Main Components of Accelerator Systems

As stated previously, one of the main components of an accelerator is the particle source injector. It can use a number of methods to produce a desired beam of particles. These methods vary wildly depending on the type of particle that is needed.

Proton and ion beams are generally generated using glow discharge columns that are then accelerated first by electrostatic accelerators like Van de Graff or Cockcroft–Walton accelerator and then Alvarez-type linear accelerators. Both mentioned electrostatic accelerators generate a high DC voltage that is used for acceleration, while the Alvarez-type accelerator uses radio frequency (RF) cavities instead. To increase the energy of heavy ion beams a thin metal foil is used to strip electrons. Generating a beam of antiprotons is more difficult and requires generating a high energy proton beam that is aimed at a heavy metal target. The resulting collision generates antiprotons along with other particles that are then focused and accelerated [3].

Electron beams can be generated in different ways including the use of a thermionic gun, laser pulses and RF guns. Positrons are generated in the same way as antiprotons, but shoot an electron beam at the heavy metal target instead.

Regardless of the method used for generating particles they do not generally have the time structure needed for further acceleration. They first need to be converted from a stream of continuous particles into a stream of short pulses as the RF field acceleration is only effective during a very short period per oscilla-tion cycle. Without proper preparaoscilla-tion most particles would end up being lost.

decelerates those bunches that arrive early and accelerates ones that arrive late.

Antiprotons and positrons do not need such compression as they are already created by a high energy beam with the appropriate time structure, but suffer from a large beam size and divergence. To reduce these negative effects they’re fed into storage rings that are a type of circular accelerator used for maintaining the speed of particles. These antiparticles can even be kept in the storage rings for longer periods of time (up to several months) and used for multiple experiments during the period [3].

Once the particle beams are created they can then be further accelerated in the main accelerator which can either be linear or circular. Linear accelerators typically consist of a linear sequence of accelerating units whose fields are timed so that the particles accumulate energy from each unit. The most common linear accelerators consist of a series of cavities excited by radio frequency sources to high accelerating fields which then provide acceleration. Another type is the induction accelerator whose accelerating units consist of transformers that generate a field on the transformer secondary formed in such a way to accelerate the particle beam. They are optimized for accelerating very high beam currents to medium beam energies.

Linear accelerators need to be very long and costly to produce very high beam energies. To save on space and cost of linear accelerators circular accelerators can be used. Here the beam will pass through the same accelerating sections multiple times gaining energy with each pass. Maintaining the trajectory of particles on a circular path is achieved with the use of bending magnets. As the particles pick up speed, the bending magnets increase the strength of their magnetic field to keep up with the beam [3].

The basic principles for accelerating particles are the same regardless of the type of particles being accelerated. Individual components will have more or less variation depending on the beam type, but the core remains the same.

After being accelerated by either a linear or circular accelerator the beam can

92 Accelerator Control System Overview

be directed onto a target. For example, the target can be made of liquid hydrogen which is used to study high energy interactions with the target protons. This type of fixed target collision has dominated the research from the first applications of artificially accelerated particle beams well into the 1970s. Additionally, heavy metal targets were used to produce secondary particles like antiparticles and mesons for further research.

To increase the center-of-mass energy, particle beams can be guided to collide head-on with another beam instead of a fixed target. These collisions are the main objective for the construction of colliding beam facilities or storage rings. Such a ring is designed to inject particle and antiparticle beams in opposite directions and cause a collision in a specific region. Due to the size of the atomic particles, any interaction between counter orbiting particles is a very rare occurrence. Because of this, storage rings allow the particles to circulate many times through and maintain the beams for several hours, thus increasing the probability of collision.

Both particle and antiparticle beams can rotate in the same magnetic fields as they have opposite charges. On the other hand, the collision of unequal particles requires two intersecting storage rings to be built [3].

Storage rings share the same design as synchrotrons, but with some adjust-ment in technical realization to optimize the desired beam parameters. Beam intensity is a property that differs greatly between the two. For a synchrotron the beam intensity is determined by the injector and is typically much smaller than the intensity needed in a storage ring. Therefore, a beam injection system is designed so that many beam pulses can be accumulated from a linear accelerator, an accumulator ring or a synchrotron. Another dividing property is the particle energy on the injector. The synchrotron generally uses much lower beam energies on the input than those desired by a storage ring. Even though the storage rings are not meant to accelerate particles, they can often be built much later than the main facility and require higher beam energy than the injector is capable of providing. In such cases, the storage ring can accumulate particles with the maximum available injection energy and then it slowly increases the energy by adjusting the strength of the bending and focusing magnets until it reaches the desired state.

Electron positron storage rings have played a great role in high-energy

re-cannot be reached by circular accelerators. [3].