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Ecool celebrates by Kurt Riesselmann
Physicist Sergei Nagaitsev and his group took the first big step toward building a cooling device for these exotic particles. The group just announced a world record for sending a large amount of their own special “cooling liquid” in a circular loop.
Like other heat-exchange systems, the cooling of antiprotons requires the right “liquid” to carry away excess heat from the material to be cooled. In the case of antiprotons, which are kept in storage rings and travel at close to the speed of light, the heat-absorbing medium is a beam of electrons, traveling at exactly the same speed as the antiprotons. To be efficient, the electron beam must contain many more particles than the antiproton beam, requiring scientists to develop a high-current electron system.
Nagaitsev’s group has created a record-breaking electron beam with a continuous current of 500 milliamps at an energy of 3.5 MeV. To the layperson, these numbers may not seem significant. After all, half an amp is the current flowing through the wire of a typical light bulb. However, the electrons in the beam travel at a much higher energy than those in a wire, leading to a beam power that even amazes the non-experts.
“We are holding a world record in DC beam power,” Nagaitsev said. “About two megawatts.”
For comparison, this amount of power is sufficient to operate two thousand kitchen refrigerators simultaneously. But in case of the electron-cooling project, only a fraction of the power is actually consumed. Most of the electrons and their power get recirculated: the electrons pass through cooling sections and return to their source, the top of a 25-foot-high Pelletron accelerator, an electrostatic device similar to a Van de Graaff generator.
The path taken by the electron beam resembles that of a stream of cold water rushing down a waterfall. At the bottom, the stream flows horizontally, eventually encountering containers with hot stuff that float along. Imagine that after about 60 feet, the stream is separated from the hot containers, turned around and sent back up the waterfall to close the loop with as little loss as possible. That’s what the electrons do. But instead of gravity, the electrons feel the presence of million-volt electric fields as they “fall down” the Pelletron, gaining energy. And while water drops usually don’t run up a mountain, the electrons can maintain their energy throughout the bottom part of their trip and are able to race back to the top of the Pelletron, from where they are re-injected to begin their cooling trip anew. Only a small fraction of electrons, about twenty in a million, is lost, allowing for stable operation of the recirculation system.
“There is nothing else like this [Fermilab system] that has so much current and so little loss,” commented scientist Mark Sundquist, of National Electrostatics Corporation, which manufactured the 2.5-million-dollar Pelletron now at Fermilab. “There are some keV systems, but they are much different. This is the only one at energies significantly above a few hundred keV.”
A versatile machine
NEC, a Wisconsin-based company that received a Department of Energy Small Business Innovation Award in 1984, has manufactured more than 140 Pelletrons with sales in 38 countries. Clients use the systems, which get their name from the chains of pellets that are used to create high voltage, to accelerate charged particles to energies ranging from a few keV to hundreds of MeV. Pelletrons are used in such important applications as surface analysis and doping of computer chips. Even archaeologists rely on Pelletrons to determine the age of tiny samples, smaller than one milligram. The machines provide a fast and extremely accurate way of carbon-dating material by determining the proportions of various carbon isotopes in a process called accelerator mass spectrometry.
Most Pelletrons operate as non-recirculating accelerators, typically featuring one-way beams of less than 50 microamps. In contrast, Fermilab’s electron cooling project relies on a continuous high-current beam, which can only be achieved through recirculation.
“People in this business know how hard it is,” said Nagaitsev. “Everybody is pushing the envelope. People working on related projects in this country and in Europe are waiting for our results. Our success or failure means quite a bit at other laboratories.”
Boosting innovation
Although Nagaitsev and his colleagues are aware of the tremendous potential of their electron recirculation system, they focus on the particle physics application of their device. With the help of electron cooling, scientists will be able to shrink the size of antiproton beams, creating higher beam densities. The reduced beam size leads to a larger number of collisions when scientists make two beams collide head on.
“The goal of our R&D project is simple: construct and commission an electron cooling device that is ready to be moved to the Recycler,” Nagaitsev said. “There is a risk involved, and the project may or may not lead to a final result. We believe it will work. We are already working with Fermilab engineers on the design of the new building next to the Recycler, into which we will move all electron-cooling equipment. We hope to complete the design this summer.”
Recycler is the name of Fermilab’s antiproton storage ring. Right now scientists build, test and improve the electron cooling system in a building about a mile away from the Recycler. So far, electrons haven’t mingled with a single antiproton as the team is still making improvements on operating the Pelletron, producing an electron beam in stable mode for long periods of time.
Fighting sparks
“Everything sparks to everything,” Saewert said. “One hurdle early on was to design the electronics to survive the mini-lightning strikes. We ran the machine to five million volts and sparked the heck out of it.”
To communicate with the high-voltage interior of the Pelletron, scientists obviously cannot use electrical cables. They rely on fiberoptic cables to avoid short-circuiting the high voltage. On top of it, all electronics located inside the accelerator must have their own power supplies. A 25-horse-power motor at the bottom of the Pelletron rotates a long Plexiglas shaft to crank six generators at different voltage levels.
Avoiding shorts and disruptive sparks is only one of many challenges. Currently, scientists are preparing to study the quality of the electron beam as it travels through a special cooling section – initially without the presence of antiprotons.
“Some of the difficult things aren’t done yet,” Nagaitsev pointed out. “The three perhaps most important issues are: Can we produce enough electron beam? Our recent results show the answer is yes. Can we produce beam of high enough quality? That is still subject to R&D. Can we cool antiprotons? We think so. Depending on the efficiency of the Recycler, maybe we can increase luminosity by a factor two, maybe more.”
Nagaitsev hopes that the test beam line with a nine-module cooling section will be ready in May. It will enable his team to carefully study the properties of the electron beam within the environment of the cooling section. Scientists will spend a lot of time on determining the energy and dimensions of the high-current electron beam with great precision.
The final step, anticipated to occur in 2003 or 2004, will be to mix the cold and hot “stuff,” sending electrons and the two thousand times heavier antiprotons through the cooling section at the same time. If everything works well, each antiproton will find itself surrounded by ten or more electrons, which is sufficient according to calculations done by Alexey Burov. Antiprotons going too fast will slow down as they bump against electrons in front of them. Antiprotons going too slow will speed up as electrons in the back kick them. With each collision, the lighter electrons will reduce the heat—the spread of energy—within the antiproton beam. All of this will happen in a gentle way, as the collisions will resemble ping-pong balls bouncing off a bowling ball.
Some day, cooling antiprotons may be as easy as putting a bottle in a fridge. Although Nagaitsev’s team still has a long way to go, it might be time to start chilling some champagne.
On the web:
The Electron Colling Project at Fermilab
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Cooling is a wonderful invention. It keeps your food fresh in a refrigerator, it prevents your car engine from overheating and it maintains the temperature inside your living room at a pleasant seventy degrees on a hot summer night.
At Fermilab, scientists now would like to cool something completely different: antiprotons. As you can imagine, a hose with cold water won’t do the job.
The development of a high-current electron beam has applications beyond cooling antiprotons. Scientists around the world are eager to have powerful electron beams to create the best Free Electron Lasers, powerful light sources that have plenty of applications as analytical tools in molecular biology, material science and chemistry. Rather than throwing away the electrons and their energy, like it is done in many conventional FELs, scientists can reuse and replenish the recirculated beam again and again to produce light of unprece-dented power. Using a continuous electron beam, scientists can even build a continuous FEL light source, rather than the pulsed-mode FELs available today.
Greg Saewert, an electrical engineer working on the project since 1996, has spent the last few years developing electrical equipment that can withstand the powerful sparks that occur inside the high-voltage environment of the Pelletron. A few weeks ago, a spark was powerful enough to melt a bundle of fiberoptic cables.
