Principal Software Engineer (m/f/d)

Focused Energy is building the future of clean energy through laser-driven inertial fusion. Backed by our $240M Series A — the largest fully secured Series A in the global fusion industry — we are scaling rapidly across multiple geographies. Based in Germany and the US, we have brought together the top scientific and engineering minds using proven technologies to unlock fusion power at commercial scale. Focused Energy is working to realize game-changing scientific innovation that promises to deliver a clean, sustainable and abundant source of power globally. YOUR ROLE Focused Energy is looking for a Principal Software Engineer to define and own the Core Software architecture that will operate and integrate our fusion power plant. Sitting within the Software Department and reporting to the Head of Software, this is a foundational technical role: you will set the direction for how controls, hardware interfaces, calibration systems, data infrastructure, and human-machine interfaces come together into one coherent system. You will design and implement critical parts of the Core Software stack, stand up reliable automated calibration workflows, and set the engineering standards the rest of the department will build on. This is a rare opportunity to shape the software backbone of a fusion power plant and operate as a technical lead within the Software Department. WHAT YOU'LL DO - Define and maintain the end-to-end Core Software architecture for the fusion power plant, including device interfaces, calibration, data handling, data storage, and HMI. - Design and implement critical parts of the Core Software system using languages such as C++ and Python. - Define clear interfaces between software, control hardware, diagnostics, plant equipment, data systems, and operator tools. - Design reliable automated calibration facilities and workflows that produce traceable and repeatable results. - Set practical standards for architecture, coding, testing, observability, documentation, release, reliability, security, and fault tolerance. - Lead technical decisions across teams, mentor engineers, resolve difficult trade-offs, and grow into a lead within the Software Department. WHO YOU ARE MUST-HAVES - Extensive experience designing and implementing control software for complex physical or industrial systems. - Extensive experience with hardware and software interfaces, including device integration and system-level troubleshooting. - Experience designing and implementing automated calibration facilities or automated calibration workflows. - Strong experience with data handling, data pipelines, metadata, and reliable data storage. - Experience designing and implementing HMI for operators or technical users. - Strong software design and implementation skills in C++, Python, or similar languages. - Ability to define architecture for complex, mission-critical systems under changing requirements and strict physical constraints. - Strong communication skills — you can explain technical decisions, risks, and trade-offs to software, hardware, science, and operations stakeholders. - Strong leadership skills — you can guide decisions, mentor engineers, build alignment, and take ownership without relying on formal authority. - Experience applying thorough testing, observability, reliability, fault tolerance, security, and documentation practices. NICE-TO-HAVES - Experience with software for plasma, nuclear power, high-energy physics, or scientific facilities. - Experience with real-time or distributed control systems and high-throughput data acquisition. - Experience defining shared frameworks, API conventions, configuration management, or communication systems for multiple teams. - Experience writing architecture decision records or technical proposals and leading architecture reviews. - Experience growing into a department-level technical lead or coaching senior engineers. - Fluency with AI-assisted software development, or strong enthusiasm for using approved AI tools responsibly to improve quality and delivery. Focused Energy is an equal opportunity employer committed to creating an inclusive environment. Qualified applicants will receive consideration for employment without regard to race, color, religion, sex, sexual orientation, gender perception or identity, national origin, age, marital status, protected veteran status, or disability status. Pursuant to the San Francisco Fair Chance Ordinance, Focused Energy will consider for employment qualified applicants with arrest and conviction records. Compensation offered will be determined by factors such as location, level, job-related knowledge, skills, and experience. Certain roles may be eligible for incentive compensation, equity, benefits.