Could we create a perfect vacuum? In a universe filled with matter and energy, we often think of deepest outer space as a vacuum, empty of everything. But it is far from it, with a multitude of particles and electromagnetic radiation zooming through it. This new animation, made in collaboration with TED-Ed, explores why CERN’s accelerators need to be one of the emptiest spaces in the universe and asks if there is such a thing as totally empty space.
Read more about the content in this animation on the TED-Ed website.
View of the LHC. It took only five weeks for the operators of the LHC to reach 2256 particle bunches circulating in each direction of the accelerator. (Image : Maximilien Brice/CERN)
An unprecedented number of particles has been reached in record time. Just five weeks after physics resumed, the Large Hadron Collider (LHC) is already running at full throttle. On Wednesday 28 June 2017 the LHC established yet another record-breaking high, with 2556 proton bunches circulating in each direction of the accelerator. The beams in the LHC are made up of bunches of protons, spaced seven metres (25 nanoseconds) apart, with each one containing more than 100 billion protons. 2556 is the maximum possible number of bunches that can be reached with the beam preparation method currently used.
The particle bunches that are delivered to the LHC are prepared and accelerated by a chain of four accelerators. Since last year, a new method to group and split the bunches enables the particles to be squeezed even closer together. With an equal number of protons, the beam diameter was reduced by 40 per cent. Denser bunches means a higher probability of collisions at the centre of the experiments.
This success has led to a new luminosity record for the LHC of 1.58x1034 cm-2s-1. This figure may not mean much to most of us, but it’s crucial for the accelerator’s experts. It measures the number of potential collisions per second and per unit of area . This new peak luminosity surpasses initial expectations defined by the original designs for the LHC, which hoped it could reach a maximum of 1x1034cm-2s-1.
A higher luminosity means more collisions for the experiments collecting data: in just a few weeks ATLAS and CMS stored more than 6 inverse femtobarns, over an eighth of the total anticipated for the whole year.
Nevertheless, the operators cannot sit on their hands. Many parameters can be tuned to further improve the luminosity.
Next week, the LHC and its experiments will take a short break for the first of the two technical stops planned for the year. This will be an opportunity to carry out maintenance.
This plot shows the values of the luminosity reached during the last few weeks by the LHC, with the record of 1.58x1034 cm-2s-1 achieved on Wednesday 28 June.
This image shows a simulation of the electron clouds development when the proton beam passes through the vacuum chamber. (Image: CERN)
Protons are jostling for space in the Large Hadron Collider. Since the start of the physics run on 23 May, the operators of the huge accelerator have been increasing the intensity of the beams, injecting more and more protons in order to increase the number of collisions.
“Trains” of proton bunches have been circulating in the machine for the past week. Consisting of up to 288 bunches, each containing more than 100 billion protons, the trains are formed by the accelerator chain and then sent into the large ring. They are then accelerated to a speed close to that of light for around twenty minutes, before they collide with each other in the centre of each experiment. Recently, 600 bunches have been circulating in each direction. The aim is to reach 2500 bunches in each beam within a few weeks.
To achieve this, the machine specialists must first improve the surface conditions of the vacuum chambers in which the protons circulate. Obtaining the best possible vacuum is an essential prerequisite to make an accelerator work. Molecules remaining in the vacuum chamber are obstacles to the circulation of the protons – it is like sending Formula 1 cars around a track full of parked cars. Hence, before starting up the accelerator, the vacuum specialists pump the air out of the beam pipes, obtaining a high-quality vacuum, almost as good as on the surface of the moon (10-10 or even 10-11 millibar). This is enough to allow the circulation of a few hundred proton bunches, but beyond that, things get harder.
Despite the ultra-high vacuum, residual gas molecules and electrons remain trapped on the walls of the vacuum chambers. When the beam circulates, these electrons are liberated from the surface of the walls due to the impact of lost particles or photons emitted by the LHC proton beams. They are accelerated by the beam’s electrical field and hit the walls on the opposite side of the chamber, detaching trapped molecules and freeing more electrons. If the number of liberated electrons is larger than the number of impacting electrons, it may initiate an avalanche of electrons, which will destabilise the beam. This phenomenon, known as the “electron cloud”, is amplified by the large number of proton bunches and the short distance between the bunches in the beam.
To mitigate the impact of these clouds, the vacuum chamber can be conditioned with the beam itself. Increasing the number of circulating bunches frees as many molecules of gas as can be sustained and causes a massive release of electron clouds. Experience has shown that, once this operation, called "scrubbing", has been carried out, the production rate of gas molecules and electrons progressively falls. This allows the beam intensity to be increased stepwise until the LHC can be filled completely.
So it’s time for spring cleaning at the LHC. For five days, starting today, the LHC operators will carry out scrubbing of the vacuum chambers with beam. The physics run will take a short break, starting again in much better conditions mid-June.