During the 1950s, he built two of the earliest magnetic mirror experiments, Toy Top I and II, which demonstrated the concept of magnetic compression for heating plasma, resulting in one of the earliest demonstrations of producing neutrons from a fusion experiment. These were documented in a classified report in 1955 and later reported at the early unclassified international conferences on fusion. He extended these results during the 1960s in the famous 2XII-B experiment at Livermore, in which fusion temperatures in excess of 200 million degrees were demonstrated at fusion-relevant plasma densities, with plasma pressure approximately equal to the magnetic pressure. These results, combined with theory, led to the ill-fated Magnetic Fusion Test Reactor (MFTF-B), which was constructed at Livermore in the late 1970s and early 1980s, only to be terminated as it was ready to operate due to government budget cuts.

Dick Post, another famous Livermore magnetic mirror fusion pioneer and still active, says of Fred, "When practical fusion power is eventually achieved, it will be as the result of notable researchers such as Fred Coensgen in laying the foundation on which fusion power systems will rest."

Fred was an unassuming person, with a cryptic sense of humor, which sometimes resulted in others not recognizing sufficiently his talents as an innovative experimental research scientist and his outstanding managerial skills and judgement.

He received his doctorate in physics from the University of California, Berkeley and worked for 45 years at Lawrence Livermore National Laboratory, retiring in 1994. Throughout his distinguished career he published numerous papers and received multiple awards for his pioneering work in Plasma Physics. In 1989 he received the Fusion Power Associates Distinguished Career Award, recognizing his many career contributions to fusion development.

He enjoyed hiking and photography. In recent years he was an active member in the Livermore Valley Photo Club and taught Photoshop at the Pleasanton Senior Center. He is survived by Charlene Coensgen, his wife of 43 years, 6 children, and 8 grandchildren.

Expression of sympathy may be sent to his wife Charlene at 856 Abbie Street, Pleasanton, CA 94566 or to his daughter Marian Coensgen-Luna, 1882 Kaweah Drive, Pasadena, CA 91105 (MLuna@chla.usc.edu).