||3.5 TeV||4||2||3.5m||100 Gp||0.51 MHz/b|
|08/07/2010||3.5 TeV||8||4||3.5m||100 Gp||1.0 MHz/b|
|15/07/2010||3.5 TeV||20||10||3.5m||100 Gp||2.5 MHz/b|
|01/08/2010||3.5 TeV||24||24||3.5m||100 Gp||6.1 MHz/b|
|01/09/2010||3.5 TeV||48||48||3.5m||100 Gp||12 MHz/b|
|15/09/2010||3.5 TeV||96||96||3.5m||100 Gp||24 MHz/b|
|01/10/2010||3.5 TeV||144||144||3.5m||100 Gp||36 MHz/b|
|15/10/2010||3.5 TeV||192||192||3.5m||100 Gp||49 MHz/b|
|01/11/2010||3.5 TeV||240||240||3.5m||100 Gp||61 MHz/b|
|01/02/2011||3.5 TeV||796||796||2.5m||70 Gp||140 MHz/b|
|01/05/2013||6.5 TeV||720||720||1m||110 Gp||1.3 GHz/b|
|01/03/2014||7 TeV||796||796||0.55m||110 Gp||2.9 GHz/b|
|01/04/2016||7 TeV||2808||2808||0.55m||110 Gp||10 GHz/b|
|01/07/2018||7 TeV||2808||2808||0.55m||170 Gp||24 GHz/b|
Steve Meyers who directs the beams has shown that his plans can be very flexible depending on how well the process goes, but we expect the plan for the rest of this year at least to be not unlike the first part of the above. As to when interesting new physics will emerge, that depends on what nature has in store for us.