LHC attempt another luminosity milestone despite problems

The Large Hadron Collider may not be producing any new physics yet, but they still keep us interested by setting new luminosity records every now and again. Now that they have high intensity bunches working, the goal for this week is to surpass a peak luminosity of  1 inverse micro barn per second. That’s 1030 cm-2 s-1 if you prefer such units. To do this they just need to increase the number of bunches and have just made a first attempt to inject 7 per beam. Sadly the beams were dumped as the energy ramp started due to a collimator problem. They will try again shortly so keep an eye on the “Page 1” status screen if you want to follow the excitement, unless you are reading this after the event.

A report from yesterday summarises the latest events and plans. The good news is that they expect to reach an integrated luminosity of 100 inverse nano barns ahead of schedule. Currently they have about 30 inverse nano barns. However, progress has not been without its problems. All the runs with high intensity bunches so far have seen sudden heavy losses of protons due to beam instabilities. This is leading to the luminosity falling off quicker than desired. The beam lifetimes should be around 330 hours based on measurements when they are stable, but the instabilities are reducing that to 10 hours. One theory for the losses is loss of Landau damping, perhaps they are still sensitive to The Hump whose mysterious cause has not yet been uncovered. In any case, the losses are all on the collimators so they are considered safe for now. Even with so much instability they can increase luminosities by a factor of 25 before the situation becomes a concern. This gives them time to try to understand the problem and correct it.

During the next week they also plan to start testing multi-bunch injections. That means they will circulate several bunches in the SPS and inject them all at once into the main collider ring. This is an important step towards running with much larger numbers of bunches in the LHC, firstly because it speeds up the injection process, but also because it is the only way they can inject bunches closely enough to pack them in.

Update: Beams with 7 bunches each were successfully ramped overnight. One of the bunches in each beam does not collide and is there only to help isolate the cause of instabilities. The other bunches provide 4 collisions per turn in each experiment. Stable beams were reached but apparently a beam dump was triggered before good luminosities were reached.

Update: On Friday evening a record luminosity of 1.1 x 1030 cm-2 s-1 was recorded, however it was not possible to sustain stable beams for very long. In fact there have now been five attempts to run with 7×7 bunches and so far none of them have lasted well before the beams dumped. It looks like they will continue trying over the weekend.


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