FPN21-09

Laser Fusion Pioneer John Trenholme Passes, Age 81

January 31, 2021

John Trenholme, a pioneer in laser physics who left a lasting impact on the field, passed away from a rare neurological illness on Nov. 23, 2020. He was 81.

Much of John's life's work came to fruition in the National Ignition Facility at Lawrence Livermore National Laboratory, where his years of basic research yielded mathematical descriptions of fundamental optical physics, nonlinear optics, and optimized system designs capable of delivering the requested performance of the world's largest and most energetic laser. NIF's operating hardware is the product of his commitment to the pursuit of laser fusion in the laboratory.

John was born in Eugene, Oregon, to Edward Curtis Trenholme and Jessie Alice Trenholme. As a child, he loved science, especially physics. Upon graduating from high school, he received a National Merit Scholarship that enabled him to study at the California Institute of Technology, where he earned his B.S., M.S., and Ph.D. degrees in physics. During graduate school, he spent a year working in India to set up a low-temperature laboratory at the Indian Institute of Technology. John's wife Keren describes this as a deeply meaningful experience that changed how he viewed the world and his relationship with other people. Prior to working in India, he had not traveled far or experienced different cultures. After India, John journeyed around the world before returning to Cal Tech. He enjoyed meeting people and developed an understanding of the complexities of other cultures. His travels later in life were always enjoyable but they never had the same impact as his year in India or his travels as a young man.

In 1969, John joined the Optical Sciences group at the Naval Research Laboratory in Washington, D.C. His analytic work on infrared and ultraviolet sources of pulsed light laid the groundwork for his subsequent analyses of flashlamps as pumps for solid-state lasers.

In 1972, John joined LLNL in Livermore, CA. His collaboration with Ken Jancaitis and Geoffroy Le Touzé led to the ray-tracing code AMP 3D, used to design the pump cavities for both NIF and Laser M├ęgajoule, the French laser facility subsequently constructed in Bordeaux, France. John's interest in modeling the architecture of large laser facilities began with modeling of pre-NIF master-oscillator, power-amplifier chains (Janus, Argus, Shiva, and Nova). He created CHAINOP, a Visual Basic optimization code used to sift through hundreds of thousands of alternative configurations, leading finally to selection of the multi-pass laser architecture now seen in NIF. His interest in performance and cost optimization did not stop with NIF, however. As NIF was beginning to operate, John turned his attention to building TORRID, a code written for laser system optimization of a diode-pumped laser fusion power plant in the future.

Detailed understanding of laser amplifier behavior took a huge step forward with the combination of John's mind and the availability of desktop computers, leading to several generations of PROP, a laser propagation code capable of describing the phase and amplitude along a laser amplifier chain. He conceived of and directed development of the Virtual Beamline (VBL), PROP's modern-language follow-on, which is now the Laser Performance and Operations Model (LPOM) computational code that sets up every shot taken on NIF. A few years before retiring, John also contributed to the inception of VBL++, the future generation of broadband laser physics code that now executes on Livermore High Performance Computing.

John also made significant contributions in the study of optical damage. He wrote Equivinit, a small piece of code now subsumed within LPOM that provides a common basis for predicting damage initiation, regardless of the pulse shape of a potentially damaging beam. John's insight also drove selection of the rapid ablation mitigation (RAM) cone design, the goal shape for CO2 laser mitigation used by the NIF Optics Recycle Loop to repair growing damage sites.

John's most important and enduring contribution is likely the concept - named by him - of the "B-integral." At the high energy and power levels required from a laser designed for pursuit of ignition, the index of refraction of the laser material becomes nonlinearly modified during transit of the high-intensity beams, leading potentially to self-focusing and unrepairable damage to the laser media. Described for the most general case by the 81-element c3 susceptibility tensor, such behavior was devilishly difficult to predict. John devised a very basic metric, the B-integral, a measure of the intensity induced phase push-back along an amplifier (with units of radians) that remarkably simplified calculation of this effect. As an amplified beam moves through the isotropic media of laser slabs and focusing optics and is reflected by turning mirrors, small imperfections in these optics transfer energy into parasitic modes. A well-designed laser propagation code can keep track of these imperfections, and since their gain, or importance, is linearly proportional to the B-integral, the risk posed by these modes can be readily identified. John's insight was critical for the design of the NIF amplifier chains and remains an important consideration today as efforts to push NIF to higher output energy and power continue. The definition and application of the B-integral is used by all international efforts as they design other high-power laser systems around the world.

Beyond his scientific contributions, John was also a leader and mentor for multiple generations of laser physicists, modelers, and engineers. His legacy persists today and into the foreseeable future.

In his free time, John was an avid reader with a large personal book collection. Towards the end of his life, his disease caused paralysis of most of his body except his head. He continued reading and communicating with the world through an app that allowed him to control a computer with his eye and head movements.

He is survived by his beloved wife, Keren; his son Samuel Trenholme; Samuel's daughter, Citlalli Trenholme; Keren's daughter, Ellen Perling Skinner, and grandchildren Sarah and David Perling; his stepchildren from his first marriage, Judy Hazen and Jim Long; and his sister Alice Isaacman and brother Peter Trenholme.

A celebration of his life will be held at a future date when it is safe to gather. Contributions in John's memory can be sent to the Wikimedia Foundation, the Internet Archive, and the Food Bank of Contra Costa and Solano County.

Remembrances may be sent to Mark Herrmann at LLNL: Herrmann9@llnl.gov