Proxima Fusion is proud to announce the open-source release of VMEC++, a modern version of the Variational Moments Equilibrium Code (VMEC). VMEC++ isn’t only an important new tool ready for the next generation of stellarators—it’s our first major contribution to the open-source software community and our encouragement to other members of the fusion community to share source code for physics software that can accelerate the entire field.

The journey from legacy code to modern tool

The original VMEC code has long been the workhorse of 3D magnetic equilibrium analysis, the most essential piece of software in the stellarator modeling toolchain. Originally written in Fortran by S.P. Hirshman and colleagues in the early 1980s, it has been adopted and modified by scientists across the world for decades, accumulating a significant amount of technical debt—that is, redundant features making the code harder to maintain and advance. Not only is VMEC hard to understand and modify, it’s also not code anyone would want to use as the foundation for building a first-of-a-kind fusion power plant!

Enter Proxima Co-Founder Jonathan Schilling, who decided to address this problem during his PhD at the Max Planck Institute for Plasma Physics, where he was working on data analysis for Wendelstein 7-X (W7-X)—the most advanced stellarator in the world. Jonathan developed a deep understanding of the numerics of VMEC by porting parts of it to Java and Python.

Plasma equilibrium calculations are at the core of stellarator design. That’s why Proxima decided early in its journey to leverage Jonathan’s know-how of VMEC to rewrite the code in C++, dedicating amazing software engineers to the project (wink, wink—Enrico Guiraud). We placed special emphasis on ensuring the code has state-of-the-art test coverage and is developed according to modern best practices—making it a tool you can trust and understand on the path toward building a fusion power plant.

Open sourcing for the future of fusion

For years, VMEC has been publicly available under an MIT License, maintained by ORNL on GitHub. The MIT License is commonly recognized as one of the most permissible licenses for public and private entities, permitting private companies to modify the code at will while keeping those modifications entirely private. At Proxima, we recognize that our work on VMEC++ can be extraordinarily useful to the wider stellarator physics community. We are therefore open-sourcing it under the same MIT License, releasing it to both public and private entities around the world to accelerate humanity’s progress toward fusion energy.

And that’s not all: we’re additionally releasing our internal documentation of the physics and numerics of both VMEC and VMEC++, sharing know-how of the code base and opening the black box for colleagues working across the field of fusion research.

Proxima works on the principle that physics inherently requires external validation, and the principle of peer-review is fundamental to ensure that we can build fusion power plants faster. That applies to scientific code every bit as much as publications—we hope VMEC++ will be used, criticized, modified, and eventually made redundant by better tools that the community will create using this new shared knowledge as a foundation.

Applications and advantages

As it becomes available to the entire fusion ecosystem, we expect VMEC++ to find plenty of applications, including:

• Optimization of stellarator configurations, both in fixed- and free-boundary mode.
• Experimental data analysis of stellarators, including W7-X.
• Inference of experimental stellarator equilibria—the key objective that originally pushed Jonathan to dive into the complexity of VMEC for his research on W7-X.
• 3D equilibrium analysis for tokamaks, particularly for tokamak disruption analysis and for scenarios using Resonant Magnetic Perturbations.

At Proxima, we’re already using VMEC++ for our own stellarator design purposes, taking full advantage of its cleaner code, faster runtimes and full Python integration. Additionally, VMEC++’s easy-to-use “hot-restart” feature dramatically decreases runtimes when analyzing many similar equilibria, as it often happens in the context of stellarator optimization.

We look forward to collaborating with public and private fusion researchers across the globe to further advance VMEC++ and move us all, together, toward clean energy, for good. 

Resources

• Source code of VMEC++ on GitHub, tested for installation on major platforms like Ubuntu and Mac
• Code documentation for VMEC++
• Documentation of the numerical details of VMEC++