1 – Introduction
Hello and welcome to the ACO experiment room. ACO is an acronym meaning Anneau de Collision d’Orsay – Orsay Collider Ring in English – and is the name of the circular apparatus in front of you. This machine operated from the 1960s to the 1980s with 2 main stages of operation.
ACO’s first goal, from the 60’s to the 70’s, was to study the different properties of the particles constituting the world around us which was a standard goal for particle accelerators at the time. In order to carry out this study, two types of particles were chosen: electrons and positrons. An electron is one of the fundamental constituents of matter and its movements cause electrical currents. A positron is a particle almost identical to an electron with an opposite charge. These particles were chosen because their natures were well-known already. They were accelerated to speeds close to the speed of light inside the ring to increase their energies and then they were sent against each other. During this collision, all the energy accumulated by the particles is released which allows for the creation of new particles that can not be observed in nature. Nowadays, larger and larger particle accelerators are built in order to reach higher and higher energies and thus discover new particles. This is the case for the LHC, a 27 km long proton accelerator located at the France-Switzerland border in the CERN (European Council for Nuclear Research), for instance.
The second use of ACO’s ring was as a source of synchrotron radiation. This radiation is emitted by the rotation of the particles in the ring and can be used to study different materials. Nowadays, synchrotrons like SOLEIL in Saclay continue studing different materials this way.
More details will be given in the rest of this guide as to how this accelerator operates by describing its major parts (injection zone, dipole, quadrupole, beam line, radiofrequency cavity, window, vacuum pump and detector).
If you would now move to the left of the ring to the injection zone, the injection of the particles into the accelerator will be explained.
2 – Injection Zone
Here is the electron and positron injection zone in ACO. You can see, on the right, coming from a white wall, and on the left, coming from a grey wall, two large tubes going towards the ring. The electrons and positrons are injected through these tubes. The tube on the right conveys the electrons that were accelerated beforehand in a linear accelerator located behind the wall. As for the tube on the left, it is used to transport the positrons. These particles do not exist in nature, hence they need to be created in advance by interactions between the electrons of the linear accelerator and a tungsten sheet.
3 – Dipole
The big red and blue numbered units you can see just right of the injection zone are electromagnets called dipoles. These dipoles are constituted of two magnetic poles: a north pole and a south pole, and they produce a magnetic field which is used to curve the charged particles’ trajectory inside the ring. Each dipole induces a 45° rotation for the particles’ beam. Since there are 8 dipoles spread across the inside of the ring, a complete rotation of the beam can be performed. Dipoles are relatively simple objects. They are made of two coils – the red parts – through which an electric current is passed, generating a magnetic field. The blue part is constituted of a steel block and is used to confine this magnetic field.
4 – Quadrupole
Now, if you would please move counterclockwise around ACO until you find a unit made of three green boxes. These boxes are 3 of the 12 quadrupoles ACO has. If a beam of particles circulates inside ACO without any constraints, it will naturally widen until it collides with the walls of the accelerator and be lost. In order to avoid that, quadrupoles are used. They operate in a similar manner to lenses in optics. Indeed, the quadrupoles stop the beam from diverging by focusing it. As their names indicate, they are made of 4 magnetic poles: two north poles and two south poles alternatively. Four red coils of conductor wire similar to those of the dipoles can be seen. When an electric current is run through these coils, a magnetic field, used to focus the beam, is generated. With this system, each quadrupole compresses the beam in one plane and widens it in the perpendicular plane. With more than one quadrupole, a global focusing effect can be achieved. That is why in ACO, triplets of quadrupoles are mounted. In total, there are 4 triplets in ACO.
5 – Beam Line
I now ask you to move around ACO clockwise until you see overhead the cross section of a tube going from one of the dipoles to a little room located a bit further away. This apparatus is called a beam line. When the trajectory of charged particles is curved, energy is lost in the shape of a beam of light called synchrotron radiation. This light can be very intense when a lot of particles are stored and it covers a large range of wavelengths – from the infrared to the X-rays. This radiation is problematic for particle physics since it causes the beam to lose energy. However, for other scientific fields, like biology or solid-state physics, the synchrotron radiation can be used to probe the structure of matter at an atomic or molecular scale. Thus, the beam line’s purpose is to guide this radiation, from the dipole where it is emitted, to an experimentation zone situated a little further away where it will be used.
6 – Radiofrequency Cavity
Taking a few steps and looking to your right, you should arrive in front of what is called a radiofrequency cavity – or accelerating cavity. This cavity’s goal is to increase the energy of the stored particles using an electric field. The electric field oscillates at a frequency of 27 MHz, meaning it changes direction 27 million times per seconds. This oscillation frequency – that is inside the radio waves range – is twice the passage frequency of the particles inside the cavity. When a particle goes through an electric field, the direction in which it is accelerated depends on its charge. Thus, the particle is accelerated in the direction of the electric field if it is positively charged and in the opposite direction if it is negatively charged. As the electrons and positrons go through the cavity at the same time but in opposite directions, they will be accelerated simultaneously each in the right direction. Therefore, one cavity can accelerate two types of particles with opposite charges at the same time.
7 – Window
Please turn around ACO clockwise until you see overhead yellow cylinders mounted on red structures. They are part of the system used to check the behaviour of the particles stored inside the ring. Inside each cylinder is a camera which – thanks to a 45° mirror – receives the light coming from a sapphire window located at the end of a dipole. In the same way that the windows on a boat enable one to see what is happening inside, these windows allow for the observation of the beams and the confirmation of their good quality. They operate using the synchrotron radiation – a light emitted when charged particles rotate – principle. This light is captured by the cameras giving an indirect image of the beam, allowing us to verify its intensity and stability.
8 – Vacuum Pump
Looking just above the ring, you can see one of the crucial systems of ACO: one of its vacuum pump. Inside a particle accelerator, vacuum is essential. Indeed, if a particle beam circulates inside a gaz – for example the air – these particles would encounter numerous molecules into which they would collide, thus the beam would entirely disappear in mere seconds. Hence, different pumping systems are used to decrease the pressure to 10-13 bar – 10 trillion times smaller than the atmospheric pressure around 1 bar. This is called ultra-high vacuum.
9 – Detector
Please move once again around ACO clockwise so that you face a faithful copy of the ACO detector in its first operating period. By annihilation between the electrons and the positrons during their collision, new particles are created. Thanks to this detection device, composed of 4 spark chambers, the trajectory of these new particles can be traced and thus, information about the collision can be obtained. The charged particles created pass through the spark chambers and ionise the gas contained inside, meaning they extract electrons from the gaz which becomes more conductor. A high voltage is then applied between each steel plate in front of you. Thus, when a particle passes through, an electrical breakdown occurs, meaning that a spark forms between the plates along the trajectory where the gas was ionised. The information about this trajectory is retrieved using an automatic photographic system.