The first successful demonstration of a new technique for improving particle accelerator beams has been reported by researchers at the U.S. Department of Energy’s Fermi National Accelerator Laboratory. This demonstration could be used in future particle accelerators. Physicists could potentially use the method to create better, denser particle beams, increasing the number of collisions and giving researchers a better chance to explore rare physics phenomena that help us understand our universe.
Particle beams are made of billions of particles travelling together in bunches.
Condensing the particles in each beam so they are packed closely together makes it more likely that particles in colliding bunches will interact — just like several people trying to get through a doorway at the same time are more likely to jostle one another than when walking through a wide-open room.
Bundling particles together in a beam requires something similar to what happens when you put an inflated balloon in a freezer. Cooling the gas in the balloon reduces the random motion of the molecules and causes the balloon to shrink. “Cooling” a beam reduces the random motion of the particles and makes the beam narrower and denser.
Integrable Optics Test Accelerator
At Fermilab, scientists used the lab’s newest storage ring, the Integrable Optics Test Accelerator, known as IOTA, to demonstrate and explore a new kind of beam cooling technology with the potential to speed up that cooling process.
“IOTA was built as a flexible machine for research and development in accelerator science and technology. That flexibility lets us quickly reconfigure the storage ring to focus on different high-impact opportunities. That’s exactly what we’ve done with this new cooling technique,”
said Fermilab’s Jonathan Jarvis.
The new technique is called optical stochastic cooling and this cooling system measures how particles in a beam move away from their ideal course using a special configuration of magnets, lenses and other optics to give corrective nudges.
This form of cooling system measures how particles in a beam move away from their ideal course and then uses a particular configuration of magnets, lenses and other optics to give corrective nudges.
It works because of a specific feature of charged particles like electrons and protons: As the particles move along a curved path, they radiate energy in the form of light pulses, giving information about the position and velocity of each particle in the bunch. The beam-cooling system can collect this information and use a device called a kicker magnet to bump them back in line.
Optical Stochastic Cooling
Conventional stochastic cooling, which earned its inventor, Simon van der Meer, a share of the 1984 Nobel Prize in physics, works by using light in the microwave range with wavelengths of several centimetres.
In contrast, optical stochastic cooling uses visible and infrared light, which have wavelengths around a millionth of a meter. The shorter wavelength means scientists can sense the particles’ activity more precisely and make more accurate corrections.
To ready a particle beam for experiments, accelerator operators send it on several passes through the cooling system. The improved resolution of optical stochastic cooling provides more exact kicks to smaller groups of particles, so fewer laps around the storage ring are needed.
With the beam cooled more quickly, researchers can spend more time using those particles to produce experimental data.
10X Cooling Rate Increase
The cooling also helps preserve beams by continually reigning in the particles as they bounce off one another. In principle, optical stochastic cooling could advance the state-of-the-art cooling rate by up to a factor of 10,000.
This first demonstration at IOTA used a medium-energy electron beam and a configuration called “passive cooling,” which doesn’t amplify the light pulses from the particles. The team successfully observed the effect and achieved a tenfold increase in cooling rate compared to the natural “radiation damping” that the beam experiences in IOTA.
They could also control whether the beam cools in one, two or all three dimensions. Finally, in addition to cooling beams with millions of particles, scientists also ran experiments studying the cooling of a single electron stored in the accelerator.
“It’s exciting because this is the first cooling technique demonstrated in the optical regime, and this experiment let us study the most essential physics of the cooling process. We’ve already learned a lot, and now we can add another layer to the experiment that brings us significantly closer to real applications,”
With the initial experiment completed, the science team is developing an improved system at IOTA that will be the key to advancing the technology. It will use an optical amplifier to strengthen the light from each particle by about a factor of 1,000 and apply machine learning to add flexibility to the system.