This latest approval by DOE, known as Critical Decision 2 or CD-2, marks the completion of the preliminary design stage of the project. It also authorizes a $590 million budget and funding profile for the project, and outlines the scope and schedule.

In addition to brighter X-ray beams, the ALS Upgrade (ALS-U) project will enable the ALS to deliver light with a more ordered “coherent” structure – like evenly spaced ripples in a pond – that will better reveal nanoscale (billionths of a meter) details in complex chemical reactions and in new materials.

“For nearly three decades, the ALS has developed innovative X-ray tools and used these to support a world-renowned portfolio of user science and collaboration,” said ALS Director Steve Kevan. “The ALS upgrade will allow us to vastly sharpen our tools and to accelerate that work for several more decades as we learn to design chemical, material, and biological systems that will solve the pressing energy and environmental challenges we face.”

“I am proud of the talented engineers, scientists, technicians, and support staff who helped the Lab achieve CD-2 approval for the Advanced Light Source Upgrade project during these challenging times. This upgrade will make it possible for Berkeley Lab to continue its leadership in soft X-ray research for another 30 years – but none of that could happen without the ALS-U team’s hard work and continued commitment to the Lab mission,” said Berkeley Lab Director Mike Witherell.

Probing new materials at the nanoscale with brighter, more focused light

The ALS is a type of particle accelerator known as a synchrotron that generates extremely bright beams of light ranging from infrared through X-rays. There are only a few dozen synchrotron light sources worldwide.

The ALS’ light is directed through 40 highly specialized instruments called beamlines to experimental endstations, where scientists from around the world conduct simultaneous studies in fields ranging from materials science and biology to physics and chemistry. The facility is optimized for science conducted with lower-energy “soft” X-rays that have the ideal energy range to probe the chemical, electronic, and magnetic properties of materials.

For many experiments, the quality of the data collected depends on the number and regularity of light particles – known as photons – that can be concentrated in a small spot. The upgrade currently underway is intended to make the ALS the brightest storage ring-based source of soft X-rays in the world.

The ALS-U project will replace the electron storage ring, the part of the accelerator where light is produced. The ALS’ X-rays are produced by electrons that race around a ring 200 meters (600 feet) in circumference at nearly the speed of light. Along the way, powerful magnets steer and focus the electron beam to keep it on its circular orbit, while additional magnets bend the beam, generating a broad spectrum of light that’s guided through beamlines. Better focus of the electron beam translates to better focus of the light produced and higher-quality data about the samples being studied.

The new electron storage ring will leverage a next-generation magnet technology known as multibend achromats. Whereas today each one of the 12 arcs that make up the accelerator ring includes three bending magnets, after the upgrade each arc will include nine bending magnets, allowing for more precise steering and tighter focusing of the electrons.

As a result, X-ray beams that today are about 100 microns (thousandths of a millimeter) across – smaller than the diameter of a human hair – will be squeezed down to just a few microns after the upgrade.

These more precise beams will make possible many applications including the study of magnetic properties in multilayer data-storage materials at smaller scales and the observation of battery chemistry and other reactions as they occur. The increase in brightness will be akin to the crisp, clear resolution that comes from taking a photograph in vivid daylight versus the fuzzy image that results when the lighting is dim.

A particularly challenging feat of the upgrade will be building a second concentric ring, called an “accumulator,” inside the already-cramped concrete tunnels that house the storage ring. This unique feature, developed by Berkeley Lab scientists, is another critical aspect for better focusing the electron beam. It enables a technique called “on-axis, swap-out injection,” which allows the electron beam to be injected into the storage ring with minimal perturbation.

Whereas in today’s ALS the electron beam is injected from an initial accelerating ring called the “booster” directly into the storage ring, the upgraded ALS will use the accumulator ring as an intermediary between the booster and storage rings, squeezing the electron beam and preparing it to be injected into an extremely confined space while preserving its tight focus and coherence. In late 2019, the project received approval for the early procurement, construction, and installation of the accumulator ring so this critical piece of the project could be installed and commissioned before the facility is shut down for a year to replace the storage ring.

In addition to the replacement of the storage ring and construction of the accumulator ring, the ALS-U project will upgrade two existing beamlines and build two new beamlines with features optimized to take full advantage of the upgraded beam. The project will also provide for the realignment of existing beamlines and a seismic and shielding upgrade of the storage ring tunnel – all while leveraging approximately half a billion dollars in existing infrastructure.

ALS-U is the biggest construction project Berkeley Lab has undertaken in more than 30 years – the last being the construction of the ALS itself under the leadership of former Berkeley Lab Directors David Shirley and Charles Shank. Shirley served as Berkeley Lab Director from 1980 through 1989; Shank, from 1989 through 2004

“This is a big deal,” said ALS-U Project Director David Robin. “Having federal CD-2 approval during uncertain times is a major step.”

Progress under ‘new normal’

Last year, Berkeley Lab, like much of the world, was thrown into uncertainty when word of a deadly new coronavirus was gaining ground. When California Bay Area counties issued a stay-at-home order to slow the spread of COVID-19 on March 17, 2020, Berkeley Lab quickly transitioned to minimal staffing for essential services and curtailed operations at its scientific user facilities – including the Advanced Light Source.

Under this “new normal,” Robin said that he and Project Manager Roberta Leftwich-Vann decided that the No. 1 priority should be doing the best they could under the unusual circumstances of a pandemic to keep the project on track toward CD-2 review and approval – which, like all things in the times of COVID, wasn’t easy. Robin credits the project’s 100-person team of engineers, scientists, technicians, and support staff for the outstanding job they did in accomplishing all of this under very challenging circumstances.

When California’s stay-at-home orders extended into the summer, the ALS-U project team encountered yet another wrench thrown into their plans: The synchrotron’s summer shutdown – a scheduled “dark” period in preparation for the ALS upgrade project – was shortened, pushing some work into the future, creating a domino effect on the project’s schedule.

“As we approached CD-2, one of the tricky things we needed to get right was developing an informed estimate of potential cost-and-schedule risks represented by the COVID-19 pandemic,” Leftwich-Vann said. “This was new territory, of course, but the team came up with a good solution that helped us solidify our CD-2 cost and schedule: an estimate of future COVID risks based on our experience to date and incorporating likely outcomes based on the best available information.”

Robin said that the ALS-U design is slated for completion in 2022. After that, Berkeley Lab will apply for the next stage of the project: federal “CD-3” approval to start construction on the storage ring replacement, beamline upgrades, realignment, and construction, and begin the seismic and shielding upgrade of the concrete tunnel that houses the storage ring and the accumulator.

The iconic dome of the building that houses the ALS – which was designed in the 1930s by Arthur Brown Jr., the architect for the San Francisco landmark Coit Tower – will be preserved in the upgrade project. The dome originally housed an accelerator known as the 184-inch cyclotron.

“The upgrade will enable the Department of Energy to provide a research tool at Berkeley Lab that is really unique: highly coherent, bright beams of soft X-ray light to probe functional materials for new applications in energy, the environment, and health that isn’t yet possible,” said Robin. “It’s very exciting to think about how this upgrade could help researchers develop new materials and technologies that improve our way of life.”

The Advanced Light Source is a DOE Office of Science User Facility.

Berkeley Lab’s Ashley White contributed to this news release.

More on ALS-U:

• Overview: The Advanced Light Source Upgrade (ALS-U) project
• “Milestone in Advanced Light Source Upgrade Project Will Bring in a New Ring,” news release announcing CD-3A approval, published January 8, 2020
• “Toward a New Light: Advanced Light Source Upgrade Project Moves Forward,” news release announcing CD-1 approval, published September 26, 2018
• VIDEO: Toward a New Light: ALS Upgrade Project Moves Forward
• “Transformational X-Ray Project Takes a Step Forward,” news release announcing CD-0 (mission need) approval, published October 3, 2016

The Berkeley Lab Laser Accelerator (BELLA) Center has long been a leader the development of LPAs. Past results include the world record for the highest electron energies produced by an LPA (8 GeV obtained in 2019). Among the many applications pursued in the Berkeley Lab Laser Accelerator (BELLA) Center is the development of an FEL driven by an LPA. This effort is led by Jeroen van Tilborg, the Deputy Director for Experiments in the BELLA Center.

FELs are a particularly promising application of LPAs, because LPAs are capable of providing extremely high electron beam quality, which is characterized by the electron beam brightness (a measure of the electron beam density in 6D phase space). The high peak brightness required to drive the FEL micro-bunching process can intrinsically be met through self-injection into the sub-50-µm transverse and longitudinal plasma wave structure in an LPA, resulting in ~1 µm transverse and longitudinal beam sizes. This means that straight from the compact (plasma) source, (sub)micron emittances and kilo-ampere peak currents are generated, avoiding additional lengthy acceleration and beam-manipulation sections. The laser-plasma-accelerated electron beams and FEL pulses are also intrinsically synchronized with femtosecond precision to other pump-probe photon and particle sources.

As identified by the DOE Basic Energy Sciences Workshop on the Future of Electron Sources, the next generation of FELs will need orders of magnitude improvement in electron beam performance, phrased as “Evolutionary advances in the existing electron gun paradigm will provide increased beam brightness, extending the reach of existing X-ray and electron instruments. However, order-of-magnitude, or greater, improvements in electron beam brightness will require novel techniques outside the present electron gun paradigm.” This creates a need for new injectors to advance performance of facilities like LCLS. At the same time, the 2018 National Academy of Sciences, Engineering, and Medicine (NASEM) report Opportunities in Intense Ultrafast Lasers: Reaching for the Brightest Light identifies laser-plasma accelerators as promising novel technology to drive FELs. The need for compact plasma-based FELs is echoed in reports such as the 2021 Decadal Assessment of Plasma Science by NASEM, and the 2014 DOE HEP report Building for Discovery: Strategic Plan for U.S. Particle Physics in the Global Context by the Particle Physics Project Prioritization Panel (P5). Plasma-based FELs are thus seen as not only a unique light source in terms of compactness, intensity, and femtosecond pump-probe capabilities, but also as a key milestone demonstrator in the path towards brighter electron beams.

The promising future of plasma-based accelerators in general, and LPAs in particular, has spurred broad international interest. Just like the BELLA Center at LBNL, facilities at DESY in Germany, SOLEIL in France, ELI in the Czech Republic, INFN-Frascati in Italy, and SIOM in China, among others, have active LPA FEL programs. Owing to decades of a robust dedicated program, the BELLA Center at LBNL is recognized as one of the key leaders in the LPA community and in photon applications of LPAs. In parallel to plasma-based accelerator and photon-source development, a global push to higher repetition rate laser systems is underway, moving the high-power laser landscape from the few-Hz to kHz rates, and allowing the peak-power performance to be supplemented with high average powers. The aforementioned 2018 NASEM report on intense ultrafast lasers and the National Science Foundation-sponsored 2019 Brightest Light Initiative report highlight how this laser revolution is critical to future success. The reports also emphasize the impending loss of US leadership due to well-funded research programs abroad. Berkeley Lab has been active is setting up its own R&D platform to regain leadership on high-average-power ultrafast laser technology, pursuing both solid-state and fiber-based solutions. Collaboration between LBNL’s BACI program (Berkeley Accelerators Control & Instrumentation) and the BELLA Center, and partnerships including Lawrence Livermore National Laboratory and the University of Michigan, have been the basis of this effort.

While kHz-repetition-rate laser systems are under development, the BELLA Center is working with its current 1-5 Hz systems to pursue a two-fold approach towards the LPA FEL application: (1) Develop a dedicated program to further increase the beam brightness to serve as 100-MeV-class injector for both plasma-based accelerators and upgrades to conventional accelerators, and (2) work with the state-of-the-art LPAs available at BELLA, couple the beam to advanced transport and phase-space manipulation concepts, and make key demonstrations on FEL lasing. For the latter thrust, a dedicated LPA FEL facility was constructed, including the drive laser, electron beam line, and diagnostics. The team has commissioned this new beamline and is now conducting experiments towards FEL gain. This article presents the history of FEL development at BELLA Center and our vision of the future where such sources have the potential to both improve existing facilities and create compact new sources.

2016: the stars align

BELLA Center’s Jeroen van Tilborg had previously gained experience and expertise on LPA development, femtosecond beam diagnostics, and molecular dynamics studies using coherent XUV and soft X-rays, making him well-positioned to lead an in-house LPA FEL program. In 2016, with a 5-year Early-Career Research Program (ECRP) grant from the DOE Office of Basic Energy Sciences (BES), Jeroen found himself with both the means and opportunity to take Laser Plasma Accelerators (LPAs) into this new territory. The Gordon & Betty Moore Foundation played a critical role in the new experimental effort with a $2.4M grant dedicated to the procurement of the laser and beamline hardware. Furthermore, the common theme of advanced accelerator concepts allowed the FEL project to leverage long-term investments to the BELLA Center from the DOE High-Energy Physics (HEP) GARD program. These efforts were well supported by BELLA and ATAP leadership, including Cameron Geddes (ATAP Director), Eric Esarey (BELLA Center Director), and Carl Schroeder (BELLA Deputy for Theory). In particular, Carl was instrumental in developing the theoretical and conceptual framework for LPA FELs, including core concepts for mitigation of the intrinsic energy spread challenge.

Previously, pursuing the DOE Office of High Energy Physics (HEP) General Accelerator R&D (GARD) mission, successes in LPA “performance” experiments were measured in terms of beam energy and charge. This method of acceleration intrinsically produces photons, which could be used as beam diagnostics; for example, terahertz radiation from the plasma-vacuum boundary (as led by Jeroen in previous work) to measure the bunch length, and betatron X-ray emission during off-axis electron motion in the plasma to measure the beam source size. The newfound financial support from BES and the Moore Foundation made it possible to explore a revolutionary post-accelerator application: producing, stabilizing, transporting, and “re-conditioning” LPA electron beams to make them suitable for driving an FEL, a gold standard in ultrashort-pulse, high-intensity photon sources. With Jeroen as principal investigator and Carl providing theory support, the core scientific team was expanded: recent PhD recipient Sam Barber (UCLA) brought his expertise in accelerator and undulator modeling to the team, and PhD student Fumika Isono (UC Berkeley) took intellectual ownership of the laser system and advanced control capabilities. This core team is still together to this day, with Sam since promoted to Research Scientist, and Fumika just months away from graduation.

A dedicated beamline for an LPA FEL

The laser system, called “HTU” for “Hundred Terawatt Undulator,” was designed to incorporate years of lessons-learned into a major upgrade to previous systems. Single-mode fiber lasers were employed to permanently guide alignment and stabilization; the aggressive cryo-cooling amplifier requirements were replaced by a more-gentle water-cooled multi-amplifier design; the whole laser system was housed for the first time on a single optical table; vacuum chambers were upgraded with mechanical isolation from the chamber walls; and noisy and vibrating equipment was placed in adjacent rooms. A mid-system Pockels cell improved laser temporal contrast (including “live” monitoring), and a post-compressor deformable mirror optimized the laser mode at focus, all while the kHz front-end laser correlation to the amplified 5 Hz pulse was maintained by keeping the pump mode large on the amplification crystals. Advanced safety features kept people and equipment safe, and the BELLA Center advanced control system, developed by Anthony Gonsalves, was implemented to provide shot-tagged data acquisition and controls for over 50 experimental devices. The mechanical and electrical team did an outstanding job integrating all system components in a safe and robust manner. After 2-3 years of bringing all this new construction together, seeing the laser system operate and perform as envisioned represented a critical milestone in the project.

Left: activity in the new laser lab, with SULI intern Manfred Ambat in front presenting new data to van Tilborg. Just over two
years earlier this lab had been an empty storage room (top right). Bottom right: integration of a single-mode fiber tip
at the LPA target (gas jet with shock-driving blade), providing in-situ wavefront-sensitive alignment and fiducialization to the
post-LPA line.

Simulations for better-performing, better-understood LPAs

Meanwhile, using advanced LPA FEL simulation, optimized electron beam transport and phase-space manipulation concepts were integrated into an executable design. To work with point-source-like electron beams with larger divergence, and few-femtosecond beams with larger energy spread, conventional accelerator concepts needed to be re-evaluated. This modeling covered electron transport and collective effects from the LPA source to undulator, coupled to an FEL code. The new beamline concept included an active plasma lens (APL) or high-gradient quadrupole triplet for rapid capture of the diverging LPA electron beam, an electromagnet triplet for fine-tuned delivery of the e-beam into a strong-focusing undulator (an undulator with embedded quadrupoles for “electron-beam guiding”), five steering magnets at critical beamline locations, and a chicane to decompress the beam. The latter is critical for achieving percent-level energy spread in LPA electron beams: by decompressing the electron beam in a chicane (lengthening through chirping), the time-sliced energy spread is reduced to acceptable levels (at the small cost of decreasing the peak current), thus strongly benefitting LPA FEL performance. Simulations and design favored a two-phase approach for the new system: first, using 100 MeV electrons to produce 3-eV undulator photons for key FEL demonstrations, followed by 27-eV photons from 300-MeV electrons for an extreme-ultraviolet (XUV) FEL. The insight gained from these groundbreaking campaigns would then lay the foundation for a future X-ray LPA FEL concept.

Layout of LPA undulator beam line (from right to left). The zoomed-in inset shows the LPA source, with the electron beam going left towards the Visible-infrared SASE Amplifier (VISA) undulator.

With a design in place, construction of the beamline started in the shielded A and B caves at the BELLA Center in 2018. The chicane itself was commissioned through a collaboration with Prof. Rosenzweig’s group at UCLA, whose team recognized the unconventional requirements for the LPA FEL chicane: instead of needing a large energy dispersion for small transverse e-beams, the LPA FEL beams need the opposite: a small energy dispersion (since the beam is ultrashort to start with) but for larger-size beams. Through a DOE Office of Science Graduate Student Research (SCGSR) fellowship, UCLA graduate student Nathan Majernik worked out an optimized design with curved poles, and oversaw the fabrication and installation onto BELLA’s LPA FEL beamline.

Through a collaboration with UCLA, Nathan Majernik and colleagues designed as low aspect ratio chicane for BELLA Center’s LPA FEL line. Right: integration of the four chicane magnets onto the beamline, with a center chamber for laser diagnostics, e-beam manipulation, and FEL seed laser integration.

Early on in the ECRP project, the VISA undulator itself was transferred, under DOE BES guidance, from Brookhaven to LBNL. The strong-focusing undulator represents a key ingredient to BELLA’s approach, since it has the capability to keep the electron beam dense and focused over the full 4 meters of undulator length. However, the presence of quadrupole magnets (with a well-defined axis) does impose a significant precision constraint on the alignment of the four undulator sections to each other and of the LPA e-beam onto this VISA quadrupole axis. The alignment that existed at Brookhaven certainly got lost during transportation, especially at the <50-micron accuracy level that we needed. In collaboration with the Berkeley Center for Magnet Technology (Diego Arbelaez and team), Sam Barber spearheaded efforts to develop an in-house pulsed-wire alignment system. The wire was matched to the undulator axis, after which LBNL’s advanced metrology team was brought in to fiducialize the wire position to new undulator markers. Using novel laser metrology technology not available during VISA’s early conception, the undulator was assembled to its full 4 m length and fiducialized beam profile monitors with virtual target positions were included along the LPA beamline.

Left images show one undulator segment on the pulsed-wire setup, with the ball-shaped fiducial nests and retro-reflectors on the top of the undulator (center image). The plot on the right shows the unwanted pulse wire deflection (“FFT FODO peak”) when varying the position of the wire. Sub-10-micron accuracy on retrieval of the magnetic axis was demonstrated.

Mechanical integration and the commissioning of the laser, LPA beamline, and undulator, was just the start. Only high-quality electron beams would be good enough for FEL lasing, and critical transport elements like novel active plasma lenses and special triplets were needed to preserve the low emittance. Some of our critical successes in this ECRP project can be traced back to these transport and diagnostics challenges.

Left: single-shot image taken on a triplet-focused electron beam, following dispersion through a bend magnet. The energy-dependent beam size can yield the single shot emittance. Right: A parameter scan of the emittance versus charge density while comparing two LPA injection schemes. Results demonstrating sub-micron emittances, as measured downstream in an energy-sliced manner, were key drivers to the LPA FEL project.

The team is proud to have been at the forefront of completely novel techniques, such as single-shot emittance diagnostics, application of these diagnostics to evaluate various LPA injection schemes, and execution of performance comparisons between the active plasma lens (APL) and the high-gradient triplet. While APLs require specialized hardware (high-voltage pulser and gas delivery system), wakefield mitigation, and damage considerations, they can excel in compactness, radial symmetry, and ultra-strong focusing gradients. For future X-ray LPA FEL, APLs are predicted to be a key advantage and potentially unique enabler. Since APLs were re-invented in 2015 at BELLA, the FEL team has written 5 manuscripts on this APL topic, and co-authored numerous others. Labs such as DESY, SLAC, Rutherford, and Frascati have since implemented APLs in their research portfolio. In the framework of cross-project collaboration within the BELLA Center, the FEL team applied their experience on plasma lenses and emittance diagnostics to the BELLA PW system as well, joining the PW team during a LaserNetUS campaign (co-sponsored by DOE’s Fusion Energy Sciences) to enable a single-shot high-resolution energy and emittance diagnostic on a footprint of 60 cm.

BELLA Center research scientist Sam Barber (l.) and van Tilborg (r) holding components of the active plasma lens used in an advanced electron-beam diagnostic. The setup enabled measurements of electron-beam energy with range and resolution comparable to what is achieved using the multi-ton dipole magnet located behind them. (Marilyn Sargent/LBNL)

Another critical task, complementing hardware integration, LPA source quality control, and transport R&D, is the shot-to-shot stability and long-term reproducibility in LPA performance. Until recently this was largely unexplored, with community achievements typically focusing on single-shot parameter demonstrations. However, in the last 12 months, we have established on our LPA FEL beamline that the variations in electron beam source location (transverse and longitudinal, linked to laser focal location) and source angle (linked to laser propagation angle at focus) were largely prohibitive to the alignment control and stability needed to send beams into the undulator. The presence of six quadrupole magnets, 5 dipole steering magnets, a chicane, and a quadrupole-embedded undulator in the post-LPA line, imposes tight tolerances on electron-beam acceptance.

We have dedicated a majority of our recent efforts to new techniques to establish a high-power laser diagnostic that works “live” — without interrupting LPA acceleration or electron-beam transport. While this seems near-impossible, commissioning of a two-surface-coated wedged final-steering mirror proved to be the key. A low-power identical copy of the high-power laser focus was created, offset by angle to allow for in-situ monitoring. Fumika Isono carried out the integration of all components, and most excitingly, performed successful high-power demonstrations described in a paper accepted by the journal High-Power Laser Science & Engineering. With an eye toward a future high-bandwidth active feedback system, she and the team validated that the unamplified on-target 1 kHz background laser carried the same 100 Hz active stabilization even on 5 Hz high-power systems. This concept is now considered for integration across all BELLA Center laser systems.

Comparison of the high-power main-beam at focus (at full power), as measured with a temporarily-inserted post-focus mode imager (“main beam”), to the on-line non-destructive witness beam. Excellent correlation is observed, opening the path to online laser monitoring and fast feedback integration. Right: schematic of the wedged final steering mirror concept, producing a lower-power fully-correlated non-perturbative copy of the main laser at focus.

What’s next for the project

Due to COVID vendor delivery challenges, the laser active feedback hardware integration has been delayed to June 2021. However, the diagnostics and support components are embedded and ready for final integration with the new fast-feedback mirrors. Our goal is having the same laser focus position and angle available every shot, every scan, and every day. With this in place, the next step is to combine our extensive suite of laser, plasma, and beam diagnostics with the novel active control on laser delivery, in order to optimize key LPA FEL parameters such as spectral-charge density and emittance.

The brightness optimization efforts are anticipated to drive high-quality transport through the undulator, and demonstrate 3-eV undulator emission and FEL gain. A 3-eV FEL seed laser line has already been integrated onto the beamline, which could ease the conditions to observe FEL gain and will stabilize the FEL through a controlled microbunching “jump-start.” Timing and synchronization are less of a concern here: this is one advantage of using a purely laser-based accelerator as an FEL driver, with a small fraction of the laser split off for the frequency-doubled 3eV seed beam. Operating the undulator in seeded operation will be pursued in coming months.

Operating among the first groups to demonstrate an LPA FEL, starting first with 3 eV photons and using that as a platform to push onward to higher energies, would be a tremendous achievement. After an intense period of construction and commissioning, we are now uniquely positioned for this goal, enabled by the incredible support from Berkeley Lab scientific, technical, administrative, engineering, and operations support staff. And let us not forget where this all started, with the Gordon & Betty Moore Foundation and the BES Accelerator & Detector Research program led by E. Lessner, recognizing the feasibility, relevance, and high impact of compact laser-driven FELs. The future is bright!

Color centers are microscopic defects – departures from the rigorous lattice structure of a crystal, such as diamond. The type of defect that is of specific interest for qubits is a nitrogen atom next to a vacancy, or empty space, in a diamond lattice. (Nitrogen is commonly found in the crystal lattice of diamond, which is primarily a crystalline form of carbon, and can contribute to the color of the stone.)

When excited by the rapid energy deposition of a passing ion, nitrogen-vacancy centers can form in the diamond lattice. The electron and nuclear spins of nitrogen-vacancy centers and the adjacent carbon atoms can all function as solid-state qubits, and the crystal lattice can help protect their coherence and mutual entanglement.

The result is a physically durable system that does not have to be used in a cryogenic environment, which are attractive attributes for quantum sensors and also for qubits in this type of solid-state quantum computer. However, making enough qubits, and making them close enough to each other, has been a challenge.

When swift (high-energy) heavy ions such as the beams this team used – gold ions with a kinetic energy of about one billion electron volts – pass through a material, such as nitrogen-doped diamond, they leave a trail of nitrogen-vacancy centers along their tracks. Color centers were found to form directly, without need for further annealing (heat treatment). What’s more, they formed all along the ion tracks, rather than only at the end of the ion range as had been expected from earlier studies with lower-energy ions. In these straight “percolation chains,” color-center qubits are aligned over distances of tens of microns, and are just a few nanometers from their nearest neighbors. A technique developed by Berkeley Lab’s Molecular Foundry measured color centers with depth resolution.

 

ATAP Division staff scientist Arun Persaud, principal investigator of this effort. (Credit: Marilyn Sargent/Berkeley Lab)

The work on qubit synthesis far from equilibrium was supported by the Department of Energy’s Office of Science. The next step in the research will be to physically cut out a group of these color centers – which are like a series of beads on a string – and show that they are indeed so closely coupled that they can be used as quantum registers.

Results published in the current article show that it will be possible to form quantum registers with up to about 10,000 coupled qubits – two orders of magnitude greater than achieved thus far with the complementary technology of ion-trap qubits – over a distance of about 50 microns (about the width of a human hair).

“Interactions of swift heavy ions with materials have been studied for decades for a variety of purposes, including the behavior of nuclear materials and the effects of cosmic rays on electronics,” said Schenkel.

He added that researchers worldwide have sought to make quantum materials by artificially inducing color centers in diamond. “The solid-state approaches to quantum computing hardware scale beautifully, but integration has been a challenge. This is the first time that direct formation of color-center qubits along strings has been observed.”

The stars, like diamonds

On a miniscule and ephemeral scale (nanometers and picoseconds) the deposition of energy by the ion beams produces a state of high temperature, which Schenkel likens to the surface of the sun, in the 5000 K range, and pressure. Besides knocking carbon atoms out of the crystal lattice of diamond, this effect could enable fundamental studies of exotic states of transient warm dense matter, a state of matter that is present in many stars and large planets and which is difficult to study directly on Earth.

It might also enable formation of novel qubits with tailored properties that cannot be formed with conventional methods. “This opens a new direction for expanding our ability to form quantum registers,” said Schenkel.

Currently, color-center strings are formed with beams from large particle accelerators, such as the one at the German laboratory GSI that was used in this research. In the future, they might be made using compact laser-plasma accelerators like the ones being developed at the Berkeley Lab Laser Accelerator (BELLA) Center.

The BELLA Center is actively developing its ion-acceleration capabilities with funding by the DOE Office of Science. These capabilities will be used as part of LaserNetUS. Ion pulses from laser-plasma acceleration are very intense and greatly expand our ability to form transient states of highly excited and hot materials for qubit synthesis under novel conditions.

Clockwise from bottom left: ATAP Division postdoctoral scholars Sahel Hakimi and Lieselotte Obst-Huebl, and staff scientists Kei Nakamura and Qing Ji, at the target chamber of the iP2 beamline. A high-intensity, short-focal-length beam line, now under construction with DOE Office of Fusion Energy Sciences support, iP2 will be used for laser-based ion acceleration at the Berkeley Lab Laser Accelerator Center (BELLA). Laser-plasma ion acceleration offers the hope of performing many functions using a facility substantially smaller than conventional accelerators. (Credit: Thor Swift/Berkeley Lab.)

The process of creating these color centers is interesting in its own right and has to be better understood as part of further progress in these applications. The details of how an intense ion beam deposits energy as it traverses the diamond samples, and the exact mechanism by which this leads to color-center formation, hold exciting prospects for further research.

“This work demonstrates both the discovery science opportunities and the potential for societally transformative innovations enabled by the beams from accelerators,” says ATAP Division Director Cameron Geddes. “With accelerators, we create unique states of matter and new capabilities that are not possible by other means.”

For information about licensing or collaboration, contact Berkeley Lab’s Intellectual Property Office.

Cameron Geddes (second from right) and colleagues at the BELLA Hundred Terawatt Thomson-scattering (HTT) laser system, which enables precision, low-dose mono-energetic photon sources for nonproliferation and medicine, and multipulse, high-energy-density science for LaserNetUS.

“I am honored to be named to this position,” says Cameron. “ATAP has leading capabilities that work together for the benefit of everything we do. We have a unique combination of expertise in lasers, plasma science, beam physics, photon sources, controls and sources, as well as supporting methods such as advanced computation and magnetics. We also have deep connections to user needs in both high-energy physics and broad applications, including photon science. This is what allows us to create new capabilities. The kBELLA initiative is an example, and has the potential to realize a transformative combination of high average and high peak power lasers.”

At the BELLA Center, Cameron most recently led the creation of a quasi-monoenergetic gamma-ray source that can bring new capabilities to nuclear security applications (as well as medical and industrial imaging) based on compact laser-plasma accelerators. He oversaw the experimental portfolio within the BELLA Center, which develops plasma accelerators to extend the energy frontier of future high-energy physics experiments and for photon sources and applications. It includes two projects: the petawatt second beamline for high-energy physics and a high-intensity, tight-focus beamline for ion acceleration, oriented toward fusion energy sciences. It also supports user experiments under the LaserNetUS program that open the Division’s capabilities to international users in areas ranging from hydrodynamics to advanced imaging.

“Cameron provided outstanding leadership to our wide array of experimental activities,” says Eric Esarey, Director of the BELLA Center. “In addition to his keen scientific insight, he brought positive energy and created an atmosphere of enthusiasm and inclusion within the BELLA Center.”

Recognition for Cameron’s work included the U.S. Particle Accelerator School Prize, the American Physical Society Division of Plasma Physics (APS-DPP) John Dawson Award for Excellence in Plasma Physics Research, and Fellowship in the APS. His graduate work was recognized with the APS-DPP’s Marshall N. Rosenbluth Outstanding Doctoral Thesis Award, as well as the Hertz Foundation Dissertation Prize.

 

In addition to these laboratory endeavors, Cameron is well known for work on the steering committees that build the future of team science. These efforts gather diverse voices together into community consensus, expressed in reports that guide agency decisions. Presently he is a co-convener of the Advanced Accelerators topic in the “Snowmass” meeting—a high-energy physics community study, held every several years, that provides key input to the strategic direction of US investment in high-energy physics.

Cameron was a contributor to 2019’s multi-agency Basic Research Needs Workshop on Compact Accelerators for Security and Medicine, which gave rise to a crosscutting Office of Accelerator R&D and Production within DOE’s Office of Science. His publications are cited by others in multiple chapters of the workshop report.

 

Other related efforts include the DOE Fusion Energy Sciences Committee’s Subcommittee on Long-Range Planning; the APS-DPP Community Planning for Fusion Energy Sciences; and the Brightest Light Initiative, which defined a path forward for ultra-intense lasers in the US. He was also a chapter lead for the National Academies’ recent Decadal Assessment of Plasma Science.

Toward new horizons throughout ATAP

Cameron started his career at Berkeley Lab as an undergraduate, performing high-energy physics detector work under James Siegrist, who at the time was in Berkeley Lab’s Physics Division. Since then, Cameron’s career has ranged through both magnetic and inertial confinement fusion and plasma physics, many experimental aspects of laser-plasma accelerators and their applications, laser science and computer modeling, all of which gives him a special appreciation of the breadth and mutually reinforcing character of ATAP’s research portfolio. “The breadth of our expertise is one of the key reasons why I’m at Berkeley Lab,” he says. “I see ATAP, with these interlinked capabilities, as being crucial to US leadership in major scientific areas.”

ATAP will play an important role in addressing grand challenges for science society in the coming decade and beyond. The division continues to advance the frontiers of fundamental science ranging from particle physics, to high energy density science, to photon sources and new laser technology. From these cutting edge techniques, we develop new capabilities ranging across security, medicine, and society. “The Division is well positioned to help address the challenges of carbon cycle and clean energy, ” Geddes says.

The quest for abundant fusion energy through magnetic fusion will require high-field magnets and advanced computation methods that the Division develops, and there is renewed interest in the inertial fusion energy alternative, which is historically an area of Division expertise. At the same time our light sources are critical for clean-energy research including development of solar materials and advanced batteries, while our compact particle sources can bring capabilities to the field for carbon cycle analysis and clean industry. ATAP is also a leader in modeling on the eve of the opportunities that exascale computing will open, and in applying the promising new technologies of artificial intelligence, machine learning, and feedback controls. “Across all these areas, ATAP research creates tools that enable discovery science and new capabilities for society, ” he adds.

The human element

Cameron has a long history of commitment to developing both the quality and the diversity of ATAP’s workforce. “I see IDEA [the Labwide commitment to inclusiveness, diversity, equity, and accountability] as integral to both our workplace climate and our scientific leadership,” he said, adding, “It’s critical that we bring the best people to the Lab from all backgrounds, break down barriers, and create a culture in which they can thrive. That benefits both our progress and our engagement with the communities that we serve.”

Outreach to future colleagues: Cameron at the APS-DPP booth at the Oakland Unified School District science fair

He helped organize the recently established Pride Committee in APS-DPP, which envisions a scientific community that is open, welcoming, and supportive of all scientists within the gender and sexual-orientation minority communities, and has championed these issues as part of Berkeley Lab’s Lambda Alliance, an employee resource group for sexual-orientation and gender minority (SGM) members of our workplace.

Cameron’s plans to further our workplace progress include recruitment from and retention of under-represented groups, mentoring and training, and engaging IDEA experts (as he says, “most of us are not IDEA experts, but all of us are responsible for it — that’s the ‘A’ in IDEA”) from both within and outside the Lab, and also through our influence in the scientific community.

BELLA Center staff scientist Jeroen van Tilborg, who has led several of the Center’s initiatives, including a compact free-electron laser (FEL) based on a laser-plasma accelerator, will fill Geddes’s former position as the Center’s Deputy Director for Experiments.

Thomas Schenkel, who has served ATAP as Interim Director since January 2019, will resume full-time duties as head of ATAP’s Fusion Science and Ion Beam Technology Program. He is excited to pursue new opportunities in that diverse and highly collaborative program on themes of qubits, beams and fusion, including his work in the hardware foundations of approaches to quantum information science applications with spins and color centers.

“I leave the Division in good hands,” said Schenkel. “Cameron is impressive in both breadth and depth as a physicist, and also has interpersonal skills that will lead us to further success.”

“What we do all across ATAP is important to meeting the nation’s scientific needs,” said Cameron. “I’m excited to work with our talented staff, and our partners at other labs and universities, to build next-generation technologies and applications.”

He was chair of an Office of Fusion Energy workshop on the synergies of quantum computing with fusion energy sciences, and served as one of the Lab’s points of contact for DOE’s first Quantum Internet Blueprint Workshop.

New energy level for quantum information science

Thomas led a Berkeley Lab team in a multi-institutional study revisiting cold fusion, an effort funded by Google. He is Berkeley Lab principal investigator in a project with Cornell University that is developing an unprecedentedly compact and inexpensive multi-beam ion accelerator, based on technologies familiar from the electronics and microfabrication industries, to accelerate many low-current beams in parallel.

His tenure as Interim Director since January 2019 was notable for progress at the BELLA Center as well. He was the key figure in bringing LaserNetUS to Berkeley Lab. With his background in ion beam technology, he was a natural proponent of ion acceleration with BELLA and now serves principal investigator of the project, now underway, to build a beamline that provides tightly focused, high-intensity laser beams for it.

As the Laboratory and the entire DOE complex cast about for ways to use their expertise to address the pandemic, Thomas helped launch a pilot study that uses BELLA’s lasers to examine the structure and components of viruses like the one causing COVID-19, and how viruses interact with their surrounding environment.

Very favorable reviews of BELLA operations from the Laboratory Director and DOE during his interim directorship affirmed the BELLA Center’s near-term directions and helped set the stage for the proposed kBELLA.

Meanwhile he emphasized synergies among state-of-the-art ATAP efforts in superconducting magnets, accelerator controls and instrumentation, accelerator modeling, and support for the Advanced Light Source and design of its future upgrade.

Thomas also brought a deeply humane management style and personality to this job. He is a champion of both ideas and IDEA, and his interests extend throughout physics and beyond; a chance encounter with him in the coffee nook (back when we could do that) might touch upon the fine arts or philosophy as well as on science and technology. It was a combination that proved well suited to the unprecedented challenge of the pandemic.

“There was no playbook for doing lab research when the few people who are allowed to come in at any given time have to stay six feet apart,” recalls BELLA Center Director Eric Esarey.” “We were all learning as we went along, and Thomas was imaginative and supportive about how to meet this challenge.”

“Thomas’s years as Interim Director leave us with vibrant scientific programs throughout the Division, healthy budgets, forward-looking strategies, a strong safety culture, and most of all, high morale despite a year of the pandemic,” says Cameron Geddes, who takes over as permanent ATAP Director. “I’d like to thank him for his service to the Division and the Laboratory, and look forward to many years of his innovative ideas.”

Jeroen is also a leader in the community-planning processes that build and summarize consensus for scientific program directions. In the ongoing “Snowmass” particle physics community planning exercise, he serves as co-coordinator of Near-Term Applications in the Advanced Accelerator Concepts topical group, and is the liaison between the Advanced Accelerator Concepts and the Community Engagement frontiers. In the American Physical Society’s Division of Plasma Physics community-planning process, he contributed to the Disruptive Technologies topical group.

Research scientist Sam Barber (l.) and Jeroen hold the plasma mirror used in an electron-beam diagnostic experiment. The setup enabled measurements of electron-beam energy with range and resolution comparable to what is achieved using the multi-ton dipole magnet located behind them. (Marilyn Sargent/Berkeley Lab)

Jeroen’s research interests include ultra-intense laser physics, laser-plasma accelerators, nonlinear optics, AMOS (atomic, molecular, and optical sciences), undulator and FEL physics, high harmonic generation, high-energy physics, plasma diagnostics, ultrafast phenomena, advanced electron beam transport, and novel radiation sources. The unifying theme of his work has been, as he puts it, “to measure changes that happen very quickly — picoseconds, femtoseconds — that you can study with very fast pulses from high-powered lasers.”

Jeroen is co-author of 47 publications in the refereed literature and lead author of another 15, including two Physical Review Letters, on topics covering nonlinear optics, plasma diagnostics, X-ray phenomena, molecular dynamics, and accelerator physics.

Jeroen’s association with BELLA Center — then known as the Laser Optics and Accelerator Systems Integrated Studies (LOASIS) Program — began with an internship when he was in a bachelor’s/master’s degree program at the Technical University of Eindhoven. Two years later, in 2001, LOASIS leaders Wim Leemans and Eric Esarey suggested that he do his PhD research at LOASIS. There followed a transatlantic program of academic work at Eindhoven and experimental work at Berkeley Lab, culminating in a PhD, cum laude, in applied physics in 2006. The American Physical Society Division Physics of Beams recognized his work through the 2007 Outstanding Doctoral Thesis Research in Beam Physics Award.

After earning his doctorate, Jeroen spent three years as a postdoctoral scientist in Berkeley Lab’s Chemical Sciences Division, performing science based on soft-x-ray beams. This enriching phase and beam-user perspective allowed him to understand and appreciate the quality that the science community expects from advanced particle and light sources. He then joined BELLA Center in 2009.

“The people as well as the projects at the BELLA Center are really diverse,” Jeroen says. “My goal is to make sure that everyone’s talent is utilized in the best way, continuing the cross-team collaboration, always with a focus on science. I’m really excited to be selected to lead these efforts.”

The award cites her “outstanding experimental work in laser-driven proton acceleration establishing unprecedented performance with high-repetition rate cryogenic hydrogen jet targets, opening the path to real-world applications, and for the discovery of an all-optical method to shape proton beam profiles.”

Lotti earned her PhD in laser-plasma acceleration from Technische Universität Dresden, performing her experimental work primarily at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) DRACO laser. She also conducted research for her doctoral studies at Laboratoire pour l’Utilisation des Lasers Intenses (LULI) of École Polytechnique, and at the Matter in Extreme Conditions instrument of the Linac Coherent Light Source at SLAC.

Lotti at work in the target area at the DRACO petawatt laser. (© Christian Essler/HZDR)

Her undergraduate and Masters-level studies, which included working one to two days a week with the HZDR group since her fourth semester, prepared Lotti to “dive right in” as an experimental physicist at a laser facility. Then, “During the time when I did my PhD, the group that I worked in was starting to take on very fruitful collaborations with SLAC and with the Ohio State University,” she said, adding, “I benefitted a lot from these collaborations, not only with some very nice data sets, but also because they helped me to start a network in the community.”

After Lotti received her doctorate in 2019, BELLA Center research scientist Sven Steinke (now head of experimental physics and lasers at the German firm Marvel Fusion) told her of a postdoctoral opportunity at Berkeley Lab. “I had a good seven months to settle in, meet people at the Lab, and do some experiments before the pandemic,” she said. She took part in experiments with the BELLA petawatt laser, including a collaboration with experimental users from the Ohio State University through the LaserNetUS program; studies of the radiobiological effects of laser-accelerated ions led by Antoine Snijders of Berkeley Lab BioSciences; a double plasma mirror experiment; and electron acceleration runs with discharge capillaries.

Lotti at work (and complying with COVID precautions) near a BELLA experimental chamber. (Photo courtesy Lotti Obst-Huebl)

Presently she is primarily working on experimental planning for IP2, a project that will add a new intensity-boosted laser delivery system to the BELLA petawatt laser to support highly-nonlinear laser-matter interaction studies and laser-ion acceleration. Fortunately this has been a pandemic-friendly endeavor, especially with the BELLA petawatt laser being shut down for the second-beamline and IP2 project work, and recently she has been able to come onsite two to three days a week to work with Kei Nakamura on preparing the BELLA laser for the upcoming experiments.

Lotti was also honored last year with an alumni talk and a poster presentation at the Lindau Nobel Laureates Meeting.

I enjoyed reading science fiction when I was little. It was from those stories that I first heard about quantum mechanics, entropy, wormholes, and so forth. They were mysterious and fascinating. One of my favorite books was Ted Chiang‘s 1998 novel Story Of Your Life, the one they made into the movie Arrival a few years ago.

I start taking physics classes at school in 8th grade. Since then, physics has no longer been a matter just of fancy concepts but also of numbers, equations, and rigorous logic. It also meant lots of homework and exams. Luckily, I did not badly in those exams, which boosted my confidence and my affection towards physics, though my idea of what “physics” might be was still very primitive at that time.

In college I was majoring in chemistry in my freshman year, but later switched to physics, answering the calling of “Schrodinger’s cat” and the “twin paradox” from my childhood SF reading. College involved a lot more classes with hands-on experiments than high school, and I had great fun with that. My thesis project was on making luminescent thin films with rare earth materials.

“In accelerators, there are so many things you can do in experiment as well as in theory. It’s good to have a solid and broad foundation.”

After earning my bachelor’s degree in 2006 at USTC, I wanted to see some of the world and try serious research work, so I went to graduate school at Indiana University. I first went into condensed matter theory — specifically, the quantum state of 2D materials such as graphene. It was an exciting area, but I still felt like doing something more hands-on. At that time, IU was building a small electron accelerator for radiation testing, as well as to explore the inverse Compton scattering in a low energy storage ring. So I switched to this project and joined S.Y.’s group, and started my journey on particle accelerators.

I visited CBP for two weeks in 2010 to learn how to design a traveling-wave kicker that we needed for the storage ring at Indiana. I met and worked with Stefano de Santis, Derun Li, John Byrd, John Staples and others. Everyone was super nice. I also enjoyed the beautiful views on the hill and the delicious foods around downtown. So when there came a postdoc position on the Muon Ionization Cooling Experiment (MICE), supported by Don Summers at the University of Mississippi but based at Berkeley, I jumped in immediately.

I then did another postdoc, this time working directly for Berkeley Lab, and was hired as a Research Scientist in 2015. My work at the Lab started with building components for MICE. Later I was involved in other projects such as PIP-II (the Proton Improvement Plan for a high-intensity accelerator for neutrino experiments at Fermilab), LCLS-II (the Linac Coherent Light Source upgrade at SLAC), and Berkeley Lab’s own Advanced Light Source Upgrade. In recent years my work has mainly focused on the design and analysis of RF components, RF measurements and tests, and simulation of electromagnetic fields for other components such as kickers and beam position monitors.

If this chimes with a young person who is thinking about a physics career, what advice would you give?
Do the best you can in the core physics classes (classical mechanics, electrodynamics, quantum mechanics and thermodynamics) and the core math classes (calculus, complex analysis, differential equations). This builds a strong foundation for your future work. Keep an open mind and be willing to try something you haven’t done before or are not familiar with.

For students particularly interested in RF design, electrodynamics and differential equations are particularly important. Also, nowadays RF design relies more and more on computation, so good coding skill will be a big plus.

In accelerators, there are so many things you can do in experiment as well as in theory. It’s good to have a solid and broad foundation.

Left to right: S.Y. Lee of Indiana University (white jacket), Brookhaven National Laboratory’s Yichao Jing ( in black sweater), and Tianhuan (standing), along with ATAP postdoctoral scholar Dan Wang, were the instructional team for the Accelerator Physics course at the summer 2019 USPAS session.

A great way to advance your education is the US Particle Accelerator School. I have been both as a student and as an instructor and enjoyed it a lot. Even though it is only two weeks long, it is very concentrated and one can learn a lot. It is also a good opportunity to catch up with old friends and meet new ones. From time to time, USPAS offers on-site classes at accelerator labs, where students can learn with real accelerators. These fill up quickly, and I was very lucky to have such a class at the Jefferson Lab energy-recovery linac in 2011.

How does an experimental scientist’s day play out in COVID times?
Since the delivery of the LCLS-II injector gun, my work has focused mainly on simulation and analysis, as well as AI/ML, so the COVID work-from-home scenario hasn’t impacted my work very much so far. I only go back onsite once a while to check our computation server. There might be some hardware work coming in soon. We will see. Regardless of the type of work, I prefer to get back to lab and meet people face to face like before.