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So to get a proton beam for the LHC, CERN prob has to make a plasma and siphon off the moving protons with a magnet. Are the electrons stored somewhere? How? I don’t mean to sound stupid but when they turn off the LHC, all those protons are going to be looking for their electrons. And that’s going to make a really big spark.

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    $\begingroup$ The Baltic Cable pushes about $5\times10^{21}$ electrons per second from Sweden into Germany, and they swim back by themselves through the sea. They don’t seem to mind. $\endgroup$ – Roman Odaisky May 15 '20 at 11:27
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    $\begingroup$ Sell 'em for a charge. $\endgroup$ – mrblewog May 15 '20 at 13:33
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    $\begingroup$ We wouldn't want a surplus of electrons to accumulate in Geneva. Otherwise Switzerland wouldn't remain neutral. $\endgroup$ – Michael Seifert May 15 '20 at 13:50
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    $\begingroup$ @mrblewog probably on e-bay? $\endgroup$ – Hagen von Eitzen May 16 '20 at 20:22
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    $\begingroup$ @HagenvonEitzen they sell the positrons on e+bay. $\endgroup$ – Brian Drummond May 16 '20 at 21:43
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You're right that CERN gets its protons by ionizing matter and collecting them. But the number of electrons & protons CERN deals with is far smaller than you might think. They get about 600 million collisions a second at CERN. So call it 1.2 billion protons used per second. $1.2 \times 10^9$. That'd be a large number in dollars, but it's not much in Coulombs.

For comparison, a wire carrying a 30 amp current has about $2 \times 10^{19}$ electrons flowing through it every second. That's a factor of 10 billion. So there's not really any issue disposing of CERN's unneeded electrons. You probably make a bigger spark when you rub your feet on the carpet.

If memory serves, the LHC has been running for its whole history off of a single canister of hydrogen gas.

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    $\begingroup$ thank you. I get that the number of protons is small but I am concerned about conservation of charge. When you get all these protons without their electrons in a tube, that seems to be a big charge. Even 1.2x10^9 electrons is a lot. do they accumulate somewhere and make a static charge ? or is it so trivial that it cannot even be measured? Common static electricity involves charges ranging from nanocoulombs to microcoulombs - 1 nanoC would have ~10^10 electrons. $\endgroup$ – aquagremlin May 14 '20 at 19:34
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    $\begingroup$ I'd guess they just flow down a wire to ground. We're talking like a nano-amp of current. Earth won't notice. $\endgroup$ – user1504 May 14 '20 at 19:43
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    $\begingroup$ @Logarr If you don't count the electric energy that goes into making it work though. $\endgroup$ – Jannik Pitt May 15 '20 at 11:36
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    $\begingroup$ According to some Googling, the LHC could run for hundreds of millennia off a single, normal-sized can of H2. However, judging from the fact that one of its H2 cans (the caption to the right in the image says this one was in use until 2015) is currently on display in a museum, I don't think it actually has been running off a single can. $\endgroup$ – HTNW May 15 '20 at 15:17
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    $\begingroup$ Not really right about running the LHC off one gas canister for years. Although each LHC fill has about 7x10^14 protons, and the bottle of gas at the source contains 2000 L of hydrogen (at STP) which is 10^26 protons per bottle, making the protons for LHC is very inefficient, so in reality the bottle was changed twice a year - I know as I used to do it! $\endgroup$ – Tricky May 15 '20 at 20:06
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The usual thing for a shutdown is to 1) stop injecting fresh particles into the beam tube, and 2) deflect any remaining particles in the main tube and any storage rings into a beam dump which is a very large chunk of metal, a very very large chunk of concrete, or a very very very large pile of earth. Take care not to be standing next to the beam dump- the radiation it produces while stopping the beam will kill you.

If your beam is working with electrons, you make them by stripping them off a hot piece of metal or ionizing you some hydrogen. In this case you steer the unwanted protons out of the resulting beam and run them into a dump. There they will find themselves some loose electrons lying about and get happy again.

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    $\begingroup$ The beam dump is there to absorb the kinetic energy, not so much the charge, which is negligible. $\endgroup$ – user1504 May 14 '20 at 17:03
  • $\begingroup$ yep yep........ $\endgroup$ – niels nielsen May 14 '20 at 17:14
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    $\begingroup$ thank you. from your answer i infer that the static charge of even 10^10 electrons is trivial. so they just hang around in the device used to make them or in the plasma where they are born. $\endgroup$ – aquagremlin May 14 '20 at 19:36
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Just to add an electrical engineering answer to these good physics answers, insulators are never perfect. In school we talk about perfect insulators, but in practice everything has some level of conductivity. Air itself has a resistivity of somewhere on the order of $10^{16}\Omega-m$, for example.

Usually when doing a process that generates ions is done, one side is "grounded," which means we let the charge flow into the ground. The earth can take an enoremously large number of these electrons before the electrostatic forces between them start to add up. And that gives time for these electrons to re-pair with protons.

Failing that, combining the electrical charges into the earth means our effect is combined with the electrostatic effects at the planetwide level. For example, the aurora borealis is a product of a massive stream of charged particles coming from the sun at a magnitude most of us cannot even fathom!

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An average lightning flash contains an amount of about 27 Coulomb negative charge (see here), provided by the flashing electrons. One Coulomb of negative charge contains about $6\times10^{18}$ electrons (see here).
So in typical lightning flash about $27\times6\times10^{18}=162\times10^{18}=1,62 \times10^{20}$ electrons are present. Now one can see in this article, using the list with facts and figures, that in an average proton beam experiment $1,2\times 2808\times 10^{11}$ or about $3\times 10^{14}$ protons are used.
This means that there are half a million electrons times as much more electrons involved in an average lightning flash than in an average proton experiment at CERN.

The energy contained in an average lightning flash is about $10^9\times6\times10^{18}=6\times10^{27}eV$ while the energy contained by all protons in an average experiment (see the facts and figures list) is about $6,5\times10^{12}\times2808\times1,2\times10^{11}eV$ or about $2\times10^{27}eV$. So while the number of electrons (protons) involved in both differs substantially, the energies involved are about the same! (Which might be the source of your confusion).

While the electrons in a lightning flash give a huge spectacle, the protons that recombine with the electrons (for which their energy has to be highly reduced first) from which they are separated give a spectacle not worth a penny!

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The beam has a positive charge, so there's an electric field that surrounds it. But the beam pipe is metal, conductive. At the surface where the field intersects the metal, electrons flow to cancel the field. There is thus a layer of electrons on the metal surface, with equal but opposite charge to the beam.

The beam pipe in turn is "grounded", connected to all the other structural metal around to avoid hazardous voltage build-up. The proton injector's electron collector is also grounded. So, electrons flow into grounded structure, and the beam pulls an equal number out of grounded structure. Charge balances.

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