What does CLIC stand for? Why is this accelerator “compact”?

CLIC stands for the Compact Linear Collider. Despite a main accelerating part of 44 km in length, the accelerator is “compact” due to its high accelerating gradient of 100 MV/m. To achieve the same centre-of-mass energy with LHC acceleration (5 MV/m) would require a distance of 840 km! Or alternatively, 700 km of LEP2 acceleration (6 MV/m).

Why linear acceleration?

When particles change direction (as they must in a circular collider) they emit photons and lose energy. This effect, called synchrotron radiation, can be avoided by accelerating particles in a straight line. The challenge of linear acceleration is to achieve a very high acceleration gradient, because, unlike in circular machines, the particles pass through a linear accelerator only once.

Is CLIC necessary after the LHC?

The LHC is a fantastic accelerator to make discoveries with - as proven by the Higgs observation. The LHC will hopefully give even more answers or hints to unanswered questions in particle physics, such as: What is the nature of dark matter? Why is the universe made of matter, while antimatter must have initially existed in equal amounts? Do all forces of nature unify? However, it is unlikely that the LHC will give us a complete understanding of all these topics. After the LHC, CLIC will be able to measure the Higgs properties with superior accuracy. This is needed to fully understand the connection between the Higgs field and matter, or to obtain indirect indications of new physics effects at even higher energies. CLIC also has the possibility to see new particles that may escape detection at the LHC. As CLIC collides electrons with positrons (anti-electrons), it provides complementary information to the LHC that mostly collides protons with protons. Electron-positron collisions will yield more accurate information. In addition, proton-proton and electron-positron collisions are actually governed by different elementary forces.

Why should CLIC be built in stages?

Building CLIC in stages, from the middle out, allows physicists to begin performing research at lower centre-of-mass energies (with a shorter accelerator), whilst engineers continue to construct the rest of the accelerator. Tuning the accelerator to a few key energy settings, at the lower, middle and higher ends, allows to study a wealth of interesting physics signals. If the accelerator would be tuned immediately to the highest energy, one would miss out on some interesting physics signals that are guaranteed to exist along the way (for example some Higgs-related processes).

What are the centre-of-mass energies going to be for the different stages?

The energies of the three stages are not yet fixed. The first stage will be around 350 - 380 GeV, in order to profit from known interesting physics processes in this region. The second stage is planned to be around 1.5 TeV. This is the highest energy possible with one drive beam system, so it is attractive from a cost perspective. However, future findings at the LHC may give strong arguments for a slightly higher / lower energy. The third stage will be the top energy: 3 TeV.

How much power will CLIC use?

Designed to be a high luminosity, high energy linear collider, CLIC will inevitably need high power. Compared to an accelerator using superconducting technology, CLIC nevertheless has very low power consumption in stand-by or "waiting-for-beam" mode. A preliminary analysis of the overall CLIC energy consumption per year for the various stages shows that the first stage of CLIC would be similar to LHC, and the second stage similar to the total CERN energy consumption. However, work is on-going in several domains (overall re-baselining, permanent magnets, air-handling etc.) to further reduce the anticipated power consumption of CLIC.

How much will CLIC cost?

To build the first stage of the accelerator is estimated to cost about 30% more than the cost for the LHC. Most of this cost is in excavating the tunnels, installing general services, in the main- and drive-beam production and in the two-beam modules. The LHC construction cost was comparatively cheap because it used, to a large extent, the pre-existing LEP tunnels and infrastructure. To build a detector for CLIC is estimated to cost approximately the same as one of the LHC experiments ATLAS or CMS. Most of the detector cost is in the calorimeters, the superconducting coil and the yoke.

When will CLIC be built?

The LHC is currently foreseen to take data until ~2035. This includes a major investment for an intensity upgrade, which is due to be completed by 2026. After 2026 CERN could possibly start investing in the construction of CLIC.

Where will CLIC be built?

It is presently assumed that CLIC will be built underground, near to CERN in the area close to Geneva. However, the CLIC design could also be implemented elsewhere.

What length will the CLIC tunnel be?

The length of the accelerator complex varies according to the centre-of-mass energies:

• Stage 1 (350-380 GeV): 11 km
• Stage 2 (1.5 TeV): 29 km
• Stage 3 (3 TeV): 50 km

What diameter will the CLIC tunnel be?

The inner diameter of the CLIC tunnel is planned to be 5.6 m. This allows for a two-beam module to be moved past a module which is already installed in the tunnel, to aid installation and maintenance. This diameter is bigger than most one-way underground subway system tunnels (London Underground tunnels are ~3.5 m diameter) but smaller than each Channel Tunnel tunnel (7.6 m diameter). The LHC tunnel has an inner diameter of 3.8 m.

How long will it take to build CLIC?

To build Stage 1 is estimated to take 7 years. This includes three years to excavate the tunnel and shafts, two years to survey and install pipes, electricity etc., one year to install the accelerator and one year to commission the complex. To build Stage 2 is estimated to take a further 5 years. This can happen while the accelerator is running at Stage 1. The LHC took approximately 6 years to install, counting between the end of dismantling LEP (2002) and the first LHC beams (2008).

What is special regarding the CLIC accelerating scheme?

In a classical approach, the linear accelerators used to accelerate the beams would be powered by Radio Frequency (RF) power supplies, called klystrons. In the CLIC acceleration scheme, the klystrons are replaced with an intense particle beam, called the drive beam. The kinetic energy in the drive beam is converted into RF power, which in turn is used to accelerate the main beams for collision. This scheme is called two-beam acceleration.

How does two-beam acceleration work?

An intense beam of electrons is accelerated to a comparatively low energy (2.4 GeV for stage 3) using conventional klystrons. This ‘drive beam’ is injected into a series of Power Extraction and Transfer Structures (PETS), which decelerate the dense beam and extract its energy. This energy is fed via an RF field in a waveguide to a second beam, which is much less intense. Since there are far fewer particles in this ‘main beam’, each one is accelerated to higher energy.

Why a two-beam scheme ?

The luminosity of the accelerator scales as the wall-plug-to-beam efficiency. So one needs at the same time a high-gradient acceleration and an efficient energy transfer. The use of high-frequency RF maximizes the electric field in the RF cavities for a given stored energy. However, standard RF sources scale unfavorably to high frequencies, both in maximum delivered power and in efficiency. A way to overcome such a drawback is to use standard low-frequency RF sources to accelerate the drive beam and use it to produce RF power at high frequency. The drive beam is therefore used for intermediate energy storage.

Why does CLIC have sectors?

The two main CLIC linacs are each divided into 25 sectors (stage 3). Each sector is 878 m long, and contains around 3000 accelerating structures. A fresh drive beam is injected into each sector to accelerate the main beam. At the end of the sector, the spent drive beam is dumped and a new drive beam is injected into the next sector. Each sector accelerates the main beam by 62 GeV.

What happens to the drive beams after they have been used?

Each spent drive beam will already have lost 90% of its power in the PETS. After each of the 50 decelerating sectors the drive beam must be bent away from the linac, leaving enough space for a new drive beam to be injected to the next sector. The old beam is bent away using a dipole magnet, into a beam dump about 20 m away. The power to be absorbed per dump is 0.5 MW.

How many two-beam modules are needed for CLIC? How many accelerating structures?

Each 22 km main linac has 10,800 two-beam modules. Each two-beam module contains up to four PETS. Each PETS generates the RF power for two accelerating structures. CLIC therefore has 74,400 PETS and some 149,000 accelerating cavities in total. The LHC uses 8 accelerating cavities per beam.

Why is the number of accelerating cavities in LHC so much smaller than in CLIC?

As the LHC is a circular accelerator, the acceleration of the beams can happen gradually, over many revolutions. This is the ‘Ramp’ phase of LHC running, which takes around half an hour. CLIC is a linear accelerator, and therefore all of the collision energy must be delivered to the beam in one passage through the accelerator, so in 0.00007 seconds. Many cavities are therefore necessary.

What is special about CLIC accelerating cavities?

CLIC accelerating structures are designed and built to run very stably at a very high accelerating gradient (100 MV/m). The structures are built to micron-level tolerances to ensure that the beam quality is not degraded by beam-to-structure misalignment effects.

What is the accelerating gradient in a CLIC cavity?

The accelerating gradient in a CLIC cavity is 100 MV/m. The cavities in the LHC provide an accelerating gradient of 5 MV/m.

How was it decided to accelerate particles in CLIC with 12 GHz RF waves?

The 12 GHz radio frequency of the main linac accelerating structures was chosen for overall performance and cost optimisation reasons. The LHC RF frequency is 400 MHz.

How does the 12 GHz frequency reflect in the dimension of the waveguides?

The short wavelength of the 12 GHz RF wave means that the waveguides have to have small transverse dimensions - of the order of a centimetre. The LHC RF waveguides are larger due to the longer RF wavelength - of the order of a metre in the transverse plane.

How many quadrupoles are to be installed in the CLIC accelerator?

As the beam travels through the accelerator, it has a tendency to increase in size (emittance). Quadrupole magnets are used to focus the beam and control the emittance. In the two main beams there will be 4200 quadrupoles in total. In the drive beam there will be two quadrupoles per CLIC module. That makes 43,250 drive beam quadrupoles in total. In the LHC there are 392 quadrupoles. 

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.

What is the general design of the CLIC detector?

The CLIC detector is actually a series of sub-detectors, working together to record the event. The different detectors fit together in concentric layers, with the collision happening at the centre. The inner layers are for particle tracking. They are very thin and light, so as not to disturb the particles. The next layers are for calorimetry. Their purpose is to absorb the particles to measure their energy, so these layers are very dense. All these layers are placed in a strong magnetic field created by a superconducting coil. Outside the coil are some more tracking layers, to detect the particles which escape from the calorimeters.

Will there be two detectors? Why (not)?

Two detectors can’t take data simultaneously at a linear collider, as there is only one interaction point. However, two detectors could be built next to each other, and then pushed and pulled one at a time into the collision point. This has advantages; cross-checks with different detector technologies, competition between two collaborations, the opportunity to perform maintenance on the detectors without losing beam time, but also disadvantages; increasing costs, losing time when switching over, compromising on technology. Currently only one detector is planned for CLIC.

How will the CLIC detector be different from the LHC detectors?

The CLIC detector most closely resembles the CMS detector. The CLIC magnetic field will be 4 T, similar to CMS. While CMS and the CLIC detector have similar diameters of 15 m and 13 m respectively, the CMS detector with its 21 m length is significantly longer than the 11.4 m CLIC detector. CLIC will profit from newer detector and electronics technologies and will have many more individual detection channels. While CMS has some 100 million detection elements, CLIC will have more than 3 billion detection elements. This will allow for more accurate measurements at CLIC.

What is particle flow and why is it important?

Particle flow is a reconstruction technique in which every particle is identified and tracked throughout the whole detector. When short-lived particles decay into jets, 60% of the energy is charged particles, 30% photons and 10% neutrons. In traditional calorimeters, it is difficult to reconstruct individual particles within a jet, as the particles are very close together. CLIC will have very fine-grained calorimeters, allowing for the separation of showers from individual particles. Once the particles are separated in the reconstruction, one can use the best available information for each of them. In particle flow, the charged particles are measured most accurately in the tracker, the photons in the ECAL to quite good precision, while the neutrons are measured in the HCAL. The less-precise HCAL therefore contributes to only 10% of the jet energy measurement. The particle-flow approach yields optimal jet energy resolution. CMS successfully uses particle flow, though with less refinement as the CMS calorimeter cells are not small enough for a complete particle separation.

What is the most important physics measurement for CLIC to make?

There are many interesting physics measurements that can be made at CLIC, and we might not know which ones are important until afterwards! Unexplained new physics from LHC data would certainly be a priority. Higgs measurements complementary to LHC results are also particularly interesting, for instance rare processes involving the Higgs, such as Higgs decays into two muons and top-top-Higgs production. Also, at 3 TeV the trilinear Higgs self-coupling can be measured, an important step in verifying the Standard Model Higgs mechanism.

What are the physics goals at the different energy stages?

• Stage 1: 350-380 GeV. The total decay width of the Higgs (including Higgs to invisible decays) can be measured using the Higgsstrahlung process and the recoil of the Z boson. This is a model independent method available only at lepton colliders. Also, a top resonance scan can be performed to achieve a theoretically clean measurement of the top mass, and electroweak physics measurements can be done.
• Stage 2: 1.5 TeV. Targeted at physics beyond the Standard Model. The top-Yukawa coupling can be measured using Higgs bosons production in association with top-quark pair production.
• Stage 3: 3 TeV. Targeted at physics beyond the Standard Model and the Higgs self-coupling.