A particle beam is an ensemble of charged or neutral particles that move together in close proximity, in many cases moving at near the speed of light. A beam is characterized by a few basic parameters such as the particle species and the average energy of the particles. The beam of particles must be well collimated so that all particles stay in close proximity throughout its motion. Deviation of the motion of individual particles from the average motion of the beam must be kept sufficiently small.
The species of particles in the beam is most commonly electrically charged. Most common examples of particle species are electrons and protons. Less common particles include charged particles such as muons, pions, or neutral particles such as some specific atoms or molecules. The physics of accelerators also significantly overlaps with the physics of light optics and lasers, as light can be treated as a beam of photon particles.
A particle accelerator is a device that manipulates the motion of charged particle beams. The most common manipulation is to increase the energy of the particles, thus the term “accelerators.” In modern times, however, other variations which do not accelerate or for which acceleration plays only a minor role have been introduced. All these devices are also customarily considered as particle accelerators. Some devices manipulate neutral particles. The physics of these devices sometimes is considered as part of accelerator physics because they can be described by very similar physical principles. Atomic beam devices and particle traps are examples of this category.
Parameters characterizing the particle beam
Other than the particle species, the final energy of the accelerated particles is the most important parameter of a particle accelerator. The particles have electric charge equal to the electron charge e or a multiple of it, and they are accelerated by potentials measured in volts (V). Therefore, a natural unit of energy is the electron volt (eV), the energy acquired by one electron charge in passing through a potential difference of 1 V.
The electron volt is a very small unit of energy (1 eV = 1.6 × 10−19 Joule), more directly applicable to energy levels in atoms than to accelerators. The energies of particle accelerators now being operated range from a few hundred keV to 1 TeV. The sizes of particle accelerators range from table-top devices to devices stretching over several miles (or kilometers).
A second important parameter used to characterize an accelerator is its intensity, usually the number of particles moving together in a bunch, or, in the case of a continuous flow of particles, the number of particles accelerated per second. Other parameters characterizing a particle beam include parameters that specify the degree of collimation of the beam. In particular, the spread of particle energies around its average value is one such parameter. This energy spread, or momentum spread, which is related, is denoted by \(\Delta E/E\), and typically ranges from 10−2 to 10−4. Two more parameters, called transverse emittances, specify the degree the beam is bunched into a tight bundle throughout its motion.
The emittances are denoted by \(\varepsilon_x\) and \(\varepsilon_y\) and are in units of millimeter–milliradians. A tightly bundled beam will require small values of the emittances. These three parameters, \(\Delta E/E\), \(\varepsilon_x\) and \(\varepsilon_y\), will have to meet the requirements of the accelerator application in hand. Still, other parameters may characterize special beam properties. One example is the beam polarization, which characterizes the degree of alignment of all the spins of the particles in the beam. A high degree of polarization is a very useful tool in analyzing some of the high-energy physics experiments.