John Edmond Walsh, Francis and Mildred Sears Professor of Physics at Dartmouth College and a pillar of the free electron laser (FEL) community, died on 5 December 2000 in Bradford, Vermont, of complications following surgery to repair a broken tibia.
Born in New York City on 20 August 1939, John grew up in Montauk, New York, graduating in 1962 from Nova Scotia Technical College with a BS in electrical engineering. He then joined the US Army Signal Research Laboratory at Fort Monmouth, New Jersey. His early training there, with its practical work in electrical engineering coupled with his own interest in basic science, helped to give him the unusual range of curiosity and ability discernible throughout his career.
John claimed to have come to physics by chance. His old ragtop convertible, used for commuting between Montauk and Fort Monmouth, broke down in front of the Columbia University physics department. He went in to seek help and emerged several years later with MS (1965) and ScD (1968) degrees. Although he was part of a research group in plasma physics, the degrees were in electrical engineering. His thesis, under the supervision of Perry Schlesinger and Tom Marshall, was a study of cross-magnetic field diffusion induced by nonthermal kilohertz fluctuations in a plasma. With his doctorate completed, John immediately joined Dartmouth College as an assistant professor of physics.
At Dartmouth, John continued in plasma studies and quickly became known as an outstanding teacher and research scientist. His first experiment on the scattering of electrons from electrostatic plasma fluctuations led him and his students on to extensive studies of the interactions between electron beams and plasma at higher beam currents and energies.
The focus of his research shifted following a 1975–76 sabbatical at Columbia University. Working again with Marshall, he took up the investigation of the stimulated Čerenkov effect. Their new idea was to use an intense relativistic electron beam in which a fast-growing beam instability would occur in a waveguide if the electromagnetic waves were slowed to the electron speed using a dielectric liner near the beam. This method resulted in exponentially developing bunching, which was the source of megawatt-level microwave and millimeter radiation.
On his return to Dartmouth, John greatly expanded on this theme using a series of FEL-type devices involving the interactions of electron beams with both dielectrics and gratings. His characteristic style of work was to use the minimum of hardware and experimental beam time to achieve useful results and to interpret the findings with outstanding physical insight. The resulting papers and conference contributions were models of clarity.
A nice example of John’s imaginative approach to research was his recent (1998) production of a far-infrared source based on the Smith–Purcell effect. He used a redundant scanning electron microscope to provide an electron beam that could be passed very close to a grating. The interaction of beam and grating produces useful power in the 200–1000 µm region, where there are few alternative sources.
John’s research group at Dartmouth was quite small, consisting of one permanent research associate (Robert Layman), the occasional postdoc or visiting scientist, and both graduate and undergraduate students, attracted not only by his preeminence in research but also the happy atmosphere in the laboratory. John found time for his students, even during those periods when he was head of the department (1980–84) or associate dean of the science faculty (1994–98). He was a most able administrator, helped by his abhorrence of unnecessary meetings and paperwork, which kept him from his beloved research. His legacy to the college as associate dean is a splendid extension to the Wilder Physics Laboratory and major improvements to the chemistry teaching labs. His legacy to science is about 40 doctoral students, nearly all still active in research.
For experiments at higher energies, John required electron beams not available at Dartmouth. His enthusiasm and expertise made him very welcome elsewhere: He led successful experiments at ENEA (Agency for New Technologies, Energy, and the Atmosphere) in Frascati, Italy, on Cerenkov lasers and at Oxford University and Brookhaven National Laboratory on Smith–Purcell research. Each of those projects led to significant advances in the field of FEL research. His interests extended to more applied activities, including microwave hyperthermia for cancer therapy and studies on power lines. He also held two patents on soil moisture probes, developed for the US Army Corps of Engineers, which are now used extensively in agriculture.
In recognition of the significance of his experimental research and a number of important theoretical papers, he was awarded the International Free Electron Laser Prize in 1998.
John had a great sense of humor and a fund of good stories. In his youth, he had worked on fishing boats out of Montauk; it was probably there that he acquired the happy knack of treating all people equally. Pomposity was anathema to him; as associate dean, he still drove to work in his ancient flatbed truck—flatbed because, at some stage, the sides had fallen off. A considerable athlete in his younger days, he remained an excellent skier until his untimely death. Science, his close-knit family, the Dartmouth community, and his countless friends and colleagues in many countries have suffered a grievous loss.
