Berkeley Lab

ATAP News, April 2020

Director’s Corner

As we move into the second month of the Bay Area’s shelter-in-place order (recently extended through May) and the Lab’s safe and secure status, I’d like to thank everyone for compliance with the need for remote work and social distancing.

At this writing, California has had some 41,000 confirmed cases of COVID-19… an alarming number, but remember that our state’s population of nearly 40 million is three orders of magnitude greater. Plainly we are doing a good job of protecting each other by staying home whenever we can, and by wearing cloth face coverings, paying attention to hygiene, and maintaining social distance when we must go out.

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Our leadership is just beginning evidence-based planning for how things might progressively re-open, and there is no way to predict how or when this will happen. It is safe to say that we will have to live and work in a new way for some time, and as Lab Director Mike Witherell pointed out in the recent online town hall, those of us who can telecommute should prepare to keep doing so for the foreseeable future. True, complete normalcy might not resume until a vaccine has been developed, proven safe and effective, and deployed widely.

Throughout, we all need to continue protecting each other.

California For All logo

Asked if the worst was over for California, Governor Gavin Newsom said, “If we all pull back, we could see a second wave that makes this pale in comparison. {…} Honestly, that’s determined by the act of 40 million Californians stepping in, continuing to meet this moment… “

Our research and development activities continue. In this issue of ATAP News, I would like to bring to your attention the progress our superconducting magnet program has helped bring to the LHC Luminosity Upgrade as part of a tri-lab collaboration, as well as an innovative electron source being developed into a unique materials probe together with the Molecular Foundry. And BELLA Center’s Jeroen van Tilborg was invited to write a perspective article on a recent breakthrough in laser-plasma acceleration.

I must end on a sad note: the passing of our colleague Max Zolotorev after a long illness. Max leaves a legacy of both physics insight and personal warmth that touched many of our lives and careers. His was truly the scientific life well lived.


FEATURED SCIENTIFIC RESEARCH

Three National Labs Team Up for Record-Setting HL-LHC Quadrupole Magnet

Adapted by Glenn Roberts, Jr., of Berkeley Lab Strategic Communications from a Fermilab original.

In a multiyear effort by three U.S. national laboratories, researchers have successfully built and tested a powerful new focusing magnet that represents a new use for niobium-tin, a superconducting material.

The U.S. Department of Energy’s Fermi National Accelerator Laboratory, Brookhaven National Laboratory, and Lawrence Berkeley National Laboratory (Berkeley Lab) designed, built, and tested the new magnet.

This video highlights magnet-making efforts to support the High-Luminosity Large Hadron Collider upgrade at CERN in Europe. Three U.S. Department of Energy national labs — Berkeley Lab, Fermilab, and Brookhaven National Laboratory — are building superconducting magnets that can produce far stronger magnet fields than the magnets now in place at the LHC. This will enable more particle collisions and data to help us learn more about exotic particles and their properties. (Marilyn Sargent/Berkeley Lab)
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The eight-ton device, about as long as a semitruck trailer, set a record for the highest field strength ever recorded for an accelerator focusing magnet, and raises the standard for magnets operating in high-energy particle colliders.

It is one of 16 that the three partners will deliver for operation in the High-Luminosity Large Hadron Collider (HL-LHC) at CERN in Europe, which is an upgrade of the existing LHC – already the world’s most powerful particle accelerator. The 16 magnets, along with another eight produced by CERN, will focus beams of protons to a tiny spot as they approach collision inside two different particle detectors. The U.S.-based team will also deliver four spare magnets.

Photo - a Magnet for the High-Luminosity LHC upgrade project. (Marilyn Sargent/Berkeley Lab)

This magnet was built by teams at Berkeley Lab, Fermilab, and Brookhaven National Laboratory for the High-Luminosity LHC upgrade project.
(Marilyn Sargent/Berkeley Lab)

In all, the upgrade will require 130 new magnets of 11 different types, produced by more than a dozen international partners. The project will replace about three-quarters of a mile of equipment at the LHC.

Niobium-tin is the ingredient that sets these U.S.-produced magnets apart. It is a superconducting material that produces strong magnetic fields. These will be the first niobium-tin quadrupole magnets ever to operate in a particle accelerator.

Superconductivity is a state achieved by extremely cooling the magnets to a temperature hundreds of degrees below freezing. In this state, the magnets can pass electrical current with virtually no electrical resistance in order to maintain a tight focus of the particle beams.

The LHC is already the planet’s most powerful particle accelerator, and its high-luminosity successor will smash together beams of protons cruising around the 17-mile ring at close to the speed of light.

The High-Luminosity LHC will pack an additional punch: With its more intense beams, it will provide 10 times the collisions that are possible at the current LHC. With more collisions there are more opportunities to uncover new physics. The new focusing magnets will help it achieve that leap in luminosity delivered to the experiments.

Berkeley Lab’s work is focused on winding wires into thin cables, measuring and analyzing those cables to ensure they meet exacting requirements, testing the quality of the magnetic fields generated by the cable-formed magnet coils, assembling the magnets into support structures, and ensuring their proper alignment and uniform compactness.

Giorgio Apollinari, head of the three-lab U.S. LHC Accelerator Upgrade Project and a scientist at Fermilab, said, “We’ve demonstrated that this first quadrupole magnet behaves successfully and according to design, based on the multiyear development effort made possible by DOE investments in this new technology.”

Kathleen Amm, Brookhaven Lab’s representative for the Accelerator Upgrade Project, said, “It’s a very cutting-edge magnet, really on the edge of magnet technology.”

The new magnets, with higher field strength than those of the existing LHC, can bring particle beams to a tighter focus, resulting in more collisions that generate more data.

Focus, magnets, focus

In circular colliders, two beams of particles race around a ring in opposite directions. An instant before they reach the collision point, each beam passes through a series of magnets that focus the particle beams into a tiny spot, much like a magnifying glass focusing light rays to a point. Now packed as tightly as the magnets can get them, the beams collide.

Even with the new magnets, most of the particles won’t collide; they continue their paths around the ring until they get another collision opportunity at the next detector. But many particles do smash into each other. That number, and the scientific fruitfulness of that smash-up, depends on how dense the beam is. The more particles that are crowded together at the collision point, the greater the chance of collisions.

You get those tightly packed beams by sharpening the magnet’s focus. One way to do that is to widen the lens.

Photo - Magnet testing at Berkeley Lab. (Marilyn Sargent/Berkeley Lab)

A magnet-testing instrument (center) is prepared for moving through the center of a magnet assembly at Berkeley Lab. (Marilyn Sargent/Berkeley Lab)

Consider the magnifying glass example: “If you try to focus the light from the sun using a magnifying glass at a small point, you want to have a more ‘powerful’ magnifying glass,” said Ian Pong, a Berkeley Lab scientist who oversees cable fabrication for the U.S. labs’ magnet effort. A larger lens has more light-gathering ability and stronger light-ray-bending power at its outer rim than a smaller lens.

In this analogy, the size of the lens is like a magnet’s aperture — the opening of the passageway the beam takes as it barrels through the magnet’s interior. If the beam is allowed to start wide before being focused, more particles will arrive at the intended focal point — the center of the particle detector. The U.S. focusing-magnet team widened the aperture to 150 millimeters, more than double the current LHC focusing magnet aperture of 70 millimeters.

But a wider aperture isn’t enough. There must be a strong magnetic field to actually focus the beam.

“The magnet has to squeeze the beam more powerfully than the LHC’s present magnets in order to create the luminosity needed for the HL-LHC,” Apollinari said.

To meet the demand, scientists designed and constructed a muscular focusing magnet. They calculated that, at the required aperture for the upgrade project, each focusing magnet would have to generate a field of between 11.4 and 12.4 teslas. This is up more than 50% from the 7.5-tesla field generated by the current niobium-titanium-based LHC magnets.

“So what do you do? You need to go to a different conductor,” Apollinari said.

Niobium-tin for the win

Magnet experts have been experimenting with one of those conductors, niobium-tin, for decades. Electrical current coursing through a niobium-tin superconductor can generate magnetic fields of 12 teslas – about 250,000 times stronger than the Earth’s magnetic field at its surface – and beyond.

But niobium-tin calls for entirely different magnet-construction techniques than the ones used with niobium-titanium, because the heat treatment that makes niobium-tin superconductive also makes it brittle.

“Once they’re reacted, it becomes a beautiful superconductor that can carry a lot of current, but then it also becomes brittle,” Apollinari said.

“If you bend it too much, even a little bit, once it’s a reacted material, it sounds like corn flakes,” Amm said. “You actually hear it break.” Niobium-titanium was easier to work with because it is pliable.

Over the years, scientists and engineers have figured out how to produce a niobium-tin superconductor in a form that is useful. Guaranteeing that it would hold up as an HL-LHC focusing magnet was another challenge altogether.

State-of-the-art physics and engineering makes the magnets

To produce large, intricate magnets that meet the demands of a collider, the group of three U.S. labs teamed up — each with its own areas of expertise and specialized facilities — under Fermilab’s leadership.

Photo - Spools of wire are wound into thinly pressed strips of cable at Berkeley Lab. (Marilyn Sargent/Berkeley Lab)

Spools of wire are wound into thinly pressed strips of cable at Berkeley Lab. The cable is wound to form niobium-tin magnet coils. (Marilyn Sargent/Berkeley Lab)

At Berkeley Lab, the magnet-making process begins by fabricating cable from 40 spools of wire — a copper matrix that contains niobium and tin. The machine-wound wires are rolled into thin, rectangular cables that must be defect-free and meet specifications within hundredths of a millimeter.

“Making these accelerator magnet cables is like directing 40 ballet dancers doing 5,000 pirouettes nonstop in a synchronized manner, where a single misstep would cost as much as crashing a few Tesla sportscars,” Pong said.

Scientists at Fermilab and Brookhaven wind these cables into coils, taking care to avoid excessively deforming them. Then comes a three-stage, weeklong heat treatment, causing a chemical reaction that makes the cables superconductive.

The magnet coils must be heated evenly, inside and out. “You have to control the temperature well. Otherwise the reaction will not give us the best performance,” Pong said. “It’s a bit like cooking. It’s not just to achieve the temperature in one part of the coil but in the entire coil, end to end, top to bottom – the whole thing.” Researchers make the coils strong and solid by embedding them in a resin.

It takes several months to yield a coil that is ready for service as one of a focusing magnet’s four poles. Together, the coils conduct the electric current that produces the magnetic field.

The magnet coils are assembled within an aluminum and steel support structure at Berkeley Lab to form a single magnet, and researchers test the magnetic fields produced by these coils and ensure that the support structure provides uniform pressure along the length of the coils.

Temporary water-pressurized metal bladders are used to provide tension to the support structure during assembly and alignment. The support structure is designed to withstand more than 20 million pounds of force during operation. Researchers use models and instruments to verify that the support structure can withstand the strength of the magnetic field.

Photo - This completed niobium-tin magnet coil will generate a maximum magnetic field of 12 tesla, about 50 percent more than the niobium-titanium magnets currently in Cern’s LHC. (Alfred Nobrega/Fermilab)

This completed niobium-tin magnet coil will generate a maximum magnetic field of 12 tesla, about 50 percent more than the niobium-titanium magnets currently in Cern’s LHC. (Alfred Nobrega/Fermilab)

“Because these coils are very powerful when they are energized, there is a lot of force trying to push the magnet apart,” Pong said. “Even if the magnet is not deforming, at the conductor level there will be a strain, to which niobium-tin performance is very sensitive. The management of the stress is very, very important for these magnets.”

Alignment of the four coils within each magnet is also critical to performance. “You need very high field precision, so we have to have very high precision in how they align these to get good magnetic-field uniformity,” Amm said.

The magnets are shipped to Brookhaven for testing, then to Fermilab for installation in their cryogenic containers, followed by another round of testing before shipment to CERN.

“This will be the first use of niobium-tin in focusing accelerator magnets, so it will be pretty exciting to see such a complex and sophisticated technology get implemented into a real machine,” Amm said. “These magnets are currently the highest-field focusing magnets in accelerators as they exist today.”

Pong added, “Finally we are coming to it, and we really want to make sure it is a lasting success.”

The many moving parts of an accelerator collaboration

Ensuring lasting success has as much to do with the operational choreography as it does with the exquisite engineering. Conducting logistics that span years and a continent requires painstaking coordination.

“For example, transportation communication: We have to make sure that things are well protected,” Pong said. “Otherwise these expensive items can be damaged, so we have to foresee issues and prevent them.”

Amm, Apollinari, and Pong acknowledge that the three-lab team has met the challenges capably, operating as a well-oiled machine.

“The technologies developed at Fermilab, Brookhaven, and Berkeley helped make the original LHC a success. And now again, these technologies out of the U.S. are really helping CERN be successful,” Amm said. “It’s a dream team, and it’s an honor to be a part of it.”

The team’s achievements are made possible by many years of R&D. The U.S.-based Accelerator Upgrade Project for the HL-LHC, of which the focusing-magnet project is one aspect, began in 2016. It grew out of a 2003 predecessor, the U.S. LHC Accelerator Research Program (LARP), that developed LHC-related accelerator technology. And these efforts drew upon past experience in pushing the frontiers of magnet and materials performance for a variety of applications.

From now until about 2025, the U.S. labs will continue to build the LHC magnets, from fine strands of niobium-tin to the hulking finished products. In 2022 they plan to begin delivering the magnets to CERN. Installation is planned in the following three years.

“People say that ‘touchdown’ is a very beautiful way to describe the landing of an airplane, because you have a huge metal object weighing hundreds of tons, descending from the sky, touching a concrete runway very gently,” Pong said. “Our magnets are massive superconducting devices, focusing tiny, invisible particle beams that are flying close to the speed of light through the bore. It’s quite magical.”

The magic starts in 2027, when the High-Luminosity LHC comes online.

The work, Apollinari said, is “a generational passing of the baton.”

“The upgrade project exemplifies what can be achieved through co-operation among the laboratories,” said Associate Laboratory Director James Symons. “The U.S. contribution to the luminosity upgrade has been made possible by the very successful LARP collaboration, which developed the design concept over the past decade.”

This work is supported by the DOE Office of Science.

To Learn More…

•    View the original Fermilab release.
•    CERN Ceremony Marks Groundbreaking for Accelerator Upgrade Involving U.S. National Labs
•    Successful Test of New U.S. Magnet Puts Large Hadron Collider on Track for Major Upgrade
•    An Advance in Superconducting Magnet Technology Opens the Door for More Powerful Colliders
•    World Record Magnet 300,000 Times Strength of Earth’s Magnetic Field



A Next-Generation Electron Source: Multi-Facility Collaboration Hits the Bullseye

—Brooke Kuei, Molecular Foundry
Selected as a Department of Energy Science Highlight on August 21, 2020

Conceptual diagram of bullseye lens

A schematic describing the geometry of the bullseye lens, showing the grating period (p), groove width (w), groove depth (d), and the center plateau radius (r)

Scientists have been using electrons to probe the structure of materials for decades. In recent years, researchers have manipulated electron beams to become small enough to study materials at the scale of individual atoms — and to pulse fast enough to capture subtle atomic movements.

However, it has been difficult to generate a beam that is both small and fast.

In a collaboration between Berkeley Lab researchers in the Accelerator Technology and Applied Physics Division (ATAP) and the Molecular Foundry, scientists have created a new kind of electron source that has the potential to overcome this hurdle.

The work, which was published Nov. 25, 2019, in Physical Review Applied, demonstrates the potential for a source made from a bullseye plasmonic lens that fires electrons as quickly as existing ultrafast electron beams but with a beam size that is hundreds of times smaller.

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“This research is the first part of a more complex and intriguing instrument that I’m thinking about, which will allow us to create ultrafast bursts of electrons emitted from a very small and confined space,” said Daniele Filippetto, a staff scientist at Berkeley Lab’s Accelerator Technology and Applied Physics Division and corresponding author of the study. Such ‘ultrafast’ bursts are quicker than the fastest chemical reaction ever studied.

Typical beam probes are relatively large; “it means your sample has to look the same over this entire field of view,” said Andrew Minor, director of the National Center for Electron Microscopy, a facility within the Molecular Foundry, and professor at UC Berkeley. “This works well for materials like graphene sheets that look the same everywhere, but we need a much smaller beam to look at things like individual nanowires and nanoparticles.” In materials with multiple components, this large beam size also prevents scientists from looking at the interfaces between different materials.

Atomic force microgram of bullseye lens

Atomic force microscopy (AFM) measurements of the surfaces of the bullseye lenses made using the typical focused ion beam method (left) and the new e-beam lithography method (right)

The electron source developed in this study is a gold surface with concentric circular grooves arranged in a bullseye pattern. When a laser hits the gold, the laser transfers energy to surface plasmons — electrons on the surface of the gold that move collectively — and the grooves cause them to ripple inward and outward. When the waves simultaneously hit the center of the bullseye, a small but intense beam of electrons is fired out from the center.

The problem with more conventional ultrafast electron sources is that the size of the beam when it hits the sample is approximately the width of a human hair, which is enormous compared to the nano-scale structures studied at the Molecular Foundry.

“It’s like when you throw a stone into a lake,” explained Filippetto. “You see these waves, but a few seconds after you throw that stone, you see a peak of water coming out where you have interference of the waves.”

The response time of the source is less than 10 femtoseconds (the time it takes for light to travel 3 micrometers), while its size is approximately 100 nanometers (approximately the width of a cell membrane).

Scanning electron microgram of bullseye lens

Scanning electron microscopy (SEM) images of the bullseye lenses made using the typical focused ion beam method (left) and the new e-beam lithography method (right)

For the emitted beam to be as bright as possible, the surface has to be extremely flat. In this case, an atomically flat surface was made using a new nanofabrication technique developed at the Molecular Foundry.

“A couple of years ago, we developed a method to make very smooth surfaces of gold with a peel-off process,” explained Stefano Cabrini, director of the Nanofabrication Facility at the Molecular Foundry. “So we’re able to put a pattern on silicon, deposit the gold onto the silicon, and then peel off the gold. We used this process on the bullseye and it worked.”

Although the electron source is designed to shoot out electrons when it is hit with light, its ability to do so was tested using the inverse technique: By shining electrons onto the source, the researchers were able to confirm its promising plasmonic behavior by measuring how light came out of it.

We have a very good collaboration between a group of people who understand each other, and we all put our best efforts forward.
— Daniele Filippetto

The key to the success of this project was the collaboration between different facilities within Berkeley Lab, Filippetto noted.

“Real breakthroughs come when you’re working at the boundaries between different disciplines,” he said. “This project wouldn’t have been possible without the Molecular Foundry. We have a very good collaboration between a group of people who understand each other, and we all put our best efforts forward. We’re seeing the results now.”

The possibility of an ultrafast, tiny electron beam opens up the doors for understanding chemical reactions and structural changes at molecular length scales, particularly for materials that do not look the same over a large area — such as nanoparticles — or for interfaces between different materials.

Moving forward, the team is working on improving the electron source with an improved design. The team also plans to measure electron yield and brightness in a test system, with the final goal of using the source as electron emitter in the HiRES beamline, an ultrafast electron scattering beamline at the Advanced Light Source.

Other directions include designs of nanostructure arrays for emission of large electron currents, exploring the temporal response of the structure to push to sub-femtosecond resolution, and plasmonic-based electron acceleration.

The Molecular Foundry is a DOE Office of Science User Facility.


NEWS IN BRIEF

Lining Up for Laser Plasma Acceleration

On March 31, 2020, Physics Review Letters published a paper by Palastro and colleagues from the University of Rochester, where a dephasingless laser-wakefield accelerator (DLWFA) was proposed. This proposed design for plasma accelerators would use line-focused laser pulses to overcome the problem of particles outrunning the acceleration region. To highlight this work, the American Physical Society asked Jeroen van Tilborg of ATAP Division’s Berkeley Lab Laser Accelerator (BELLA) Center to write a Viewpoint perspective, reproduced here, in their online magazine Physics. The DLWFA presents a clever way of shaping the spatial and temporal laser energy delivery onto a plasma, aimed at realizing a compact (room-size) high-energy electron accelerator. This goal is very much in-line with research in BELLA Center, where, over recent years, key community milestones on laser-plasma acceleration were demonstrated. This Viewpoint article and Palastro et al., Physical Review Letters 124, 134802 (31 March 2020), will help in appreciation of the various approaches being employed to pursue the same objectives: the compact production of multi-GeV electron beams and the development of novel “table-top” radiation sources.

Conceptual diagram of dephasingless wakefield acceleration

The proposed dephasingless wakefield acceleration would utilize two optical elements: an echelon (blue) and an axiparabola (green). A laser pulse would first strike the echelon, producing a series of time-staggered rings. The axiparabola would then focus these rings onto a line, consisting of multiple Rayleigh-length segments. This spatiotemporal laser control could generate a wakefield that travels at the speed of light, overcoming one of the main limitations to wakefield acceleration. (Illustration courtesy APS/Alan Stonebraker)

 

One way to accelerate particles is to fire an intense laser pulse into a plasma, creating a density wake whose electric field pushes charged particles like electrons to high speeds [1]. The accelerating gradient from such a plasma wakefield is much higher than can be achieved in conventional radio-frequency-based technology. However, a central difficulty with employing laser wakefield accelerators (LWFAs) is that the electrons eventually outrun the accelerating region of the laser-driven wakefield. Because of this dephasing, a single stage of wakefield acceleration is typically limited to a few tens of centimeters in length, and it is forced to operate at a low density, which constrains the accelerating gradient.

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Several ideas have been proposed to deal with this dephasing limit. A new idea from John Palastro and colleagues from the University of Rochester, New York, uses special optical components to focus the laser into points along a line, thus extending the wake region so electrons are accelerated for longer and at a higher gradient [2]. This method faces some hurdles, such as increasing the laser power above what’s currently available, but it has the potential to accelerate electrons to TeV energies over just a few meters.

The high gradients offered by laser wakefield acceleration open the path for tabletop accelerators at universities, hospitals, and smaller R&D labs, with applications such as x-ray bio-imaging, active nuclear interrogation, and medical treatments. LWFA also enables progress towards future TeV-scale particle colliders that promise to extend our understanding of the basic structure of the Universe [3]. In the past 20 years, the LWFA community has evolved from small-scale proof-of-principle demonstrations to milestones in the production of beams with high current, low emittance, and narrow (percent-level) energy spread. Precision control of the laser-plasma interaction with multiple lasers or density profile shaping is a central thrust in ongoing research. Efforts are also underway to increase the number of laser shots from a few per second to more than a thousand per second, which will benefit stabilizing feedback procedures and applications that require high flux.

The maximum achievable electron energy for LWFA is determined by the accelerating field strength and the length of acceleration Lacc, both of which are limited by laser and plasma physics [1]. The field strength scales with the plasma density as Ez∼n1∕2. At currently used densities (1016–1019 cm−3), the field strength can exceed 10–100 GV/m, which is about a thousand times the acceleration in traditional radio frequency cavities used at places like CERN. Ez also depends on the laser, whose intensity is determined by the laser energy, wavelength, pulse duration, and spot size.

The effective accelerator length depends on the laser as well. Specifically, the acceleration is limited to the region over which the laser remains focused. In vacuum, this focus region is characterized by the Rayleigh length zR, which is calculated with the laser wavelength and spot size. For most setups, the Rayleigh length is in the millimeter to centimeter range, but researchers can extend the focusing region by adding light-guiding structures, as was demonstrated in a recent LWFA experiment that achieved 8-GeV acceleration energies with a 20-cm-long waveguide [4]. However, guiding structures are not without challenges, including the need for precise alignment and transverse mode control to avoid damage from the high-power laser pulses.

Guiding structures also don’t alleviate the problem of dephasing. Dephasing occurs because the wakefield region travels behind the laser at a speed slower than the vacuum speed of light, so electrons accelerated to relativistic energies will eventually get ahead of it. As such, the wakefield speed places an upper limit on the acceleration length, which scales with density as n−3∕2. To increase this limit—and correspondingly increase the maximum electron energy—traditional setups use a low plasma density.

Recently, a group proposed a scheme to overcome this dephasing limit by obliquely intersecting two tilted-pulse-front lasers [5]. The interference between the lasers would generate a wakefield region that travels at the speed of light in vacuum. However, this approach requires precision control of two lasers, with potential challenges from transverse wakefield asymmetries.

Palastro et al. [2] have a similar idea for speeding up the wakefield region that uses a single laser and special optics rather than interfering lasers. Specifically, their proposed method combines an echelon (a mirror with specially designed steps) and an axiparabola (a recently developed curved mirror) [6]. As the team conceives it, a laser pulse would first strike the echelon, which divides the light into a number of concentric rings (Fig. 1). These rings would be separated in time, such that the outer rings arrive at the axiparabola ahead of the inner rings. The curved reflective surface of the axiparabola would focus the rings at successive points along a line. This spatiotemporal shaping of the laser pulse offers a way to generate a wakefield traveling at the vacuum speed of light, which would circumvent dephasing.

To see how this “dephasingless” LWFA compares to previous schemes, we can imagine the system is tuned so that each ring produces the same acceleration effect as a single laser pulse in the traditional nonguided LWFA. In other words, each ring generates a focal segment that is one Rayleigh-length long, and if there are N rings, the total acceleration length would be N times the Rayleigh length. In addition to increasing the acceleration length, dephasingless LWFA could operate at high plasma density, which means a higher accelerating gradient. The bottom line is that dephasingless LWFA could potentially reach higher electron energies than previous methods with the same acceleration length.

One potential hurdle to this proposal is the laser energy requirements. If each of the N rings is to produce a separate wakefield, then the single laser pulse would need roughly N times more energy than in a traditional LWFA of comparable laser intensity. For TeV-scale electron acceleration, the number of rings would need to be in the thousands, and the corresponding femtosecond-duration laser energy would be multiple kilojoules, which may pose a challenge in terms of the availability of laser systems and power consumption limitations (especially for kHz operation). There are other issues that may require further investigation, such as how well does the line-focus scale to large ring numbers, and how will the spatiotemporal laser evolution affect wakefield generation and laser-electron overlap. In addition, questions remain over the inverse accelerated charge scaling with plasma density [7].

Despite these uncertainties, Palastro et al. have provided a clever variation of staged acceleration, with independently timed laser “beamlets” driving acceleration without the need for intrastage optics or complex guiding structures. As such, dephasingless LWFA enables new opportunities in the choice of density and laser-plasma accelerator architecture. This could be of particular importance for leveraging future multipetawatt lasers to compactly create high-energy electron beams that can probe nonlinear quantum electrodynamics [8]. Next on the dephasingless LWFA to-do list: performing detailed simulations of laser delivery and wakefield production at multi-GeV scale and testing the waters regarding practical limitations.

Their research was published in Physical Review Letters 124, 134802 (March 31, 2020).

References

E. Esarey et al., “Physics of laser-driven plasma-based electron accelerators,” Rev. Mod. Phys. 81, 1229 (2009).
J. P. Palastro et al., “Dephasingless laser wakefield acceleration,” Phys. Rev. Lett. 124, 134802 (2020).
W. P. Leemans and E. Esarey, “Laser-driven plasma-wave electron accelerators,” Phys. Today 62, No. 3, 44 (2009).
A. J. Gonsalves et al., “Petawatt laser guiding and electron beam acceleration to 8 GeV in a laser-heated capillary discharge waveguide,” , Phys. Rev. Lett. 122, 084801 (2019).
A. Debus et al., “Circumventing the dephasing and depletion limits of laser-wakefield acceleration,” Phys. Rev. X 9, 031044 (2019).
S. Smartsev et al., “Axiparabola: A long-focal-depth, high-resolution mirror for broadband high-intensity lasers,” Opt. Lett. 44, 3414 (2019).
W. Lu et al., “Generating multi-GeV electron bunches using single stage laser wakefield acceleration in a 3D nonlinear regime,” Phys. Rev. Special Topics: Accelerators and Beams 10, 061301 (2007).
A. Di Piazza et al., “Extremely high-intensity laser interactions with fundamental quantum systems,” Rev. Mod. Phys. 84, 1177 (2012).

About the Author

Jeroen van Tilborg

Jeroen van Tilborg is an experimental staff scientist at the BELLA Center at Lawrence Berkeley National Laboratory (LBNL), California. Jeroen moved to Berkeley in 2001 for a combined Ph.D. program from LBNL and the Eindhoven University of Technology, Netherlands. For his work on femtosecond bunch length measurements he received the outstanding thesis award from the APS Division of Physics of Beams. Following a postdoctoral appointment in LBNL’s chemistry division (studying molecular dynamics), he returned to the BELLA Center in 2009. In 2016 Jeroen received a five-year DOE Early Career Research Program grant, which is funding his current pursuit of laser-plasma accelerator applications towards novel radiation sources.



 

In Memoriam: Max S. Zolotorev, 1941-2020

By Dmitry Budker, Roger W. Falcone, Derek F. Jackson Kimball, Fernando Sannibale, Valeriy V. Yashchuk, and Alexander Zholents

Max Zolotorev

Photo courtesy Felix Izrailev

Retired Berkeley Lab senior scientist Max Samuilovich Zolotorev, a Fellow of the American Physical Society, a pioneer of experimental studies of atomic parity violation, and a generator of ideas across the spectrum of modern physics, passed away on April 1st, 2020 in his home in Eugene, Oregon.

Max was born on October 27, 1941 in Petrovsk, a small town not far from the Russian city of Saratov (on the Volga River), where his mother found herself evacuated from the advancing German army. Max grew up in Kiev, Ukraine. Upon graduating from secondary school, despite showing unusual talent and ability from an early age, since he was Jewish he was not admitted to an institute or even a vocational school. With a friend, he then ventured to Tselina — as part of the Soviet virgin-steppe agricultural development campaign. Upon returning, he worked for about three years at a musical-instrument factory in Kiev, after which, in 1961, he tried his luck in Siberia with the Novosibirsk Electro Technical Institute, where this time he was accepted. After demonstrating outstanding academic performance in his first year, he was able to transfer to the newly founded Novosibirsk State University, from which he graduated in 1966 and assumed a research position at the Institute of Nuclear Physics of the Siberian Branch of the Academy of Sciences of the USSR. There he obtained his first and second doctoral degrees in 1974 and 1979, respectively.

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Max started his career as a particle physicist working on measurements of the hyperon magnetic moments. However, in the early 1970s, following a proposal by theorist Iosif B. Khriplovich (who had been four years ahead of Max in the same elementary school in Kiev, and had the same teacher), Max was drawn into studying fundamental physics using the methods of atomic, molecular, and optical physics. Together with his mentor and colleague Lev M. Barkov, he was the first to discover parity violation in atoms by observing optical rotation of the plane of polarization of light propagating through a bismuth vapor. Atomic parity violation — a consequence of the neutral weak interaction between electrons and nuclei — is a key prediction of the Glashow-Weinberg-Salam electroweak unification theory, the core of what is known today as the standard model of particle physics.

Zolotorev and Barkov’s 1978 measurement came at a crucial time in the history of the standard model. While observations of high-energy neutrino scattering on nuclei at CERN in 1973 provided evidence of neutral weak currents, there was no evidence that the neutral weak current violated parity, as predicted by the Glashow-Weinberg-Salam model, at the time of Zolotorev and Barkov’s experiment. Furthermore, earlier atomic parity violation experiments had produced null results, in contradiction to theoretical predictions. The observation of parity violation in bismuth, followed later by measurements of parity violating electron scattering at SLAC and measurements of atomic parity violation in thallium by Eugene Commins and colleagues at the University of California at Berkeley, was crucial evidence that the Glashow-Weinberg-Salam theory was indeed the correct description of the electroweak interaction.

Max and colleagues also established the foundation for some of today’s most sensitive magnetometers with their measurements in the late 1980s of nonlinear Faraday rotation, clearly identifying the crucial role of quantum coherences.

In 1989, Max and his family emigrated from the USSR and, with support from Max’s friend and future collaborator Eugene Commins, found their way to California (via Austria and Italy). After a brief appointment at UC Berkeley, Max assumed a research position at the Stanford Linear Accelerator Center (SLAC). In 1996, he moved to Berkeley Lab, where he worked in the Accelerator Technology and Applied Physics Division until his retirement in 2018.

Upon arriving at SLAC, Max proposed using lasers for cooling hadrons in colliders (a revolutionary idea for that time) as a variation on van der Meer’s stochastic cooling method. Max was the first to foresee that a laser working in tandem with magnetic undulators would be capable of broadening the bandwidth of van der Meer’s microwave system by a factor of a thousand, correspondingly reducing the cooling time. The “optical stochastic cooling” concept formulated by Max, together with Alexander Mikhailichenko and Alexander Zholents, will soon be put to a test at Fermilab by a group led by Max’s former Novosibirsk University student Valeri Lebedev.

Another of Max’s major inventions (in collaboration with Zholents) is the “slicing method” to produce ultrashort pulses of x-rays, essential for time resolved studies of the properties of condensed matter. In slicing experiments, an ultrashort laser pulse “tags” a portion of an electron bunch circulating in a storage ring, and this results in emission of correspondingly short x-ray pulses when the electrons propagate in a periodic magnetic structure. Joined by Robert Schoenlein and other LBNL colleagues, they were first in the world to obtain ~100 femtosecond x-ray pulses with appreciable intensity. Later on, similar capabilities were developed at x-ray facilities in Germany and Switzerland.

Max was an inspiring mentor and teacher who always set the highest expectations for his students. He taught at Novosibirsk University, and had several research students (including Dmitry Budker). Later on, he played a pivotal role in “bringing up” many of his “scientific grandchildren” (including D. F. Jackson Kimball). While working at SLAC and LBNL, Max actively collaborated with Budker’s group at UC Berkeley (including Valeriy V. Yashchuk) and often visited their lab, typically over the weekend. His teaching and mentoring occurred during these visits. His ability to find “weak spots” in one’s scientific logic was legendary.

ATAP senior scientist Fernando Sannibale describes Max as “a reference standard for me, not only as a scientist but also as a person.” Max was one of those (extremely rare) physicists with extremely broad knowledge comprising pretty much all fields of physics, able to verify, in ten minutes on his office board, if an idea was feasible, and to estimate (within a factor of 2, as he usually remarked) all the consequent results. He did this with warmth and a sense of humor, not only clarifying a physics question, but wrapping it in a joke or a whimsical story.

One particularly memorable dialog between Max and a student started with Max announcing: “Physicists are 3% of rats.” After pondering this for a few moments, the student replied, “Max, what do you mean?” “Look. They did experiment. They put rats in a cage with a high voltage electrode. 27% of rats touch the electrode one time, get shocked, never touch again. 70% of rats watched 27% of rats touch electrode, never touch in first place. But 3% of rats go up to electrode, touch from bottom, get shocked. Then they touch from side, get shocked. Then they touch from other side, get shocked. Then they touch from top, get shocked. They try to figure out what is going on. They are physicist rats.”

One of Max’s great insights was that as physicists, we should never design our experiments around what was sitting in our labs or in our heads. Max would remind us that “We should not be tied to iron!” We should choose deep and important problems, think hard about them, and develop the cleverest way to approach them that we can, learn new subjects, build new apparatus, and push our boundaries and limits as far as we can. Max’s work exemplified the curiosity, creativity, and rigor of physics at its best.

Max Zolotorev is survived by his wife of 55 years Alya, their children Irina and Yakov, and grandchildren Gersh and Giora.



US Plasma Science Strategic Planning Reaches Pivotal Phase

Cover of A Community Plan for Fusion Energy and Discovery Plasma SciencesUS efforts in fusion energy and other aspects of plasma sciences took a key step forward this month with the publication of A Community Plan for Fusion Energy and Discovery Plasma Sciences.

Organized by the American Physical Society’s Division of Plasma Physics, the effort is part of a decadal study, modeled upon the successful community-based consensus-building processes that yielded the 2014 Particle Physics Project Prioritization Panel (“P5”) report for high-energy physics and the 2015 Long-Range Plan for Nuclear Science. The report will serve as input for a subcommittee of the Fusion Energy Sciences Advisory Committee (FESAC) tasked with advising the DOE’s Office of Fusion Energy Sciences.

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ATAP has a long history of diverse R&D in these areas. Four of our people were lead authors of initiatives and white papers that served as input to the process and provided content to the Strategic Plan:

•  Xiaorong Wang, Superconducting Magnet Program, “National Fusion Magnet and Conductor Development Program.”
•  Carl Schroeder, Berkeley Lab Laser Accelerator (BELLA) Center, “Physics and Applications of Ion Acceleration Driven by High-Repetition-Rate PW Lasers.”
•  Peter Seidl, Fusion Science & Ion Beam Technology Program, “Low-Cost, Scalable Power Plants Based on Heavy Ion Fusion.”
•  Jeroen van Tilborg, BELLA Center, “Light sources from Laser-Plasma Accelerators.”

Meanwhile, another consensus study is being conducted by the National Academies: the Decadal Assessment of Plasma Science takes into account fusion and other aspects of plasma science across the Federal agencies. It complements a recent National Academies study that focused on burning plasma research for fusion.



WORKPLACE LIFE

Stay In Touch with Online Seminars

Screen shot of Interdisciplinary Instrumentation Colloquium homepageFeeling a bit disconnected from your colleagues these days? You’re not alone, but dropping on the Lab’s online seminars can help. The ATAP Seminar series has brought Zoom talks by BELLA Center’s Sam Barber, Hai-En Tsai, and Tobias Ostermayr, and Fermilab’s Valeri Lebedev, thus far during April. Late March saw BELLA’s Cameron Geddes present an Interdisciplinary Instrumentation Colloquium, a series held the last Wednesday of the month (next up: Rikky Muller of UC-Berkeley on “Instrumenting the Brain,” April 29).

Employee resources for everything from morning stretches to career development are also available as virtual seminars. To expand your horizons, make new virtual friends, and, as Shelter-in-Place enters its second month, catch glimpses of your colleagues’ quarantine haircuts, add the ATAP and Labwide events calendars to your Google Calendar.

Make Your Online Meetings More Inclusive and Effective

Logo of LBNL's Inclusion, Diversity, Equity, and Accountability OfficeThe social dynamics of online interaction are different, and that includes making everyone feel welcome and valued and getting the most out of your team. The Lab’s Diversity, Equity, and Inclusion office has posted ideas about how to run inclusive and effective virtual meetings. Their recent virtual brown bag seminar on the subject was recorded and is available online.

The Employee Resource Group All Access join forces with the DEI Office for a virtual brown bag on “Remote Meeting Best Practices” at noon Tuesday, April 28. The event will be co-chaired by ATAP Outreach and Education Coordinator Ina Reichel and Facilities Division Senior Electrical Engineer Doug Burkhardt.

Logos of social media platforms

Show The World What Doing Science From Home Looks Like

Are you an adept social-media user? The Laboratory is encouraging staff to use these tools to reach out to the public and explain how science is done at home. Don’t forget to use the hashtags #LBNLwfh and #sciencefromhome.


RECENT PUBLICATIONS AND PRESENTATIONS

Please see the Publications tab of this website for a complete listing.

S. Steinke, J.H. Bin, J. Park, Q. Ji, K. Nakamura, A.J. Gonsalves, S.S. Bulanov, M. Thévenet, C. Toth, J.-L. Vay, C.B. Schroeder, C.G.R. Geddes, E. Esarey, T. Schenkel (LBNL); W.P. Leemans (DESY), “Acceleration of high charge ion beams with achromatic divergence by petawatt laser pulses”, Physical Review Accelerators and Beams 2, 021302 (19 February 2020), https://doi.org/10.1103/PhysRevAccelBeams.23.021302

C. Sun, Ph. Amstutz, T. Hellert, S.C. Leemann, C. Steier, C. Swenson, M. Venturini, “Optimizations of nonlinear kicker injection for synchrotron light sources”, Physical Review Accelerators and Beams 23, 010702 (3 January 2020), https://doi.org/10.1103/PhysRevAccelBeams.23.010702

S.R. Yoffe (Scottish Universities Physics Alliance, University of Strathclyde, and LBNL); R. Lehe (LBNL); B. Ersfeld, E. Brunetti, G. Vieux, A. Noble, B. Eliasson SSUPA, University of Strathclyde); M.S. Hur (UNIST); J.-L. Vay (LBNL); D.A. Jaroszynski (SUPA, University of Strathclyde); “Particle-in-cell simulation of plasma-based amplification using a moving window”, Physical Review Research 2, 013227 (28 February 2020), https://doi.org/10.1103/PhysRevResearch.2.013227

Invited talks without publication opportunity

A. Huebl, “GPU-Powered Particle-in-Cell Community Frameworks for Laser-Plasma Interaction,” Society for Industrial and Applied Mathematics PP20 Exascale Particle Mini-symposium (Seattle, WA, Feb. 12-15, 2020).

R. Lehe, “WarpX: An advanced electromagnetic Particle-in-Cell code,” 2020 DOE Exascale Computing Project Industry Council Deep Dive Workshop (via Zoom, March 10, 2020).

M. Thévenet, “WarpX: Electromagnetic particle-in-cell with adaptive mesh refinement for advanced particle accelerators,” Society for Industrial and Applied Mathematics PP20 Exascale Particle Mini-symposium (Seattle, WA, Feb. 12-15, 2020).


SAFETY: THE BOTTOM LINE

photo illustration of laptop user with wrist pain

[email protected]: Office Work Shouldn’t Hurt

As we settle in for the long haul in a remote workstyle, let’s make sure our ergonomics at home are as sound as our customary set-ups at the Lab.

Most important of all: please do not work directly on your laptop in the long term. An external keyboard, mouse, and monitor, all well positioned on a desk with a good chair, is far more healthy.

Ergonomic assessment is available online, and many ergo accessories can be drop-shipped to your home. (Note: this must be done by Divisional buyers; eBuy items can only be shipped to the Lab, and of course we are trying to avoid unnecessary site visits.)

The Berkeley Lab Ergonomics website has many resources that you can explore, including COVID-specific information, to make your home office safe and comfortable.

After reading all this, do you need to get up and move around? No matter where you are and how you’re equipped, that is a key aspect of being happy and healthy when doing computer work.

Ultraviolet light emitting diode

Consider the source: UV light intense and energetic enough to kill something as small and hardy as a virus can certainly damage living cells; eyes are especially vulnerable

Considering a UV Sanitizer? Select And Use It Safely

As people become worried about contracting the novel coronavirus from contaminated surfaces, sanitizers that use ultraviolet light have spiked in popularity.

These devices, which come in many physical forms, use UV-C radiation to “kill bacteria and viruses, along with just about anything else that is living,” as LLNL’s Sharon Cornelious and LBNL’s Greta Toncheva put it in the latest edition of LLNL’s Laser Safety News Letter (available in versions optimized for screen viewing and for printing).

UV sanitizers are best reserved for special laboratory needs. At home, handwashing with soap and warm water, and avoidance of touching your face unless you know your hands are clean, along with use of ordinary home and office cleaning products in the usual way, will offer safe and effective protection. If you do choose to buy one for home use, please familiarize yourself with its proper usage and safety precautions so that you and your family members (and pets and even houseplants) are protected.

For official use at the Lab, UV generators are subject to our safety rules for non-ionizing radiation. Before buying and using such devices for official purposes, update your Work Planning and Control activity and complete a Hazard Assessment, and ensure that the desired product meets our requirements. Contact Laser Safety Officer Greta Toncheva or Deputy LSO Robert Fairchild when planning your purchase.

Whether the topic is UV sanitizers or other purchases these days, buyer beware! Watch out for products that don’t meet the expected quality and safety standards, especially if the source is unfamiliar or the deal looks too good to be true… and keep an eye on your credit-card statements in search of unexpected charges.

CDC Advice on Prevention: Face Coverings, Social Distance, Handwashing

The virus that causes COVID-19 can spread from a person before they have symptoms. Take action to slow the spread by wearing a cloth face covering in public spaces, keeping at least 6 feet of physical distance, and frequently washing your hands.

For more information, visit cdc.gov/coronavirus

A cloth face covering can be made from items around your home, such as a scarf or cloth napkin. Make sure the covering reaches above your nose and below your chin.

The primary goal is to protect others by keeping your respiratory particles to yourself, in case you are infected (many COVID-19 patients have mild symptoms or none at all, especialy early on).

COVID Oops

Because we could all use a laugh these days…

Social distancing illustrated with alligator and hockey stick

Locally meaningful attempts to help people visualize six feet or two meters of social distance run the gamut from an alligator (Leon County, Florida) to a hockey stick (Canada, eh).

Shutterstock image of VW microbus with surfboard

Might a California equivalent involve the length of a surfboard, or the width of a car? (Why not both?)