Beam-Beam Effect

Beam-beam effect, or beam-beam interaction, is a recent research being conducted over many areas throughout the world, from European laboratories to American Institutes. Basically the issue involves the passing through of proton bunches which results in many different types of effect, sometimes including the unexpected. Aside from the shooting of particles across one another, usually the collision rate of the protons generates a certain amount of energy. Today, the issue is to focus on modifying new colliders to enhance the effectiveness of the machines.

The study of the beam-beam effect involves particle colliders which determines the collision rate of protons. One of the commonly used collider is the LHC, which stands for Large Hadron Collider. The LHC is an accelerator which brings protons and ions into collisions at higher energies than ever achieved before. This will allow scientists to penetrate still further into the structure of matter and recreate the conditions prevailing in the early universe, just after the “big bang”. It is represented by a thick gaussian lens, and the ring is described by a 9th-order Taylor map. At the single-particle level we compare the dynamic aperture with and without the beam-beam effect. At the multiparticle level, using a “strong-strong” description of the beam-beam interaction, we compare the collision rate with a linear lattice map and with the full nonlinear map. The LHC is being used at CERN, the world’s largest physics center for particle physics exploring what matter is made of, and what forces hold it together.

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Many different models for this effect have been conducted before, but most of them often ignored longitudinal motions of the particles, meaning that they only look for steady-state solutions, or assumed one beam contained a larger number of particles. These are usually known as “weak-strong” simulations. The LHC being developed is a “strong-strong” simulation in that it treats both beams equally and allows them to have arbitrary relative strengths. It is dynamic in that it models the motion on a turn-by-turn basis looking for coherent oscillations in the beam shape. It uses a variety of methods for computing the electric fields so that it can run as quickly as possible in each situation it encounters.

Inside the LHC, usually there is a storage ring acting as accelerator that collide the bunches of particles repeatedly by storing the bunches in the collider throughout a certain amount of time. When shooting out the particles, the collider aims for a very high collision rate, or luminosity. Inside the collider ring, the counter-rotating beams are able to “sense” the electromagnetic effects of each other before the particles collide. On the other hand, each time the bunches pass through each other at the interaction point, only a small fraction of the particles collide producing high energy effect, while the rest of the proton particles are simply influenced by the fields of the opposing field. As the beams of bunches pass through each other over many turns, the shape of the beam changes. This then turns out different effects that could easily degrade performance, such as decreased collision rates through beam blow-up, and orbit distortions and instabilities.

In order to minimize the deleterious beam-beam effect, the LHC uses many bunches to make them cross each other at an angle rather than colliding head-on. Also, as the role played by the beam-beam effect in machine performance is determined by the collision rate, the parasitic crossing is then introduced. The parasitic crossings take place mostly in the arcs where the particle beam separation is great and the crossing occur only at the experimental collision halls.

A visual interpretation to the understanding of beam-beam effect may be the using of a coordinate system indicating the direction of the proton beams colliding at desired angles.
The figure illustrated includes the three kinds coordinates systems moving together with beam-1 and beam-2. All the three coordinates are collinear in the z-dimension. In the presence of crossing, the origins of the moving systems are shifted from the centers of the beams, except in the negative z1 and negative z2 directions.

Beam-beam effect is an issue that’s still in the process of investigation. As being one of the recently designed colliders, the LHC enables researchers to eliminate many unwanted factors resulting from beam-beam interaction. It enhances the efficiency, or the performance of the machine. The researchers have learnt to avoid head-on collisions of the proton particles as the beams pass through each other and become influenced by the magnetic field produced by each other. The LHC uses additional bunches of particles and makes them cross at angles, while putting to use parasitic crossing. This helps to work out the desired luminosity and thus produces higher energy level. The introduction of new colliders has led the physicists a huge step further in getting a better grasp of the beam-beam effect, which in the long run understand more about the structure of matter.

Works Consulted
McClintock, Jeffrey E. Scientific American.

CERN Courier. wysiwyg://129/
Furman, Miguel A.Beam-beam effect and dynamic aperture of the collider.
Ruggiero, F.A Symplectic Beam-Beam Interaction with Energy Change.