ACCELERATOR

The heart of the future machine features a superconductor linear accelerator, delivering some of the most intense beams in the world, which are then used to bombard a matter target. The resulting reactions, such as fission, transfer, fusion, ... will generate billions of new nuclei.

Once extracted, sorted and accelerated, the most useful nuclei are assembled into beams, allowing for groundbreaking experiments and paving the way for new prospects in nuclear physics, as well as other scientific topics, thanks to this new, cutting-edge, multi-beam platform.
 

LINAC

Deuteron source

SPIRAL2 will not use the ion source presently available at GANIL, but rather a neutron source, which breaks up matter very efficiently. 


Hydrogen: 1 proton in the nucleus and 1 orbiting electron.
Deuterium: heavy hydrogen atoms, with 1 proton and 1 neutron in the nucleus, and 1 orbiting electron.
 

Deuterium atoms are inserted into the ECR source (Electronic Cyclotron Resonance), which creates an electromagnetic field. The only electron in the Deuterium atom then gains energy and separates from the nucleus, creating a deuteron. Because an electromagnetic field of about 60mW is created at the source output, nuclei escape and then form a plasma.

Diagnostics

Particles pass through a racket-shaped detector known as the "diagnostics" box. Each string of the racket reacts with a passing particle. It provides the operators with a density/range beam image over which the beam is spread.
With this first view, engineers can check whether the beam is shaped correctly, in accordance with the needs of their experiment.

Tubes

A high vacuum is generated by pumps placed at different locations within the machine. These allow plasma to be displaced, with a minimum likelihood of colliding with air particles or the residues from previous experiments.

Magnets

Solenoid magnets: these focus the beam, both vertically and horizontally.

Quadrupole magnets: these focus the beam, either vertically or horizontally, according to their settings. To control the beam, two of these must be positioned one after the other, and be adjusted in different manners. This technique is more efficient and cost-effective, because it requires less energy.

The creation of a static magnetic field, using one of these two types of magnet, does not influence the acceleration, but redirects the beam, which could otherwise tend to deviate and impact the tube walls, thus making them radioactive and therefore unusable by humans.

Dipole magnets: these exert a deviating force on particles, and thus provide the beam with a curved trajectory.


Along the curve, particles are sorted according to their electric charge and mass, because the displacement, velocity and trajectory of ions are proportional to these quantities. This step leads to a purification of the beam, through the deviation of excessively or insufficiently charged ions, as well as residues from previous experiments, or elements remaining after creation of the vacuum.
 

RFQ, the radiofrequency quadrupole

The beam reaches the RFQ which, every 11 nanoseconds, divides it up into smaller particle packets. This beam configuration is required for its later acceleration.
Electrodes with a specific shape are placed within the RFQ, thus creating a non-continuous electromagnetic field, and separating the beam into packets.
 The velocity of the latter then increases to 4% of the speed of light.
 

LME

The medium energy line (LME, which stands for Ligne Moyenne Energie)

After the RFQ, the beam tends to dissociate. Two different tools are thus used to maintain it in the form of packets.

With quadrupole magnets, the beam is focused vertically. 


A buncher generates a sinusoidal electric field, which gathers the particles horizontally. The leading particles in the packet are slowed down, whereas the trailing particles are accelerated.



A slow chopper is also introduced, in order to suppress certain packets from the beam, whenever the final experiment does not require a large number of particles.

LINAC

The LINAC comprises two types of accelerator cavities, A and B.
The so-called type A cavity has a low-energy acceleration area, whereas B cavities comprise two high-energy acceleration areas. Each cavity is placed inside a cryomodule at a temperature as low as -269°C, to prevent any increase in temperature of the equipment, and allow the use of a very strong electric field.
 

Cavities A, then B
 

The LINAC is an assembly of 112 cavities of type A having their cryomodules each, followed by 14  cavities of type B arranged in pairs in a cryomodule  Acceleration is gradual; the intensity is smaller in the first portion, but increases in the B cavities. Together they can reach energies of 40 000 000 electron volts.

The beam then accelerates to 20% of the speed of light.

 

The story of a cavity's birth

After the physicists have designed them with millimetric accuracy, over a period of many years and after many tests, the cavities are finally manufactured in partner laboratories. They are made in several portions, designed for later assembly.
When they leave the assembly line, the cavities are immersed in an acid bath, which removes their inner roughness. This is because each micro-spike or residue resulting from the manufacturing process may later become an obstacle during the acceleration phases.
 

Once they have been descaled, the cavities are placed in a clean room, which contains less than 10 dust particles per m3. After this, they are rinsed with a high-pressure water jet, in the range between 80 and 100 bars, to remove any dust grains still adhering to the cavity walls. They take 24 to 48 hours to dry. Whilst still in this room, in order to create a vacuum within the cavities, they are assembled and sealed to prevent any "contamination".

 
Considerable care must then be taken to make forthcoming accelerations as safe as possible. To protect them from outside "hazards", screens are placed around the cavities. A first thermal screen prevents radiation from passing through the cavity walls. A second copper mattress is positioned in order to cool the cavity with gaseous helium. A third magnetic screen absorbs any magnetic fields, even those of terrestrial origin, which might deviate the beam.

 Finally, the cryomodule is placed around these numerous layers to prevent uncontrolled heat conduction and exchange. It is also placed within a high vacuum for a better efficiency.