About Marathon Fusion

Marathon Fusion is building the technologies needed to make fusion practical. Our work spans high-flux fusion neutron sources for radioisotope production, fuel cycle technologies, and plant-level systems that will enable fusion power at climate-relevant scale.

We are developing compact, beam-driven fusion systems to produce radioisotopes for targeted cancer therapies and to test materials for future fusion power plants. Alongside this work, we develop stable isotope enrichment, tritium processing and recycling, and advanced blanket technologies to close the fusion fuel cycle and enable both energy generation and high-value co-products.

Founded in 2023, Marathon Fusion is advancing near-term applications of fusion in medicine and materials development while building the foundation for abundant, reliable energy in the 21st century.

Culture at Marathon Fusion

We are a startup environment characterized by a need for focus, adaptability and prioritization in a shifting landscape. Thriving in this environment requires excellent proactive communication across roles and backgrounds, the ability to build and maintain trust between team members, and the willingness to learn and develop new skills on the job. We place particularly high weight on each team member's ability to be self-directed.

Please review these points carefully before applying:

• This application is exclusively for the Plasma Modeling Intern role.
• We provide both Internship and Co-op placements through this single posting.
• Application Deadline: December 2nd, 2026

Role Description:

You will join our science team as a physics modeling intern, building simulations that guide the design of our fusion fuel cycle hardware.

Much of our hardware operates in regimes where fluid models stop being valid and particles have to be tracked directly. Your main project will likely involve Direct Simulation Monte Carlo (DSMC) and Particle-in-Cell (PIC) modeling of rarefied gases and partially ionized plasmas.

Once you have ramped up, you will also be expected to help with smaller modeling projects across the company, which may involve different physics and different methods from your main project.

Role Responsibilities:

• Build, run and validate particle-based simulations (DSMC, PIC) of rarefied gases and plasmas
• Present results and turn them into takeaways that inform real hardware and design decisions
• Take on smaller modeling projects alongside your main one, and keep track of several at once

Minimum Requirements

• Pursuing or holding an undergraduate degree in physics or a similar field
• Experience with numerical physics modeling

Preferred Requirements

• Research experience in computational plasma physics
• Experience with Particle-in-Cell modeling or Direct Simulation Monte Carlo
• Skilled at using AI tools for research and coding.

Soft Skills:

• Excellent written and oral communication
• Ability to think holistically about systems
• Capacity to learn new technical subjects quickly, collaborate cross-functionally, and consider problems from inter-disciplinary perspectives
• Organization and project management
• A high degree of conscientiousness and integrity
• Long-term vision while executing short-term goals
• Comfort with the inherent uncertainty of breakthrough technology
• Experimental mindset, ability to form and test hypotheses

Marathon Fusion considers all qualified applicants equally for employment. We do not discriminate on the basis of race, color, national origin, ancestry, citizenship status, protected veteran status, religion, physical or mental disability, marital status, sex, sexual orientation, gender identity or expression, age, or any other basis protected by law, ordinance, or regulation.