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

[mfb]

"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.

 

[Drakkith]

Why does it take several weeks to reach the design collision rates? Just safety checks and such?

 

[Vanadium 50]

Yes. The stored energy in the beam is enormous (or it is when they circulate thousands of bunches) so they creep along slowly.

 

[mfb]

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.

 

[sandy stone]

The full beam is powerful enough to heat several tons of graphite by a few hundred Kelvin.

(!) How quickly?

 

[Vanadium 50]

Instantaneously.

 

[sandy stone]

Great googly moogly! The graphite is the beam dump? And all that energy is contained in a tiny amount of hydrogen nuclei?

 

[mfb]

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):

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).

 

[Vanadium 50]

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.

 

[sandy stone]

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!

 

[mfb]

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.

 

[sandy stone]

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.

 

[nikkkom]

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?

 

[mfb]

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.

 

[mheslep]

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?

 

[mheslep]

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.

 

[Vanadium 50]

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.

 

[mheslep]

It looks like there might be a bellows in your picture - above the leftmost green post.

You mean MFB's LHC picture?

 

[Vanadium 50]

Yes, in post #14,

 

[mfb]

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.

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.

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.

 

[mheslep]

Thank you. Fascinating machine.

 

[mheslep]

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?

 

[mfb]

Figure 12.10 looks like the beam pipe could move on some supports (along the beam pipe direction).

 

[Vanadium 50]

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.

 

[mfb]

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.

 

[mfb]

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.

 

[nikkkom]

Here is the design report.

"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?

 

[mfb]

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.

 

[nikkkom]

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?

 

[mfb]

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.

 

[houlahound]

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.

 

[Vanadium 50]

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.

 

[Adrian59]

To put this in perspective, at what time after the big bang would these sort of energies be seen, theoretically?

 

[mfb]

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.

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.

 

[Vanadium 50]

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.

 

[houlahound]

Thanks explanations and links, most interesting.

If money wasn't a factor what would be the most optimal config to get the beam up to energy, how is this determined. I guess I could ask the same about rocket stages - is it the same physics principles based in thermodynamics?

How does the 20% figure come about?

LHC fanboy here.

 

[mfb]

20%? Do you mean the factor 20? The magnets have to adjust their magnetic field according to the beam energy very accurately (10^-5 precision) to keep the particles on track, at very low fields (relative to the maximum) that can be challenging. You also have to take into account if your particle speed still changes notably during the acceleration process.

If money wasn't a factor you could build a 15 km long linear accelerator directly leading to the LHC. Then you can fill it in two steps (one per ring), in seconds instead of 20 minutes, and with more bunches. Or, if we get rid of any realism, make the linear accelerator ~300 km long and directly put the protons in at their maximal energy. Then you also save the 20 minutes of ramping up and 20 minutes of ramping down.
The beam dump would need some serious upgrades to handle a higher turnaround.

 

[houlahound]

Construction, design, beam steering, beam intensity and collision geometry...etc would be optimal with a LINAC in a world of no constraints?

Rings are the compromise solution to real world constraints?

Is there any possibility of building a research facility that would then become a alternative structure post research, eg build a big LINAC straight thru the Alps north and south which could then become a commercial transport tunnel when the research is competed.

 

[fresh_42]

Construction, design, beam steering, beam intensity and collision geometry...etc would be optimal with a LINAC in a world of no constraints?

Rings are the compromise solution to real world constraints?

Is there any possibility of building a research facility that would then become a alternative structure post research, eg build a big LINAC straight thru the Alps north and south which could then become a commercial transport tunnel when the research is competed.

They have degrees up to 50°C or something in the stones of the new Gotthard base tunnel. I just try to imagine how you would cool the entire tunnel to 0.3K or so, on 57 km! And this is just one mountain. My guess is it would be easier to construct a linear accelerator in Death Valley than under the Alps.

 

[Vanadium 50]

A circular machine has two advantages over a linac. The first is cost - it let's you use the small part that actually accelerates again and again on the same proton. Superconducting magnets are expensive, but accelerating structures are even more expensive. The second is beam quality - by requiring each proton to return to the same spot (within microns) every orbit you get a very high quality beam. This is done by setting up a complex negative feedback scheme: if a particle drifts to the left, it feels a force to the right, and vice versa. Linacs don't do this - a beam particle that drifts to the left keeps going to the left, and if your accelerator is long enough to be useful, it's likely that this drifting particle hits a wall.

Proposals for future linacs include something called "damping rings" so that before the final acceleration, you can get the beam going in a very, very straight line.

The factor of ~20 comes about for several reasons. One is, as mfb said, problems with persistent fields. If your magnets are good to 10 ppm at flattop, and the ring has an injection energy 10% of flattop, at injection it's only good to 100 ppm. Make that 5% and now it's 200 ppm. The lower the energy, the harder it is to inject. And even without this problem, it would still be harder to inject because the beam is physically larger (we say it has more emittance). Finally, there is some accelerator physics that makes you want to keep this ratio small. There is something called "transition", where you essentially go from pulling on the beam to pushing on it. At the exact moment of transition, you can't steer the beam, so you lose it after a fraction of a second. The bigger the energy range, the more likely you have to go through transition. The LHC is above transition, but if you injected at a low enough energy, you'd have to go through transition. That number is probably of order 50-100 GeV.

 

[mfb]

This is done by setting up a complex negative feedback scheme: if a particle drifts to the left, it feels a force to the right, and vice versa. Linacs don't do this - a beam particle that drifts to the left keeps going to the left, and if your accelerator is long enough to be useful, it's likely that this drifting particle hits a wall.

I guess you mean quadrupole (+potential higher order) magnets? Long linear accelerators do this as well.
They just keep the beam together, they don't reduce the emittance (like damping rings do for electrons), but the LHC doesn't reduce that either.

Is there any possibility of building a research facility that would then become a alternative structure post research, eg build a big LINAC straight thru the Alps north and south which could then become a commercial transport tunnel when the research is competed.

500 km through the Alps to replace about 35 km of LHC plus preaccelerators, built at convenient spots near Lake Geneva? Even if the tunnels would be wide enough to be used for transport afterwards (they are not), and even if there would be demand for a 500 km tunnel, that project would be way too expensive for particle physics or transportation. And that is just the tunnel - you need 500 km of accelerating structures. There is absolutely no way to fund that.

 

[Vanadium 50]

I guess you mean quadrupole (+potential higher order) magnets?

That, plus things like stochastic cooling. Yes, you can add correctors to linear accelerators, but the ratio of corrector lengths/accleration lengths is much higher in a circular accelerator. Perhaps the two most directly comparable accelerators are LEP and SLC at the Z pole. Despite the fact that the electrons underwent significant synchrotron radiation, LEP still ended up with a smaller beam energy spread than SLC.

So I think my statement that the requirement that the beam makes it around the ring at the same point that it started gives you better beam quality is an advantage that a circular design has over a linear design is borne out.

 

[mfb]

For electrons, synchrotron radiation is a great cooling method. For protons it is not - protons in the LHC have a cooling time of days but they don't stay in the machine that long. The FCC would be the first proton machine where synchrotron cooling gets relevant.
They tried to get collisions with 600 bunches over the night, but didn't achieve it due to powering issues. The plan is to get 600 bunches next night.

 

[houlahound]

What does a simple conservation of energy equation look like at LHC at point of collision.

Proton energy = ionisation energy + rest mass + brehmstalung losses + relativistic energy + coulomb energy + nuclear binding energy + ...?

 

[mfb]

Apart from the rest mass, all these things don't apply to protons colliding in a vacuum. The rest mass contributes 0.94 GeV to the total proton energy of 6500 GeV. You can call the remaining 6499.06 GeV "kinetic energy" if you like.The machine operators are preparing the machine for collisions with 600 bunches now.

 

[houlahound]

Huh?

To have the proton smash surely you need to overcome both coulomb & binding energy at least?
They are not zero.

 

[mfb]

Binding energy of what? There is nothing bound.

The coulomb potential between the protons is of the order of 0.001 GeV, completely negligible. Nuclear binding energies, if anything would be bound, would be of the same order of magnitude.

 

[houlahound]

Binding energy to break the nucleus apart in collision.

 

[mfb]

There is just one nucleon in the nucleus, there is nothing to break apart.
The protons are not broken into pieces in any meaningful way. Completely new particles are created in the collision.
Stable beams with 600 bunches, 30% of the design luminosity for ATLAS/CMS, 125% of the (lower) design luminosity for LHCb.

 

[nikkkom]

Binding energy to break the nucleus apart in collision.

This would apply only to ion beams (Pb).
But anyway, please realize that at some 3-7 TeV energies per nucleon, any binding energy of nucleus is utterly insignificant. Even the entire rest energy of the nucleus is much lower than the "kinetic" energy of that magnitude (it's about 0.03% of it).