The Future of Charged Particle Beams at the Center for Bright Beams

Across the United States, over 20 graduate students, 20 professors and several postdoctoral researchers from seven universities and two national labs are working on making charged particle beams brighter. These physicists, chemists, materials scientists and others have been brought together by the National Science Foundation's Science and Technology Center: The Center for Bright Beams (CBB). Led by Cornell University, the Center also includes the University of Chicago, Arizona State University, the UCLA, the University of Florida, Brigham Young University, Clark Atlanta University, Lawrence Berkeley National Laboratory and Fermilab.

Created in 2016, CBB aims to increase the brightness of electron beams by a factor of 100 in order to open the door for many areas of research. For example, brighter beams for ultrafast electron diffraction will open access to larger, more complex molecules or the imaging of atomic motion. Compact, high-flux, hard X-ray sources from Compton backscattering will enable precision microscopy of structural materials. Coherent, continuous-wave hard X-ray sources with brighter beams will enable condensed matter physicists to study nanoscale phase separation in correlated electron systems. Brighter beams in electron microscopes could lead to better, faster imaging and to new techniques in semiconductor manufacturing and quality assurance. Linear, circular, and energy-recovered colliders with brighter beams will allow particle physicists to probe nearer the big bang, and nuclear physicists to peer deeper inside the proton.

In order to reach these goals, members of CBB work in three small, intensive teams, labeled themes. The Beam Production theme studies photoemission electron sources, focusing on developing better photocathode materials that produce electrons with near-zero momentum transverse to the beam direction. By gaining the fundamental understanding needed to improve the performance of superconducting accelerating cavities, the Beam Acceleration theme is improving the energy efficiency and increasing the accelerating gradient. Lastly, to maintain the quality of high-brightness beams, the Beam Storage and Transport theme is developing methods to transport ultra-bright beams and to minimize the impact of instabilities and non-linear resonance in storage rings. This theme also addresses transport in electron microscopes to improve aberration correction. While these themes organize the research, they are interconnected, and the Center meets regularly as a whole for seminars, smaller graduate student meetings and topical meetings at the theme intersections.

In the two short years since CBB's inception, it has already contributed insights. For example, superconducting RF cavities are the gold-standard for beam acceleration in high power accelerators, but the factors that drive their energy efficiency and accelerating gradient–their key performance parameters–are poorly understood. In 2012, Anna Grassellino of Fermilab discovered that nitrogen doping the interior surface could improve performance, but the reasons are unknown. To find the answer, Center for Bright Beams surface scientists and microscopists are providing detailed information about the surface composition and structure that complements performance measurements, and condensed matter physicists are developing tools to predict the RF performance as a function of impurity doping level and profile, offering a route to optimizing the niobium surface. Already, Center for Bright Beams insights have led to an improved treatment for a cavity for the NSLS-II X-ray facility at Brookhaven National Laratory. Efficiency has doubled.

Future accelerating cavities are likely to use compound superconductors, which have better intrinsic properties than niobium, and could operate at 4.2 K, saving millions of dollars in cooling costs (niobium cavities operate at 2 K). Liepe's group at Cornell and others have achieved promising results using Nb3Sn, but fully capitalizing on its advantages requires better understanding of the growth properties and the impact of defects. Thanks to ab initio calculations of the growth process combined with the surface analysis of Nb3Sn cavities grown by vapor diffusion, new growth techniques and other improvements developed by the Center for Bright Beams have halved the surface resistance, and further advances are on the horizon.

Graduate student Joshua Paul of the University of Florida finds the Center's dialogue between theory and experiment exciting and productive. "Working with experimentalists lets me refine my research to focus on what experimentalists care about," he said. "This lets me have a direct impact with my work and lets experimentalists streamline their workflow."

A combined theoretical and experimental investigation into photoemission has shown that there are many factors degrading the beam quality, including body effects, surface roughness and band structure. The investigation suggests strategies to avoid or mitigate these factors. However, when CBB started, there were no tools available for measuring such pristine beams, so CBB had to invent them. Confirmation comes from CBB's successful production of a beam whose electrons' mean energy transverse to the photocathode surface (at low current) is below 6 meV using cryogenic copper photocathodes. An important goal is to demonstrate record coherence-length beams for ultrafast electron diffraction.

The Beam Production team is working hand-in-hand with the Beam Storage and Transport team, whose goal is to preserve the quality of these exquisite beams as they travel to their target. A first step is to distinguish the effects of various sources of emittance dilution, and interestingly, early indications are that the electromagnetic repulsion between electrons (space charge) is important. This is well known for high-charge-density beams but was not previously known for low emittance beams at low current. This theme is also applying accelerator know-how to electron microscopes, where they have come up with a technique to rapidly calculate aberrations that could eventually lead to real-time microscope tuning. This research also looks into the source of nonlinear resonances introduced into storage rings due to nonlinear magnets such as sextupole magnets and octopule magnets.

Ritchie Patterson of Cornell and CBB's director, is pleased by the progress. "The combination of talents in the Center have led to rapid advances in understanding–far beyond what we imagined," she said.

Advancing the field of accelerator physics, however, is not enough. CBB is preparing graduate students and postdoctoral researchers for a range of careers, including accelerator science. As University of Chicago graduate student Darren Veit notes, "Working with collaborators within CBB has been an invaluable aspect of my graduate career. By working with scientists from other disciplines, I have been able to hone skills that allow me to communicate important information to people outside my field and learn how to successfully work with others on complex issues."

A key CBB priority is the recruitment and education of new accelerator physicists, chemists, materials scientists and engineers from diverse backgrounds. These students get the experience of working in an interdisciplinary team, opportunities for hands-on accelerator training and training in communications, entrepreneurship and other career skills.

Many CBB graduate students participate in outreach events which demonstrate simple concepts underneath the complex machinery of particle accelerators. University of Chicago graduate student Lipi Gupta attempted to simplify and present the physics of electromagnets to young girls from all over the greater Chicago area through the national Expanding Your Horizons workshop held yearly at campuses across the country. At the March 2018 workshop, each young middle-school girl got to build a small speaker out of a cup, a few magnets and some wires. They were able to plug it into their phones and use instantly to play their favorite music. Another outreach activity supported by CBB is STEP UP!, which creates "design experience" kits for use by middle-school science teachers in their classrooms. This year, CBB will take the STEP UP! kits on the road, offering training workshops for teachers in Chicago and Atlanta.