| An ordinary hydrogen atom has one
electron and one proton. Its electric charge is zero.
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We could strip off the electron and
accelerate the protons. |
Instead we add an
electron, making a negatively charged hydrogen-ion. The H- ions are accelerated in the Cockroft-Walton and the
Linac to 400 MeV.
Then the two electrons are stripped off and the bare
protons injected into the Booster for acceleration in the rest of the chain.
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| An ordinary "C"
size cylinder of compressed hydrogen gas (H2) is used. When full it
contains 6.9 liters of hydrogen at 2000 psi pressure. Remember Avogadro's
number? Using it, one can calculate that the gas bottle contains 5 x 1025
hydrogen atoms. If the Fermilab chain of accelerators ran "flat out" it
could accelerate 1020 protons each year. In fact due to maintenance,
unscheduled failures, or turning off the machine to save on the power bill, this number is
less. So the gas in the bottle could last 500,000 years!! In fact due to
inefficiency in the ion source, it is replaced about once/year. |
Production
of H- ions |
Place Hydrogen atom in an electric
field and strip away electron
Protons will congregate on metal surface with cesium.
The metal has free electrons and the cesium
makes it easier to steal electrons from the metal (low work function.)
Every once in awhile, an incoming
proton will smack a proton with two electrons attached off the metal wall. 
Because of its negative charge, the H-
ion will move away from the negative surface. |
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Multi-Turn
Injection in the Booster |
Because the Linac is a
single pass accelerator, the amount of power that the klystrons can “pour” into
the beam and still achieve 400 MeV limits the beam current. (beam loading).
The total charge in the Linac is the beam current x pulse length. To get a lot of charge,
the pulse length in the Linac must be made very long (20 msec).
Since the Booster circumference is only 2.2 msec long, the Linac beam must be
wrapped around the Booster many times (turns). |
How is more than 1 turn
added to the Booster without “kicking” out the protons that are already inside
the Booster?
The Linac accelerates hydrogen ions, protons (+charge) with two electrons (-charge)
attached. Because electrons are very light, the ion has nearly the same mass
as a proton but with opposite charge.
Because the H- ion has the opposite charge of a proton, a proton beam and H-
beam would “merge” when passed through a single magnet. The
“merged” beams would diverge again if passed through a second magnet.
The electrons in the H- ion are weakly bound to the proton and can be easily
stripped if passed though a thin Carbon foil. Because the foil is thin and the
protons have high energy, the foil will not bother the protons. |
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