ATAP News, April 2020

“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 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 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.

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 L_acc, both of which are limited by laser and plasma physics [1]. The field strength scales with the plasma density as E_z∼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. E_z 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 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.

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.

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.