## Archive for November, 2014

### Cool Little Neutrinos

Thursday, November 6th, 2014

A lot of us working in experimental neutrino physics think that these strange and tiny particles are pretty cool. Here are some fun facts about them from Fermilab physicist, and old classmate of mine, Tia Miceli.

Neutrinos change their flavor just as chameleons can change color. The observer needs to make sure their instruments are prepared to detect these changing beasts.

We don’t know much about neutrinos, but what we do know points to renegade particles that, despite their prevalence, are hard to pin down. Here are, in a nutshell, nine neutrino nuggets that scientists have figured out so far.

1. Neutrinos are super abundant. The shining sun sends 65 billion neutrinos per second per square centimeter to Earth. Neutrinos are the second most abundant particle in the universe. If we were to take a snapshot, we’d see that every cubic centimeter has approximately 1,000 photons and 300 neutrinos.

2. Neutrinos are almost massless. No one yet knows the mass of neutrinos, but it is at least a million times less massive than the lightest particle we know, the electron. We do know that each is so lightweight and so abundant that the total mass of all neutrinos in the universe is estimated to be equal to the total mass of all of the visible stars.

3. Neutrinos are perfect probes for the weak force. All other fundamental particles interact through the strong, electromagnetic or weak force or through some combination of the three. Neutrinos are the only particles that interact solely though the weak force. This makes neutrinos important for nailing down the details of the weak force.

4. Neutrinos are really hard to detect.On average, only one neutrino from the sun will interact with a person’s body during his or her lifetime. Since neutrino interactions are so rare, neutrino detectors must be huge. Super Kamiokande in Japan is as tall as Wilson Hall and holds 50,000 tons of ultrapure water. IceCube is buried between 1.5 and 2.5 kilometers under pure and clear ice in Antarctica, instrumenting a full cubic kilometer of ice.

5. Neutrinos are like chameleons.There are three flavors of neutrinos: electron, muon and tau. As a neutrino travels along, it may switch back and forth between the flavors. These flavor “oscillations” confounded physicists for decades.

6. Neutrinos of electron flavor linger around electrons. When neutrinos travel through matter, they see dense clouds of electrons. Electron neutrinos will have trouble traversing these dense clouds, effectively slowing down while muon and tau flavors travel through unimpeded. The NOvA experiment is using this phenomenon to deduce more information about the neutrino masses.

7. Neutrinos let us see inside the sun.The light that reaches Earth takes 10,000 to 100,000 years to escape the thick plasma of the sun’s core. When light reaches the solar surface, it freely streams through open space to our planet in only 8 minutes. Neutrinos provide us a penetrating view into the core, where nuclear fusion powers the sun. They take only 3.2 seconds to escape to the solar surface and 8 minutes to reach Earth.

8. Neutrinos may have altered the course of the universe. Why is everything in the universe made predominantly of matter and not antimatter? Cosmologists think that at the start of the universe there were equal parts of matter and antimatter. Neutrino interactions may have tipped this delicate balance, enabling the formation of galaxies, stars and planets like our own Earth.

9. Neutrinos dissipate more than 99 percent of a supernova’s energy.Certain types of stellar explosions lose nearly all of their energy through neutrinos. These “core collapse” supernovae end as either a black hole or a neutron star. Neutrinos are used to understand how supernovae explode and tell us more about other astronomical objects like active galactic nuclei.

Tia Miceli

### New technique for generating RF power may dramatically cut linac costs

Monday, November 3rd, 2014

A team from the Accelerator Division has successfully powered this small SRF cavity with a magnetron. Now they aim to power a large, application-specific model. Photo: Brian Chase, Fermilab

If you own a magnetron, you probably use it to cook frozen burritos. The device powers microwave ovens by converting electricity into electromagnetic radiation. But Fermilab engineers believe they’ve found an even better use. They’ve developed a new technique to use a magnetron to power a superconducting radio-frequency (SRF) cavity, potentially saving hundreds of millions of dollars in the construction and operating costs of future linear accelerators.

The technique is far from market-ready, but recent tests with Accelerator Division RF Department-developed components at the Fermilab AZero test facility have proven that the idea works. Team leaders Brian Chase and Ralph Pasquinelli have, with Fermilab’s Office of Partnerships and Technology Transfer, applied for a patent and are looking for industrial partners to help scale up the process.

Both high-energy physics and industrial applications could benefit from the development of a high-power, magnetron-based RF station. The SRF cavity power source is a major cost of accelerators, but thanks to a long manufacturing history, accelerator-scale magnetrons could be mass-produced at a fraction of the cost of klystrons and other technologies typically used to generate and control radio waves in accelerators.

“Instead of paying $10 to$15 per watt of continuous-wave RF power, we believe that we can deliver that for about \$3 per watt,” Pasquinelli said.

That adds up quickly for modern projects like Fermilab’s Proton Improvement Plan II, with more than 100 cavities, or the proposed International Linear Collider, which will call for about 15,000 cavities requiring more than 3 billion watts of pulsed RF power. The magnetron design is also far more efficient than klystrons, further driving down long-term costs.

The magnetron project members are, from left: Brian Chase, Ed Cullerton, Ralph Pasquinelli and Philip Varghese. Photo: Elvin Harms, Fermilab

But the straightforward idea wasn’t without obstacles.

“For an accelerator, you need very precise control of the amplitude and the phase of the signal,” Chase said. That’s on the order of 0.01 percent accuracy. Magnetrons don’t normally allow this kind of control.

One solution, Chase realized, is to apply a well-known mathematical expression known as a Bessel function, developed in the 19th century for astronomical calculations. Chase repurposed the function for the magnetron’s phase modulation scheme, which allowed for a high degree of control over the signal’s amplitude. Similar possible solutions to the amplitude problem use two magnetrons, but doubling most of the hardware would mean negating potential savings.

“Our technique uses one magnetron, and we use this modulation scheme, which has been known for almost a hundred years. It’s just never been put together,” Pasquinelli said. “And we came in thinking, ‘Why didn’t anyone else think of that?'”

Chase and Pasquinelli are now working with Bob Kephart, director of the Illinois Accelerator Research Center, to find an industry partner to help them develop their idea. Inexpensive, controlled RF power is already needed in certain medical equipment, and according to Kephart, driving down the costs will allow new applications to surface, such as using accelerators to clean up flue gas or sterilizing municipal waste.

“The reason I’m not retired is that I want to build this prototype,” Pasquinelli said. “It’s a solution to a real-world problem, and it will be a lot of fun to build the first one.”

Troy Rummler