Electron Cooling of RHIC
By Ilan Ben-Zvi
Discoveries at the Relativistic Heavy Ion Collider (RHIC) have captured worldwide attention. They’ve also raised compelling new questions about the theory that describes the interactions of the smallest known components of the atomic nucleus. To address these questions, we need to study rare processes and thus to increase the collider’s luminosity, or the rate at which ions collide inside the accelerator. The Collider-Accelerator Department is pursuing various upgrades, including the investigation of a luminosity upgrade through electron cooling of RHIC. The electron cooled RHIC, known as RHIC-II, would use cool, energetic and intense bunches of electrons to keep the ion bunches at high density, thus increasing the rate at which the ions collide. This requires the application of advanced accelerator techniques such as high-brightness, high-current energy recovery linacs, which are also needed for other applications, such as eRHIC (energetic electron ion collider at RHIC) and future light sources. As RHIC operates, the luminosity goes down. That’s because the gold ion bunches “heat” up and become more diffuse through a variety of mechanisms, including intra-beam scattering, instabilities of the ions’ motion, mechanical vibration of the magnets, and the collisions themselves. More diffuse beams produce fewer collisions. To improve luminosity, RHIC accelerator physicists aim to eliminate or reduce the buildup of heat within bunches through a process called electron cooling. Cooling would reduce the random movement of ions within the bunches, allowing them to pack more tightly. The more tightly packed the ions in each bunch, the higher the collision rate. To cool the bunches without removing them, Brookhaven proposes adding an “electron cooler” to RHIC. The idea is to bring cold electrons into contact with the ions so that heat can flow from the warmer ions to the colder electrons. The cold electrons would be produced by an electron source, then accelerated to match precisely the speed of the ions in the ring. Over a short (100 meters) section of the ring, the two beams would overlap and have a chance to exchange heat. The electrons would be discarded after one pass and replaced by fresh electrons to continue the cooling process. One of the big challenges in doing this at RHIC is its high energy — about ten times higher than any previous electron cooler. This slows down the electron cooling rate by more than the energy cubed, requiring an electron beam that not only has a high energy, but also a high current and low temperature. So the RHIC electron cooling group members and C-AD engineers are working to develop special hardware: an Energy Recovery Linac (ERL) electron accelerator that will produce cool electrons (emittance smaller than 3 microns) in large quantities (5 nC bunches) at a high energy (54 MeV for 100 GeV/A ions), and some equipment to precisely match the electrons to the ions in position, speed and direction of motion.
Even more difficult is the task of producing such “cold” electrons, also known as low-emittance (or high-brightness) electron beams. The Brookhaven team is now working on a laser-photocathode superconducting radiofrequency source to produce a high-brightness electron beam continuously, on a superconducting accelerator cavity developed to enable a very high current ERL as well as other technologies for accelerating a very high current very efficiently.
Following several years of intensive R&D, we are confident that these techniques will increase the luminosity at RHIC according to our calculations, allowing more sensitive, precision studies of the substructure of matter. The accelerator technologies we are developing may also have applications at Brookhaven beyond the RHIC-II upgrade, for example, in the eRHIC upgrade, which would add electrons produced by an ERL to collide with RHIC ions, and possibly also at future “light source” facilities using very high brightness X-rays to study the properties of materials and biological samples. More information about the Collider-Accelerator Department’s electron cooling group can be found here.