Racing Ahead at the Speed of Light

Imagine trying to catch up to something moving close to the speed of light -- the
fastest anything can move - and sending ahead information in time to make mid-path flight corrections. Impossible? Not quite. Physicists at the Relativistic Heavy Ion Collider (RHIC), a particle accelerator at the U.S. Department of Energy's Brookhaven National Laboratory, have achieved this tricky task - and the results may save the Lab money and time in their quest to
understand the inner workings of the early universe.

The physicists have developed a way to measure subtle fluctuations in RHIC's particle beams as they speed around their 2.4-mile-circumference high-tech racetrack - and send that information ahead to specialized devices that smooth the fluctuations when the beam arrives.

"These corrections help to keep the beams focused and colliding, recreating thousands of times a second the conditions that existed just after the Big Bang," says Steven Vigdor, Brookhaven Lab's associate laboratory director for nuclear and particle physics, who manages the RHIC program.

Already, RHIC scientists have learned that mere microseconds after the Big Bang, the universe was more interesting than imagined - a nearly "perfect" liquid with virtually no viscosity and strong interactions among its constituents. With the ability to race ahead of RHIC's beams and keep them focused, the scientists will be able to create many more "mini-Bangs" for study. The increase in data will help them investigate and measure the detailed properties of this "perfect"
liquid, and test certain predictions stimulated by an unanticipated link between RHIC findings and "string theory," an appealing approach to incorporate gravity into a unified theory that describes all of Nature's forces.

The beam-correcting technique, called stochastic cooling, has been implemented at accelerators where the beams are made of a continuous stream of particles - but never before at a facility where the particles travel in discrete bunches, as is necessary for the beam-on-beam collisions that take place at RHIC. "Its successful demonstration at RHIC provides an alternative path to the goal of increased collision rates, which would be much more costly and take longer to
achieve via other proposed means," Vigdor says.

How it works

RHIC circulates two beams of ions - electrically charged particles - moving in opposite directions in two separate rings at 99.995 percent the speed of light. Within each beam, the ions travel in discrete groups, or "bunches," each containing more than a billion ions.

In the highest-energy experiments, the ions are the nuclei of gold atoms, composed of protons and neutrons that slam together where the two beams cross to produce a tiny speck of extremely hot, dense matter that mimics the conditions of the early universe.

But like all charged particle beams, RHIC's ions tend to spread out (heat up) as they circulate. As the ions spread, the number of protons and neutrons colliding - and the amount of useful data - declines.

So RHIC physicists are taking advantage of RHIC's circular shape and the ability to send signals as fast as the near-light-speed beam to cool the ions down - that is, keep them tightly bunched.

The technique includes the term stochastic (derived from statistics, meaning random) because it relies on measuring the random fluctuations in the beam shape and size. The measurements are made at one point on the accelerator by devices that generate signals proportional to how far the particles are straying from their ideal positions. These devices then send the signals via fiberoptic or microwave links to a position ahead of the speeding beam, where electric fields are generated to "kick" the charged particles back toward their ideal positions.
The result: more tightly squeezed, cooler ion bunches.

The signals stay ahead of the beam by taking one of two shortcuts - either traveling from one point to another across the circular accelerator, or by backtracking along the circle to meet the speeding beam about halfway around on its next pass.

So far the RHIC physicists have tested stochastic cooling in the longitudinal direction - along the direction of the beam - in one of RHIC's two rings. Longitudinal cooling compensates for the ion bunches' tendency to lengthen as they circulate. This improvement has already increased RHIC's heavy-ion collision rate by 20 percent. Next, the physicists will test the same cooling
system in RHIC's other ring.

With $7 million in additional funding, the physicists will design and build a similar system for correcting the tendency of RHIC's ion bunches also to become "fatter" as they circulate. Computer simulations, which have accurately predicted the achievements of the longitudinal cooling system, predict that combining this new transverse cooling system with longitudinal cooling in both rings and some additional equipment could increase collision rates overall by
500 percent.

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R.I. Physicists Use Big Computers To Gain Knowledge of Particles

What if, before designing a car, an engineer could use physical knowledge of atomic particles to design materials and know exactly how they would react in a collision? What if fundamental physical equations that predict how those particles will behave in various energy states (cold, heat, stress) could be used to design spacecraft? To solve the physics equations that will lead to the engineering designs of tomorrow, physicists at the University of Rhode Island are investigating the fundamental quantum mechanics of particles using computers from SGI.

The SGI Altix system, installed in August, is used by the Physics Department to solve one of the basic equations of quantum mechanics, the Schrodinger equation,which describes how matter behaves at the atomic scale. While the equationhas been solved in several simple cases, SGI Altix technology was chosen because systems involving more than two or three particles cannot bedirectly solved, and require instead the use of computationally intensive numerical methods.

"We selected the Altix because of the speed of the connectivity of the various nodes, the speed of the exchange of information between the nodes,the speed of the individual processors, and the flexibility that that offers," says Peter Nightingale, professor of physics, University ofRhode Island. "That is important because a lot of what we do involves linear algebra with matrices, and the matrices tend to be spread out over different nodes. There is communication necessary to do a coordinated calculation for a matrix that is spread out over these systems. It's the speed of that communication that really is a bottleneck at times. Our cluster system was simply becoming unreliable and the SGI Altix offers muchstronger connectivity and therefore we are able to do bigger calculations faster, which require a lot of compute power."

Purchased in July through James River Technical, Inc. (JRTI), SGI's exclusive higher education reseller, the SGI Altix 350, with 10Intel Itanium 2 processors running Novell SUSE Linux Enterprise Server 9, is connected to the older cluster as well as to numerous desktops in the Physics Department. The SGI Altix system is also connected to the Internet, allowing anyone with access to use it, typically students who log in from home.

The SGI Altix 350 system will be used by Nightingale and his students as part of an ongoing program, supported by the National Science Foundation (NSF), to study the behavior of small van der Waals complexes and to develop their own applications. These van der Waals systems consist of a small number of weakly interacting atoms, and the research addresses the fundamental problem of solving the Schrodinger equation for these systems. The University of Rhode Island researchers will develop new Monte Carlo methods to solve this equation, and the SGI Altix system will supply the power to speed up study of these particle systems and compare them with experiments.

As Nightingale explains, the development of these methodsalso has implications for future engineering design.

"The energies of ground and excited states tell us about how the particles interact with each other, but what is not really known is how complicated systems interact, what the strength of the interaction is, as the distance changes," adds Nightingale. "For instance, if you can accurately predict where the energy levels are, you can figure out what the interactions between the particles are and that is important for all sorts of applications. Ultimately, and we may not be there for quite a while, the idea is to write down these fundamental physical equations and then designmaterials on the basis of the fundamental properties of this material. Right now, engineers design products based on what are called phenomenological models, which are a mixture of things that are known, things that are guessed, and things that are measured. But it would be much more elegant if you could start from fundamental physics, from the Schrodinger equation, and on the basis of that predict how your car will behave in a collision, for example. That's quite a stretch, but that's the ultimate goal."



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