Researchers have characterized a prototype Uranium-238 fission chamber (FC) intended for the SPARC tokamak's neutron diagnostics suite, according to a new preprint. The diagnostic is designed to monitor neutron yield rates from high-performance deuterium-tritium (DT) plasmas, covering a dynamic range from approximately 10¹⁵ to over 10¹⁹ neutrons per second. The successful validation confirms the detector's suitability for accurately measuring fusion power output in SPARC's high-field, compact configuration. This work is critical for providing the real-time data necessary for machine protection and physics studies during the project's most demanding operational phases. Source: arXiv plasm-ph

The FC design utilizes a parallel-plate detector geometry with a $^{238}$U coating, which has a high energy threshold for neutron-induced fission, making it primarily sensitive to the 14.1 MeV neutrons from DT reactions. To further isolate the signal from unscattered fusion neutrons, the system incorporates borated polyethylene collimation. Experimental testing was conducted using both DD (2.5 MeV) and DT (14.1 MeV) neutron generators to benchmark the device's performance. The results corroborated the vendor-specified efficiencies and demonstrated excellent detector linearity across the intended operational flux range, a key requirement for reliable power measurement. Source: arXiv plasm-ph

To further isolate the signal from unscattered fusion neutrons, the system incorporates borated polyethylene collimation.

Validation efforts included extensive modeling to predict the detector's response. The measured count rates from the prototype showed good agreement with predictions from OpenMC neutronics simulations, lending confidence to the integrated design of the detector and its shielding. This computational verification is essential for interpreting the on-machine data from Commonwealth Fusion Systems' SPARC device, where the complex geometry and high neutron flux create a challenging measurement environment. The simulations help deconvolve the direct neutron signal from background noise caused by scattered neutrons and gamma rays. Source: arXiv plasm-ph

The study also addressed SPARC-specific environmental factors that could compromise diagnostic performance. The prototype FC was tested for robustness against stray magnetic fields up to 14 mT, showing no significant impact on its operation. This is a crucial validation given SPARC's powerful high-temperature superconducting magnets. Additionally, the potential for signal degradation over the approximately 30-meter-long cable runs required for installation was assessed and found to be manageable. These tests confirm the design's resilience within the tokamak hall's harsh conditions. Source: arXiv plasm-ph

With this validation complete, the $^{238}$U fission chamber is confirmed as a reliable component of the comprehensive neutron diagnostics planned for the SPARC program. The system's ability to provide accurate, real-time measurements of the total neutron source rate is fundamental for calculating the fusion power and the resulting plasma energy gain. The findings support the deployment of this diagnostic to track performance during SPARC's mission to demonstrate net energy gain, providing essential data for both machine operation and the design of the subsequent ARC power plant. Source: arXiv plasm-ph