Symposium celebrates Claudio Pellegrini, pioneer of SLAC’s X-ray laser

Claudio Pellegrini, inventor of SLAC’s X-ray free-electron laser, as a 1-year-old and on his 90th birthday at a SLAC symposium in his honor. 

Pellegrini received his academic education at Sapienza University of Rome, Italy. From 1958 to 1978, he worked at the Italian National Laboratory of Frascati. He then moved to DOE’s Brookhaven National Laboratory to become associate chairman of the National Synchrotron Light Source (NSLS) and co-director at the Center for Accelerator Physics. In 1989, he became a professor of physics at UCLA. He is currently a distinguished professor emeritus of physics at UCLA and adjunct professor of photon science at SLAC. Pellegrini is a Fellow of the American Physical Society and a member of the National Academy of Sciences. In 2001, he received the Robert R. Wilson Prize, and in 2014 the Enrico Fermi Award.

In the early 1970s, John Madey at Stanford University invented a new type of light source, the free-electron laser (FEL). Unlike a regular laser, in which light gets amplified in a medium where it interacts with light emitted by electrons bound to atoms, amplification in an FEL happens through interaction with light emitted by a beam of “free,” or unbound, electrons.

The schematic shown here is taken from Madey’s FEL patent, issued in 1992. It shows the three principal components of an FEL (from right): a source of electrons (electron gun), a linear accelerator (RF linac) that turns those electrons into an energized electron beam, and a magnetic structure (wiggler magnet), in which the FEL radiation is extracted from the electron beam.

Madey’s FEL produced infrared light with small amplification (low gain). In 1984 Pellegrini, in a paper with Bonifacio and Narducci, extended the FEL theory and realized that, in principle, FELs can produce X-rays with high amplification (high gain), provided the quality of the electron beam is good enough. Other authors had also studied the high-gain regime in the late 1970s.

In 1989, Pellegrini moved to UCLA. There, his team put together an FEL, using an electron gun and a small linear accelerator built at UCLA, and an undulator magnet, built by Alexander Varfolomeev at the Kurchatov Institute in Moscow. While still producing infrared radiation, the experiment was the first verification of the high-gain, wavelength-independent FEL theory. Subsequent UCLA-led experiments done at Los Alamos National Laboratory measured the very high gain needed to enable an FEL that produces “hard,” high-energy X-rays.

This photo shows the 0.6-meter-long undulator used in the first UCLA experiment. It was a gift from Varfolomeev and is now on display in Pellegrini’s office at UCLA.

In 1992, researchers met at SLAC at the Workshop on Fourth Generation Light Sources to discuss the future direction of the lab’s Stanford Synchrotron Radiation Lightsource (SSRL). While many of the attendees were in favor of continuing the current direction of the facility, which produces bright X-rays with a ring-shaped accelerator, Pellegrini proposed a different road. In his talk, he laid out his idea of turning SLAC’s linear accelerator (linac) into an X-ray free-electron laser (XFEL).

Such a machine would take the electron beam from the linac and send it through a 100-meter-long undulator magnet. Pellegrini’s calculations showed that as the electrons are forced on a wiggly flight path through that undulator, they emit light that would interact with the electron beam in a specific way that would generate extremely powerful X-ray pulses at the undulator’s exit. This process of light production is called self-amplified spontaneous emission (SASE).

This photo shows Pellegrini with SLAC and Stanford Professor Emeritus Herman Winick (left), who, together with Max Cornacchia, organized the 1992 workshop, at a celebration of the 20th anniversary of the meeting.

Pellegrini’s proposal received mixed reactions at the 1992 workshop and in the X-ray community. While there was interest, a number of people thought the idea would never work or it would be technically too challenging to implement.

However, by 1998, Pellegrini and colleagues had succeeded in demonstrating the validity of the FEL SASE theory in the two key experiments described earlier (third image in this series). The final compelling demonstration experiment, a collaboration between SLAC, UCLA, and Brookhaven and Lawrence Livermore National Labs, followed within a few years, showing saturation of the FEL amplifier. By that time demonstrations had also been done at Argonne National Lab and at the German research center DESY.

With the experimental proof-of-principle established, the Department of Energy began funding the construction of the LCLS XFEL in 2001. The graphic shown here is the cover art for a design study report, released in 1998, for the LCLS proposal that defined engineering parameters and their feasibility.

In 2009 – 25 years after Pellegrini and collaborators had developed their wavelength-independent theory for a high-gain FEL – SLAC turned on LCLS, the world’s first XFEL producing high-energy (“hard”) X-rays. A few years earlier, researchers at the German research center DESY had fired up FLASH, the first XFEL producing low-energy (“soft”) X-rays. 

LCLS opened the door to doing science in a completely new way. It provided researchers with a powerful tool to study processes in atomic detail that happen on the extremely fast time scale of millionths of a billionth of a second.

At left: the 100-meter-long string of undulators of the original LCLS. At right: screenshot of “first light” produced by LCLS on April 19, 2009.

On May 9, 2025 – Pellegrini’s 90th birthday – SLAC celebrated the inventor of the lab’s XFEL with a symposium in his honor. Addressing the 150 attendees, including many leaders in accelerator and XFEL science, LCLS Director Mike Dunne said, “LCLS would never have been built without Claudio’s vision and expertise, and today X-ray free-electron lasers are driving a scientific revolution.” Dunne then shared congratulatory messages from XFEL facilities around the world.

SLAC Director John Sarrao (not shown) thanked Pellegrini in his remarks. “SLAC is a different and special place because of Claudio. He continues to be a source of inspiration for us,” Sarrao said.

Claudio Pellegrini with Mark Hogan (at left) and Agostino Marinelli (at right), who both met Pellegrini when they were graduate students at UCLA. Hogan worked directly with Pellegrini as PhD supervisor while Marinelli did his PhD thesis work in James Rosenzweig’s group.

Hogan, who is now the director of SLAC’s Facility for Advanced Accelerator Experimental Tests II (FACET-II), remembered his time at UCLA: “We were inventing and learning things we’re now pretty much taking for granted, and I loved it. I’m profoundly grateful to Claudio, who was always a sage and mentor to me.”

Marinelli, assistant professor of photon science and of particle physics and astrophysics at SLAC, called Pellegrini a wonderful mentor and a very dear friend – to himself and his family. “Claudio taught me not to be afraid of being too ambitious and to work on important problems. He taught me to think about the big picture and how technological developments can drive scientific discovery. Finally, he taught me the importance of being a good mentor and giving students a front seat to the amazing science happening at SLAC,” Marinelli said.

From left: James Rosenzweig, distinguished professor of physics at UCLA; Claudio Pellegrini; Uwe Bergmann, Martin L. Perl Endowed Professor of ultrafast X-ray science at the University of Wisconsin, Madison.

Bergmann, who organized the symposium and was the LCLS deputy director from 2009 to 2013, said, “Claudio is not only a theorist. From the beginning, he was closely involved in the practical and experimental aspects of FELs, and that makes a big difference.” In recent years, Bergmann and Pellegrini have begun to increasingly collaborate, including on projects that could shape the future of XFELs.  

Rosenzweig, who joined Pellegrini at UCLA as an assistant professor just when the SASE experiments were initiated, told the audience about “the great adventure of the proof-of-principle campaign that validated the SASE FEL concept” and paved the way for LCLS construction.

The symposium concluded with a talk by Professor Emeritus Joachim Stöhr, first director of LCLS from 2009 to 2013. “I think it’s time for the first X-ray free-electron laser Nobel Prize in Physics,” he said, arguing that Pellegrini and DESY researcher Evgeny Saldin would be strong contenders for such a prize. Both are pioneers who took Madey’s FEL concept and extended it into the X-ray regime, Stöhr said, and both stuck with it and helped turn their ideas into actual facilities – Pellegrini with LCLS at SLAC and Saldin with FLASH at DESY.