What are the challenges faced by LHC in the initial data-taking of 2017?

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In summary, stable beams were declared 30 minutes ago, but the initial collision rate is low, at only 0.2% of the design rate. This is because the machine operators must ensure safety and check for any potential dangers before filling in more protons. It will likely take a few weeks to reach the same collision rates as last year. Meanwhile, experiments are starting to collect initial data, and the low collision rate of 0.2% is actually ideal for certain analyses. However, at design values, this means that 99.9975% of collisions are discarded due to limitations in data processing. The beam dump, which is a block 70 cm x 70 cm x 7 meters in size and surrounded by
  • #1
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"Stable beams" has been declared 30 minutes ago.
Similar to 2016, the initial collision rate is low (0.2% the design rate). The machine operators have to check that everything works and nothing presents a danger to the machine before more protons can be filled in. It will probably take a few weeks to reach the same collision rates as achieved last year.

Meanwhile, the experiments start collecting some initial data. 0.2% sounds like nothing, but for some analyses this is ideal. The LHC experiments are not only limited by the number of collisions, they are also limited by the amount of data they can read out and process. This is about 1 kHz for ATLAS and CMS (about 13 kHz for LHCb, 200 Hz for ALICE). At the design values, this means 99.9975% of all collisions are discarded: Only the most collisions with the highest particle energies can be kept. The other collisions are still interesting, however. Currently the high-energetic collisions are rare, which means there is more space to record other processes.
 
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  • #2
Why does it take several weeks to reach the design collision rates? Just safety checks and such?
 
  • #3
Yes. The stored energy in the beam is enormous (or it is when they circulate thousands of bunches) so they creep along slowly.
 
  • #4
The full beam is powerful enough to heat several tons of graphite by a few hundred Kelvin. You want to be really sure it doesn't hit anything it is not supposed to hit.

Safety is the main point, but not the only one. There are always stray electrons in the beam pipe, and they can heat the magnets. Starting at lower intensities reduces this issue and prepares the machine to go to higher intensities. See this and this post in the 2016 thread for details. We might see a few days of dedicated "scrubbing" runs, but last year it worked without them.
 
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  • #5
mfb said:
The full beam is powerful enough to heat several tons of graphite by a few hundred Kelvin.
(!) How quickly?
 
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  • #6
Instantaneously.
 
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  • #7
Great googly moogly! The graphite is the beam dump? And all that energy is contained in a tiny amount of hydrogen nuclei?
 
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  • #8
The graphite is the beam dump, yes (well, both beam dumps - one per direction, every following number is per beam dump).
A block 70 cm x 70 cm x 7 meters, with a mass of 7.5 tons. Water-cooled and surrounded by more than 750 tons of steel, iron and concrete.

The bunches gets 600 meters of flight distance to spread out, and kicker magnets at the start make sure different bunches impact the block at different places. You see the time-structure of the beams here (axes=position at beam dump):

swept-beam.jpg


All that energy (320 MJ, the energy of 80 kg of TNT) in 0.5 ng of hydrogen ions (that much hydrogen wouldn't even fill the volume of a grain of sand at room temperature+pressure). If the beam is dumped, it hits the absorber within one revolution (90 microseconds).
 
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  • #9
And one of the things they are doing is examining the pattern in mfb's plot very, very carefully to ensure that they understand exactly where the beam is going before they add more beam to the machine.
 
  • #10
Thank you. Up till now, I never had an intuitive feel for what 10 Tev actually meant in macroscopic terms. When you talk about high energy physics, you're not exaggerating!
 
  • #11
Well, 6.5 TeV is a tiny energy - in macroscopic terms, per proton it is huge. We get a large macroscopic energy if we consider that the LHC has up to 2800 bunches per beam with 110 billion protons per bunch.
 
  • #12
Of course you're right, what I meant was that you're throwing a totally insignificant amount of hydrogen ions (in macroscopic terms) at a graphite block, hard enough to raise its temperature hundreds of degrees. Extremely impressive, and a good real-world indication of how much energy it takes to "see" (make?) something like a top quark.
 
  • #13
I read about beam dump before. One surprising thing is that despite all this whacking with TeV-scale protons (more than enough to knock many neutrons off or outright disintegrate carbon nuclei), beam dump block does not become dangerously radioactive afterwards. (It _is_ radioactive, but not to the point where you can't stand near it).

One question I did not find answer to, is the entire beam dump assembly in vacuum?
 
  • #14
It has hundreds of tons of shielding around it. Without that shielding, I would avoid standing next to it. Graphite doesn't get activated much, but still a bit.

The vacuum pipe goes into the shielding. I guess it ends somewhere and the protons shoot through the endcap.

dump.jpg
 
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  • #15
mfb said:
The vacuum pipe goes into the shielding. I guess it ends somewhere and the protons shoot through the endcap.
What endcap material is used/suitable, that has the structural strength to support the vacuum yet not significantly absorb beam energy?
 
  • #16
I'm unable to quickly find any mention of how thermal expansion is handled for the 27 km vacuum tube. I imagine tight temperature control in the tunnel is used, though a loss of thermal control allowing ~3degK change leads to a meter of length change in steel.
 
  • #17
mheslep said:
I'm unable to quickly find any mention of how thermal expansion is handled for the 27 km vacuum tube.

It looks like there might be a bellows in your picture - above the leftmost green post.
 
  • #18
Vanadium 50 said:
It looks like there might be a bellows in your picture - above the leftmost green post.
You mean MFB's LHC picture?
 
  • #19
Yes, in post #14,
 
  • #20
mheslep said:
I'm unable to quickly find any mention of how thermal expansion is handled for the 27 km vacuum tube. I imagine tight temperature control in the tunnel is used, though a loss of thermal control allowing ~3degK change leads to a meter of length change in steel.
Thermal expansion is a major issue. Not so much for the outermost tube, where you can control the temperature, but for the beam pipe with the magnets. You have to install them at room temperature, and then cool them to 2 K. There are many bellows to handle the shrinking magnets.
The beam dump has bellows every 12 meters (no magnets in that region), the image in post 4 shows one of them. Vacuum design report, page 13.
mheslep said:
What endcap material is used/suitable, that has the structural strength to support the vacuum yet not significantly absorb beam energy?
Here is the design report. The graphite is kept in an inert gas environment. A vacuum was considered but not used: The graphite is designed for a temperature of up to 1250 °C, and an air leak shortly after a beam has been dumped could lead to a fire.
Design report page 18 said:
The window at the end of the extraction line, before the dump block, will be able to withstand this differential pressure and the gas pressure in the TDE will be slightly above atmospheric.
Unfortunately they don't mention the window material.

Here the high energies are an advantage. Most protons will pass through as minimally ionizing particles. A small fraction will interact with a nucleus, and produce several minimally ionizing particles - that is still fine as long as the seal is short compared to the hadronic interaction length. The peak heating rate occurs deeper into the absorbers.

There is also an interesting comment on activation:
Only 1 hour after dumping the beam, the dose-rates will be typically below 300 μSv/h. However, most of this will be due to the 24Na in the concrete shielding and walls, so allowing several days for this to decay would be preferable. The dismantling of the dump to exchange the core will require strict control and remote handling.
We had a nice stable run over night, 0.4% of the design luminosity with 12 bunches in the machine. We might get collisions with 50-100 bunches in the night to Saturday or Sunday.
 
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  • #21
Thank you. Fascinating machine.
 
  • #22
mfb said:
The beam dump has bellows every 12 meters (no magnets in that region), the image in post 4 shows one of them. Vacuum design report, page 13...
Thanks, yes, the bellows is visible. The LHC then has many parts moving with respect to each other in expansion: inner tube with crogenics, outer tube, green supports. Tube travel (in the mm range per 12 m section but it's there with delta T) still has to occur with respect to either the tube wrt supports, or the supports wrt to the floor. The vacuum report you supplied references the "supports" for the tube, both fixed and "mobile", without elaboration as to what mobile means. In the post 14 photo I can't pick up any indication of a travel mechanism (e.g bearings) between tube and green support. Does the base of a "mobile" support travel (seems unlikely)? Or does a support simply flex, wrt the nearest fixed support?
 
  • #23
Figure 12.10 looks like the beam pipe could move on some supports (along the beam pipe direction).
 
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  • #24
The idea is to minimize the motion of the shells, which are roughly at room temperature, and let the cold internals adjust via an expansion bellows. There are constraints which make this an idealization rather than a strict rule, but that's the idea.
 
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  • #25
We had two long runs with stable beams in the last 24 hours, 75 bunches, up to 3% the design luminosity.
More than 0.01/fb worth of data collected for ATLAS and CMS, about a trillion collisions per experiment.

The main focus is still on commissioning, but in parallel they increase the number of bunches.Edit Monday morning: We got another run over the night, 336 bunches, 15% the design luminosity. More than 0.05/fb collected.

Up to 1236 bunches per beam have been tested, but only at low energy.

The plan now is to do scrubbing. The beam pipe condition is that bad.
 
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  • #26
The main commissioning part is done.
2100 bunches in beam 2, not too far away from the design value of 2800. Scrubbing will be done with nearly the full beams. Can take a few days, but it is not always easy to predict how fast it works. Afterwards the plan is to increase the intensity with stable beams, which means we'll start collecting many collisions.
 
  • #27
mfb said:

"An optical transition radiation (OTR) beam monitor located in front of the dump [33] will detect off-normal dilutions."

Hmmm. There exist _photos_ and maybe even _videos_ of these dumps?
 
  • #28
The devices produce figures like the one in post 8. I don't think that is very photo-like, although it shows the distribution as function of the 2D position.
 
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  • #29
What about taking actual pics or videos of the dump, as beam leaves the vacuum tube and travels through "air" (nitrogen, I guess) into the TDE? Will it be visible in air? How much Cherenkov radiation? Or you think it will look "dangerous" and thus be a bad PR?
 
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  • #30
The beam dump elements are close together, I don't think their is an air gap to take pictures. The beam is dumped within 0.1 milliseconds, I doubt you would see actual beam effects. The glowing hot beam dump element surfaces: maybe (if there would be an air gap).The short scrubbing runs yesterday helped a lot already.
Stable beams with 315 bunches right now, initial luminosity was close to 20% the design value, about 0.1/fb collected in total. It is planned to go to 600 bunches on Thursday. Going beyond that might require more scrubbing.
 
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  • #31
How did they determine the specific number of stages of rings to build (three I think) and the specific diameter plus length of linac eg why not more smaller rings or fewer big ones. I know its optimal but is there a simple way to explain the physics or just simulation came up with this configuration?

Also what angle do the counter rotating beams collide at, doesn't seem to be head on the way the geometry looks at the beam cross over points.
 
  • #32
Most of the accelerators used as boosters today were front-line research machines in the past, now repurposed. IKt's not optimal. But it's a lot cheaper than ripping out the old accelerator and putting in a new one that is 10% bigger or smaller.
 
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  • #33
To put this in perspective, at what time after the big bang would these sort of energies be seen, theoretically?
 
  • #34
houlahound said:
Also what angle do the counter rotating beams collide at, doesn't seem to be head on the way the geometry looks at the beam cross over points.
It depends on the running conditions, typically 300 µrad, or 0.017 degrees. The angle is necessary to avoid collisions with the previous / following bunch (relative to the bunch they should collide with) - see the first image here, marked "long range". 300µrad for half the bunch spacing leads to a separation of 1.1 mm at 3.75 m distance to the collision point.
Adrian59 said:
To put this in perspective, at what time after the big bang would these sort of energies be seen, theoretically?
Somewhere in the first pico- to nanoseconds, depending on the process studied.After some problems with power supplies and other hardware, we had another run with stable beams this morning, 300 bunches, 17% the design luminosity.
We might get collisions with 600 bunches during the night.
 
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  • #35
The crossing angles are around 300 microradians. One important aspect of designing an accelerator complex is that you don't want huge increases in energy at a single stage. keeping it a factor of 20 or less is good practice.
 
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<h2>1. What is the purpose of the Large Hadron Collider (LHC)?</h2><p>The LHC is a particle accelerator located at the European Organization for Nuclear Research (CERN) in Switzerland. Its main purpose is to collide particles at high speeds in order to study the fundamental building blocks of matter and the forces that govern them.</p><h2>2. What were the challenges faced by the LHC during its initial data-taking in 2017?</h2><p>Some of the main challenges faced by the LHC during its initial data-taking in 2017 included technical issues with the accelerator, such as magnet failures and vacuum leaks, as well as unexpected behavior of the proton beams that caused disruptions in the data collection process.</p><h2>3. How were these challenges addressed and overcome?</h2><p>The technical issues were addressed by the LHC team through regular maintenance and repairs, as well as implementing new safety protocols. The unexpected behavior of the proton beams was studied and analyzed in order to make adjustments and optimize the data collection process.</p><h2>4. What impact did these challenges have on the research being conducted at the LHC?</h2><p>The challenges faced by the LHC in 2017 resulted in a delay in data collection and analysis, which impacted the timeline for research projects. However, the LHC team was able to address these challenges and continue with their research, resulting in significant discoveries and advancements in the field of particle physics.</p><h2>5. How has the LHC improved and evolved since its initial data-taking in 2017?</h2><p>The LHC has undergone several upgrades and improvements since its initial data-taking in 2017, including the replacement of faulty magnets, upgrades to the accelerator's systems, and the implementation of new data collection and analysis techniques. These improvements have allowed for more efficient and accurate research at the LHC.</p>

1. What is the purpose of the Large Hadron Collider (LHC)?

The LHC is a particle accelerator located at the European Organization for Nuclear Research (CERN) in Switzerland. Its main purpose is to collide particles at high speeds in order to study the fundamental building blocks of matter and the forces that govern them.

2. What were the challenges faced by the LHC during its initial data-taking in 2017?

Some of the main challenges faced by the LHC during its initial data-taking in 2017 included technical issues with the accelerator, such as magnet failures and vacuum leaks, as well as unexpected behavior of the proton beams that caused disruptions in the data collection process.

3. How were these challenges addressed and overcome?

The technical issues were addressed by the LHC team through regular maintenance and repairs, as well as implementing new safety protocols. The unexpected behavior of the proton beams was studied and analyzed in order to make adjustments and optimize the data collection process.

4. What impact did these challenges have on the research being conducted at the LHC?

The challenges faced by the LHC in 2017 resulted in a delay in data collection and analysis, which impacted the timeline for research projects. However, the LHC team was able to address these challenges and continue with their research, resulting in significant discoveries and advancements in the field of particle physics.

5. How has the LHC improved and evolved since its initial data-taking in 2017?

The LHC has undergone several upgrades and improvements since its initial data-taking in 2017, including the replacement of faulty magnets, upgrades to the accelerator's systems, and the implementation of new data collection and analysis techniques. These improvements have allowed for more efficient and accurate research at the LHC.

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