RHIC Electron Cooling Group

Ion Storage Ring and Cooling explained

The purpose of a particle accelerator is in most cases to collide elementary particles. By carefully measuring the “wreckage” one can learn about the construction of matter. We use charged particles (electrons, positrons, protons and ions), so that we can use electromagnetic forces for the acceleration. The particles are produced in bunches. An ion bunch in RHIC contains a billion (10⁹) ions. The radius of an Ion is tiny, approximately 3×ּ10^-16 m. This means that the distance between two ions in a bunch is many ion radii.

Imagine how a particle collider works. Bunches of charged atoms (ions), for example gold ions, are counter-rotating in two rings. The rings are evacuated to an extremely good vacuum to prevent collisions of the ion bunches with the molecules of gas in the rings. We want to collide the two streams of counter-rotating ion bunches in particular points. This is no simple task since the rings are very large: about 2.4 miles in circumference each. The Relativistic Heavy Ion Collider RHIC has 6 special points where the rings intersect and collisions can made to take place. We call these the Interaction Points (or IP for short). Some of these IPs have large detectors in them to observe the results of the collisions – that is where the physics of the nuclear matter is being studied.

The bunches are moving very fast, edging the speed of light very closely. To bend their trajectories so that they will follow the circular path of the rings takes very strong magnets of a few types: Bending magnets (or dipoles) bend the trajectory. Focusing magnets (or quadrupoles) herds the ions together to prevent them from drifting apart from each other and focusing them to small cross-sectional areas. Other specialized magnets help in this task, correctors, sextupoles. Radio-frequency systems accelerate the ions to their final energy and keep the bunch length from drifting. The magnets must be very strong, so we use superconducting magnets, devices that can carry very large electrical currents with no electrical loss (other than the connecting wires). We cool the magnets to a very low temperature to be superconducting. This is a fascinating story by itself but not the cooling we are going to discuss here.

To get the physics results fast, we want the gold ions to collide as frequently as possible. Luminosity is the rate at which collisions may take place for a given reaction. We can increase the luminosity by having more ions in each bunch. The number of gold ions we can place in each bunch is very large, about a billion. Another obvious way to increase the luminosity is to place many bunches in each of the two counter-rotating rings. We use about 120 bunches per ring. Finally, to increase the luminosity one makes the size of each of the bunches very small, to increase their density. The luminosity is proportionally larger by the degree that the ion bunches cross sectional area is small.

What is limiting the size of the ion bunches and why we cannot make them arbitrarily small? To understand this let us imagine that we are traveling alongside one of these bunches, going at 99.995 of the speed of light, or racing around the huge rings about 78 thousand times per second. From this vantage point, we see the bunch of ions as a highly elongated cigar-shaped object, almost 100 meters long. The diameter of this object is changing as it goes through the various magnets, but is about a few millimeters or so in diameter at the largest point, a fraction of a millimeter at the IPs. Within this volume, the individual ions move back and forth in random directions, in a way that is characteristic of particles possessing a temperature. This temperature of the ions keeps them oscillating in the confines of the bunch, with the focusing magnets preventing them from flying too far from the bunch’s center. Thus, the size of the bunch is determined by the temperature of the ions in the bunch. Obviously, for a high luminosity we want a low ion temperature.

How can we make bunches that have a small temperature? Of course, a lot of care is being taken in preparing the bunches to be ‘cold’ (or concentrated) and prevent their heating (or dilution) by various mechanisms as they are being accelerated towards the final energy in the chain of accelerators that prepare them, the Tandem van de Graff, the booster and the AGS and then, finally RHIC. However, even as bunches are stored in the rings at their final energy for the collisions to take place, there is a variety of mechanisms that can heat the bunches. One is Intra-Beam Scattering, or IBS. There are other mechanisms: Instabilities of the ions’ motion, mechanical vibration of the magnets, the collisions in the IP and more. Because of these, the ions heat up while they are stored in the rings and lose their effectiveness in a few hours: The luminosity goes down and we must replace them with freshly made cool bunches.

We can increase the effective luminosity and the useful time that we can use the bunches by cooling them. This is the story that we want to tell here: How does one cool ion bunches, hurling about the speed of light in an evacuated pipe?

Electron cooling has been invented in Russia by Gersh Itzkovich Budker of the Institute of Nuclear Physics in Novosibirsk in 1966. The idea is intuitively simple to understand in not to apply. When two volumes of gas or fluid, having two different temperatures, are brought into contact, heat flows from the hotter ensemble to the colder ensemble. Budker, who was a highly inventive scientist, suggested that ions can be cooled by being brought into contact with cold electrons. It is relatively easy to produce cold electrons. The tricky part is to bring them into contact with the ions. To do so, Budker and his associates built an ‘electron cooler’: An electron source (called electron gun) produces cool electrons. They are accelerated to just the right velocity (to match precisely the speed of the ions in the ring), guided and focused by a solenoid magnet, and made to overlap the ions over a small part of the storage ring. There, the two streams (ions and electrons) have a chance to exchange heat. The ions are cooled down and the electrons heat up. The electrons are discarded after one pass to be replaced by fresh electrons from the gun, thus we do not worry about their heating up. Thus, we have made an electron cooler: Electron soak up heat from the ions and cool the ions down.

The special problems that we face in cooling RHIC have to do with its high energy. The electrons have to be accelerated to energy about one hundred times larger than previous electron cooler, even a factor of ten higher than the energy of the Tevatron recycler cooler, the highest energy of any cooler so far. This is because the energy at which we want to cool RHIC is much higher than any storage ring to be electron cooled. Therefore we must develop special hardware to do the cooling: An electron accelerator that will produce cool electrons in large quantities (a current of about a tenth of an ampere) and at a high energy (50 million electron-volts), some highly specialized equipment to match the electrons to the ions. Even more difficult is the task to produce “cold” electrons, also known as low emittance (or high-brightness) electron beams.

The technologies that we are developing for electron cooling are innovative accelerator science. The laser-photocathode superconducting RF gun must produce a high-brightness electron beam continuously. The superconducting electron linac must accelerate a very high current very efficiently, using a technique called energy recovery. Incidentally, these techniques are important for future BNL user facilities such as eRHIC and beyond BNL such techniques are being contemplated for future light sources of the higher brightness X-rays.

Electron cooling is a fascinating example of accelerator research, in which we pick up very small random motions of tiny ion bunches moving at incredible velocities and smooth them out. Electron cooling exercises control over these motions using clever non-intercepting medium and requires cutting-edge hardware. We are carrying out research and development of electron cooling and its underlying technologies at the Collider-Accelerator Department, hoping to increase the luminosity of RHIC in the future to increase its luminosity and enable the scientific experiments to study the structure of quark-matter with more sensitivity and higher precision.

Last Modified: January 31, 2008
Please forward all questions about this site to: Ilan Ben-Zvi

One of ten national laboratories overseen and primarily funded by the Office of Science of the U.S. Department of Energy (DOE), Brookhaven National Laboratory conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies and national security. Brookhaven Lab also builds and operates major scientific facilities available to university, industry and government researchers. Brookhaven is operated and managed for DOE's Office of Science by Brookhaven Science Associates, a limited-liability company founded by Stony Brook University, the largest academic user of Laboratory facilities, and Battelle, a nonprofit, applied science and technology organization.

Privacy and Security Notice  | Contact Web Services for help