Faculty, Staff, Student and Alumni Awards & Notes

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Category: Department News

Published: Friday, September 02 2022 00:01

We proudly recognize members of our community who recently garnered major honors, began new positions and more.

Compact Electron Accelerator Reaches New Speeds with Nothing But Light

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Category: Research News

Published: Friday, September 02 2022 00:01

Scientists harnessing precise control of ultrafast lasers have accelerated electrons over a 20-centimeter stretch to speeds usually reserved for particle accelerators the size of 10 football fields.

A team at the University of Maryland (UMD) headed by Professor of Physics and Electrical and Computer Engineering Howard Milchberg, in collaboration with the team of Jorge J. Rocca at Colorado State University (CSU), achieved this feat using two laser pulses sent through a jet of hydrogen gas. The first pulse tore apart the hydrogen, punching a hole through it and creating a channel of plasma. That channel guided a second, higher power pulse that scooped up electrons out of the plasma and dragged them along in its wake, accelerating them to nearly the speed of light in the process. With this technique, the team accelerated electrons to almost 40% of the energy achieved at massive facilities like the kilometer-long Linac Coherent Light Source (LCLS), the accelerator at SLAC National Accelerator Laboratory. The paper was published in the journal Physical Review X on September 16, 2022

“This is the first multi-GeV electron accelerator powered entirely by lasers,” says Milchberg, who is also affiliated with the Institute of Research Electronics and Applied Physics at UMD. “And with lasers becoming cheaper and more efficient, we expect that our technique will become the way to go for researchers in this field.”  An image from a simulation in which a laser pulse (red) drives a plasma wave, accelerating electrons in its wake. The bright yellow spot is the area with the highest concentration of electrons. In an experiment, scientists used this technique to accelerate electrons to nearly the speed of light over a span of just 20 centimeters. (Credit Bo Miao/IREAP)

Motivating the new work are accelerators like LCLS, a kilometer-long runway that accelerates electrons to 13.6 billion electron volts (GeV)—the energy of an electron that’s moving at 99.99999993% the speed of light. LCLS’s predecessor is behind three Nobel-prize-winning discoveries about fundamental particles. Now, a third of the original accelerator has been converted to the LCLS, using its super-fast electrons to generate the most powerful X-ray laser beams in the world. Scientists use these X-rays to peer inside atoms and molecules in action, creating videos of chemical reactions. These videos are vital tools for drug discovery, optimized energy storage, innovation in electronics, and much more.  

Accelerating electrons to energies of tens of GeV is no easy feat. SLAC’s linear accelerator gives electrons the push they need using powerful electric fields propagating in a very long series of segmented metal tubes. If the electric fields were any more powerful, they would set off a lightning storm inside the tubes and seriously damage them. Being unable to push electrons harder, researchers have opted to simply push them for longer, providing more runway for the particles to accelerate. Hence the kilometer-long slice across northern California. To bring this technology to a more manageable scale, the UMD and CSU teams worked to boost electrons to nearly the speed of light using—fittingly enough—light itself.

“The goal ultimately is to shrink GeV-scale electron accelerators to a modest size room,” says Jaron Shrock, a graduate student in physics at UMD and co-first author on the work. “You’re taking kilometer-scale devices, and you have another factor of 1000 stronger accelerating field. So, you’re taking kilometer-scale to meter scale, that’s the goal of this technology.”

Creating those stronger accelerating fields in a lab employs a process called laser wakefield acceleration, in which a pulse of tightly focused and intense laser light is sent through a plasma, creating a disturbance and pulling electrons along in its wake. 

“You can imagine the laser pulse like a boat,” says Bo Miao, a postdoctoral fellow in physics at the University of Maryland and co-first author on the work. “As the laser pulse travels in the plasma, because it is so intense, it pushes the electrons out of its path, like water pushed aside by the prow of a boat. Those electrons loop around the boat and gather right behind it, traveling in the pulse’s wake.”

Laser wakefield acceleration was first proposed in 1979 and demonstrated in 1995. But the distance over which it could accelerate electrons remained stubbornly limited to a couple of centimeters. What enabled the UMD and CSU team to leverage wakefield acceleration more effectively than ever before was a technique the UMD team pioneered to tame the high-energy beam and keep it from spreading its energy too thin. Their technique punches a hole through the plasma, creating a waveguide that keeps the beam’s energy focused.

“A waveguide allows a pulse to propagate over a much longer distance,” Shrock explains. “We need to use plasma because these pulses are so high energy, they're so bright, they would destroy a traditional fiber optic cable. Plasma cannot be destroyed because in some sense it already is.”

Their technique creates something akin to fiber optic cables—the things that carry fiber optic internet service and other telecommunications signals—out of thin air. Or, more precisely, out of carefully sculpted jets of hydrogen gas.

A conventional fiber optic waveguide consists of two components: a central “core” guiding the light, and a surrounding “cladding” preventing the light from leaking out. To make their plasma waveguide, the team uses an additional laser beam and a jet of hydrogen gas. As this additional “guiding” laser travels through the jet, it rips the electrons off the hydrogen atoms and creates a channel of plasma. The plasma is hot and quickly starts expanding, creating a lower density plasma “core” and a higher density gas on its fringe, like a cylindrical shell. Then, the main laser beam (the one that will gather electrons in its wake) is sent through this channel. The very front edge of this pulse turns the higher density shell to plasma as well, creating the “cladding.” 

“It's kind of like the very first pulse clears an area out,” says Shrock, “and then the high-intensity pulse comes down like a train with somebody standing at the front throwing down the tracks as it's going.” 

Using UMD’s optically generated plasma waveguide technique, combined with the CSU team’s high-powered laser and expertise, the researchers were able to accelerate some of their electrons to a staggering 5 GeV. This is still a factor of 3 less than SLAC’s massive accelerator, and not quite the maximum achieved with laser wakefield acceleration (that honor belongs to a team at Lawrence Berkeley National Labs). However, the laser energy used per GeV of acceleration in the new work is a record, and the team says their technique is more versatile: It can potentially produce electron bursts thousands of times per second (as opposed to roughly once per second), making it a promising technique for many applications, from high energy physics to the generation of X-rays that can take videos of molecules and atoms in action like at LCLS. Now that the team has demonstrated the success of the method, they plan to refine the setup to improve performance and increase the acceleration to higher energies.

“Right now, the electrons are generated along the full length of the waveguide, 20 centimeters long, which makes their energy distribution less than ideal,” says Miao. “We can improve the design so that we can control where they are precisely injected, and then we can better control the quality of the accelerated electron beam.”

While the dream of LCLS on a tabletop is not a reality quite yet, the authors say this work shows a path forward. “There’s a lot of engineering and science to be done between now and then,” Shrock says. “Traditional accelerators produce highly repeatable beams with all the electrons having similar energies and traveling in the same direction. We are still learning how to improve these beam attributes in multi-GeV laser wakefield accelerators. It’s also likely that to achieve energies on the scale of tens of GeV, we will need to stage multiple wakefield accelerators, passing the accelerated electrons from one stage to the next while preserving the beam quality. So there’s a long way between now and having an LCLS type facility relying on laser wakefield acceleration.” 

This work was supported by the U.S. Department of Energy (DE-SC0015516, LaserNetUS DE-SC0019076/FWP#SCW1668, and DE-SC0011375), and the National Science Foundation (PHY1619582 and PHY2010511).

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Story by Dina Genkina

In addition to Milchberg, Rocca, Shrock and Miao, authors on the paper included Linus Feder, formerly a graduate student in physics at UMD and now a postdoctoral researcher at the University of Oxford, Reed Hollinger, John Morrison, Huanyu Song, and  Shoujun Wang, all research scientists at CSU, Ryan Netbailo, a graduate student in electrical and computer engineering at CSU, and Alexander Picksley, formerly a graduate student in physics at the University of Oxford and now a postdoctoral researcher at Lawrence Berkeley National Lab. 

Recent Physics Grad Sees Many Roads Ahead

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Category: Department News

Published: Friday, September 02 2022 00:01

As Jeffrey Wack (B.S. ’22, physics; B.S. ’22, mathematics) walked across the graduation stage in May 2022, he carried with him a lot of uncertainty about where to go next. His trepidation came from his voracious curiosity for a broad range of things, primarily within physics and math—the subjects of his two degrees—but also from his interests in teaching, outreach and music. The prospect of having to pick just one path forward felt confining to Wack. But that same curiosity served him extremely well during his time at the University of Maryland, and it left him with many opportunities for next steps.Jeffrey Wack (courtesy of same)

Wack collected an impressive resume at UMD. He taught an introductory course on nuclear physics and reactor operations, studied physics in Florence, participated in an optomechanics research project that resulted in a publication, made significant contributions to experimental research with coplanar waveguides, and co-taught a self-designed course on music theory and math. Since graduating, he began working as a fellow at the Museum of Math in New York City, sampling the working world while contemplating graduate school.

“The four years I spent at UMD were the best four years of my life this far,” Wack says. “I’m already having a blast living in New York, but I’m going to miss all the great people I met in College Park.”

Born and raised in Carroll County, Maryland, Wack attributes his broad scientific curiosity to his upbringing and the influence of his father.

“My dad is a pediatrician, but he's very interested in all sorts of science,” Wack says. “I have memories of playing the ‘why’ game with him and just asking him why. You know, you ask why, and then no matter what the answer is, you can always ask why again, and you sort of end up down this rabbit hole.”

Although the younger Wack asked questions about everything, from why fruit grows to what an immune system is, his earliest fascination orbited around astronomy. Then, during high school, his curiosity shifted gears, landing on the curiously strong connection between physics and mathematics.

“There was something about physics and calculus in particular that I really enjoyed,” says Wack. “Those relationships between position and velocity and acceleration, there's something about them that really caught me. Like ‘that's awesome!’”

Following in his older sister’s footsteps, Wack chose to attend UMD, drawn in by the opportunities for learning all things physics and math at a large university. In the fall of 2019, Wack studied abroad in the Maryland-in-Florence program, specifically designed for physics students to continue taking required courses while exposing themselves to a foreign culture and language. He was particularly inspired by the instruction of Luis Orozco, now professor emeritus at UMD and a Fellow at the Joint Quantum Institute (JQI). After the semester abroad ended, Wack reached out to Orozco to see if he could work with him on a research project. Orozco agreed, and during the summer of 2020 invited him to join a nanofiber project. 

Orozco’s research interests include optomechanics, the study of interactions between mechanical systems and electromagnetic waves. The project Wack joined was a multi-national collaboration, with an experimental group at Shanxi University in China and a collaborator at the University of Conception in Chile. The goal was to use light to cool an optical fiber as it travels through it.

Optical fibers are used to confine and direct light, whether it’s for carrying internet signals to homes or aiding in research. The fibers Orozco’s team used are stretched incredibly thin, about a hundred times thinner than human hair. These nano-fibers guide light, but they hardly confine it—some of the light actually travels outside the fiber. This is particularly useful for studying the interaction of light with atoms and ions, which can be brought close to (but remain outside of) the fiber. The downside is that the fiber is quite fragile and prone to tiny vibrations that shake and twist it, disturbing the light as it travels.

To minimize these tiny twists, the team sent in a laser beam of a particular intensity. The interaction of the beam with the material inside the fiber counteracted the fiber’s twisting, minimizing that particular vibration and thus cooling down the fiber overall. To detect this cooling, the team sent a second, probing laser beam and observed how much the fiber’s twists and turns perturbed that beam.

Wack’s role was to analyze the raw photodetector data from the probing laser and use it to extract information about the fiber twists. He analyzed the data and concluded that the method was successful, as detailed in a recent paper published in Photonics Research. But Wack wasn’t satisfied with simply analyzing data. He played the ‘why’ game, trying to understand the deeper physics of what was going on. He made his own, simplified model of the cooling mechanism—not to put in the paper, but enough to model the system to his own satisfaction. “I did that just to entertain myself,” Wack explains.

"Jeffrey contributed crucially in understanding the cooling process, thanks to his analysis of the distribution of the temperature fluctuations,” Orozco says. “The plots he produced made it into figure two of the publication."

By the summer of 2021, COVID-19 restrictions were easing up, and Wack was itching to try hands-on lab work. He joined the group of one of UMD’s most mathematically minded experimentalists, Chesapeake Assistant Professor of Physics and JQI Fellow Alicia Kollár. Kollár’s research concerns coplanar waveguides—little paths printed on a circuit board that microwaves can travel through—to create never before seen geometries and interaction patterns between bits of quantum information known as qubits.

Kollár’s creation of novel geometries relies on a peculiar theoretical property of coplanar waveguides: that stretching or scrunching them up does not change the frequency of microwaves they carry. Wack’s role was to make careful measurements to test how well this property holds in practice.

To investigate this, Wack had to get his hands dirty with several different lab skills. He had to learn to solder and assemble electronics, work with graduate students to create coplanar waveguides of different lengths, analyze data, and model the system using purpose-built software.

“Jeff did really phenomenal work,” Kollár says. “He was really just sort of diving into research, almost like a senior graduate student.”

Wack automated some simulation steps that had previously been done manually and used the new process to quantify a confounding effect—that the frequency change depended on the number of times that the waveguide was bent. If this pattern is confirmed experimentally, Kollár says, it will be used in many future experiments and theoretical studies alike.

On top of his studies and research, Wack also found ways to participate in outreach and teaching throughout his time at UMD. He volunteered to film a slinky demonstration of wave propagation. He taught an introductory course on nuclear physics and reactor theory to undergraduates for the Maryland Undergraduate Training Reactor (MUTR) program, where undergrads can become certified reactor operators. He also has an interest in music, having sung and performed in musicals in high school and having picked up electric bass during his college years. “And also, because I'm such a geek for computers, I do some digital synthesis,” Wack says. He found a way to weave this in with his math interest by creating a co-teaching a course on the math of music for the Student Initiated Courses (STIC) program.

Upon graduating last spring, Wack decided to take a gap year. This summer, he started a fellowship at the Museum of Math, combining his passion for mathematics and outreach. As a docent there, he talks to visitors about the exhibits and thinks a lot about math. As part of the fellowship, he’s also pursuing a personal project: planning a live performance that combines music, physics demos and lectures on math and music theory.

“So many of the paths forward seem appealing to me,” Wack says. “I'm going to go to grad school at some point, but this is part of why I wanted to do a gap year. I'm hoping that over the next two years, it'll come to me like ‘Aha! This is exactly what I want to do.’”

Written by Dina Genkina

Women in Physics Group Changes its Name to Physicists of Underrepresented Genders

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Category: Department News

Published: Friday, September 02 2022 00:01

Women in Physics (WiP) has officially been renamed Physicists of Underrepresented Genders (PUGs) at the University of Maryland.

According to UMDPUGs (Physicists of Underrepresented Genders) physics graduate student Ina Flood, the group’s new president, the change reflects the organization’s ongoing commitment to fostering a supportive and encouraging community for all.

“Changing our name was a group decision initiated under Mika Chmielewski, our previous president,” Flood said. “The rationale behind this decision was to make it obvious that we’re committed to supporting people who might feel like they are underrepresented in the physics community. The name change is to help people feel that they’re included and welcome from the get-go.”

For more than a decade, WiP has provided physics undergraduate and graduate students with resources such as a mentoring program and networking opportunities. In addition to professional development events led by physics faculty members and professionals, the club also offered social programming like group study sessions, where members mingled and made new friends.

PUGs plans to continue the group’s ongoing programs and opportunities while taking a more proactive approach to supporting all members of the physics community. 

“As a university club, we’re already open to all people and sincerely welcome anyone who is interested in physics,” said incoming physics graduate student Kate Sturge (B.S. ’22, physics; B.S. ’22, astronomy), who was an active undergraduate member of WiP and is currently the PUGs webmaster and social media manager. “But this name change is our way of making ourselves more deliberate and explicit in supporting everyone in physics.”

Physics Chair Steve Rolston echoes the sentiment: "We value the contributions of everyone who shares our love of physics. We appreciate PUGs’ efforts to make that crystal clear."

Flood, Sturge and other PUGs members plan to do more to coordinate with other LGBTQ+ student organizations on campus. Flood said she hopes increased communication and collaboration will also help PUGs connect mentors with mentees and share more institutional knowledge about STEM and physics. The group also plans to develop more opportunities for safe in-person gatherings, including “study hours,” during which physics students gather to discuss and do homework together.

“Our biggest goal after our name change is to expand our accessibility and availability to members who may need guidance or community support during the school year,” Flood said. “It’s really important to our organization that we get people together, facilitate meaningful conversations and celebrate our shared identity as physicists.”