ABOUT SUBCRITICAL SYSTEMS

Subcritical Systems, Inc. is a venture-backed startup in Austin, Texas building accelerator driven systems and the only US commercial manufacturer of superconducting accelerator cavities. What Space X did for rockets, we plan to do for accelerators and commercialize across applications of power production, medical isotopes, and semiconductors.

We are in the process of building the Accelerator Driven System — an inherently safe, novel nuclear fission system in which a particle accelerator drives a subcritical core, making meltdown physically impossible for clean power production. Our founding team includes successful entrepreneurs with a track record of building several multibillion dollar companies in deep-tech, and scientists who were leading U.S. national labs with deep experience in accelerator & nuclear technologies.

We are actively hiring to expand our team as we work toward licensing, regulatory submissions, and construction of our first energy amplifier. More information is available at subcritical.com.

ROLE SUMMARY

Subcritical Systems Inc. (SSI) is hiring an LLRF/HPRF Engineer to develop the RF systems that power and control the cavities of its superconducting proton linac. The linac drives a subcritical nuclear core and is comparable in class to the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory.

This engineer owns cavity field and resonance control and the high-power RF (HPRF) systems that feed the cavities, at two sites: SSI's factory, where cavities, couplers, and cryomodules are conditioned and tested on RF test stands, and the accelerator facility, where HPRF stations power the operating linac. They specify, build, commission, and maintain the amplifiers, high-voltage supplies, and RF distribution at both sites, and define how the low-level RF (LLRF) system drives, linearizes, monitors, and protects them. Owning both sides lets them make the trade-offs between RF power overhead, control bandwidth, and cavity behavior that set beam stability, efficiency, and RF availability.

KEY RESPONSIBILITIES

LLRF System Design

• Derive fast feedback requirements, the LLRF system, from RF field amplitude and phase stability requirements.
• Design or specify LLRF hardware: analog RF front ends, down-conversion, digitizers, FPGA-based controllers, master oscillator, and phase reference distribution.
• Develop and tune feedback and feedforward algorithms for cavity field regulation and beam-loading compensation.
• Implement cavity startup, frequency tracking, self-excited loop, and calibration of forward, reflected, and probe signals.

Resonance Control

• Implement piezo and stepper tuner control, including Lorentz force detuning compensation and active microphonics suppression.
• Characterize cavity mechanical modes and detuning sources with SRF and cryomodule engineers.

HPRF Systems: Factory Test Stands

• Specify, procure, install, and maintain HPRF test stands for vertical cavity tests, coupler conditioning, and cryomodule (horizontal) tests: amplifiers, power supplies, circulators, loads, directional couplers, and waveguide or coaxial lines.
• Run high-power coupler conditioning and cryomodule RF tests with the SRF and cryomodule teams; write test stand procedures and own their interlocks and safety controls.
• Plan test stand capacity and uptime to keep pace with the cavity and cryomodule production schedule.

HPRF Systems: Accelerator Facility

• Develop requirements and specifications for the linac HPRF stations (klystrons, IOTs, solid-state amplifiers, or magnetrons, with their high-voltage supplies or modulators), and support vendor selection, factory acceptance, and site acceptance testing.
• Design or specify RF power distribution from source to cavity: waveguide and coaxial lines, circulators, loads, directional couplers, and windows, with their cooling, vacuum, and arc detection.
• Install, commission, and maintain the HPRF stations; track failures and spares, and drive improvements in RF availability.

LLRF–HPRF Integration

• Define the LLRF–HPRF interface in ICDs: drive levels, signal paths, timing, status and interlock signals, and calibration points.
• Characterize amplifier gain and phase versus drive and supply voltage, bandwidth, noise, and saturation; build amplifier models for control design.
• Implement amplifier linearization, predistortion, and operating-point control, such as high-voltage or drive optimization for efficiency, and phase-locking schemes where needed (e.g., injection-locked magnetrons).
• Set the RF power budget and overhead for beam loading, detuning, and control margin.
• Monitor forward and reflected power, arcs, and amplifier status, and implement fast RF trips and drive inhibits that protect the amplifiers, couplers, and cavities.

Protection, Diagnostics, and Integration

• Implement quench detection and RF interlocks; integrate them with the machine protection system.
• Build waveform capture, fault post-mortem, and cavity diagnostic tools (e.g., Q_L, detuning, field-emission indicators).
• Integrate LLRF with the control system (e.g., EPICS) and timing system.

Test, Commissioning, and Operations

• Provide LLRF and HPRF for vertical tests, coupler conditioning, and cryomodule tests in the factory; use test data to refine control and amplifier models.
• Commission LLRF with the HPRF stations at the accelerator facility, then with beam; tune loops for beam stability and RF availability.
• Write specifications, test procedures, and operating documentation; support operations and troubleshooting at both sites.

QUALIFICATIONS

Required

• B.S. in Electrical Engineering or Physics; M.S. or Ph.D. preferred.
• 5+ years of experience developing RF control, measurement, or high-power RF systems, with hands-on lab work.
• Strong grounding in control theory (feedback, feedforward, loop stability) and digital signal processing.
• Experience with FPGA-based signal processing (VHDL/Verilog or HLS) or close work with firmware engineers.
• Hands-on experience operating and troubleshooting high-power RF amplifiers and transmission lines, including their nonlinear behavior.
• Working knowledge of high-voltage and high-power RF safety, interlocks, and lockout/tagout practice.
• Experience with RF instrumentation: vector network analyzers, spectrum and signal analyzers, power meters, and phase-noise measurement.
• Programming skills in Python or MATLAB; experience with Linux.
• Ability to troubleshoot across RF hardware, firmware, and software.

Preferred

• LLRF or HPRF experience at an accelerator facility with SRF linacs (e.g., ORNL SNS, Jefferson Lab, Fermilab PIP-II or LCLS-II, FRIB, ESS, DESY).
• Direct experience with SRF cavity field control, microphonics and Lorentz force detuning compensation, and piezo tuner control.
• Experience building or operating RF test stands for SRF cavity, coupler, or cryomodule testing.
• Experience specifying, commissioning, or maintaining klystron, IOT, solid-state, or magnetron transmitters and their high-voltage supplies or modulators, including vendor acceptance testing.
• Experience with waveguide and coaxial RF distribution, circulators, high-power loads, and RF windows.
• Experience integrating LLRF with high-power amplifiers, including linearization and injection locking.
• Experience with MicroTCA.4 or similar LLRF platforms and with phase reference distribution.
• Experience with EPICS integration, RF machine protection, and fault post-mortem analysis.
• Experience with system identification, model-based or adaptive control, or machine learning applied to RF control.
• Publications or conference contributions (LLRF Workshop, SRF, LINAC, IPAC).

BENEFITS

Benefits: We offer health, dental, and vision insurance.

LOCATION & WORK POLICY

This role is fully in person in Austin, Texas. Our facility is located next to Austin Airport.

HOW TO APPLY

Apply on LinkedIn or submit your resume and a brief cover letter to careers@subcritical.com with the subject line "RF Engineer - Low-level and High-power- Application". Applications are reviewed on a rolling basis.