Beams

What particles make up the beams? Why these particles? Why polarise electrons?

The CLIC beams will be made of point-like particles: electrons (in one beam) and positrons (in the other beam). These two particles are matter-antimatter partners. When they come into contact in the collision they will annihilate each other, liberating all their energy for the production of new particles. The electron beam will be polarised, because this increases the probability of producing certain (interesting) physics interactions. Polarisation also helps to collect additional knowledge about the particles produced in the interaction.

Where do the polarized electrons come from?

To produce the polarised electrons for CLIC, a circularly polarised laser shines on a GaAs type cathode. This moves electrons with the correct helicity from the valence band into the conduction band. Then, the negative electron affinity surface is activated to extract the polarised electrons in the conduction band into the vacuum. They can then be collected and accelerated.

Where do the positrons come from?

Since positrons are anti-matter particles, they do not exist stably in the world around us. The positrons for CLIC are created by sending a 5 GeV electron beam onto a two-target system. The first target is a tungsten crystal, which produces photons via the coherent bremsstrahlung process. The photons enter a second tungsten target, where they create e+e- pairs. Downstream of the second target, the positrons are collected and accelerated.

What is the structure of the CLIC main beams?

Each main beam is formed of a series of dense bunch trains, separated from each other by 20 ms gaps. This means that the repetition rate of the CLIC accelerator is 50 Hz. Each bunch train is composed of 312 bunches, one separated from the next by a 0.5 ns gap.

Why is the repetition rate at CLIC 50 Hz?

This value is chosen so that the beam pulses are in phase with the mains power, which also oscillates with a frequency of 50 Hz. This reduces the effect of electric and magnetic stray fields on the beam or any equipment. As any stray fields will have the same frequency as the accelerator, they will have the same effect on the beam every time it passes. They can therefore be corrected for more easily.

Why have such a short bunch separation of 0.5 ns?

About one third of the power that flows into the accelerating structure to generate and maintain the accelerating field is dissipated in the copper walls. Therefore for efficiency reasons the bunches in the train should follow each other as quickly as possible, so as to minimise the power lost. The bunches can’t be closer together than 0.5 ns due to long-range wakefields. These are disruptive electromagnetic fields that each bunch induces into the accelerating structure, and must be removed before the next bunch arrives. The quickest this can currently be done is 0.5 ns.

A similar reasoning limits the amount of charge in a single bunch. It must be small enough not to generate too high levels of disruptive fields that would affect the bunch itself (called short-range wakefields). Therefore each bunch is limited to 109 particles.

Why have 312 bunches per train?

Using more bunches would require increasing the length of the RF pulse into the accelerating structure. This would increase the probability of the structure to have a break-down (to produce a spark). Break-downs affect the steering and acceleration of the beams, causing them to not collide properly. To limit the number of badly colliding beam pulses to under 1%, trains are limited to 312 bunches.

How big is the crossing-angle at CLIC?

A crossing angle between the two beams is necessary to have collisions at the interaction point only, and avoid that the beams collide at other locations. At CLIC a 20 mrad crossing angle has been chosen as the optimum between necessary beam separation and achievable luminosity. Crab cavities will be used to twist the bunches at the last moment before collision, to ensure a maximum possible luminosity. However, crab cavities cannot ‘undo’ any angle larger than 20 mrad. This is therefore the largest angle that can be used without decreasing the luminosity.

What is the total power per beam in CLIC?

At 3 TeV, the total power per beam in CLIC is 14 MW. The total power per beam in the LHC is 4 TW. The main reason for this difference is ‘repetition rate’; the LHC beam circulates every 90 microsecond, the CLIC beam frequency is 50 Hz or every 20 ms. The smaller number of bunches at CLIC (312 vs 2808 at LHC) and smaller number of particles per bunch (109 vs 1011 at LHC) also play a role. The energy contained in the CLIC beam is of the order kJ, compared to MJ at the LHC. The difference in power caused by the different centre-of-mass energies (3 TeV vs 14 TeV) is negligible.

What is done with the main beams after the collision?

Due to the crossing angle, the beams continue through post-collision beam-lines towards a water beam-dump. Each of the two dumps is located next to the main linac, 300 m downstream from the interaction point. They consist of a cylindrical titanium vessel, 10 m long and 2 m in diameter. The water inside is pressurised to 10 bar, to prevent it from boiling. It is continuously circulated through a heat exchanger. The front window of the water dump is made of a titanium alloy. The water dump must absorb 14 MW of power.

The two LHC beam dumps are 7 m long cylinders of carbon, with a 70 cm diameter. These cylinders are contained inside steel and concrete, and are water-cooled. The beam must be defocussed to avoid burning through the beam dump. Each beam dump must dissipate the 362 MJ beam energy in the 90 ms circulation time, which is equivalent to a power of 4 TW. However, while LHC beam dumps might be separated by several hours, CLIC beam dumps happen every 20 ms.

Overall what drove the parameter choices of the CLIC accelerator?

The CLIC parameters were derived in an optimisation, taking into account limitations from the RF, the beam and the physics. The aim was to find the cheapest machine that could provide the specifications requested by the experiments. There were designs which would have needed fewer bunches per train to reach the luminosity goal, and some which would have used more bunches, but they would have been less cost effective.