Chopping beam to your taste

For lunch, do you enjoy having your tomato whole, in slices or finely chopped? It probably depends on the dish: The tomato could serve as a standalone fruit snack, as part of a sandwich or as a salad topping. Similarly, different Fermilab experiments may want particles beams coming to them in different chunks. Now they have a chopping “knife” to prepare the beam to their taste: the new beam chopper for the future Fermilab linear accelerator.

Scientists can create custom particle beams by removing select particle bunches — or packets of particles each containing about 200 million particles — from a longer, continuous queue. For example, experts can adjust settings to eject every third bunch, with the remaining bunches forming a particular pattern that’s optimized for an experiment. The beam chopper is the device that carries out this high-precision bunch removal.

Beam choppers are ubiquitous in ion accelerators. But when, several years ago, a team of Fermilab accelerator experts set out to design the chopper that would become part of the lab’s upgraded accelerator complex, nothing even close to what was needed existed.

The program of upgrades called Proton Improvement Plan-II (PIP-II) will meet the demands of Fermilab’s future experiments for more and more intense particle beams. While the new PIP-II linear accelerator uses superconducting resonators to increase the particles energy to 800 million electronvolts, the chopper is placed upstream in the section called the medium-energy beam transport, MEBT. It will pass for acceleration only those bunches that are requested by the experiments and remove all others.

A beam chopper has two main parts: kicker and absorber. The kicker removes unneeded bunches. The absorber receives them.

The kicker is essentially two parallel plates between which voltage can change quickly, and it is situated at an optimal point in the beam path for bunch removal. So when a bunch to be removed flies between the kicker plates, a voltage is applied — a nanosecond-long, roughly 1,000-volt burst of electric potential. That pulse kicks the bunch out of the main queue and into the beam absorber downstream.

PIP-II asks a great deal of the chopper. The MEBT chopper must act rapidly, able to remove bunches at a high frequency. It must be able to generate a pulse strong enough to completely eject entire bunches. It has to be precise enough to pick off single bunches without perturbing its neighbors — removing the unneeded bunches and only the unneeded bunches. The propagation of the voltage through the kicker plates has to sync perfectly with the path of the hydrogen ion bunches. And with record-power ion beams barreling through the accelerator, the chopper must be hardy enough to handle the increased power and radiation.

It was a collection of demands beyond the current state of the art.

The Fermilab accelerator team experimented with various designs, timings, electromagnetic variables and beam parameters. They developed two different kicker prototypes — a 50-ohm and a 200-ohm kicker, each requiring a specific way to create the electric signal. For example, the 200-ohm version would use a switch: open the switch and close it quickly back, and a short voltage pulse runs into the kicker. (It is almost like making a flash of the light by pushing a button of your flashlight, except the voltage is more than 100 times higher and “pushing” needs to be done in a billionth of second, tens of millions times per second.)

Ding Sun began development, prototyping, and eventually testing of the 50-ohm kicker version. Greg Saewert led the development of both the 200-ohm kicker and its switch driver, later joined by Daniil Frolov. Alex Chen led the mechanical design of both kicker versions.

Even with kickers redefining the state-of-the-art and combining deflection of two of them, the orientation and size of the beam are such that there was not much room to negotiate the placement of the absorber, which would have to deal with a high power density from the beam. The absorber is a piece of metal that intercepts the removed portion of the beam. Curtis Baffes sketched contours of the future absorber and then made a quarter-size prototype. Of course, the rapid expansion of the absorber material under fast heating from the tightly focused beam was the biggest issue. Eventually, the team came up with a workable design, and another quarter-size prototype was successfully tested to maximum power density envisioned for the future accelerator, ready to be exposed to ions.

Their final PIP-II chopper design comprised two kickers of the same type and one absorber. Each kicker fires as the unneeded bunch flies through the first and then the next, deflecting the bunch from the main beam. The absorber, like an outfielder on a baseball team, readily receives these high-frequency, high-power bunches.

When tests of the chopper components were conducted, to everybody’s excitement, several instruments showed how the beam envelope was splitting into two distinctive trajectories! Then, remove one of them, and a monitor downstream showed a sequence of bunches selected according to a predetermined pattern.

The excitement was followed by multiple shifts answering multiple questions. Does each kicker deflect the beam as expected? Does the velocity of electric pulse propagation through the kicker match the ion speed? Can the deflections by two kickers be efficiently summed? Do kickers survive if the beam is passed through them for a day? Does the absorber prototype behave with the ion beam as it was predicted from experiments with electrons? Each “yes” was adding confidence that the chopper will work, and heart of future PIP-II can beat as it has been envisioned eight years ago.

In 2018, the team plans to demonstrate operation of PIP2IT with a high-power beam, manufacture and install a full-size absorber, and order two final-design kickers, which will be slightly modified version of the present 200 Ohm kicker. In a year or so, the team hopes to have a chopper capable of fulfilling the first tasks foreseen for PIP-II, injection into the Booster and supplying the beam for the upgraded version of the Mu2e experiment.

Learn more in proceedings from the recent HB2018 conference or from a recent IEEE conference.

For four years, three laboratories on two continents have prepared the ICARUS particle detector to capture the interactions of mysterious particles called neutrinos at the U.S. Department of Energy’s Fermi National Accelerator Laboratory.

On Tuesday, Aug. 14, ICARUS moved into its new Fermilab home, a recently completed building that houses the large, 20-meter-long neutrino hunter. Filled with 760 tons of liquid argon, it is one of the largest detectors of its kind in the world.

With this move, ICARUS now sits in the path of Fermilab’s neutrino beam, a milestone that brings the detector one step closer to taking data.

It’s also the final step in an international scientific handoff. From 2010 to 2014, ICARUS operated at the Italian Gran Sasso National Laboratory, run by the Italian National Institute for Nuclear Physics. Then the detector was sent to the European laboratory CERN, where it was refurbished for its future life at Fermilab, outside Chicago. In July 2017, ICARUS completed its trans-Atlantic trip to the American laboratory.

“In the first part of its life, ICARUS was an exquisite instrument for the Gran Sasso program, and now CERN has improved it, bringing it in line with the latest technology,” said CERN scientist and Nobel laureate Carlo Rubbia, who led the experiment when it was at Gran Sasso and currently leads the ICARUS collaboration. “I eagerly anticipate the results that come out of ICARUS in the Fermilab phase of its life.”

Since 2017, Fermilab, working with its international partners, has been instrumenting the ICARUS building, getting it ready for the detector’s final, short move.

“Having ICARUS settled in is incredibly gratifying. We’ve been anticipating this moment for four years,” said scientist Steve Brice, who heads the Fermilab Neutrino Division. “We’re grateful to all our colleagues in Italy and at CERN for building and preparing this sophisticated neutrino detector.”

Neutrinos are famously fleeting. They rarely interact with matter: Trillions of the subatomic particles pass through us every second without a trace. To catch them in the act of interacting, scientists build detectors of considerable size. The more massive the detector, the greater the chance that a neutrino stops inside it, enabling scientists to study the elusive particles.

ICARUS’s 760 tons of liquid argon give neutrinos plenty of opportunity to interact. The interaction of a neutrino with an argon atom produces fast-moving charged particles. The charged particles liberate atomic electrons from the argon atoms as they pass by, and these tracks of electrons are drawn to planes of charged wires inside the detector. Scientists study the tracks to learn about the neutrino that kicked everything off.

Rubbia himself spearheaded the effort to make use of liquid argon as a detection material more than 25 years ago, and that same technology is being developed for the future Fermilab neutrino physics program.

“This is an exciting moment for ICARUS,” said scientist Claudio Montanari of INFN Pavia, who is the technical coordinator for ICARUS. “We’ve been working for months choreographing and carrying out all the steps involved in refurbishing and installing it. This move is like the curtain coming down after the entr’acte. Now we’ll get to see the next act.”

ICARUS is one part of the Fermilab-hosted Short-Baseline Neutrino program, whose aim is to search for a hypothesized but never conclusively observed type of neutrino, known as a sterile neutrino. Scientists know of three neutrino types. The discovery of a fourth could reveal new physics about the evolution of the universe. It could also open an avenue for modeling dark matter, which constitutes 23 percent of the universe’s mass.

ICARUS is the second of three Short-Baseline Neutrino detectors to be installed. The first, called MicroBooNE, began operating in 2015 and is currently taking data. The third, called the Short-Baseline Near Detector, is under construction. All use liquid argon.

Fermilab’s powerful particle accelerators provide a plentiful supply of neutrinos and will send an intense beam of the particle through the three detectors — first SBND, then MicroBooNE, then ICARUS. Scientists will study the differences in data collected by the trio to get a precise handle on the neutrino’s behavior.

“So many mysteries are locked up inside neutrinos,” said Fermilab scientist Peter Wilson, Short-Baseline Neutrino coordinator. “It’s thrilling to think that we might solve even one of them, because it would help fill in our frustratingly incomplete picture of how the universe evolved into what we see today.”

The three Short-Baseline Neutrino experiments are just one part of Fermilab’s vibrant suite of experiments to study the subtle neutrino.

NOvA, Fermilab’s largest operating neutrino experiment, studies a behavior called neutrino oscillation. The three neutrino types change character, morphing in and out of their types as they travel. NOvA researchers use two giant detectors spaced 500 miles apart — one at Fermilab and another in Ash River, Minnesota — to study this behavior.

Another Fermilab experiment, called MINERvA, studies how neutrinos interact with nuclei of different elements, enabling other neutrino researchers to better interpret what they see in their detectors.

“Fermilab is the best place in the world to do neutrino research,” Wilson said. “The lab’s particle accelerators generate beams that are chock full of neutrinos, giving us that many more chances to study them in fine detail.”

The construction and operation of the three Short-Baseline Neutrino experiments are valuable not just for fundamental research, but also for the development of the international Deep Underground Neutrino Experiment (DUNE) and the Long-Baseline Neutrino Facility (LBNF), both hosted by Fermilab.

DUNE will be the largest neutrino oscillation experiment ever built, sending particles 800 miles from Fermilab to Sanford Underground Research Facility in South Dakota. The detector in South Dakota, known as the DUNE far detector, is mammoth: Made of four modules —  each as tall and wide as a four-story building and almost as long as a football field — it will be filled with 70,000 tons of liquid argon, about 100 times more than ICARUS.

The knowledge and expertise scientists and engineers gain from running the Short-Baseline Neutrino experiments, including ICARUS, will inform the installation and operation of LBNF/DUNE, which is expected to start up in the mid-2020s.

“We’re developing some of the most advanced particle detection technology ever built for LBNF/DUNE,” Brice said. “In preparing for that effort, there’s no substitute for running an experiment that uses similar technology. ICARUS fills that need perfectly.”

Eighty researchers from five countries collaborate on ICARUS. The collaboration will spend the next year instrumenting and commissioning the detector. They plan to begin taking data in 2019.

In 1998, scientists discovered that the universe’s expansion is accelerating. Physicists don’t know how or why the universe is accelerating outward, but they gave the mysterious force behind this phenomenon a name: dark energy.

Scientists know a great deal about the effects of dark energy, but they don’t know what it is. Cosmologists approximate that 68 percent of the universe’s total energy must be made of the stuff. One way to get a better handle on dark energy and its effects is to create detailed maps of the universe, plotting its expansion. Scientists, engineers and technicians are currently building the Dark Energy Spectroscopic Instrument, or DESI, to do just that.

DESI will help create the largest 3-D map of galaxies to date, one that will span a third of the entire sky, stretch back 11 billion light-years, and record approximately 35 million galaxies and quasars.

It will measure the spectra of light emanating from galaxies to determine their distances from Earth. Other surveys have created maps that locate galaxies’ lateral positions in the sky, but scientists using DESI will be able to take more precise measurements of their distance from us, creating high-resolution, 3-D maps.

DESI is currently being installed at the Mayall 4-Meter Telescope at Kitt Peak National Observatory in Tucson, Arizona. Once installation is complete, it will run for five years.

The DESI project is managed at the U.S. Department of Energy’s Lawrence Berkley National Laboratory (Berkeley Lab) in California, and the U.S. DOE’s Fermilab is contributing to the ambitious effort with specialty systems for collecting and analyzing the galactic light.

“The collaborative effort to build DESI is an example of how science draws on expertise from multiple institutions toward a common goal, one that humanity is always moving toward: understanding the fundamentals of our universe,” said Berkeley Lab’s Michael Levi, DESI project director.

One of the largest pieces Fermilab is contributing is the DESI corrector barrel. Fermilab collaborators designed, built and tested the barrel, which is roughly the size of a telephone booth. It plays a critical role: holding DESI’s six giant lenses in perfect alignment. To ensure spot-on precision, the barrel is designed so that the lenses are accurately positioned to within the width of a human hair. Collaborators at University College London recently finished installing the lenses in the barrel, and the whole ensemble will soon be lifted onto the telescope.

“The barrel needs to be extremely precise,” said Gaston Gutierrez, Fermilab scientist managing the corrector barrel construction. “If there is any misalignment of the lenses, the error will be highly magnified, and the images will be blurred.”

Fermilab also designed and built large structures that will support a cage surrounding the barrel. These were delivered to the Mayall in April, and their installation has begun.

To convert the light from galaxies into digital information for analysis, DESI will use high-tech versions of the familiar components in typical hand-held cameras — charge coupled devices, or CCDs. Fermilab packaged and tested these sensitive devices before delivering them to Tucson.

The job of collecting the galactic light belongs to DESI’s 5,000 fiber-optic cables, which will help record the spectra of each galaxy. For roughly 20 minutes, each one of the fibers will aim at a single galaxy and record its spectrum. Then the telescope will move to a new position in the sky, and all 5,000 fibers will be moved to point at new galaxies. Fermilab is developing the software that tells the instrument where in the sky to point those fibers. Without this automation, DESI would not be able to measure the millions of objects it plans to study.

To fully understand the spectra that DESI will collect, scientists need to keep detailed information about the instrument and telescope status. In addition to the DESI barrel, Fermilab is creating an electronic logbook and a database to store the instrument control systems operational data. These will be used to keep track of the information on the systems required to operate DESI, such as how to read the CCDs, direct the telescope and ensure the apparatus for recording the spectra is working properly.

DESI’s predecessor, called the Dark Energy Camera (DECam), is currently mounted on Chile’s Victor Blanco telescope, the sister telescope of the Mayall. In 2012, researchers and technicians completed DECam’s construction for use in the five-year Dark Energy Survey, hosted by Fermilab. The same scientists who designed DECam are bringing their expertise and knowledge to DESI.

The Dark Energy Survey and DECam serve as stepping stones to DESI. The DESI project will improve our understanding of the nature of dark energy by using the Dark Energy Survey’s results as a baseline. DECam’s data will also help DESI find the galaxies so the latter can take more precise spectra measurements to determine the galaxy’s redshift: The farther away a galaxy is from us, the more its light is stretched and shifted in the direction of redder (longer) wavelengths, by the expansion of the universe.

“For the Dark Energy Survey, we are just taking images, but for DESI we are pointing fibers at galaxies and measuring spectra,” said Fermilab’s Brenna Flaugher, project manager of DES and one of the leading scientists for DESI. “So, it is sort of the next level of resolution in redshift.”

DESI’s final pieces are planned to be installed by April 2019, with first light planned for May of that year.

“DESI will help us understand the nature of dark energy,” Flaugher said. “And that will lead to a better understanding of the evolution of our universe.”

Work on DESI is supported by DOE’s Office of Science along with several international partners.