Donald Hartill, a professor of physics emeritus in the College of Arts and Sciences and a driving force behind decades of experimental research in particle physics, died on April 16. He was 86.

Hartill’s legacy at Cornell spans more than five decades. His work included some of the most transformative developments in the university’s physics research infrastructure, researchers at Cornell’s Wilson Synchrotron Laboratory said. As a physicist, engineer and leader, he played a central role in the design and operation of the Cornell Electron Storage Ring (CESR), helped shape the success of the CLEO high-energy physics program, and contributed to the founding and growth of the Cornell High Energy Synchrotron Source (CHESS), they said.

“Don’s development of CESR’s uniquely tunable magnet system extended the facility’s lifetime by decades and enabled its transition across multiple generations of research,” said Ritchie Patterson, the Helen T. Edwards Professor of Physics (A&S) and former director of the Cornell Laboratory for Accelerator-based Sciences and Education (CLASSE). “Beyond the lab, Don’s steady leadership – especially during times of uncertainty – left an enduring mark on the culture and strength of Cornell’s physics community.”

After earning his bachelor’s degree from the Massachusetts Institute of Technology in 1961 and a Ph.D. from the California Institute of Technology in 1967, Hartill held a research appointment at the European Organization for Particle Physics (CERN). He arrived at Cornell in 1968 as an assistant professor in what was then the Laboratory for Nuclear Studies.

The lab’s 10 GeV synchrotron had just come online and Hartill became immersed in experimental work that helped define the lab’s trajectory for decades. He began with experiments using high-energy photon and electron beams before becoming a key figure in the development of CESR and the CLEO particle detector.

Hartill contributed significantly to much of the experimental work at Cornell, said David Cassel, a professor emeritus of physics (A&S) and a collaborator on the CLEO project. “Many of us used the chambers and electronics that Don developed. Furthermore, he made essential contributions to the CLEO detectors,” Cassel said.

One of Hartill’s most lasting and influential achievements at Cornell, his colleagues said, was the system he helped to develop for individually controlling the focusing magnets in CESR – a defining feature of CESR’s adaptability and longevity. Unlike many accelerator storage rings, which are optimized for a single purpose, CESR’s individually tunable magnets allowed the facility to evolve over time, supporting experiments ranging from CLEO (focused on high-energy physics), to CESRTA (the CESR Test Accelerator for accelerator physics research), to CHESS (optimized for X-ray production).

During a sabbatical at Stanford University, he worked on the Mark II drift chamber, technology he later adapted and implemented at the CLEO project. Earlier, during his postdoctoral work at CERN, he studied proportional chambers, another innovation he helped introduce to Cornell’s experimental infrastructure.

Hartill held visiting appointments at institutions around the world, including the SLAC National Accelerator Laboratory and the Osservatorio Astrofisico di Arcetri in Florence, Italy. He was honored as a fellow of both the American Physical Society and the American Association for the Advancement of Science.

Hartill was a co-author of all 530 CLEO publications, a recognition of his essential role in the CLEO program.

He worked closely with the superconducting radio frequency (SRF) group at CLASSE, pioneering techniques to identify and resolve performance-limiting defects in SRF cavities, said Matthias Liepe, professor of physics (A&S) and head of the Cornell SRF group. Hartill built perhaps the world’s largest SRF cavity, and his innovative use of second sound detection in superfluid helium remains a widely respected diagnostic approach in the field, Liepe said.

“Don’s kindness, broad technical knowledge, and his seemingly unlimited availability to listen and help made Don such a wonderful leader,” Liepe said. “Over the years, Don has joined hundreds of our SRF group meetings, which greatly benefitted our research program and left a mark on all of us. He will be deeply missed.”

Hartill served on numerous review and advisory committees for organizations including the National Science Foundation, the U.S. Department of Defense, Fermilab’s Board of Overseers, CERN, and the Laser Interferometer Gravitational-Wave Observatory (LIGO). Hartill remained active on scientific panels until late in life.

According to Cassel, one of Hartill’s enduring points of pride was his involvement in National Science Foundation oversight of scientific research in Antarctica, serving on the U.S. Antarctic Program Blue Ribbon Panel, a group responsible for evaluating both research and logistics, and making multiple trips to the continent, often annually, to support U.S. programs there.

After his retirement from Cornell in 2016, Hartill remained an advisor and mentor in the Department of Physics, serving as chair and continuing to support collaborative research efforts. This culminated in his appointment as director of CLASSE in 2020.

His leadership came at a particularly difficult moment: the start of the COVID-19 pandemic. Under Hartill’s guidance, CLASSE became one of the first laboratories on the Cornell campus to reopen for critical research. “Thanks to his clear-eyed decision-making, deep institutional knowledge and commitment to safety, the lab navigated the crisis with minimal disruption while protecting the health of its staff and researchers,” said Patterson. “During a time of great uncertainty, he helped CLASSE move forward with clarity and confidence.”

In addition to his scientific contributions, Hartill was the fifth and longest-standing mayor of the Village of Lansing, serving the community as mayor for 25 years and had been involved as a trustee member and deputy mayor since 1989.

Rick Ryan is a science communicator for the Cornell Laboratory for Accelerator-based ScienceS and Education (CLASSE).