A real-time motional Stark effect diagnostic has been developed and deployed on the Korean Superconducting Tokamak Advanced Research (KSTAR). The diagnostic receives data from 25 spatial channels with analysis from either the standard motional Stark effect (MSE) system or the MSE with the background polychromator system on KSTAR. The system streams tan(γmag) (the tangent of the magnetic field pitch angle) to the real-time plasma control system with ∼10 ms time resolution for use in real-time control and equilibrium reconstruction. Here, we present initial results from the real-time system operation during the most recent experimental campaign, comparison with offline analysis, and information regarding the conditions for best MSE data on KSTAR. Real-time background subtraction will be developed during the upcoming KSTAR wall upgrade and is expected to be available in the post-upgrade run campaigns.
The National Ignition Facility (NIF) is reaching MJ fusion yields and has plans to reach tens of MJ. Diagnostic operation at these yields introduces challenges, from detector linearity at high signal levels to increased backgrounds. We present two upgrades to the NIF neutron time-of-flight (nToF) diagnostic suite that will enable measurements at high yields. First, silver neutron attenuators are being installed in the collimated nToF beamlines, reducing neutron flux and maintaining optimal signal-to-background levels. Silver was selected for its smooth neutron transmission cross section and sufficient γ-ray transmission to retain the gamma peak used for hot-spot velocity measurements. Each line-of-sight will have four insertable silver pucks, providing ∼2–20× attenuation of 14 MeV neutrons. Second, microchannel plate (MCP) photomultiplier tubes using Zener diodes to establish constant MCP-to-anode acceleration voltages have been installed in the quartz Cherenkov detectors. These provide stable instrument response functions across large gain ranges and permit operation at MCP gains near unity. The combined upgrades will allow NIF nToFs to function at all projected future yields.
The SPARC tokamak is expected to robustly enter the burning plasma regime (Q > 5) during deuterium–tritium operations and will provide a unique platform to study novel effects of alpha heating in reactor-relevant plasma scenarios. This article presents the near-final design and characterization of the high-resolution magnetic proton recoil (MPR) neutron spectrometer for SPARC and discusses the opportunities for burning plasma studies enabled by this instrument. The MPR spectrometer will be positioned in the midplane line of sight of the poloidal neutron camera and view SPARC’s core plasma. The as-built instrumental response function is calculated based on measured magnetic fields, which have been optimized to maximize resolution and sensitivity constrained by limited space and power. The spectrometer has variable resolution and sensitivity, with settings ranging from high efficiency (2.22 × 10−6 counts per incident neutron) and low resolution (1070 keV) to low efficiency (1.48 × 10−7 counts per incident neutron) and high resolution (149 keV). We also present kinetic simulations of burning plasma effects, which we use to calculate synthetic neutron spectra to better understand the diagnostic capabilities of the instrument. Based on these simulations, the spectrometer is expected to diagnose core ion temperature with less than 7% uncertainty and time resolution less than 100 ms in break-even campaign experiments. Non-thermal alpha knock-on fusion is found to be a small contribution to overall fusion power; however, it does produce a measurable feature in the neutron spectrum that can be used to evaluate alpha heating and confinement in high-gain scenarios.
High-performance gamma-ray spectrometry for fusion plasma diagnostics requires high detection efficiency in the MeV energy range, low pileup losses at high count rates, and an enhanced peak-to-background ratio. These requirements cannot be achieved simultaneously with a single large-volume scintillation detector. To address this limitation, a segmented detector consisting of a stack of three optically isolated lutetium–yttrium oxyorthosilicate doped with cerium scintillation crystals with a silicon photomultiplier (SiPM) readout was developed and tested. Each crystal was equipped with an independent, synchronized readout channel and digitized using a high-speed waveform-acquisition system, enabling operation at count rates of 200 kcps per crystal. Energy was reconstructed using charge integration, whereas event timing was determined using constant-fraction discrimination to enable coincidence measurements. System performance was evaluated using a Ga-66 gamma-ray source that emits multiple lines at energies of up to 5 MeV. A proof-of-principle demonstration of pileup deconvolution was performed using a pattern-based algorithm applied to raw waveforms. The method enabled overlapping events to be identified and reconstructed at a count rate of ∼200 kcps per scintillator. Coincidence selection identified a subset of events corresponding to ∼21% of the 2752 keV full-energy peak counts observed in the individual detector spectra, providing additional information on correlated interactions within the segmented detector. The results demonstrate that detector segmentation, combined with coincidence techniques and pileup deconvolution, enables efficient high-count-rate gamma-ray spectrometry using SiPM-based detectors and offers a promising solution for fusion plasma diagnostics in high-magnetic-field environments.
