Vacuum

[Unica]

Hi,

i was wondering if the vacuum is the same everywhere in the accelerator complex from
Linac to LHC.
If not, how are the several accelerators separated and how does it work during beam transfer from Linac -> PSB .... -> LHC.
I guess they use something more sophisticated than simple valves.

tia

Unica

[pcatom]

Hi Unica
There are sector valves (sometimes called gate valves) which can be closed in order to split the machines and transfer lines into vacuum sectors. These are normally open. They can be closed manually - for example it is often a routine procedure to close them when giving access to the machine. They will also close automatically if the level of the vacuum degrades in the regions adjacent to the valve. Most valves take a few seconds to close. They would protect other parts of the machine in case a vacuum leak occurs.

In answer to your question all the vacuum systems in the machines are interlinked as normally all valves are open. However, the pressure is not exactly the same everywhere as it depends strongly on the number and type of vacuum pumps installed locally.

There are places where there are vacuum windows. These are used where the beam has to pass from vacuum to air. The experimental areas of the PS and SPS have several such windows.

In the LHC there are vacuum windows in the beam dump lines, just in front of the dump itself.

[Unica]

Hi Pcatom,

thank you for the explanation.

I guess when the valves are opened for a beam dump,
it causes a significant pressure wave in the dump line.

unica

[oxodoes]

I guess when the valves are opened for a beam dump,
it causes a significant pressure wave in the dump line.

There are no valves opened for a dump!! (Otherwise they would need many days of pumping before injecting a new beam.) A vacuum windows is simply a piece of very thin metal-foil that seals the vacuum but allows the beam to pass through pretty much undisturbed.

[Kasuha]

The dump is just (basically) a block of graphite located in the side branch of beam pipe. When beam is to be dumped it is just kicked by magnets to this pipe branch towards the graphite block. There are no valves and no vacuum windows involved.

I'm also not sure about vacuum windows anywhere on LHC or between LHC and SPS. The beam is strong enough to drill a hole in the beam pipe if it is not steered properly so I'd expect the thin metal to get punctuated too...

Edit: okay I stand corrected. The graphite block actually is in a pressurized nitrogen atmosphere and there is a vacuum window in the way to it designed to withstand the beam intensity and protect the main pipe from radiation and debris.

[oxodoes]

There are no valves and no vacuum windows involved.

You might want to have a look at http://accelconf.web.cern.ch/AccelConf/ ... PLS123.PDF and http://proj-lbds.web.cern.ch/proj-lbds/ ... ystems.pdf.

[Unica]

Hi oxodoes,

thanks for the links.
Pretty cool stuff these vacuumwindows.

[Tau]

A am not a physicist, but I'll try to clarilfy with what I know. If I understand it correctly, the pressure differences between the different parts of the accelerator complex will probably not cause as much "wind" as intuition would suggest.
I see two reasons for that:

• With so little molecules, there can never be a lot of wind.
• Since the temperatures are very low, the movement of the molecules is restricted, reducing pressure effects (wind is more about pressure that about individual molecules moving)

If you are a physicist, and you can explain this better than I do, feel free to do so.

[chriwi]

Pressure and wind are only stistical phenomenons resulting from a lot of moving particles often bouncing into eachother, in the high vascuum like in the LHC or the other accelerators connected to it even in the areas with the highest pressure there are only so little particles moving only strait and bouncing between the walls but hardly hitting eachother.
Also removing them cannot be done aymore by sucking only at a small hole, the pump rather has to wait at a bigger area of the wall for particles and try to kick them out.

did anyone find a paper about the vacuumsystem of the LHC and the other accelerators?

Its quite al ong time (15 years) since I learned this in college when we were playing with electron microscopes.

[serych]

A am not a physicist, but I'll try to clarilfy with what I know. If I understand it correctly, the pressure differences between the different parts of the accelerator complex will probably not cause as much "wind" as intuition would suggest.

Yes, you are right. In the high vacuum there are conditions totaly different from these we know from our everyday life. For example particles nearly do not interact with each other, but only with the walls of the system.

There is also elastic interaction (in which molecule behaves like the tennis ball) but much more there are non elastic interactions in which the wall absorbes the particle and it can leave (if it can leave at all) the wall again only after some time and its leaving angle is not equivqlent to reflection angle of elastic interaction.
There is much more unusual rules for the life of particles in high vacuum.

So it's realy very difficult for the particle to get from one end of the vacuum tube to the other (pro in situations of the small defects and leacages in the system but also great con for the pumping the molecules out of the system).

Jakub

[amiso]

Regarding the degree of vacuum, I can add the following:

Achieving a high vacuum is usually a real chalenge in vacuum systems, but not here !

When a surface is at low temperature, the large molecules tends to stick to it (adsorption). The lower the temperature, the smaller the molecules that will stick to the surface. The ultimate condition occurs at liquid helium temperature, where almost any gas will stick to the cold surface. The result is that in a system like LHC, there is NO residual background gas. The only exception is possibly hydrogen atoms, protons, electrons, but they quickly form chemical bonds with anything, and then adsorb to the surface.

Obviously, some pumps are needed to evacuate the system, before the cooling gas is injected, but as soon as the temperature gets low, then the vacuum becomes good, very good. At least this is one less problem for them.

Regarding the windows : I am not an expert, but since the protons are massive charged particles, I do not think that they can penetrate a metal or glass window without making a lot of damage, heat... unless it is very very very thin... My opinion is that there is NO window of any kind all the way from the LINAC to the LHC.

[Kasuha]

Not all of the beam pipe is cooled to 1.9K - the pipe does not need it only magnets and power lines do. Other parts which don't strictly require such low temperature are held above it.

There's also good overview of LHC vacuum system in LHC design report:
https://edms.cern.ch/file/445860/3/Vol_1_Chapter_12.pdf