Obituary of John McDowell Walsh

John McDowell (Mac) Walsh, noted for work in shock wave physics, died March 9, 2009 at his home in Santa Fe, NM. He was awarded, with M. Rice and R. McQueen, the first George E. Duvall Shock Compression Science Award “In recognition of pioneering contributions in the use of intense shock waves to determine high-pressure equations of state for condensed matter.”

Mac enlisted in the Naval V-12 program soon after Pearl Harbor and eventually served as a deck officer on a destroyer in the Pacific theater. After the war he entered graduate school at the University of Texas in Austin, majoring in physics with a minor in mathematics. His dissertation concerned molecular orbital theory. On completion of his PhD. in 1950 he joined Group GMX-6 at Los Alamos, which he served as group leader in the period 1956-60. During those 10 years Mac, with numerous collaborators, published research that extended the science of shock wave physics and its role in determining material equations of state at extreme pressures. Novel experimental techniques, as well as refined theoretical methods, were developed while at Los Alamos. Much of this work is summarized in the 1958 review article in Seitz and Turnbull’s Solid State Physics, Vol. 6. The Topical Group for Shock Compression of Condensed Matter of the APS, established in 1981, gave him its first Shock Wave Science Award in 1987 along with Melvin Rice and Robert McQueen.

Mac was invited to join Project Orion at General Atomic in 1960 to address the many issues in high-pressure physics. Orion was to be an enormous spaceship propelled by nuclear explosion pulses and designed to navigate the solar system. Concern about shock wave damage in the pusher plate led him to develop a criterion for shockless acceleration. Orion was eventually terminated because of the test-ban treaty, but many scientists and engineers considered the project a high point in their lives. All this was documented in Project Orion, an excellent book by George Dyson, Freeman’s son, who drew heavily on his father’s experiences and contributions.

While at General Atomic Mac was awarded a long-term contract to investigate hypervelocity impact. This led to a revolutionary approach known as Late Stage Equivalence. The concept states that it is neither the energy nor the momentum of a projectile that controls the late-stage behavior following hypervelocity impact, but an intermediate quantity mv^β, with m the projectile mass and v its velocity. It was shown with computer simulations that all impacts with the same mv^β converge toward the same late-stage behavior for β =1.74, slightly less than the value of 2 for energy equivalence. Thus, the effects of impacts far above the experimental range could be inferred. This was important because it was being argued by some that momentum governed, implying β equal to 1. At Mac’s suggestion, Dienes addressed this issue analytically and showed that, in one dimension, β is an eigenvalue of the (nonlinear) flow equations supplemented by shock wave behavior at one boundary of the flow and zero pressure at the other. Approximate analytic solutions to the multi-dimensional problem were later found by W. Rae. This result accounts for momentum multiplication in the target as a result of the backward ejecta. Walsh joked privately that the record for momentum multiplication was held by the photoelectric effect. Walsh and Dienes went on to show that strength effects, including crater size, could be accounted for with a special plastic-flow model, as reported in High Velocity Impact Phenomena, R. Kinslow, Ed.. While the first calculations were performed with a one-material Eulerian code, subsequent studies were performed with a multi-material code known as HELP, developed with L. Hageman and now used throughout the world in various incarnations.

Mac was instrumental in starting Systems, Science, and Software, a California corporation specializing in weapons effects. When it was acquired by another company, he was invited to return to Los Alamos as Associate Division Leader for M Division. Subsequently he led group M4 until his retirement. While back at LANL he initiated a variety of applied programs involving high explosives. He also investigated the interaction of oblique detonation waves with metals, the topic of his paper opening the 1987 Shock Wave Physics Meeting. In those years he also taught a variety of hydrodynamics courses both at LANL and externally.

He was widely admired for his deep insights into thermodynamics and hydrodynamics, his leadership abilities, subtle sense of humor, and humility.