Live from CERN - no more clouds in the LHC tunnel

I am feeling like it is a long time I have not written anything about the LHC, the Large Hadron Collider at CERN, which is also known as one of the most powerful machines ever built by humans to explore the phenomenology of the microscopic world.

The CERN Large Hadron Collider had undergone a technical shutdown during the last winter, and the machine is now being prepared for restarting the physics run.

In other words, protons will soon be collided with the aim of getting an even more precise understanding of the Standard Model of particle physics and of hopefully discovering new phenomena that are expected by many particle physicists.

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THE 2016-2017 WINTER LHC SHUTDOWN

During the 2016-2017 winter, the LHC was shut down for technical reasons, as during any winter by the way. During this 2016-2017 shutdown, the machine was warmed up and two sectors of the LHC ring have been opened.

As a consequence of this opening, the inner walls of the ring have been contaminated. The vacuum condition of these sectors where not realized anymore due to the opening. This is the contamination I am referring to.

This contamination stems from the fact that air is now in these two sectors of the LHC, so that plenty of gas molecules and electrons could enter (together with air).

Of course, when the machine is cooled down again, a very high vacuum is put in place and not a single molecule of any gas or electron is supposed to stay there. However, this is not totally true. Molecules or electrons could be trapped on the walls of the vacuum chamber.

This may not seem to be a problem as long as they stay calmly on the walls. However, once the LHC works, very-energetic protons are organized into two beams that will collide each others. These beams will pass next to the trapped molecules and electrons, and they could free them.

And now the issue: these freed electrons and molecules can be in the way of the beams. We are in fact in the presence of a phenomenon called electron coulds. In very few words, these freed molecules and electrons could destabilize the beams.

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PREVENTING BEAM DESTABILIZATION BY CLEANING

In order to avoid such a phenomenon, the inner walls of the vacuum chamber must be cleaned. This is done in a very specific way, that permits to inhibit any future formation of clouds.

By the way, such a cloud as the shape of a star, as illustrated on the figure below, which represented a typical electron distribution on one of the LHC magnets.

Before entering into the details of the cleaning process, it is good to remind what a beam circulating into the LHC machine is made off. It consists of bunches of protons, and each bunch is separated from each other.

The way to measure this spacing is in general expressed in nanoseconds, and the LHC is expected to handle, under the 2017 running conditions, beams made of 2556 bunches spaced by 25 nanoseconds each.

That consists of a lot of protons and we must hence be sure to keep the beams into control (and to avoid electron clouds).

In order to remove the electrons trapped in the vacuum chamber, one starts by injecting beams made of a small number of long bunches. This small number is here 288, for those interested in numbers. Which is indeed small as when compared to the 2556 number mentioned before.

Those conditions are actually quite far from the normal working conditions, but it is good to move on little by little (or at least step by step) to avoid troubles.

This special beam has the role of bombarding the inner walls of the tube in order to provoke as many electron clouds as possible. The walls are consequently less and less likely to induce, in the future, the formation of new clouds. Which is in particular important when the very-energetic and dense beams useful for physics will circulate in the tunnel.

As a result, the sectors that have been opened have started to heat a lot. This was expected, as the electrons trapped on the wall are being scrubbed and hence lead to an increase of the heat load.

After a few days, the head load decreased, as there are less and less electrons to be scrubbed. As expected, And consequently, the beam quality improved.

It is now time to start the more serious work! The cleaning is indeed not done yet.

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FINAL CLEANING WITH MORE POWERFUL BEAMS

However, as above mentioned, we are still far from the usual LHC working conditions.

A couple of weeks ago, a beam containing a huge number of 2820 bunches was injected into the machine. The resulting heat load was stil non negligible, but it was nicely taken care of by the fast-reacting cryogenic system of the machine.

At that time, there were however still a dense presence of electron clouds that were causing a slight degradation of the beam. The heat load nevertheless move on in decreasing (less and less clouds), and reaches the value that it had at the end of 2016.

Almost ready to go for physics!

After a few extra tests and a couple of days of running with test beams, the LHC is today free from electron clouds. It is hence ready to receive more and more powerful beams that will be used for physics.

This will happen very very soon.

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SUMMARY AND REFERENCES

This post was motivated by my regular reading of the CERN bulletin thar I read regularly. I have briefly summarized some of tests that have been performed around the CERN Large Hadron Collider.

The machine is today almost ready for its 2017 run for physics, and the cleaning of the electron clouds (that was the topic of this article) potentially dangerous for the beams has been finalized.

For more information, I recommend browsing the CERN LHC website. Moreover, extra details on the news I discussed can be found on these two pages:

• A brief report on the commissioning period can be found here.
• Another report about the treatment of the electron cloud issue can be found here.