A particle accelerator named HERA
by Markus Ehrenfried
HERMES is installed at HERA, the largest accelerator of the DESY laboratory in Hamburg (Germany) and also one of the biggest particle accelerators in the world. With a length of more than six kilometers underground it is quiet an impressive machine -- why don't we step into the elevator, ride downstairs and take a little walk?
Entering the elevator at ground level we notice slightly amused that these strange DESY physicists label ground floor with '7'. We pick up our radiation protection dosimeters, press '1' and disembark six floors lower into a huge hall. On the other side of this hall behind a thick concrete wall is the actual accelerator tunnel. The concrete wall serves as radiation shielding to protect people outside the accelerator tunnel: a particle accelerator like HERA produces radiation so intense that you would get hurt by a lethal dose within fractions of seconds would you be inside the tunnel. In the beginning you would only feel slight nausea but within days or weeks at most you would die in a very unpleasant way. That's the reason why radiation protection is taken very seriously and up to now at DESY never a serious incident happend. There is a sophisticated interlock system which shuts down the whole accelerator instantly if someone opens one of the doors leading towards the tunnel: by the time this person reaches the tunnel there wouldn't be any radiation anymore. That doesn't mean that this person wouldn't be in serious trouble as an emergency shutdown like this usually causes some damage to the facility.
Normally HERA runs seven days a week, 24 hours each day, but every few weeks there are 'access days' scheduled. This means that the accelerator is switched off and technicians and physicists can enter the tunnel to perform maintenance work and repair things. At the end of such an 'access' every corner of the area affected by radiation is searched carefully to prevent that somebody is locked inside when the accelerator will be switched on again. If everything is empty, the interlock can be set again.
Let's assume we're lucky and today is such an access day where we can go inside the tunnel without any danger -- there is no radiation leftover then, it's really not dangerous. Of course you could stumble and fracture your leg, but this can happen anywhere.
The picture above shows how it looks like inside the HERA tunnel. There is a pathway where you can walk all the way around the accelerator. After exactely 6,336 meters you would get back to the same point again. On your way around you would cross four experimental halls with huge detectors and other areas where the particles are injected into the accelerator from smaller accelerators.
The HERA accelerator itself is the complicated looking stuff on the right hand side in the picture. If you look at the lower right corner of the picture you can see something which looks like a metallic pipe, the thing which soon disappears in the red thing in the middle of the picture: that is the beampipe. It has a diameter of six centimeters and runs all the way around the ring. If the accelerator is in operation inside this pipe the particles fly around with almost the speed of light. That means they cross the same point 47,000 times each second, which corresponds to about 80% of the distance between the earth and the moon each second.
Don't imagine the particle beam inside this pipe like a stream of particles with six centimeters diameter. The beam is tiny compared to the pipe. The beam is thin as a hair and it is circulating in ultra-high vacuum. It's also not a continuous stream (so in fact it's not at all like a thread), it's a stream of tiny packets of particles -- we call them 'bunches'. About 200 bunches are circulating inside the accelerator and they follow each other with a distance of 96 nanoseconds - that means that 10 million times each second a bunch is passing by at any given point.
In fact HERA is not one particle accelerator but two particle accelerators: it accelerates protons in one direction and electrons in the other direction. That allows us to bang together the building blocks of atoms with enormous energy. In the picture above the second beampipe is not visible, it is enclosed in the yellow thing in the upper half of the picture, but it runs in parallel with the other pipe around the ring.
In the arial photograph at the top of this page HERA looks almost like a perfect circle but in fact it is more like a square with rounded corners -- very rounded corners, as you can see in the schematic drawing above. There are straight sections in the regions where the experiments are and arcs in between. The reason for this is that especially the electrons emit an extremely powerful radiation if you try to convince them to fly through a curve -- this radiation is called synchrotron radiation -- and it would damage the detectors. Therefore we let the particles fly straight ahead in the vicinity of detectors.
Let's have a look where the particles come from:
It is not possible to accelerate the particles inside HERA from zero to almost the speed of light. The particles need to have a certain minimum speed before they are injected into HERA. Almost all accelerators which were built in the history of DESY serve as pre-accelerators before the particles finally find their way into HERA. About 50 computers with 3000 processors in total take care that the particles get to the right place at the right time.
The protons start as normal hydrogen gas in a totaly normal gas bottle. In the first step the H2 molecules are split into single hydrogen atoms. Next they get an additional electron whereby they become negatively charged hydrogen ions, H-. Then they are accelerated to an energy of 750,000 eV and injected into the linear accelerator LINAC III which you can find in the drawing above. Linac III is only about 32 meters long but it accelerates the hydrogen ions to an energy of 50 MeV. At the end of LINAC III the hydrogen ions are shot into a thin aluminum foil which peels away the two electrons: on the other end positive hydrogen ions come out -- and positive hydrogen ions are just nude protons. DESY III (317 meters long) accelerates them up to 8 GeV and from there they are transported into PETRA (2,300 meters circumference) where they are stored for a while and accelerated up to 40 GeV. If there are enough protons with the right energy in PETRA they are injected into HERA. About three PETRA fillings are needed to fill HERA. If all the particles are in HERA, the acceleration process starts (they get 'ramped', as the accelerator experts say) up to a final energy of 920 GeV. This sounds complicated but not half as much as it is in reality!
The way of the electrons is slightly different. They are first accelerated to 450 MeV with LINAC II, then in the next step to 9 GeV with DESY II and after that up to 12 GeV in PETRA II. At this energy they are injected in HERA where they get their final acceleration to 27.6 GeV.
HERA is not only a particle accelerator but also a storage ring. Once the particles have reached their final energy they can be stored there for many hours. In principle HERA could store the particles for days and even weeks (especially the protons) but only storing particles is a bit boring: we want to do experiments with them!
Often visitors ask: "Is it true that you made antimatter here at DESY?" -- yes, it is. We do that every day and in quite considerable amounts. In fact the electrons in HERA are usually positrons, the antiparticles of electrons. That has mainly technical reasons as the positive charge makes it easier to store them for many hours.
To dig the 6.5 km long HERA tunnel 180,000 m3 soil had to be moved. On the picture above you can see the drill named HERAKLES which was custom-made for this task. HERAKLES started at the place where today the ZEUS experiment is located and returned again on August 19, 1987 -- two years and three months later -- from the opposite direction. The walls of the HERA tunnel have a thickness of 30 cm and the tunnel is -- depending on the area above -- between 10 and 25 meters below ground level.
Many elements of HERA are superconducting, especially the 416 superconducting dipole magnets of the proton ring which produce a magnetic field of 4.7 Tesla. All these elements are cooled with liquid helium to a temperature of 4.2 Kelvin (= -269°C). The cooling system contains 15 tons of liquid Helium and was at the time it was built the worlds largest helium liquefaction facility. Helium is the second smallest atom (only hydrogen is smaller) and it is able to sneak through the walls of any gas container. If you wait long enough you'll find the container empty! Even if the system has no leaks (in the sense of real holes) there will be always a loss of helium -- a considerable loss: between 5 and 8 tons of the 15 tons in the HERA cooling system get lost every year
If elektrons fly through a strong magnetic field they emit a powerful electromagnetic radiation which is named synchrotron radiation because this effect occurs in the arcs of synchrotrons when the trajectories of the electrons are bended. This radiation is emitted tangential to the flight path and causes an energy loss for the particle: part of its kinetic energy goes into the emitted light.The energy loss increases with the fourth power of energy: if the particle energy doubles the losses due to synchrotron radiation will increase by a factor of 16! This limits the highest energy which can be achieved by the particle accelerator as more and more of the energy which is invested into increasing the particles kinetic energy is lost in form of synchrotron radiation.