Subatomic Physics

20 Accelerators
Photo 1: The tandem Van de Graaff accelerator at the University of Washington, Seattle, WA.
Figure 2.6: Drift tube linac. The arrows at the gaps indicate the direction of the electric field at
agiventime.
gain is therefore twice that of a single-stage machine. Photo 1 shows the Tandem
accelerator at the University of Washington.
Van de Graaff generators in various energy and price ranges can be obtained
commercially, and they are ubiquitous. They have a high beam intensity (up to
100 µA); this beam can be continuous and well collimated and the output energy
is well stabilized (±10 keV). Until the end of the last century, they were the
workhorses of nuclear structure research and some are still in use. However, their
present maximum energy is limited to about 30–40 MeV for protons, and they can
therefore not be used in elementary particle research.
2.4 Linear Accelerators (Linacs)
To reach very high energies, particles must be accelerated many times over. Conceptually
the simplest system is the linear accelerator, (3) sketched in Fig. 2.6.
3R. Wideröe, Arch. Elektrotech. 21, 387 (1928); D. H. Sloan and E. O. Lawrence, Phys. Rev.
38, 2021 (1931).