Jonathan Citrin: “I want to help build the cathedral of fusion”

July 12th 2022

Jonathan Citrin, leader of the DIFFER group Integrated Modelling and Transport, recently was awarded a grant from the Netherlands eScience Center and was principal investigator of a consortium awarded an ITER tender for simulation framework development. In this interview he talks about the special group dynamics in the area of nuclear fusion and the beauty of his research field.

Jonathan Citrin
Jonathan Citrin.
(c) Bram Lamers/DIFFER

Short biography of Jonathan Citrin
Jonathan Citrin (1980) leads the research group ‘Integrated Modeling and Transport’ at DIFFER. He was born in Hong Kong and moved at the age of 14 with his family to Israel, where he completed high school and studied physics. In 2009 he moved to the Netherlands to start his PhD research in plasma physics, which he completed in 2012. After a postdoc in France he has been working at DIFFER since 2016. In his free time he likes playing the guitar, though, with two small kids he has less time for it than he would like. In his PhD days he and DIFFER colleagues played blues and classic rock songs in a band called ‘The Wrong Assumptions’. The band also covered songs whose lyrics were changed to make fun of science.

Element of mystery
When Jonathan Citrin was doing his MSc-research in plasma physics at the Weizmann Institute, he spent many nights working till two or three o’clock in the morning performing calibrations of photo multiplier tubes. “After that experience, I was convinced that I’d be happier working in theory and simulation”, he says. “But, do not get me wrong, I am hap-py I did it and I gained enormous respect and appreciation for experimental physics.”

From high school on, Citrin has been interested in those aspects of theoretical physics that contain an element of mystery: “My brain was tickled by things like time dilation in relativity theory, multiple dimensions, and the nature of uncertainty in quantum physics. But it was my societal interest in the promise of fusion that made me decide as a student to con-tinue in plasma physics and not, let’s say, in quantum information theory.”

Citrin did experimental MSc-research that is very useful for his present work. “For example, it gives me a better understanding of how data is acquired and processed.” Citrin leads the DIFFER-group Integrated Modeling and Transport which aims to understand the behavior of the super hot plasma inside a fusion reactor.

What is the beauty of your research field for you?

“It is the combination of a fascinating intellectual subject with the goal of reaching the ultimate energy source. Fusion plasmas show a very rich, beautiful interaction between different physical subsystems, which is the result of highly nonlinear physics. At the same time the societal promise of fusion as an ideal power source is immense.”

Commercial fusion has been a dream for many decades, but that dream has still not been realized. What keeps you motivated?

“I have confidence that in the end it will work. In my subfield it is safe to say that the plasma confinement now has a very high probability of being sufficient for a commercial reactor. Of course, before commercialization there are still challenges in certain engineering aspects, such as protection of the wall material, but those things are solvable. If suddenly there was a moon shot effort in fusion, these things could be solved in a much faster time scale. Whether it will take 20 years or 40 years, I believe it will work. And once it works, fusion will be an ideal power source. Some cathedrals took hundreds of years to build. I want to help build the cathedral. I want to be part of something bigger.”

What do you consider to be the unique selling point of your group?

“We focus on the turbulent transport in the fusion plasma, more specifically on building and validating transport models. What makes us unique is that we combine simulation models with different fidelity levels. Some models have a high accuracy, but then they are slow to run. Other models run very fast, but are less accurate. Ultimately you want a model that is both fast and accurate. To reach that aim, we validate the fast models with the more accurate but slower models. We also help build, maintain and run one of the few physics-based reduced order turbulence models in the world, used for tokamak simulation. In addition, we develop its neural network surrogate – a fast replica of the physics model using machine learning techniques. We are one of the pioneers of this technique for tokamak simulation. The neural network model provides a major acceleration in modeling the plasma.”

Realizing the dream of fusion not only takes a long time, but also a huge group effort. How do you look at this group aspect of your field?

“Collaboration is key. It is just too big for any individual or even a small group. Of course, every problem can be broken up into smaller pieces, but those pieces still need to be embedded in the larger community.”

What does this mean for you as a group leader?

“I find it very important that our work is strongly embedded in larger collaborations, such as ITER and EUROfusion. In our field it is not enough to show a proof of principle and write a paper that later collects dust on academic shelves. It is even not enough to build a prototype. It is important to have an international team that can incorporate the developments of smaller groups and turn them into workhorse community tools for everybody. This demands commitment to long-term maintenance, sustainability, and knowledge transfer.”

How do you lead your group?

“I find it very important to help develop group members’ network and visibility from an early stage, to help embed their work within the wider community. Beyond coordinating projects, it is also very important for me to remain active with technical work and analysis, to continue to discover and learn. I do not want to lose touch with the real stuff. It also makes me very happy when I learn from the skills of my group members, for example in software engineering and machine learning. As for the social aspect of leading the group, the pandemic has made it very challenging in the last two years. It became so much more difficult to maintain the social spirit that is important in the group dynamics.”

And what does the group aspect require of the individual scientists? Do they need specific skills?

“Intellectual curiosity and strong analytical skills are the foundation as in any research group. However, the nature of the fusion modeling community also means that people should be team players, have a strong collaborative instinct, be good communicators, network oriented and goal oriented. This is Big Science. Team spirit is crucial.”

Text by Bennie Mols