Some would call the Relativistic Heavy Ion Collider (RHIC) — the "atom smasher" here at Brookhaven Lab — the most modern accelerator facility in the world. Many ideas about how to accelerate, focus and collide energetic particle beams that were tried unsuccessfully elsewhere have succeeded at RHIC. And as scientists' understanding of the matter created in RHIC's light-speed collisions has evolved, so too has the collider itself —to probe ever deeper into the mysteries of how this ultra-hot primordial matter gave rise to the visible structure of the universe today.
The machine, which steers beams of billions of ions into collisions thousands of times per second, is operating at 15 times the level of performance for which it was designed.
For example, RHIC physicists have increased collision rates, or luminosity, with innovative techniques that squeeze, "cool," and nudge the beams to keep the ions within tightly packed, maximizing the chance these tiny particles will make contact when the beams cross.
Each head-on, heavy-ion smashup momentarily "melts" the ions' inner components of protons and neutrons to form a seething soup of quarks and gluons — the most fundamental building blocks of visible matter, which haven't existed in free form since just after the Big Bang. Understanding the strong force that holds these building blocks together and locks them within atomic nuclei today is one of the main scientific goals of RHIC.
Unlike any other collider in the world, RHIC can collide a variety of ions — from single protons to heavy uranium nuclei — at a very wide range of energies. This versatility allows physicists to explore the mysterious world of quark interactions and the strange and unexpected features of the strong force — including details of the transition between ordinary matter and what the universe looked like some 13.7 billion years ago.
For a separate research objective, RHIC is the only facility in the world that can accelerate and collide protons with their "spins" aligned. Collisions with such spin control allow physicists to tease out how quarks and gluons contribute to this intrinsic particle property. While spin is used every day in medical imaging, it continues to hold many secrets. RHIC is the first facility to reveal that gluons play a significant role.
The success of RHIC is particularly impressive given that, at the beginning, scientists weren't sure it would work. Gold ions colliding at nearly the speed of light at RHIC produce a "perfect" liquid quark-gluon plasma, offering insight into how matter behaved in the very early universe. "Never before was a collider made to collide heavy ions," said Thomas Hemmick, a physicist at Stony Brook University and a collaborator on RHIC research. "There were many new challenges that were absolutely enormous."
The success of RHIC is particularly impressive given that, at the beginning, scientists weren't sure it would work. No collider had ever been built to collide heavy ions, and significant challenges faced its construction. But now, gold ions colliding at nearly the speed of light at RHIC produce a "perfect" liquid quark-gluon plasma—at sun-shaming temperature of 4 trillion degrees Celsius—offering insight into how matter behaved in the very early universe.
Gold ions — the nuclei of gold atoms stripped of their electrons — carry enormous positive charge. As these highly charged particles pass closely by one another at high speed, they generate an extremely strong electromagnetic field, resulting in the production of many electrons and positrons. You can imagine that the negatively charged electrons would be very attracted to the positively charged gold ions., and capturing even one of these electrons would throw off the gold ion's total charge and allow it to escape from the beam.
Beyond that, the high charge of the ion beams exerts significant forces on the electrons in the atoms that make up the beampipe walls, generating currents that heat the material. There was a chance that this heat would dissipate outward to warm up the surrounding superconducting magnets, which could quench, or "kill," their superconductivity, bringing the accelerator to a halt.
Overcoming these challenges, said Hemmick, made Brookhaven itself a kind of magnet for accelerator physicists.
"Brookhaven is a site of particular excellence in terms of accelerator science," he said, "because the best people go to the greatest challenges."
Throughout the design and construction of RHIC in the 1980s and 1990s, nuclear and accelerator physicists — and students from all over the world — wanted to work on the project. Stony Brook, a leading research institute and the university closest to Brookhaven, was a natural partner.
Brookhaven scientists foster a climate that nurtures those attracted to the scientific and technological challenges presented by RHIC and accelerator science in general. Our physicists now direct the Center for Accelerator Science and Education (CASE), a unique joint university-laboratory graduate and post-graduate program focused on developing the next crop of bold accelerator scientists and engineers.
Accelerators play a role in many aspects of our lives. The U.S. Department of Energy estimates that there are 30,000 accelerators operating in world. Many of these are small and conduct behind-the-scenes work: producing beams of radiation used to sterilize medical equipment and keep pathogens at bay in our food supply, imprinting computer chips with ions to improve their performance, producing radioisotopes for cancer diagnosis and treatment, and scanning shipping containers for illicit materials.
Large research centers like RHIC and educational efforts like CASE prepare students who go on to work on next-generation technologies for many of these applications. The fundamental physics explorations at RHIC also lay the foundation for entirely new, unpredictable, and game-changing innovations. By passing the torch to future generations, Brookhaven scientists are ensuring future U.S. leadership and promoting research that improves lives and promotes national security.
Brookhaven is one of just two facilities in the U.S. that produces high-demand, short-supply radioactive isotopes used in heart-disease diagnosis, and Brookhaven scientists are actively exploring new applications in cancer diagnosis and treatment. The Brookhaven Linac Isotope Producer (BLIP) produces these isotopes by bombarding specific materials with protons that are accelerated through the 200-million-electron-volt (MeV) linear accelerator portion of the RHIC accelerator complex, piggybacking on ongoing RHIC operations funding.
RHIC accelerator physicists have also applied their expertise to improving the delivery of particle beams to treat cancer. Particle therapy shows great promise because, unlike conventional x-rays, particles deposit most of their energy where the beam stops, i.e., in the tumor.
Such precision treatment could lead to lower doses, less collateral damage, and more promising outcomes for patients. But existing particle beam delivery systems are large and very expensive. Advances made at RHIC have led to the design of more compact beam-delivery systems, which should make this promising therapy more affordable and available.
The RHIC research program also inspired the U.S. space agency to build and operate its NASA Space Radiation Laboratory (NSRL) at Brookhaven, using beams that come from RHIC's Booster to simulate the kind of particle radiation that permeates deep space. Studies of how these particles affect cells, DNA samples, electronics, and shielding materials are helping scientists evaluate risks and test strategies to protect future astronauts and satellites. Studying the biological effects of radiation in this manner is also offering new insight into our understanding of cancer and the body's defense mechanisms.
Materials used in accelerator design are also finding new applications. Most of RHIC's magnets, for example, are made of superconductors — remarkable materials that conduct electricity with no energy loss when kept extremely cool. The design of these magnets has made Brookhaven a world-leader in magnet technology and the study of superconducting systems, including more recently discovered high-temperature superconductors that offer enormous promise for future applications.
Brookhaven scientists are now leading a new frontier in engineering to develop high-temperature superconducting accelerator magnets, as well as superconducting magnets used for storing large amounts of energy. These superconducting magnet energy storage systems (SMES, shown above) would enable high-density energy storage — enough to keep the current flowing from a solar- or wind-powered system overnight or when the wind doesn't blow. They could also provide stable electric power off the grid — for example, at military camps deployed in the field — which explains why the Army is helping to fund this research.
Even the future of accelerator science at RHIC — a proposed Electron Ion Collider known as eRHIC — offers promise of applications beyond its role as a tool for investigating the structure of matter. Adding an electron ring to the existing RHIC tunnel will use high-speed electrons to probe the inner structure of heavy ions.
To make this a reality, we are developing a novel kind of accelerator known as an energy recovery linac. In this type of accelerator, electrons are first accelerated, used, and then decelerated, giving their energy back to the radiofrequency cavities to be used again in the next bunch of electrons.
Another use of these high-quality electron beams would be to pass them through magnet configurations that cause them to emit self-amplifying light, resulting in an enormously powerful laser. Such a laser could fuel further advances in science, including new kinds of x-ray light sources even brighter than the new National Synchrotron Light Source II.
Throughout our history, we've been on the cutting-edge of accelerator science, and eRHIC will not only keep us at the forefront of physics, but also keep attracting the brilliant individuals who will meet the challenges of tomorrow.