Anurag Maan

A liquid lithium boundary offers a possible route to improved energy confinement via reduced recycling, but the resulting low-density, high-temperature edge can become sufficiently collisionless such that standard fluid models are no longer applicable. Experimental measurements of scrape-off-layer (SOL) parameters, including density decay length ( λ n ) estimates presented here, can be used for simple analytic estimates of kinetic effects, but more complete models are needed. We present a gyrokinetic simulations, with one configuration-space dimension and two velocity-space dimensions, of low and higher-recycling LTX- β SOL conditions using Gkeyll , an open-source continuum gyrokinetic code. The simulations follow a single open field line from the low-field-side midplane to the high-field-side limiter. Gkeyll is used to evolve kinetic ion and electron distribution functions, the electrostatic potential, collisions, sources, and sheath losses with experimentally constrained density and temperature at the low field side mid-plane. The low-recycling case develops a hotter SOL, a larger electrostatic potential variation along the field line, and a density response that is not recovered with an adiabatic Boltzmann-electron closure of the field equations. This indicates that kinetic-electron dynamics and the associated ambipolar potential are important for maintaining quasi-neutrality and reducing electron end loss from the field line in the low-recycling regime. Collisionality scans show that, as the ion neoclassical collisionality is reduced below unity, the ion distribution develops a stronger non-Maxwellian structure. The corresponding SOL particle and energy confinement times increase at low collisionality, especially for the low-recycling case, because ions become mirror trapped, and electron transport is suppressed by the ambipolar electrostatic potential to maintain quasi-neutrality. These results suggest that low-recycling lithium-wall operation in LTX- β accesses a SOL regime in which mirror trapping and electrostatic modification of electron transport alter the field-line residence time relative to conventional high-recycling fluid expectations. These SOL modifications are expected to become more pronounced in larger devices.

We present the IRE Identification Multimodal Network (IIMN), a classifier to automatically detect Internal Reconnection Events (IREs) in spherical tokamaks. The IIMN utilizes data from the Lithium Tokamak eXperiment-β (LTX-β), a low-aspect-ratio spherical tokamak capable of operating with walls coated in solid or liquid lithium. As a multimodal network, the IIMN is well-suited to handling complex, multi-dimensional relationships among both image and tabular data. To train the model, 7,265 events from 2,968 shots were manually reviewed and labeled, covering nearly two years of diagnostic data from LTX-β. Each event includes key plasma parameters in identifying IREs like the time evolutions of the plasma current, average electron density, and Mirnov spectrogram. The model concatenates the results from a multilayer perceptron branch and a convolutional neural network (CNN) branch to return a one-hot encoded output that determines if the event is an IRE or not based on the event’s plasma parameters. We show that the IIMN has an accuracy of 94.4±0.9%, a precision score of 0.934±0.014, and a recall score of 0.931±0.014. IIMN is fully automated such that it could be used quickly and easily by operators in the control room for spherical tokamak operation. To realize full automation and zero human intervention, our current multimodal architecture incorporates a CNN that directly analyzes image-based data from select plasma diagnostics. By automating IRE detection, this tool can assist operators in monitoring fusion performance and developing strategies to reduce confinement losses even in between two successive plasma discharges.

We present a compact photodiode-based diagnostic array developed to study low-energy neutral beam injection in the Lithium Tokamak Experiment spherical tokamak. The in-vacuum diagnostic combines filtered soft-x-ray (SXR), narrowband Lyman-α, and unfiltered absolute extreme ultraviolet (AXUV) photodiode rows with partly overlapping, nearly coincident tangential views of the plasma, including the neutral beam path. This geometry provides simultaneous sensitivity to beam-induced SXR emission; neutral-hydrogen line radiation associated with recycling, fast neutrals, and fueling; and broadband emission that can include direct neutral impacts from fast-ion charge-exchange losses. Initial measurements from 12 to 20 keV hydrogen beam operation show beam-synchronous detector responses in all three modalities. The unfiltered AXUV signals exhibit millisecond-scale rise and fall times that vary across sightlines and depend on lithium-conditioning history. Comparison with classical slowing-down time estimates indicates that charge exchange with background neutrals contributes appreciably to the measured decay. The diagnostic can potentially be used to constrain a forward model to estimate the time-resolved balance of beam heating and fueling for small tokamaks.

We present a synthetic soft x-ray (SXR) forward-modeling framework that characterizes the emissivity structure of rotating magnetic islands in the Lithium Tokamak eXperiment- β across space and time. Magnetic islands associated with tearing modes produce modulations in line-integrated SXR brightness. Traditional tomographic methods struggle to resolve these structures in devices with limited sightlines, resulting in an under-determined inversion problem, or otherwise necessitate equilibrium reconstruction data, preventing their use in active control. Our approach removes this challenge by combining a fully three-dimensional ray-tracing model with a time-dependent emissivity prescription of a m / n = 2 / 1 magnetic island. The model is validated through helical island geometry, rotation measured by magnetic diagnostics, and equilibrium constraints from PSI-Tri reconstructions. Synthetic brightness signals are generated for each photodiode sightline from a modeled emissivity profile and directly compared with experimental data from a tangential midplane SXR array. By fitting the synthetic diagnostic output to the observed brightness across time, we simultaneously infer key geometric parameters-such as island radial location, island width, and rotation frequency-without requiring full tomographic reconstruction, all with an average absolute deviation of less than 3 % . This work demonstrates that forward modeling with a single tangential array can extract key island parameters in a spherical tokamak, providing a computationally efficient alternative to conventional SXR tomography and providing a pathway toward real-time magnetic-island characterization in future devices.

We describe the upgrades to the Thomson scattering (TS) diagnostic on the Lithium Tokamak eXperiment-beta (LTX- $\beta $ ) to provide complete radial coverage of the plasma. LTX- $\beta $ is a spherical tokamak with plasma limited at a major radius of R= 14cm and R= 66cm. Flat temperature profiles with peaked density were observed in LTX- $\beta $ using an existing multipoint TS diagnostic with radial coverage of the plasma core. There is a need for more comprehensive core and edge TS measurements to thoroughly examine beam fueling, beam heating, and transport under low recycling conditions with liquid lithium walls and a lower collisionality scrape-off layer (SOL). The core TS set up provides plasma profiles from R= 40–60cm. To extend the TS diagnostic coverage: 1)we added a TS viewing optics to cover the SOL region at five radial points from R= 57–64cm, and 2)another TS viewing optics in the HFS region from R= 15–45cm to obtain TS data from ten radial positions while making use of the original laser beam. The main focus of this article is on the new HFS TS setup that is equipped with better-matched collection optics located in the horizontal midplane where the laser polarization lies and views from a scattering angle of 130°–150° with respect to the laser path. A set of unequal-length Y-branched fiber bundles transports the TS light from two radial points to each polychromator. The new HFS view is better matched in spot size, scattering angle, and throughput to the laser and polychromators and has new V-grooved style viewing dumps. The new HFS TS view, along with the core and SOL TS views, will provide a full radial measurement coverage from R= 15–64cm to explore the flat temperature profiles.

Magnetic equilibrium reconstruction in the Lithium Tokamak Experiment-Beta (LTX-β) is complicated by strong eddy currents and toroidal asymmetries arising from its segmented, close-fitting conducting shell. In the earlier experiment LTX, these three-dimensional (3D) conductor effects significantly distorted diagnostic signals and challenged conventional axisymmetric reconstruction methods. In this work, we demonstrate that accurate plasma equilibria can be recovered on LTX-β by incorporating a small number of dominant eddy current modes derived from realistic conductor models. Using the open-source TokaMaker Grad–Shafranov solver, we reconstruct equilibria across a systematically selected set of LTX-β discharges and validate them against the legacy PSI-Tri hybrid 2D-3D code. Our results show that fully 2D TokaMaker reconstructions achieve significantly improved agreement with flux loop and Mirnov probe measurements, reducing total chi-squared fitting errors, especially during startup. Finally, we develop new hybrid 2D-3D TokaMaker reconstructions by integrating the ThinCurr 3D eddy current model, which we optimize for significant further reduction of chi-squared in most scenarios. These findings underscore the importance of realistic wall-current modeling in short-pulse tokamaks, and establish a physics-based reconstruction framework that is extensible to devices with complex passive structures and 3D wall interactions.

We investigate how growth of an oxide film will influence the deuterium recycling properties of Li on plasma facing surfaces. Lithium films on the walls or plasma-facing material surfaces of a fusion vacuum vessel improves plasma performance in part by removing residual impurity atoms from the plasma. Oxygen atoms, from residual water vapor or eroded oxide surfaces, are a prominent ubiquitous impurity that can create a layer of oxide on lithium films. The results of the work herein also indicate a possibility for determining the thickness of the oxide layer on such Li films by measuring the reflection probability of incident deuterium particles from the surface.

The fixed-frequency and frequency-modulated continuous-wave (FM-CW) reflectometers on LTX-β (Lithium Tokamak eXperiment-β) have been configured to use the same transmission lines and antenna arrays for coincident views of the core and edge plasma. The fixed-frequency channels (13.1–20.5 and 20–40 GHz, tunable between discharges) provide time-resolved measurements of density fluctuations, while the FM-CW channels (13.1–20.2 and 19.5–33.5 GHz) measure the density profile and fluctuations, with high spatial resolution and a sampling rate determined by the frequency sweep interval (5 μs). Data from both reflectometers are synchronously acquired to simultaneously leverage the wide bandwidth and high spatial resolution of the respective systems. Experiments showed that mutual crosstalk interference is momentary and does not diminish the capability of either system. Spectral analysis indicated broad power spectra (several hundreds of kHz) and suggests that the signals from the FM-CW system are consistent with under-sampled fixed-frequency signals. Radial correlations were explored using data from the two reflectometers, as well as from the FM-CW system alone. The core channels showed high levels of agreement between these two comparisons, suggesting that the data from the reflectometers are interchangeable for statistical estimates. For the edge channels, comparisons using data from the FM-CW reflectometer alone showed significant decorrelation due to time lag caused by the finite frequency up-sweep duration. Alternatively, this effect is eliminated when cross-correlating data from the different reflectometers. These results highlight the advantages of operating the fixed-frequency and FM-CW reflectometers in this manner, where the combined system can overcome the limitations of each separate system.

Both predicting and controlling disruptions are critical and urgent issues for ITER as some disruptions with a short warning time may be unavoidable. For these cases, a rapid response disruption mitigation system (DMS) referred to as the electromagnetic particle injector (EPI) is being developed. The primary advantages of the EPI are its fast response time and high velocity, which have been demonstrated in offline experiments (Raman et al., 2021). The EPI is capable of accelerating a metallic sabot electromagnetically using a rail gun to the required velocities ( $>$ 2 km/s) within 2 ms. Two high-field racetrack magnets able to generate fields over 2 T are positioned above and below the rails to permit high velocity at low rail currents, a requirement to minimize electrode erosion. At the end of the acceleration phase, a sabot capture mechanism retains the spent sabot inside the vacuum chamber that houses the EPI. At this point, it releases well-defined microspheres, or a shell pellet, of a radiative payload into the disrupting plasma. A remotely operated sabot loading system positioned behind the injector contains several pre-equipped sabots that can be loaded by an operator from the tokamak control room. The injector is interfaced to the tokamak through a guide tube attached to the front of the EPI vacuum chamber. The advantages of the EPI system over other DMSs under consideration are described in conjunction with the details of an EPI system designed for near-term test on an existing large tokamak.