Subatomic Physics

2.4. Linear Accelerators (Linacs) 21
A series of cylindrical tubes are connected to a high-frequency oscillator. Successive
tubes are arranged to have opposite polarity. The beam of particles is injected
along the axis. Inside the cylinders the electric field is always zero; in the gaps it
alternates with the generator frequency. Consider now a particle of charge e that
crosses the first gap at a time when the accelerating field is at its maximum. The
length L of the next cylinder is so chosen that the particle arrives at the next gap
when the field has changed sign. It therefore again experiences the maximum accelerating
voltage and has already gained an energy 2 eV0. To achieve this feat,
vT, wherevis the particle velocity and T the period of the
oscillator. Since the velocity increases at each gap, the cylinder lengths must increase
also. For electron linacs, the electron velocity soon approaches c and L tends
to 1
2cT . The drift-tube arrangement is not the only possible one; electromagnetic
waves propagating inside cavities can also be used to accelerate the particles. In
both cases large rf power sources are required for the acceleration, and enormous
technical problems had to be solved before linacs became useful machines.
L must be equal to 1
2
Photo 2: A view of the linac at RHIC. Its purpose is to provide currents of up to 35 milliamperes
of protons at energies ≈ 200 MeV for injection in a synchrotron for further acceleration. The basic
components of the linac include a radiofrequency quadropole pre-injector, and nine accelerator
radiofrequency cavities spanning the length of a 150 meters tunnel (shown above.) [Courtesy of
Brookhaven National Lab.]
At present, Stanford has an electron linac that is 3 km (“2 miles”) long and
produces electrons of 50 GeV energy. A proton linac of 800 MeV energy with a
beam current of 1 mA, a so-called meson factory, was constructed at Los Alamos.
It is now primarily used to bombard targets made of neutron-rich elements and
produce neutrons that are subsequently used to study properties of materials. The
Relativistic Heavy Ion Collider (RHIC) at Brookhaven has as one of its components
a linac (see Photo 2) and the planned Rare Isotope Accelerator (RIA) will produce