Uri Feldman

On 26 February 2023 we lost Uri Feldman, a pioneer in the spectroscopy of highly ionized atoms. Uri’s work focused on developing instruments, making observations, and formulating theoretical models to understand astrophysical and laboratory plasmas using high-resolution spectroscopic techniques, primarily in the extreme UV and x-ray ranges. His astrophysical studies concerned the Sun’s upper atmosphere, while the laboratory studies involved laser produced plasmas, tokamaks, and electron beam ion traps (EBITs).

Uri was born on 17 April 1935 in Kfar Ganim, a small village in Petach Tikvah, Israel, and grew up there. He entered the Israeli army in 1952, and in 1956, after his military service, he began studying physics at the Hebrew University in Jerusalem, obtaining a PhD in physics in 1965 under B. S. Fraenkel.

Uri obtained a postdoctoral position at NASA’s Goddard Space Flight Center (GSFC) in Maryland in 1966 and began identifying spectral lines of highly ionized atoms. He developed a low inductance vacuum spark that produced highly ionized atoms, resulting in an ion spark spectral line list in the 10–18 Å region that he compared with solar flare spectra obtained by orbiting spacecraft launched by NASA. Uri and coworkers at GSFC also developed a superb grazing incidence 3-meter rocket spectrometer for solar atmosphere studies. Following his postdoctoral research, Uri returned to Israel and took a position at Tel Aviv University.

In 1969, Yuval Ne’eman, head of the Physics & Astronomy Department at Tel Aviv University, asked Uri to oversee the building of the first major astronomical observatory in Israel, the Wise Astronomical Observatory in the Negev desert. Uri consulted extensively with Ira Bowen at Mount Palomar on optical design of large telescopes. Completed in 1971, Wise Observatory has been in continuous operation for more than 50 years.

In 1976 Uri joined the Space Science Division of the Naval Research Laboratory (NRL) in Washington, DC, and worked there until his retirement. With NRL coworkers, Uri began analyzing NRL solar spectroscopic data as well as laboratory extreme-UV (EUV) and x-ray spectra produced by lasers in the NRL Plasma Physics Division.

From spectra obtained by NRL instruments on Skylab, Uri and colleagues developed many diagnostics for measuring plasma temperature distributions, densities, and chemical abundances in the Sun’s chromosphere, transition region, and corona, greatly extending what was known about the Sun’s highly dynamic atmosphere. These measurements led him to be among the first to recognize that the structure of the Sun’s upper atmosphere was much more complex than originally thought.

From the NRL laser-produced plasma EUV and x-ray spectra, Uri and colleagues identified many new EUV and x-ray spectral lines of highly ionized atoms. They compiled a database of n=2-2 transitions in the B I through F I isoelectronic sequences, enabling the determination of highly-accurate energy levels for a wide range of elements.

Uri participated in designing solar physics Bragg crystal x-ray spectrometers that were flown on the Department of Defense’s P78-1 spacecraft and later on the Japanese Yohkoh spacecraft. He collaborated with scientists at the Aerospace Corporation in analyzing solar flare and active region x-ray spectra from the NRL instrument and from an Aerospace X-ray spectrometer flown on P78-1. These groups identified many of the x-ray lines in the solar spectrum between about 1.7–23 Å and determined many of the physical characteristics of x-ray solar flares. Later, Uri participated in analyzing EUV spectra from the Japanese Hinode spacecraft.

In the 1980s, more powerful lasers became available at Lawrence Livermore National Laboratory (LLNL), Los Alamos National Laboratory, and at the University of Rochester Laboratory of Laser Energetics (LLE). Uri helped develop spectroscopic instrumentation for use at these laboratories that greatly extended line identifications of highly ionized iron and heavier elements up to uranium, enabling the analysis of relativistic and quantum electrodynamics effects. The spectral line identifications were used to validate atomic computer codes that calculated energy levels and intensities in multimillion-degree plasmas. Uri and colleagues developed methods for achieving lasing in the EUV and x-ray regions using energy level population inversions achieved by collisional and photo-pumping processes.

Uri and colleagues also studied ions of iron and other solar abundant elements using plasmas created by the Princeton Large Tokamak and the EBIT at LLNL. The teams at LLNL and NRL made the first accurate measurements of the polarization of x-ray transitions in heliumlike ions of iron and scandium. Later, Uri helped establish an EBIT at NIST.

Uri was an author or coauthor of more than 400 papers published in refereed journals. He was a member of the International Astronomical Union and a fellow of both the Optical Society of America and the American Physical Society. In 1989 he was awarded the NRL E. O. Hulburt Annual Science and Engineering Award for outstanding research.

Uri Feldman was a superb scientist and a great colleague. He was always enthusiastic and eager to attack new problems in atomic and solar physics. Uri never missed an opportunity to engage colleagues in spirited discussions of ongoing solar and atomic physics problems. He will be greatly missed.

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