Ion Temperature Measurement with Neutron Counting for General Fusion Magnetized Target Fusion Devices: LM26 and PI3

We present a method for inferring deuterium ion temperature in plasmas with Maxwellian velocity distributions using neutron counts measured by fast liquid scintillators. Local neutron count rates from multiple detector positions are combined to estimate the total neutron yield, with absolute detection efficiency determined through MCNP simulations of neutron transport based on a detailed 3D model of the experimental setup.

This approach is well suited to Magnetized Target Fusion systems, offering time-resolved measurements without requiring line-of-sight access or neutron collimation. We apply the described method to General Fusion's Plasma Injector 3 (PI3) spherical tokamak setup and General Fusion's Lawson Machine 2026 (LM26) lithium liner compressed plasma setup, and describe the diagnostic configuration, including pulse-shape discrimination, pileup correction, detector calibration, and the ion temperature extraction algorithm with uncertainty analysis. Results are benchmarked against Ion Doppler Spectroscopy measurements.