Remembering the Contributions of Todd Evans

by Richard Buttery and Rick Moyer

Todd Evans was a giant amongst the fusion community – one of the great thinkers, and great innovators, with a passion for the science, but also a kind word for his colleagues and mentor to so many. He passed away on Monday October 26^th after a short illness. His death was quite unexpected and a great shock to the fusion community.

We came to know Todd as one of the most innovative thinkers in the fusion program. Perhaps his seminal contribution was his pioneering research in the use of edge-resonant magnetic field perturbations (“RMPs”) to suppress heat burst instabilities known as edge localized modes (“ELMs”) in tokamaks. Todd predicted that tiny fields, one ten-thousandth of total field, could suppress the ELMs, and successfully demonstrated this in the DIII-D tokamak in July 2003 using a set of coils originally designed for another purpose. Todd extended this technique to high confinement plasmas like those expected in ITER, the power plant scale fusion reactor now being constructed in France, leading to a seminal paper in the journal Nature Physics in 2006, and the adoption of such coils in the ITER design.

However, this was only the tip of the iceberg in Todd’s broad and pioneering contributions, which extended over a much wider range of research topics, with ground breaking work up to his final days. Recurring themes were the complexity and benefits of complex “3-D” magnetic fields and the resulting “homoclinic tangles” and other structures [https://sites.google.com/site/scitangle/]. These chaotic phenomena were exciting from a fundamental science perspective. But they could also be used to influence and regulate plasma behavior in a multitude of ways, leading to a range critical scientific advances throughout his career and driving the advancement of whole fields of plasma physics.

Todd started his work in fusion research as a graduate student on PRE-TEXT tokamak at University of Texas, where he made the first direct observation of the structure of a driven global Alfven eigenmode in a tokamak plasma using CO2 laser interferometry. He then joined General Atomics in 1985 in Applied Physics Group and spent much time at other labs studying 3D physics, including the ergodic magnetic limiter program on the TEXT tokamak, the HIDEX pumped-limiter program on JIPP TIIU in Nagoya, Japan (where he was co-leader), and the Tore Supra Ergodic Divertor Program as leader, and later helped develop the conceptual physics design of the TEXTOR Dynamic Ergodic Divertor.

In the 1980s, Todd undertook seminal research on toroidal asymmetries in divertor tokamaks at ASDEX, using divertor calorimetry to study structures in divertor heat fluxes. When he returned to GA in the early 90s and joined the DIII-D program, he designed and installed the Toroidal Tile Current Array, and used it to confirm the existence of helical, thermoelectric currents in the divertor tokamak boundary, currents often referred to as “Evans currents”. To understand these toroidal asymmetries, he developed the concept of “intrinsic topological noise”, which produces toroidal asymmetries in any poloidal divertor tokamak with discrete magnetic coils.

Todd was also the first in the tokamak research community to bring the well-established concepts of stable and unstable manifolds, and homo- and heteroclinic tangles to the tokamak community, not without some blowback. On one occasion, Todd was asked “why do you keep using this language? I don’t know what it means and don’t understand it.” To which Todd calmly and quietly replied, in the most “Todd” of ways: “because you need to learn it because it’s important”.

As great a plasma physicist as Todd was, he was always first a nonlinear and complex dynamics physicist who happened to study complexity in magnetically confined plasmas. He brought the concepts of homoclinic and heteroclinic tangles into tokamak research because he knew that these structures, which in many complex systems were abstract concepts in phase space, were quite real for the conservative Hamiltonian magnetic field in a tokamak, and were important for understanding magnetic island growth and interactions. And when the tokamak community started photographing the tangles, for example by Max Fenstermacher using the X-point tangential TV, this was quite exciting for scientists in that community who had never “seen” the tangles in their system.

And Todd was a versatile physicist. He developed the MCI Monte Carlo Impurity transport code to study physical sputtering and chemical erosion. And invented the cryogenic Argon “killer pellet” to reliably generate runaway electrons following a rapid shutdown. Killer pellets have enabled DIII-D to study runaway electron physics for nearly two decades, enabling the facility to work on their fundamental physics and control.

Todd was also an expert in magnetic reconnection in space physics. In 1994-95, when the US fusion budget collapsed, the DIII-D program made an effort to find new funding sources to “tide it over”, and Robin Snyder, then the DIII-D diagnostics coordinator, came to Todd with a NASA FOA for the “Fire and Ice Solar Probe Mission”, wanting to know if Todd could come up with an idea worth proposing in response. Todd, Robin and Rick Moyer worked together, with support from colleagues in the Center for Astrophysics and Space Sciences at UCSD, to develop the SECA concept for the mission (the Solar Event Correlation Analyzer). They wrote a white paper and Todd presented it at the mission workshop in October of 1995. [Evans, et al., “Causal Spatio-Temperal Correlations of Short Scale Length Solar Wind Acceleration and Heating Mechanisms with a Solar Event Correlation Analyzer,” in: Robotic Exploration Near the Sun: Scientific Basis, AIP Conference Proceedings 385 (American Institute of Physics, Woodbury,1996) 145.]

Here we first experienced what became a recurring theme in collaborations with Todd: most of those who saw Todd’s presentation at the meeting had been working on this mission design for 10-20 years, and they thought these new ideas were crazy! Not unlike what most at DIII-D thought of Todd’s proposal to suppress ELMs with magnetic perturbations from the C and/or I coils in 2002-2003! But a leader in solar physics theory from Los Alamos National Lab, spent the lunch break dumping his prepared talk for the afternoon, and running with Todd’s ideas. That afternoon, he stood up and said, essentially, that not only was our idea good, but it was what the mission should be about. This pattern of the community thinking Todd’s ideas were ahead of their time, but always a small group would see the physics behind them and the opportunity for a breakthrough. This “outside the box”, thinking—always focused on solving a problem facing fusion—was a hallmark of Todd’s career.

Perhaps because he didn’t start out as a DIII-D team member, but in Dan Baker’s Applied Plasma Physics Group, he was a “migrant plasma physicist” (have good idea, will travel to find tokamak to research it), he developed an extensive network of international collaborators — evident in looking at the likes and comments to announcement of his passing on the fusion and DIII-D Facebook pages, and the many remarks we have received. Todd was always highly collaborative (we were accused of recruiting international collaborators for the first RMP ELM control experiments to leverage run time—8 in total came to participate in person), and much of this time was spent collaborating with and mentoring early career scientists. He had a phenomenally successful record with undergraduate summer students, including his final SULI student who received one of two awards at the APS DPP meeting just last week, but also mentored dozens of graduate students, postdocs, and early career scientists in his 35 years in plasma physics.

Todd’s contributions have been recognized at the highest levels in our field – a Fellow of the American Physical Society, a recipient of the John Dawson Award for Excellence in Plasma Physics Research and winner of the Nuclear Fusion Award. And his scientific impact has been huge and well recognized, with a memorable front page article in Nature Physics in 2006 for RMP ELM suppression, and perhaps most significantly, his footprint on ITER, where his innovative thinking and diligent scientific work has given us new solutions to control the heat emerging from fusion plasmas – addressing maybe the greatest challenge in plasma physics. His work has led to and driven whole fields of research, and his contributions are world-renowned.

But perhaps his personal contributions to his colleagues will be most remembered. As one scientist put it, “I joined DII-D in 1990, while Todd started his professional career in about 1985. So I was a half a decade behind him, but he embraced me when I got here, encouraged and mentored me to be a part of his research, and made a huge impact on my career. I wouldn’t be here today without his support and collaboration.” That, to many, is Todd’s true legacy: a vast, international network of collaborators, many of whom were mentored by Todd on his “hairbrained” ideas, and who are now, key scientists in the international fusion community.