Return to Fermilab's Chain of Accelerators
(movie clip)
Fermilab'sChain of Accelerators
Accelerator Details: the Proton Source Accelerator DetailsReturn to Main Page
Protons are accelerated to 8 GeV kinetic energy in 3 steps:
The Cockcroft-Walton Accelerator
The Linac (Linear Accelerator)
The rapid cycling Booster Accelerator
Link to the story of why we use H- ions and not H+ (simply protons) and multi-turn injection into the Booster
How synchrotrons work (there are 6 at Fermilab)
The Cockcroft-Walton Accelerator is a 750 kV DC voltage source. The maximum voltage is limited by how much the air can "stand off" before sparking.
More technical details
Cockcroft & Walton's original accelerator (1932) produced the first artificial nuclear reaction: Li7 + p ® He4 + He4
The Linac (Linear Accelerator)
Radio frequency power makes an electric field, polarized in the same direction in all the gaps. Particles are accelerated in the gaps, and "hide" inside the drift tubes when the electric field reverses. If a particle was in the gap when the polarity was reversed it would be decelerated. As the velocity of the particles increases, the drift tubes (and the gaps between them) become longer and longer. This is called an Alvarez type linear accelerator after its inventor.
A useful analogy is pushing a kid on a swing. Each time the kid comes back to you you give it a push and on that swing the kid goes higher.
The advantage of the Linac over the Crockoft-Walton is that the DC
voltage of the Linac is zero. The beam is bunched instead of a
continuous ribbon of beam that is in the Crockoft-Walton. In the Linac, there must be “dead” spaces where there is no beam. These “dead”
spaces coincide with where the electric field is pointing in the wrong direction.
Linac Parameters
Bunches of beam are spaced 5 nanoseconds apart (1/201.25 MHz).
The maximum Linac pulse is 50 ms (microseconds) (10,000 bunches) The typical pulse length is 20 ms (4,000 bunches).
A typical Linac pulse contains 6 trillion particles.
A typical Linac bunch contains 1.5 billion particles.
The Fermilab Linac is about 130 m long (1.4 football fields)
The beam “sees” about 3
MegaVolts every meter (1MV/foot) The Fermilab Linac has five 5 megawatt triode amplifiers (old section) and seven 12 megawatt klystrons (new section).
The original Linac, built in 1971 ran at 201.25 MHz. Nine of these big blue tanks filled with drift tubes accelerated
protons to 200 MeV.
In 1993 the high energy portion of the original Linac was replaced with a more modern structure, running at 805 MHz. The remaining part of the old Linac accelerates protons to 116 MeV. The new portion, with its higher gradient (volts/meter) brings the beam to 400 MeV before injection into the Booster.
The power of the beam at the end of the Linac is 18 megawatts
The Booster Accelerator
Booster RF Cavities
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Booster Magnets
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The beam “sees” about 500 kVolts every turn in the Booster. This accelerating voltage is given to the particles by the RF cavities.
The RF frequency in the cavities must change at a rate of 1 GHz per second to match the changing velocity of the protons as they are accelerated from 400 MeV to 8 GeV.
The Booster is the first synchrotron (of six total) in the accelerator chain at Fermilab. The Booster is 475 meters in circumference accelerates protons from 400 MeV to 8 GeV in a period of 0.033 seconds. The beam goes around the Booster 16,000 times.
The inductance of the Booster magnets is balanced by banks of capacitors (inside the orange girder in the photo) to form a 15 Hz
resonant circuit with a Q of about 10. Because the Linac pulse length is 20 msec. and the Booster circumference is 2.2 msec., the Booster uses
multi-turn injection process.
Link to Proton Source Department
Questions? Contact Ernie Malamud. rev. September 5, 2000
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