A perspective by Jeroen van Tilborg, BELLA Center Deputy Director for Experiments
The reach of particle accelerators in basic and applied science is very wide, ranging from extremely large-scale particle colliders (such as the Large Hadron Collider at CERN), to a variety of small-scale accelerators for medicine and industry. This includes a class of x-ray lasers, known as free electron lasers (FELs). For example, the Linear Coherent Light Source at SLAC is an FEL driven by a kilometer-scale convention linear accelerator (linac) powered by radio-frequency technology. Because the accelerating field that can be produced in an RF linac is limited by breakdown, conventional high energy electron accelerators tend to be large and expensive devices, which can limit affordability and availability of these machines.
An emerging technology that may revolutionize the field of particle accelerators and their applications is laser-driven plasma-based acceleration. Because laser-plasma accelerators (LPAs) are capable of generating enormous accelerating fields, 100 to 1000 times larger than those in RF linacs, LPAs offers the potential for a new class of compact, less expensive accelerators. Furthermore, the technology of short pulse, high power lasers that drive LPAs is rapidly evolving, which further spurs an equally rapid growth in LPA R&D worldwide.
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.
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.
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.
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.
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.
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.
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.
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!