Particle Accelerators

Abstract

Particle or high-energy accelerators owe their development to the interest of scientists in doing physics research at high energies. Charged particles accelerated to energies of several million electron volts can be used for bombardment of target nuclides and study of their transmutations. Because electrons accelerated to high energies can be made to strike targets and produce X-rays, accelerators became useful for radiotherapy. The transformer principle used (in Chapter 8) for stepping up voltage reaches a limit at about 400 kV, where the insulation between the transformer coils breaks down. Air-core transformers extend the kilovolt range, although not significantly. Different techniques are needed for acceleration of particles to megavoltage energies. The various accelerators differ in the techniques they employ for imparting energy. Any particle of charge q is subject to a Lorentz force $\vec F_L$ in an electromagnetic field, given by

$$ \vec F_L = q(\vec E + \vec v \times \vec B) $$

where $\vec E, \vec B and \vec v$ are vectors representing the magnitude and direction of the electric field, magnetic field, and particle velocity, respectively. The accelerators can be classified into three categories, according to the acceleration technique employed, as follows:

• 1. Direct-voltage accelerators, in which the charged particles gain energy as they travel through an applied potential difference. The electric field does not change with time during the transit of the particle. Conventional accelerators belong to this class.

• 2. Induction accelerators, in which the electric field is induced by a monotonically increasing magnetic flux. Betatrons belong to this type.

• 3. Resonance accelerators, which make use of the electric field component of a radiofrequency electromagnetic wave. The particles move along either a circular or a linear trajectory. The particles can undergo acceleration steadily if they are ”in phase“ with the electric field and there is a resonance between the accelerated particle and the wave pattern. The linear accelerator (linac), cyclotron, and microtron belong to this class.