Courtesy Fermilab Visual media services                          Courtesy Fermilab Visual media services
This is an aerial view showing the smaller machines in the Fermilab accelerator complex.There is a good view of the Pre-accelerator (Cockroft-Walton), the Linac, the Booster ring, and the Antiproton source (Accumulator and Debuncher)                    This is a schematic of FNAL accelerator complex; the arrows give
                   a sense of how the beams (proton or anti proton) move from one
                   machine to the next.

The second stage of Fermilab’s accelerator system is a Linac, or Linear Accelerator. The linac accelerates a beam of H- ions from an injection energy of 750 KeV to an extraction energy of 400 MeV. Unlike the preaccelerator that uses a DC (steady) electric field for acceleration, the linac uses time-varying electric fields for acceleration. For more information on how acceleration is accomplished with time-varying electromagnetic fields, see ‘Acceleration with alternating electric and magnetic fields’ on this website.

The first linac at Fermilab was built in 1971 and originally accelerated particles up to 200 MeV. The linac was upgraded in 1993, the low energy end of the linac remaining as originally designed, but with different style accelerating structures at the high energy end. Now the linac can accelerate beam to 400 MeV. The low energy end of the Fermilab linac is an Alvarez style drift tube linac. The accelerating structures are the big blue tanks shown in the photo. The five tanks of the low energy end take the beam from 750 KeV to 116 MeV. The resonant frequency of the cavities is 200 MHz.
This is an inside view of one of the blue tanks shown in the photo above. The cylindrical tubes are called drift tubes, because when particles are inside these tubes they are shielded from the electromagnetic fields filling the rest of the space inside the tank. Since the particles do not see any field inside the tubes, they are not accelerated or decelerated, and drift along at a constant velocity. In the space between any two drift tubes, which is called an accelerating gap, the particles do see the electromagnetic field in the tank, and are kicked(accelerated) by the electric field.
The present Fermilab Linac uses a different design for the low energy and high energy ends of the accelerator. The low energy end is a drift-tube linac as described above. The high energy end shown here has a side-coupled cavity design. There are no drift tubes inside. Power is coupled from one resonant cavity structure to the next through couplers that bridge adjacent cavities and lie on top of the structure, rather than directly in the beam path.
This is a view of the high energy end of the linac. Power is pumped into the cavities using waveguides that can be seen in the photo along the length of the linac. These waveguides bring power from outside the enclosure into the accelerating structures. The frequency of the high energy end of the linac is 805 MHz rather than 200 MHz, and so klystrons instead of traditional power tubes are used as amplifiers and waveguides (less lossy at high frequencies) guide the high power electromagnetic waves from the klystrons to the cavities.