About Marathon Fusion

Marathon Fusion is building the engineering technologies needed to scale fusion from breakeven experiments into real power plants. While much of the industry is focused on proving new fusion devices, our focus is on the practical systems that will make fusion deployable at climate-relevant scale.

We develop core fuel cycle and plant-level technologies for stable isotope production, fuel cycle closure, and advanced blanket systems that enable both energy generation and high-value co-products.

Founded in 2023, Marathon Fusion is building the components and systems that will help fusion become a practical, abundant, and reliable energy source for 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.

About the Role

As an Applied Physicist on the Lithium Team, you’ll work closely with mechanical engineers, electrical engineers, and technicians to design, execute, and interpret experiments that advance the development of the fusion fuel cycle and stable isotope production. With your hands-on experience designing and testing in a reactive materials systems (e.g., lithium, liquid metals, or molten salts), you’ll play a central role in the development and iteration of a first-of-its-kind isotope separation device.

What You’ll Do

• Design and execute experiments on lithium plasma systems operating under high-vacuum and high-temperature conditions.

• Develop and apply diagnostics, instrumentation, and data acquisition systems to capture and interpret key experimental results.

• Analyze experimental data to develop physical intuition, identify governing phenomena, and drive system design and process improvements.

• Collaborate closely with mechanical and test engineers to translate experimental insights into robust hardware solutions.

• Contribute to the design and development of critical subsystems, including hollow cathodes, lithium handling, condensation, thermal management, and pumping systems.

What You Bring

• 5+ years of experience in designing, prototyping, and testing of electromechanical systems.

• B.S. or higher in Physics, Engineering Physics, or a related field.

• Hands-on experimental experience where you've design, built, and operated complex experimental apparatus in a plasma, vacuum, or similar system.

• Strong experimental and data analysis skills, enabling you to draw insights from complex physical systems.

Preferred Experience, But Not Required

• Experience working with lithium, liquid metals, or molten salts.

• Experience with plasma diagnostics or high-voltage systems.

• Experience working in startup or early-stage R&D environment.

Compensation and Benefits

Our compensation for this role is purposefully wide to encompass a variety of levels we're willing to hire. Your final compensation package will be determined by an evaluation of your experience and qualifications. All full-time employees receive equity as part of their compensation.

We're proud to offer the following benefits:

• Medical, dental, and vision insurance

• Generous paid time off and sick leave

• 401(k) plan with 4% employer match

• Commuter benefits

• Fully stocked kitchen

This role is open to applicants who require relocation to the Bay Area and is supported with a $5,000 relocation reimbursement.

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.

Compensation Range: $115K - $170K