John quickly established himself as a pioneer in the use of numerical formulations of plasma dynamics and, using the best computers available at the time, he contributed numerical models to guide and explain the numerous types of fusion "magnetic bottles" that were built in the 1960s. During the 1960s, also, John became a professor and eventually Associate Dean of the Graduate School of Engineering at the University of California at Davis.

In the early to mid 1970s, high speed computers were becoming more available and useful to the fusion effort and John became the founding Director of the Controlled Thermonuclear Research Computing Center. This Center evolved to become the major centralized computing center for all of the Department of Energy Office of Science programs, now known at the National Energy Research Supercomputing Center (NERSC). John led the Center from 1974 to 1990, when he retired following a disabling stroke. Fusion Power Associates presented John with a Special Award that year.

In addition to his many scientific contributions and leadership of the fusion computing center, John played other roles important to fusion development, including Principal Editor of the Journal of Computational Physics and founding director of the Plasma Physics Research Institute at Livermore.

Whenever a swimming pool was around, John could always be seen in it. He was a Master Swimmer, and he competed in races in the ocean, in the San Francisco Bay and in high mountain lakes.

Condolences may be sent to his daughter Kate Killeen: killeenatlaw@yahoo.com