San Diego Supercomputer Center / University of California, San Diego (1996-present)
•  Supercomputer Project Manager (2013-present). Oversee the operation and support of the Popeye supercomputer for the Simons Foundation. Previously performed the same role for the Gordon supercomputer.
•  Distinguished Scientist (2002-present). Doing research and development in bioinformatics. Analyzing DNA sequence data to study human longevity and to characterize bacteria that infect people and birds. Co-principal investigator on NSF and NIH grants to maintain and enhance the CIPRES science gateway, which is used by researchers around the world to infer phylogenetic trees using supercomputers. Improved parallel performance of RAxML, a widely used code for phylogenetic tree inference. Did research in supercomputer performance evaluation and modeling.  Led effort to acquire and evaluate the IBM Blue Gene supercomputer at SDSC.
•  Deputy Director for SDSC and its National Partnership for Advanced Computational Infrastructure (1996-2002). Responsible for computing technology research within SDSC and computational resources across NPACI. Principal investigator on NSF grant and DARPA contract to acquire and evaluate the Tera MTA, a novel computer with multithreaded architecture. Co-led the Joint NSF-NASA Initiative on Evaluation of scalable parallel computers

San Diego Supercomputer Center / General Atomics (1985-1996)
•  Deputy Director for Research (1989-1996); Manager, Applications and Research Department (1987-1991); Manager, User Services Department (1985-1987). Directed research activities at SDSC, with particular emphasis on high-end parallel computing. Co-investigator on ARPA, NIH, and NSF grants to acquire and evaluate a series of parallel computers including the Intel iPSC/860, nCube 2, Intel Paragon, Cray T3D, and Cray T3E. Organized symposia and workshops in high-performance computing. Member of the original SDSC management team. Helped write proposal in 1984 that led to the founding of SDSC in 1985.

General Atomics (1969-1985)
•  Manager, Plasma Transport Branch (1975-1985). Responsible for developing analytic and computational techniques for describing plasma transport in tokamaks. Developed ONETWO, a plasma transport code, and applied it to the analysis of tokamak experiments. Responsible for developing an energy-confinement database for the Doublet III tokamak.
•  Manager, Fuel Methods Department (1972-1975); and Physicist (1969-1972). Responsible for developing all computer codes used to design and analyze fuel for high-temperature gas-cooled reactors (HTGRs). Developed codes to solve reactor physics, flow and thermal analysis, optimization, and statistics problems. Participated in the planning, execution, and analysis of the pulsed-neutron experiments performed on the Fort St. Vrain HTGR.

Jet Propulsion Laboratory (summer 1966)
•  Programmer. Improved numerical integration subroutines in large, orbit-determination code.