The ilc travel Guide:

Follow the particles on their trip through the a­ccelerator

At the height of operation, bunches of electrons and their antiparticles (positrons) will collide roughly 7,000 times per second at a total collision energy of 500 GeV, creating a surge of new particles that are tracked and registered in the ILC’s detectors. Each bunch will contain 20 billion electrons or positrons – this means a very high rate of collisions. This high “luminosity”, when combined with the very precise interaction of two point-like colliding particles that annihilate each other, will allow the ILC to deliver a wealth of data to scientists that will allow the properties of particles, such as the recently discovered Higgs boson, to be measured precisely. It could also shed light on new areas of physics such as dark matter.

How will it work? How do the particles get from one end of the accelerator to the collision point at the centre, and what happens to them on their way? Here’s your travelguide.

Why is the linear collider linear?

Why is the linear collider linear?

When an electric charge follows a curved track, it emits X-rays and loses energy. The higher its energy, the more energy it loses. The energy loss also depends on the mass of the particle and is much more severe for electrons and positrons than for the LHC’s protons. The solution to reach high energies is to eliminate the curves, hence to build a “linear” collider.


What is a gev?

What is a gev?

An electronvolt, eV, is the basic unit of energy or mass used in ­particle physics. It refers to the amount of energy a single electron gains when it is accelerated across an electric potential difference of one volt. One eV is extremely small, and units of a million electronvolts, MeV, or a billion electronvolts, GeV, are more common. The latest generation of energy-frontier ­particle accelerators, such as the LHC, reaches up to several ­trillion electronvolts, or TeV.


1 source

Electrons are produced in the electron source. An intense laser beam shines onto a semiconductor cathode and knocks out billions of electrons. Electric and magnetic fields gather the electrons together and accelerate them to an initial energy of 5 GeV.

2 damping ring

Their next port of call is the damping ring. The density of the particles in the bunch generated by the source is far too low, and the particles need to be packed into a much small volume – or damped – to reach the high ILC performance requirements. This is done in a closed storage ring accelerator similar to many synchrotron light sources around the world.

Indeed it is the same “synchrotron light” that the particles radiate as they go around the ring that causes them to pack together more tightly. But the ILC pushes this technology still further, demanding that the entire damping process happen in a fraction of a second before the bunches are extracted and sent on the next stage of their journey to the collision point. This requires some impressive technologies to make the particles radiate as fast as possible. No easy feat, but experiments at test accelerators have shown that these requirements can be met. Experts from the fields of light sources and high-energy-physics accelerators have worked together to create a design of a high-performance damping ring that will be up to the ILC job. One kind of special magnet, the so-called wiggler, sends the -particle bunches on a serpentine course, causing them to shed some of their energy, which makes them more uniform within the bunch and thus reduces its size. Other technologies for the vacuum system needed to deal with the very high number of particles in the rings have also been the subject of intense study over the last several years.

There are two damping rings of three-kilometre circumference installed inside one tunnel – one for electrons, the other for positrons. When the electron or positron bunches leave their respective damping ring they will have gone around the rings about 20,000 times and become very compact and dense – but not quite dense enough. The bunches are still too long and need to be “compressed”, but that’s the next part of the story.

3 ring to main linac & bunch compression

The neatly packaged and tiny – but still too long – electron and positron bunches are extracted from their respective damping rings and transported in opposite directions some 15 kilometres before being turned 180 degrees in a gentle arc of magnets, ready for the race back to the collision point. Before that all-important sprint through the main accelerator, the bunch length is compressed from 6 mm down to just 0.3 mm, using special sections of the same accelerator technology used in the main linacs, together with some special magnets. While doing this compression, the bunches undergo their first acceleration from 5 to 15 GeV. Now the ultra-compact bunches are ready to be accelerated to the collision energy.

4 main linac

Now comes the fast part. Up until this point, the electrons and positrons have been accelerated to a modest 15 GeV. The energy they need to reach at the collision point, however, is 250 GeV (or even twice that at a possible later phase of the ILC). At these energies, scientists hope they can see, and study in detail, the physics phenomena that have been out of reach or only been glimpsed at by other colliders.

Superconducting microwave cavities are the key to reaching this energy. The more accelerating voltage they can apply to the electrons and positrons, the shorter the accelerator can be (which can reduce the cost quite substantially). The cavities’ core quality is their acceleration gradient – the higher the gradient, the bigger the boost they can give to the particles. Gradients are however limited by the quality of the inner surface of the cavities, which must have an ultra-clean mirror-like finish as perfect as possible. Several decades of research on superconducting cavities have resulted in a recipe for fabrication and surface treatment that can produce cavities with a gradient exceeding the average 35 Megavolts per metre required for the ILC. Despite this state-of-the-art performance, the two linacs still need to each be 12 kilometres long in order to accelerate the -particle bunches to 250 GeV. -Operating at -271 degrees Centigrade, the ILC’s main linacs will also require one of the world’s largest liquid-helium refrigeration plants like the one in operation at the LHC.

The electrons and positrons zoom through the cavities, carried along by the force of electromagnetism. An oscillating electric field inside the cavity at the frequency of radio waves – which is where the name of the acceleration technology comes from: superconducting radio-frequency acceleration – push the particles from cavity to cavity. For a beam energy of 250 GeV, they will pass through 8,000 cavities, or a total of 16,000 -cavities for both accelerators.

The production of these cavities and their surrounding equipment has been one of the greatest challenges in the R&D work for the ILC. Accelerator and -particle physicists from institutes all over the world have worked together with industry from different countries to work out the best way to produce reliable as well as affordable cavities and their cryomodules, the containing structures within which they operate. The production of accelerator parts like cavities and cryo-modules will be done by industry, almost certainly different companies in different regions of the world, whose products then have to fit together to ultra-high precision.

Once accelerated through the main linacs, the positrons are ready for the final leg of the journey to collisions. However, the electrons have one additional job to do before colliding. We have omitted to say where the positrons come from – they are in fact produced by the electron beam!

5 position source

While it is relative straightforward to rip electrons out of a semiconductor with a laser beam, making positrons presents an altogether bigger challenge. This is because, unlike their matter counterparts the electrons, positrons do not actually exist ­naturally in our universe. They have to be made.

The way the ILC will do this is by using the 250 GeV electron beam on its way to the ­collision point. The high-energy electrons will pass through a special magnet called a helical undulator, which is 200 metres long. Just as in the damping rings, the electrons will be made to radiate light in this magnet – indeed it is exactly the same phenomenon.

The difference is in the energy: the 250 GeV electron beam in the positron source’s helical undulator generates high-energy gamma rays in a very narrow cone. These gamma rays slam into a thin titanium-alloy target, knocking loose a shower of electrons and ­positrons. The accelerator sections downstream then collect the positrons and throw away the electrons. The positrons are then bunched, accelerated to 5 GeV and injected into the positron damping ring in much the same way as the electrons. In the meantime, the 250 GeV electron beam is gently bent away from the gamma ray cone and around the metal target before it proceeds on its way to meet the positrons generated on the previous machine pulse at the collision point.

6 final focus

When electrons and positrons have each reached their collision energy of 250 GeV, the beams get one last massage before they enter the hall where it all happens: the interaction point around which the detector sits ready to record the collisions.

In the last two kilometres, instead of running through accelerating cavities the beams pass a series of specially designed and arranged magnets that ultimately focus them down to a height of just a few nanometres at the collision point. The “final focus” system can be thought of as a sort of microscope in reverse, where the beams are demagnified – instead of magnified – by a factor of about 300. Rather than using glass lenses as in an optical microscope, special arrangements of magnets are used to produce the focusing “lenses”.

The tiny beams need to be collided with an accuracy of a fraction of their size, so less than a nanometre! At this level, great care has to be taken to deal with ­vibration and other tiny fluctuations in magnetic and electric fields that can easily cause the bunches to wobble as they pass through the accelerator and final focus. Ultra-fast ­diagnostics systems analyse the shape and position of the bunches and tell the magnets how to correct them to optimise the collisions: without such fast “feedback systems”, the beams would simply miss each other. Using a purpose-built final-focus test beamline in Japan, an international ­collaboration has developed the controls and ­instrumentation needed to achieve these challenging parameters.

7 interaction point and detectors

After all the squeezing, acceleration and focussing, the ­particles can finally hurtle towards their final destination: the collision between electron and its anti­particle, the positron. At full operation there are a potential 1.3 x 1014 (that’s 130 ­trillion) electron-positron collisions per second, crammed into the tiny interaction area of just 6x500 nm2 – or just 0.000000003 mm2. Despite these impressive numbers, only a tiny fraction of the electrons and positrons actually collide. The higher the density of the ­particles at the interaction point, the greater the probability that some will collide: one reason why so much effort is made to produce tiny intense beams at the collision point.

When they do collide, electrons and positrons annihilate in a burst of energy, creating an array of new ­particles that fly out from the collision point. It’s these new ­particles, their interactions, lifetime, and energy that the detectors will study.

Arranged in concentric cylinders of sub­detectors with different technologies and tasks, these detectors are awe-inspiring in speed and ­precision (and in physical volume). Years of research and ­development have already produced mature ­technologies and concepts which will be required for the unprecedented precision of the measurements that need to be made. This R&D will go on until the very last minute to get even better, faster results more cost-effectively. The way the ­detectors will take data is unique, too. Two detectors will be built as one has to verify the results of the other before physicists can claim to have made a discovery. In order to avoid building a second, very costly, final-focus system, the ILC scientists invented the push-pull system: while one detector is installed in the interaction point and takes data, the other is in servicing position. After some time they swap position, being moved on two gigantic platforms that ensures the stability of even the tiniest part within the huge detector.

What are the possible stages of construction?

What are the possible stages of construction?

The current design foresees an ILC with a maximum operating energy of 500 GeV centre-of-mass energy, i.e. 250 GeV per beam, which could be raised to one TeV at a later stage by making the ­linacs longer. The ILC could also be built as a so-called Higgs factory: at half the original design energy, requiring just half of the linacs, it could mass-produce and study the new ­particle found at the LHC in great detail. Such a scenario should be considered as a first phase of the larger machine.


What is superconductivity and superconducting RF?

What is superconductivity and superconducting RF?

Some metals become superconducting when they are cooled down to very low ­temperatures, which means that they lose all ­electrical ­resistance and can conduct electricity in an ultra-efficient way. ­Superconducting radio­frequency cavities are at the heart of the ­technology in the ­International Linear Collider. The ILC will use a voltage generator to fill a hollow structure called a cavity with an electro­magnetic field. Made out of pure niobium, the cavities will be chilled to 2 Kelvin, near absolute zero ­temperature, at which point niobium is a super­conductor and the cavities have almost no electrical resistance. Inside the cavities, the ­voltage of the field oscillates with a certain frequency – a radio frequency. Charged ­particles feel the force of the electric field and accelerate. String enough of these cavities together, and you will have a ­particle accelerator.


How much will it cost?

How much will it cost?

For the Technical Design Report, the GDE has produced a “value estimate”. Value estimates are a common form of costing large international projects that are usually constructed using mainly in-kind contributions from participating nations. The value estimate for the construction of the ILC in the Technical Design Report is 7.8 billion ILCU together with 23 million person hours (approximately 13,000 person years) of additional labour (ILCU stands for ILC value Unit. One ILCU is 1 US Dollar in January 2012. The relation of the ILCU to a currency other than the US Dollar is determined by purchasing power parity indices published by the Organization for Economic Co-operation and Development.). This estimate is averaged over three regional sample sites and represents the construction cost of a 500 GeV linear collider as described in the cost chapter of the Technical Design Report. The variance among the three regional site estimates is about 2%. The value estimate has an uncertainty of 25%. A more accurate estimate can be ­calculated when a host site is identified and the international project governance and in-kind contributions are agreed upon.