Speaker
Description
This report presents an overview of the experimental results and data processing advancements in fast-ion diagnostics using neutron emission spectroscopy (NES) on the EAST tokamak.
The EAST NES system comprises multiple spectrometers arranged along four lines of sight (LOS), including a time-of-flight enhanced diagnostic (TOFED), a stilbene crystal, three liquid scintillators, a CLYC, a LaCl$_3$ spectrometer, and a newly installed single-crystal diamond spectrometer in the 2026 campaign. These detectors, varying in viewing angles and physical principles, cover different regions of the fast-ion phase space (particularly energies > 100 keV), enabling a systematic evaluation of diagnostic capabilities.
During synergistic NBI and third-harmonic ICRF heating, a significant broadening of the neutron spectra was observed across multiple spectrometers, indicating fast-ion acceleration well beyond the injection energy [1]. TRANSP simulations, validated by synthetic spectra generated by the GENESIS code and the instrument response functions (IRFs), as well as a weight-function analysis, revealed a fast-ion tail extending up to ~600 keV with a pronounced pitch-angle asymmetry favoring co-current velocities. Orbit calculations demonstrated that this asymmetry is primarily governed by the fast-ion loss boundaries on EAST.
Furthermore, in high poloidal beta ($\beta_{\mathrm{p}}$) discharges that achieved a record fusion neutron rate ($S_{\mathrm{n}}=3.9\times 10^{14}\,\mathrm{s}^{-1}$) on EAST, fast-ion behaviors under NBI and second-harmonic ICRF heating were analyzed by combining NES and neutron flux monitors (NFMs) [2]. Analysis shows that while the synergistic effect directly contributes $\sim30\%$ to the total neutron rate via the formation of a high-energy fast-ion tail, this enhancement is partially offset by NBI-induced profile degradation. Orbit phase-space analysis using the ORBIT code further revealed that synergistic heating drives suprathermal ions into smaller orbits (e.g., stagnation orbits), leading to the spatial redistribution of fast ions and the peaking of the neutron emissivity profile.
By systematically evaluating detectors across various distances, shielding configurations, and detection principles (TOF, organic, and Cl-based inorganic scintillators), this work establishes a comprehensive benchmark of NES for fast-ion diagnostics. Ongoing efforts focus on 4-LOS NES-only tomography and the development of orbit-space weight functions to understand the pronounced orbit effects typical of medium-sized tokamaks like EAST. Collectively, these advancements provide critical insights into high-fidelity fast-ion measurements, offering a robust technical framework for the design and optimization of NES-based diagnostics in future burning plasma devices.
References
[1] A.D. Xu et al 2026 Nucl. Fusion 66 066024
[2] A.D. Xu et al 2026 Nucl. Fusion (submitted)