Abstract Reliable diagnostics are essential for understanding burning plasmas and enabling robust control strategies in future fusion reactors. However, reactor-scale devices impose severe constraints on diagnostic systems, including high neutron wall loads, limited access, and reduced component survivability. These constraints necessitate a transition from measurement-rich diagnostic approaches toward integrated frameworks that combine a reduced set of robust diagnostics with advanced modeling and inference methods. In this context, KSTAR, a long-pulse superconducting tokamak, provides a unique platform for developing and validating reactor-relevant diagnostic strategies. KSTAR hosts a comprehensive suite of diagnostics covering plasma profiles, fluctuations, radiation, energetic particles, and plasma–wall interactions, and has demonstrated advanced capabilities in real-time control, multi-diagnostic integration, and synthetic diagnostic modeling under reactor-relevant conditions. This work provides a status-oriented review that systematically maps reactor diagnostic requirements to current KSTAR capabilities, highlighting how key functionalities such as equilibrium reconstruction, density control, radiation monitoring, and divertor diagnostics can be achieved through integrated measurement and control frameworks. Representative examples include drift-resilient magnetic diagnostics, real-time density control, microwave-based profile reconstruction, and tungsten-environment radiation diagnostics. Special attention is given to fast-ion diagnostics, where a fundamental trade-off exists between measurement fidelity and robustness in reactor environments. While diagnostics such as fast-ion D-alpha (FIDA), fast-ion loss detector (FILD) and collective Thomson scattering (CTS) provide detailed information in present devices, their applicability to reactors is limited. In contrast, neutron-based diagnostics offer more robust but indirect measurements, emphasizing the need for integrated interpretation methods. Building on these insights, the study highlights the emerging role of integrated and virtual diagnostic frameworks. Approaches combining synthetic diagnostics, Bayesian inference, and machine-learning-assisted reconstruction enable reliable plasma state estimation from sparse measurements. Furthermore, the integration of diagnostics into control frameworks and the development of virtual tokamak concepts, such as Virtual KSTAR, provide new pathways for predictive and real-time plasma monitoring. These developments demonstrate that KSTAR serves as an effective testbed for the transition toward reactor-relevant diagnostics. The results provide guidance for future devices, including CPD (Compact Pilot Device) and DEMO, where diagnostic strategies must prioritize robustness, integration, and inference over completeness.
Abstract In this work, a comparative study was conducted on the optical and spectral characteristics of nanosecond discharge plasma in air with varying humidity at atmospheric pressure in a point-to-point gap. The influence of voltage pulse duration (0.7 ns and 13 ns) and relative air humidity on the formation of diffuse and constricted discharges was experimentally studied. It was confirmed that during sub-nanosecond breakdown, a diffuse plasma channel (diffuse discharge) arises after the “collision” of counter-propagating large-diameter streamers. It was established that with a relatively long pulse duration, a diffuse discharge first forms in the gap, which then constricts. The discharge channel in this case consists of individual filaments and exhibits a whitish glow. Analysis of optical emission spectra revealed qualitative differences between the diffuse and constricted modes, consisting of the appearance of a broadband continuum as well as intense lines of atoms and ions of gas components and electrode material. It was found that higher air humidity enhances the spectral energy density of radiation from metal atoms and ions, as well as the continuum. It has been shown that in constricted discharges in humid air, radiation from a second continuum of H2 is superimposed on the radiation of the recombination continuum. Data are presented that support a previously proposed hypothesis explaining the appearance of a broadband continuum during discharge constriction (filamentation) under high humidity conditions. It has been established that this additional continuum arises due to radiation emitted during transitions between bound and repulsive terms of molecular hydrogen. The excitation of levels in the second positive system of nitrogen as a result of a pooling reaction has also been demonstrated.
Abstract Soft X-ray (SXR) tomography is essential for resolving magnetohydrodynamic (MHD) instability structures in the EAST tokamak. Two tomographic methods have been established at EAST: the Fourier--Bessel (F--B) expansion method and the Gaussian Process (GP) method, and a deep-learning-based imaging approach has also been explored. The minimum Fisher information (MFI) method, with anisotropic regularization aligned to the magnetic flux surfaces, has been successfully applied on several tokamaks, including JET with only two viewing chords. In this work we adapt it to the three-camera SXR diagnostic at EAST, where the flux-surface grid implementation raises a boundary problem. We reformulate it as a flexible-boundary MFI method that introduces no additional free hyperparameters. Validation on synthetic phantoms with known emissivity, together with experimental results on EAST sawtooth, fishbone, and tearing modes, demonstrates that the flexible-boundary MFI achieves imaging quality comparable to the GP method while using fewer free hyperparameters than both GP and F--B, thus providing EAST with a physically reliable SXR tomographic tool and enabling multi-method cross-comparison of emissivity reconstructions.
Abstract Out-of-band radiation and surface photoemission can strongly affect EUV-induced low-pressure hydrogen plasmas, yet their distinct kinetic roles remain unclear. We combine a spectrally resolved response framework with particle-in-cell–Monte Carlo collision (PIC–MCC) simulations to track electrons originating from 20–40 eV gas-phase photoionization, 60–120 eV gas-phase photoionization, and surface photoemission. The 20–40 eV band produces 1.6 times more direct electrons than the 60–120 eV band, while exhibiting negligible collisional multiplication (1.007) but strong long-time retention (41% at 1 µs). By contrast, the 60–120 eV channel shows stronger multiplication (1.250), whereas only 2% of its electrons remain at 1 µs. The surface-photoelectron lineage contributes 86.3% of the collisionally generated secondary-electron yield, followed by rapid boundary loss. The three sources therefore play distinct stage-dependent roles: surface photoemission governs early plasma buildup; high-energy photons promote transient ionization; low-energy photons sustain the post-pulse electron reservoir. These results provide plasma-kinetic guidance for spectral control, surface engineering, and the assessment of ion-bombardment, charging, and plasma–surface-interaction risks in EUV systems.
Abstract Enhancing plasma sustainment under airflow conditions is an important challenge for dielectric barrier discharge (DBD) systems operating in magnetohydrodynamic (MHD)-relevant coupled-field environments. In this work, a dual-electrode DBD structure is proposed to exploit airflow-transported pre-ionization for discharge enhancement. An experimental platform incorporating magnetic field, airflow, and DC electric field was constructed to investigate the discharge characteristics and Faraday current responses under coupled-field conditions. The results show that increasing the magnetic field from 0 T to 1.2 T promotes a transition of the discharge from a filamentary mode to a more diffuse mode. At B = 0.4 T and under the same total electrode area, the dual-electrode configuration exhibits significantly enhanced discharge intensity and higher Faraday current than the single-electrode counterpart at an airflow velocity of 25 m/s, whereas the two configurations show nearly identical behavior at 5 m/s. This velocity-dependent response indicates that the enhancement originates mainly from airflow-driven particle transport rather than from electrode geometry alone. The experimental evidence supports an airflow-transported pre-ionization mechanism, in which surviving plasma-generated residual ions and longer-lived reactive products can be conveyed downstream and promote subsequent discharge development. In addition, the Trichel-pulse waveform shows that activation of the upstream coupled-field region modifies the downstream electrical and space-charge environment, while a force-scale estimate provides a physical basis f or local electromagnetic flow-field modulation. These results demonstrate that utilizing airflow transport is an effective strategy for improving plasma sustainment under coupled magnetic-field and airflow conditions.
