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Concern about adequate tritium resources to start up deuterium-tritium (DT) fusion reactors in the near term is misplaced: Proposed demonstration reactors and pilot plants will not realistically be buildable for a great many decades. More disturbing, tritium fuel self-sufficiency of eventual DT reactors may be unattainable, because it is unlikely that tritium-breeding blankets can completely replenish burned-up fuel as well as unburned tritium that is lost in numerous reactor subsystems. The concept of doubling time is unworkable. The only viable fuel solution for sustained operation demands deuterium-based reactors where the tritium is concurrently generated and burned up in the fusioning plasma. [ABSTRACT FROM AUTHOR]

Simultaneous regulation of multiple properties in next-generation tokamaks like ITER and fusion pilot plant may require the integration of different plasma control algorithms. Such integration requires the conversion of individual controller commands into physical actuator requests while accounting for the coupling between different plasma properties. This work proposes a tokamak and scenario-agnostic actuator-sharing algorithm (ASA) to perform the above-mentioned command-request conversion and, hence, integrate multiple plasma controllers. The proposed algorithm implicitly solves a quadratic programming (QP) problem formulated to account for the saturation limits and the relation between the controller commands and physical actuator requests. Since the constraints arising in the QP program are linear, the proposed ASA is highly computationally efficient and can be implemented in the tokamak plasma control system in real time. Furthermore, the proposed algorithm is designed to handle real-time changes in the control objectives and actuators' availability. Nonlinear simulations carried out using the Control Oriented Transport SIMulator illustrate the effectiveness of the proposed algorithm in achieving multiple control objectives simultaneously. [ABSTRACT FROM AUTHOR]

Using local nonlinear gyrokinetic simulations and building up on linear results from a companion paper (Volčokas 2024 Submitted for publication), we demonstrate that turbulent eddies can extend along magnetic field lines for hundreds of poloidal turns in tokamaks with weak or zero magnetic shear s ^ . We observe that this parallel eddy length scales inversely with magnetic shear and at s ^ = 0 is limited by the thermal speed of electrons v th , e . We examine the consequences of these 'ultra long' eddies on turbulent transport, in particular, how field line topology mediates strong parallel self-interaction. Our investigation reveals that, through this process, field line topology can strongly affect transport. It can cause transitions between different turbulent instabilities and in some cases triple the logarithmic gradient needed to drive a given amount of heat flux. We also extensively study a novel 'eddy squeezing' effect introduced in Volčokas (2023 Nucl. Fusion 63 014003), which reduces the perpendicular size of eddies and their ability to transport energy, thus representing a novel approach to improve confinement. Finally, we investigate the triggering mechanism of Internal Transport Barriers (ITBs) using low magnetic shear simulations, shedding light on why ITBs are often easier to trigger where the safety factor has a low-order rational value. [ABSTRACT FROM AUTHOR]

This study examines the impact of low and zero magnetic shear, along with rational values of safety factor, on linear mode behavior in tokamak plasmas. We focus on how magnetic field line topology and parallel domain length influence linear mode structures and growth rates as magnetic shear decreases. Our results show that as magnetic shear is reduced to low values, linear modes can transition between different branches, significantly affecting their characteristics. At zero magnetic shear, accurate modeling requires precisely capturing magnetic topology, as small deviations near rational surfaces can greatly impact growth rates. [ABSTRACT FROM AUTHOR]

Massive material injections in the JET tokamak have been observed to substantially affect resistive bolometer measurements, resulting in a spurious radiated power signal proportional to the quantity injected and reaching up to 8 MW. These bolometers are calibrated and designed to operate in near vacuum but certain scenarios requiring large gas injections can push the neutral pressure past nominal values. This study demonstrates that the bolometry measurement can be affected at neutral pressures above 0.1 Pa following injections with standard gas valves, shattered pellet injections, and particularly massive gas injections. The power measurement of resistive bolometers is based on the temperature difference between a measurement sensor exposed to radiation and a shielded reference sensor. We employ a thermal conductivity model to demonstrate that the conduction through the gas and the distinct geometries between the sensors can affect their cooling efficiency. This additional cooling pathway, coupled with the Joule heating from the applied voltage causes the equilibrium temperatures of the sensors to diverge. Being the very basis of the measure, this temperature difference induces a signal that is erroneously interpreted as radiated power. Experiments show large discrepancies in the response to neutral pressure among bolometer channels, attributed to variations in channel physical parameters. Nonetheless, the modeled total radiated power reproduces the experimental measurements within an order of magnitude, affirming the sensitivity of resistive bolometers to neutral pressure and gas species. [ABSTRACT FROM AUTHOR]

Understanding the confinement of fast ions is crucial for plasma heating and non-inductive current drive, i.e., for the operation of a fusion reactor. Interactions between fast ions and magnetohydrodynamic instabilities can reduce the performance of fusion reactions. Measuring the spatial shape and amplitude is crucial for constraining numerical modeling of the interaction between fast ions and these instabilities. Soft x rays can be used to study these magnetic instabilities. In particular, SXR tomography is used to reconstruct the two-dimensional profile of the SXR emissivity requiring only line integrated measurements, thus providing the spatial structure of the instabilities. This work presents SXR tomography reconstruction performed on synthetic SXR emissions from the Mega Ampere Spherical Tokamak Upgrade device. The synthetic SXR emissions are derived from time dependent tokamak transport data analysis code (TRANSP/NUBEAM) simulations. Different tomographic reconstruction models are compared, and the effect of two additional fans of intersecting lines of sight on the reconstructions' performances is investigated. The additional intersecting lines of sight greatly improve the accuracy of the reconstructions. [ABSTRACT FROM AUTHOR]

Understanding the erosion of plasma facing components in fusion devices is vital, particularly for long-pulse operations. This study presents the application of synthetic optical diagnosis on the all-W WEST tokamak. The analysis reveals reflections as significant contributors to measured emission, varying across main chamber limiters and divertor targets. Reflections at divertor locations can be up to 50% of measured emission while 95% at limiter locations. Oxygen is investigated as a proxy for low-Z species and underscores the importance of reflections in interpreting optical diagnostics, especially for validating plasma–material interactions and scrape-off layer impurity transport codes. As more fusion devices adopt full metal walls, the accurate assessment of reflections will become increasingly crucial for erosion analysis and plasma control. [ABSTRACT FROM AUTHOR]

Soft x-ray (SX) tomography is a useful diagnostic in fusion research, and a multi-channel SX diagnostic will be installed in JT-60SA, the largest elongated tokamak in the world. However, in the SX diagnostic of JT-60SA, plasmas will be only viewed from the low field side and the upper side of plasmas; the sight lines are limited, which would be common in future devices as well as JT-60SA. This kind of limited sight lines is not preferred for SX tomography to investigate the spatial structure of magnetohydrodynamics (MHD) modes because inadequate information of plasmas makes artifacts in the reconstructed SX profiles. One of the solutions to reduce the artifacts is to employ L1 regularization, which gives the essential and sparse contributions [Kaptanoglu et al., Phys. Plasmas 30, 033906 (2023)]. In this study, as a first topic, the applicability of L1 regularization to reduce the artifacts in SX tomography with limited sight lines is investigated with traditional L2 regularization for a high beta scenario of JT-60SA where MHD modes would occur. Here, as a series of basis functions, the Fourier–Bessel series (FBS) is employed because FBS has the poloidal Fourier modes explicitly. A disadvantage of FBS is that the accurate equilibrium inside the last closed flux surface (LCFS) is needed; interior measurement such as the motional Stark effect measurement is required, which is not always available during a whole discharge. The second topic of this study is to investigate other appropriate basis functions to study the spatial structure of MHD modes in elongated tokamak plasmas. Here, we introduce Saito's Laplacian eigenfunction (LEF). Saito's LEF can be calculated if LCFS is given and the LEF is expected to show the explicit poloidal Fourier mode. Because the calculation of LCFS with magnetic measurements is a basic task of plasma operations, Saito's LEF may be used anytime. Our investigation showed that L1 regularization can strongly improve the SX tomography with the traditional L2 regularization having FBS/LEF and would be effective against other tomographic problems in fusion devices. [ABSTRACT FROM AUTHOR]

In the present work, electrostatic drift kinetic simulations of parallel plasma transport within the tokamak scrape-off layer (SOL) are conducted using the COGENT code. The SOL configuration is represented in one-dimensional slab geometry, incorporating a heat source localized in the midplane. The heat source parameters correspond to those characterizing edge-localized modes observed in the Joint European Torus (JET) tokamak. The numerical model includes kinetic treatment of both ions and electrons, a simplified model for the gyrokinetic Poisson equation that allows one to step over short time scales associated with fast electrostatic shear Alfvèn waves, and the logical sheath boundary condition (LSBC) that enforces global system quasineutrality. A third-order accurate LSBC is derived to be consistent with the third-order accurate upwind advection scheme utilized in the code, and it was shown to noticeably impact the simulation results, especially parallel heat flux at the target plate. The findings of this study are in agreement with results from preceding fluid and kinetic simulations. [ABSTRACT FROM AUTHOR]

Advanced tokamak scenarios often feature equilibriums with zero and reversed magnetic shear. To isolate and investigate their impacts on the resistive wall mode (RWM) instability analytically, we construct a series of cylindrical limiter equilibriums with different magnetic shear in the core and toward plasma edge, as a prototype of the configurations in advanced tokamak scenarios. Equilibriums with zero β are considered so that the analysis focuses on the current-driven RWMs. Analytical solutions for the n = 1 resistive wall mode, derived from the reduced ideal magnetohydrodynamics (MHD) equations, consistently demonstrate key insights for both the core and the edge: the growth rate of RWMs is inversely proportional to the averaged magnetic shear when the safety factor at the edge ( q a ) is held constant. Specifically, an increase in positive averaged magnetic shear results in a decreased growth rate, whereas an increase in negative averaged magnetic shear leads to an increased growth rate. Furthermore, the influence of n q a on the mode's stability significantly surpasses the effects of magnetic shear changes previously discussed. These analytical calculation results provide a verification of NIMROD simulations. [ABSTRACT FROM AUTHOR]

An advanced tokamak operation featuring reversed magnetic shear configuration can trigger double tearing modes, a deleterious resistive magnetohydrodynamic (MHD) instability that plays a crucial role in causing major disruptions. Generally, the properties of the double tearing mode can be investigated through solving MHD equations. However, solving MHD equations is widely recognized as a challenging task due to the complex physical model, massive CPU requirements, and lengthy computation time. A surrogate model based on MHD equations can quickly provide physics-based instability information, such as the mode growth rate, serving as references for future control needs. In this work, a reliable surrogate prediction model, LTM-NN, is established using a neural network (NN) based on a numerical database. This work employs a linear tearing mode (LTM) code with a large aspect ratio tokamak configuration to construct a supervised learning dataset of equilibria and growth rates. It is found that the trained LTM-NN predicts growth rates with a high accuracy up to 97 % , and the classification accuracy for the stability of the double tearing mode reaches 100%. Furthermore, LTM-NN is applied to systematically investigate the dependence of the growth rate on the inter-resonance distance, the magnetic shear, and the magnetic Reynolds number. The results are consistent with existing theories. Thus, high accuracy, fast computing speed, and correct parameter dependencies demonstrate that LTM-NN is capable of serving as a rapid predictor of double tearing modes to provide instability information for the integrated model or the disruption warning model in the future. [ABSTRACT FROM AUTHOR]

In the low-density edge plasma of tokamaks, ion cyclotron range of frequencies actuators may parasitically emit slow waves. If the density is sufficiently low, which may be common in large future devices such as international thermonuclear experimental reactor (ITER), these slow waves take the form of so-called resonance cones. The traditional theoretical description of this wave mode relies on formally relating an electrostatic approximation of the frequency-domain wave equation to a time-domain wave equation and relating the cone angle to the wave speed in the time-domain wave equation. In the cold plasma collisional regime, that wave speed is complex. We investigate that scenario in this work. [ABSTRACT FROM AUTHOR]

Electromagnetic gyrokinetic simulation plays an important role in the research of anomalous transport driven by micro-turbulence in tokamak fusion plasmas. The development of electromagnetic gyrokinetic codes usually faces challenges including physical model, numerical instabilities as well as corresponding algorithms and accuracy; one of the typical challenges is the well-known cancellation problem. In this work, the gyrokinetic code NLT [Ye et al., J. Comput. Phys. 316, 180 (2016)] is updated to the electromagnetic version. The scheme of mitigating the cancellation problem in GYRO [Candy and Waltz, J. Comput. Phys. 186, 545 (2003)] is adopted. The simulation results including the real frequency, linear growth rate, and mode structures of the ion temperature gradient mode and the trapped electron mode as well as the kinetic ballooning mode from NLT are consistent with those from previous Eulerian and particle in cell codes. Moreover, it is found that the cancellation problem does not exist, at least for the drift waves, in the NLT simulation. [ABSTRACT FROM AUTHOR]

Two-plasmon decay instabilities (TPDIs) are a class of parametric instabilities where a gyrotron beam, polarized in X-mode, decays nonlinearly into two upper hybrid waves at approximately half frequency. Here, we present the design of a simple radiometer that was installed at Tokamak á configuration variable to diagnose TPDIs as well as a novel technique that was used to calibrate the radiometer using thermal electron cyclotron emissions. We show examples of measured radiation from TPDIs that occur when the gyrotron beam intersects blob filaments in the scrape-off layer and demonstrate that the frequency separation of the TPDI daughter waves can be used to assess the densities of the blob filaments. This technique could be utilized to diagnose density fluctuations due to both blobs, ELMs, and rotating magnetic islands. [ABSTRACT FROM AUTHOR]

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This study explores the application of machine learning techniques for detecting and tracking plasma filaments around the boundary of magnetically confined tokamak plasmas. Plasma filaments, also called blobs, are responsible for enhanced turbulent transport across magnetic field lines, and their accurate characterization is crucial for optimizing the performance of magnetic fusion devices. We present a novel approach that combines machine learning methods applied to data obtained from ultra-fast cameras, including YOLO (You Only Look Once) for object detection, semantic segmentation, and specific tracking methods. This approach enables fast and accurate detection and tracking of filaments while overcoming the limitations of conventional methods, which are time-consuming and prone to human subjectivity. A significant advance in our study lies in the development of a method for automatically labeling a large batch of data, which greatly facilitates the training of supervised machine learning algorithms. Using these techniques, we obtained promising results demonstrating a significant improvement over conventional tracking methods, achieving a detection accuracy of up to 98.8%, while reducing the inference time per frame by 15% to 31% compared to conventional Kalman filter tracking. These results open up new perspectives for investigating turbulent phenomena in tokamaks, and could have important implications for the development of controlled nuclear fusion. [ABSTRACT FROM AUTHOR]

Centrifugal confinement fusion, a promising alternative to toroidal confinement devices like tokamaks and stellarators, leverages supersonic plasma rotation within a magnetic mirror configuration to achieve simplified coil design, compactness, and enhanced stability. This brief article explores the potential of centrifugal confinement fusion for propulsion applications. A previous concern regarding the escape of energetic ions, essential for propulsion, is addressed through test-particle simulations. The results indicate that the earlier estimate based on adiabatic invariance was overly pessimistic, and the underlying physics is clarified. [ABSTRACT FROM AUTHOR]

A mechanism of nonlinear interaction between a geodesic acoustic mode and a low-frequency zonal flow has been proposed to explain the appearance of satellites of the geodesic acoustic mode observed in tokamak experiments. Interaction between eigenmodes with significantly different frequencies leads to the modulation of the amplitude of the geodesic acoustic mode at the frequency of zonal flow oscillations, which is manifested as two side harmonics near the frequency of the geodesic acoustic mode in the spectrum of oscillations. The frequencies of the indicated harmonics correspond to the sum and difference of the frequencies of the geodesic acoustic mode and zonal flow. The effect is triggered by a stationary toroidal plasma rotation. [ABSTRACT FROM AUTHOR]

A study of the effect of toroidal magnetic field (Bt) on the divertor asymmetries is carried out with a plasma transport code under BOUT++ framework with the magnetic equilibria from EAST and C-Mod. For the simulation cases with drifts, the density is larger at the inner divertor target, while the temperature and heat flux are larger at the outer divertor target. The ratio of the total particle flux at the outer target to that of the inner target increases with increasing Bt, while the ratio of the total heat flux at the outer target to that of the inner target decreases with increasing Bt. The in–out divertor asymmetries of both total particle and heat fluxes get weaker with increasing Bt. Further analysis shows that the edge radial transport induced by drifts is much weaker for the simulation case with higher Bt, indicating that drift-driven divertor asymmetries may be less important for future tokamaks with high Bt. [ABSTRACT FROM AUTHOR]

Runaway electrons, accelerated in a tokamak discharge to high energies (tens of MeV), can cause serious damage to plasma facing components. Therefore, it is important to develop effective mitigation strategies to reduce the risk of tokamak damage. To study the effects of various mitigation strategies, a dedicated diagnostic, the calorimetry probe, was developed at the COMPASS tokamak. This probe successfully measured the heat loads caused by runaway electrons directly, using a set of temperature sensors embedded in a graphite body. Deposited energy measured by the probe ranged from a few hundred joules up to (15 ± 1) kJ, while the mean deposited energy was (4.5 ± 1.1) kJ. The design of the probe and the initial results from 250 discharges covering four experimental campaigns dedicated to runaway electron studies (2019–2021) are presented in this article. [ABSTRACT FROM AUTHOR]

Power loading from plasma in the scrape-off layer limits the lifetime of plasma-facing components in tokamak-based power plants. The Mega Ampere Spherical Tokamak Upgrade [W. Morris et al., IEEE Trans. Plasma Sci. 46(5), 1217–1226 (2018)] (MAST-U) features four divertor strike points (SP) owing to its up-down symmetry. This paper introduces the five-camera infrared thermography system, covering all four SPs and their mode of operation on MAST-U. The system employs both medium and long wavelength cameras, offering high frame rates up to 4 kHz and spatial resolutions of down to 3 mm. Postprocessing involves artifact elimination and heat flux calculation using in-house analysis software that incorporates the inverse heat transfer code, THEODOR (THermal Energy Onto DivertOR). [ABSTRACT FROM AUTHOR]

A new thermal helium beam diagnostic has been implemented in the outer lower divertor of the ASDEX Upgrade tokamak. The purpose of this diagnostic is to measure two-dimensional profiles of electron density (ne) and temperature (Te) with high temporal and spatial resolution. The geometry of the lines of sight is chosen to avoid the influence of prompt recycling and to optimize the resolution without significantly impacting the divertor structure. Moreover, the effect of long-term helium recycling has been analyzed, and its amplitude compared to the active signal is negligible. Finally, the reconstruction of ne and Te is done via a collisional radiative model, while a static and a dynamic model were implemented and compared with SOLPS simulations as well as divertor Thomson scattering data. Furthermore, a new 2D parameterization of the outer divertor volume, which is required for the dynamic model, was developed. Due to its fast and local ne and Te profile measurements, the diagnostic is suitable for investigating fast processes such as divertor transitions and filaments. [ABSTRACT FROM AUTHOR]

Recently, the numerical scheme presented by Mishchenko et al. [Phys. Plasmas 21, 052113 (2014); 21, 092110 (2014)] enabled explicit gyrokinetic simulations of low-frequency electromagnetic instabilities in tokamaks at experimentally relevant values of plasma β. This scheme resolved the long-standing cancellation problem that previously hindered gyrokinetic particle-in-cell code simulations of magnetohydrodynamic phenomena with inherently small parallel electric fields. Moreover, the scheme did not employ approximations that eliminate critical tearing-type instabilities. Here, we report on the implementation of this numerical scheme in the global gyrokinetic particle-in-cell code GTS. This implementation allows for a more complete and accurate picture of interaction between small scale turbulence and MHD modes in tokamaks. Additionally, we present a comprehensive set of verification simulations of numerous electromagnetic instabilities relevant to present-day tokamaks. These simulations encompass the kinetic ballooning mode, the internal kink mode, the tearing mode, the micro-tearing mode, and the toroidal Alfven eigenmode destabilized by energetic ions, which are all instrumental in understanding tokamak physics. We will also showcase the preliminary nonlinear simulations of kinetic ballooning instabilities and (2,1) island formation due to tearing mode instability. These simulations validate the accuracy of the scheme implementation and pave the way for studying how these instabilities affect plasma confinement and performance. [ABSTRACT FROM AUTHOR]

Twisting of the spatial structure of eigenmodes by energy fluxes generated due to the spatial channeling (SC)—the fast-ion energy and momentum transfer across the magnetic field by destabilized modes—is considered. It is revealed that there is a correlation between the direction of the radial wave energy flux, orientation of the mode twist (MT), and the direction of mode rotation. The mode twist parameter is introduced and relations connecting it with the energy flux are established. It is shown the energy flux transforms zeros of the radial profile of the mode amplitude into minima (i.e., zeros disappear). It is found that the radial group velocity and phase velocity of reversed shear Alfvén eigenmodes (RSAEs) have opposite directions. These findings can be used for diagnostics of the energy fluxes during fast-ion driven instabilities and understanding whether the SC degrades or improves plasma performance. Specific calculations are carried out for fast magnetoacoustic modes, FMMs (known also as compressional Alfvén eigenmodes, CAEs), and Alfvén eigenmodes, AEs. A DIII-D experiment where RSAEs were observed is considered. It is concluded that an outward SC took place in this experiment. Peculiarities of various modes are discussed, which may explain why twisting of AEs, but not of FMMs, was observed experimentally in the DIII-D and NSTX tokamaks. [ABSTRACT FROM AUTHOR]

The kinetic effects of Neutral Beam Injection (NBI) induced energetic particles (EPs) on the stability of internal kink mode are numerically investigated in the presence of anisotropic thermal transport for the new medium-sized tokamak HL-3, utilizing the MHD-kinetic hybrid code MARS-K (Liu Y Q et al. 2008 Phys. Plasmas 15 112503). It is found that after including realistic level of thermal transport, the kinetic effect of EPs on the mode stability is stabilizing in the co-NBI case while destabilizing in the counter-NBI case, in contrast to the similar stabilizing effect with both NBI cases at vanishing thermal transport. Detailed investigation reveals that this opposite effect with the two NBI cases is due to the different non-adiabatic transit resonance contributions of passing EPs in the presence of thermal transport. Anisotropic thermal transport can indirectly affect the non-adiabatic contribution of passing EPs by modifying the mode eigenvalue, which also contributes to the resonance condition between the mode and passing EPs. The non-perturbative approach adopted in the study also enables comparison of different modifications to the mode eigen-structure between the two NBI cases. [ABSTRACT FROM AUTHOR]

A general approach of how to construct plasma equilibrium in a tokamak with n = 1 violation of toroidal symmetry is proposed. For an arbitrary axisymmetric tokamak plasma equilibrium, there exists the small n = 1 deformation of the initial magnetic configuration that keeps the nesting of the magnetic surfaces (as in the initial configuration) and provides plasma equilibrium; such deformation and final equilibrium configuration are calculated analytically. The asymmetric analogue of the Solov'ev's equilibrium with non-degenerated plasma pressure and current density profiles is presented as an example of the application of the developed algorithm. [ABSTRACT FROM AUTHOR]

In this work, we use the global electromagnetic and electrostatic gyro kinetic approaches to investigate the effects of zonal flows forced-driven by Alfvén modes due to their excitation by energetic particles on the dynamics of ITG (ion temperature gradient) instabilities. The equilibrium of the 92416 JET tokamak shot is considered. The linear, nonlinear Alfvén modes, and the zonal flow dynamics are investigated, and their respective radial structures and saturation levels are reported. ITG dynamics in the presence of the zonal flows excited by these Alfvén modes are also investigated. The zonal flows forced-driven by Alfvén modes can significantly impact the ITG dynamics. A zonal flow amplitude scan reveals the existence of an inverse relation between the zonal flow amplitude and the ITG growth rate. These results indicate that forced-driven zonal flows can be an important indirect part of turbulence mitigation due to the injection of energetic particles. [ABSTRACT FROM AUTHOR]

An understanding of the behaviour of the D or He fuel used in tokamak discharges is essential in analyses such as modelling edge and divertor transport and the erosion of the vessel walls. However, poor agreement is found between measurements made on the JET tokamak and collisional-radiative models used to predict the hydrogen-like D and He line intensities. The range of temperatures of the plasmas emitting the radiation is also limited, in contrast to that for many impurities for which a wide range is possible. This is particularly so for He II whose line intensities tend to have the same near-constant ratios in most pulses, suggesting that the emission originates in plasma regions with very similar electron temperatures. To gain understanding and to allow quantitative comparisons with theoretical models, extensive observations of the VUV Lyman series have been made, for all discharge scenarios run during three He campaigns. Those for He discharges in both JET ITER-like wall (JET-ILW) and JET C (JET-C) campaigns are presented here. He discharges have the advantage of fewer impurities resulting in less complex spectra than when D is used as the fuel. However, the characteristics of the observed discrepancies are similar in both species, allowing He to be used as a proxy for D in order to gain understanding of the discrepancy. In addition, the study of He avoids the complication of molecular species contributing to the level populations. Opacity effects are also expected to be less severe in He discharges. Nevertheless, so as to ensure that the measurements are not unduly affected by opacity, comparisons have also been made with emission from Balmer and Paschen series members. Measurements of both line intensities and their ratios are presented for all-pulse surveys and for individual pulses. In exceptional cases in which the He emission is intense a dependence on the He II line intensity is demonstrated. The discrepancy between these measurements and the theoretical models is illustrated. [ABSTRACT FROM AUTHOR]

The DIII-D tokamak has elucidated crucial physics and developed projectable solutions for ITER and fusion power plants in the key areas of core performance, boundary heat and particle transport, and integrated scenario operation, with closing the core-edge integration knowledge gap being the overarching mission. New experimental validation of high-fidelity, multi-channel, non-linear gyrokinetic turbulent transport models for ITER provides strong confidence it will achieve Q ⩾ 10 operation. Experiments identify options for easing H-mode access in hydrogen, and give new insight into the isotopic dependence of transport and confinement. Analysis of 2,1 islands in unoptimized low-torque IBS demonstration discharges suggests their onset time occurs randomly in the constant β phase, most often triggered by non-linear 3-wave coupling, thus identifying an NTM seeding mechanism to avoid. Pure deuterium SPI for disruption mitigation is shown to provide favorable slow cooling, but poor core assimilation, suggesting paths for improved SPI on ITER. At the boundary, measured neutral density and ionization source fluxes are strongly poloidally asymmetric, implying a 2D treatment is needed to model pedestal fuelling. Detailed measurements of pedestal and SOL quantities and impurity charge state radiation in detached divertors has validated edge fluid modelling and new self-consistent 'pedestal-to-divertor' integrated modeling that can be used to optimize reactors. New feedback adaptive ELM control minimizes confinement reduction, and RMP ELM suppression with sustained high core performance was obtained for the first time with the outer strike point in a W-coated, compact and unpumped small-angle slot divertor. Advances have been made in integrated operational scenarios for ITER and power plants. Wide pedestal intrinsically ELM-free QH-modes are produced with more reactor-relevant conditions, Low torque IBS with W-equivalent radiators can exhibit predator-prey oscillations in T e and radiation which need control. High- β P scenarios with q min > 2, q 95–7.9, β N > 4, β T–3.3% and H 98y2 > 1.5 are sustained with high density ( n ¯ = 7E19 m−3, f G–1) for 6 τ E, improving confidence in steady-state tokamak reactors. Diverted NT plasmas achieve high core performance with a non-ELMing edge, offering a possible highly attractive core-edge integration solution for reactors. [ABSTRACT FROM AUTHOR]

The objectives of NSTX-U research are to reinforce the advantages of STs while addressing the challenges. To extend confinement physics of low- A, high beta plasmas to lower collisionality levels, understanding of the transport mechanisms that set confinement performance and pedestal profiles is being advanced through gyrokinetic simulations, reduced model development, and comparison to NSTX experiment, as well as improved simulation of RF heating. To develop stable non-inductive scenarios needed for steady-state operation, various performance-limiting modes of instability were studied, including MHD, tearing modes, and energetic particle instabilities. Predictive tools were developed, covering disruptions, runaway electrons, equilibrium reconstruction, and control tools. To develop power and particle handling techniques to optimize plasma exhaust in high performance scenarios, innovative lithium-based solutions are being developed to handle the very high heat flux levels that the increased heating power and compact geometry of NSTX-U will produce, and will be seen in future STs. Predictive capabilities accounting for plasma phenomena, like edge harmonic oscillations, ELMs, and blobs, are being tested and improved. In these ways, NSTX-U researchers are advancing the physics understanding of ST plasmas to maximize the benefit that will be gained from further NSTX-U experiments and to increase confidence in projections to future devices. [ABSTRACT FROM AUTHOR]

A molecular pump is a high vacuum acquisition piece of equipment that provides a clean vacuum environment for the Experimental Advanced Superconducting Tokamak (EAST) device. Its running state affects the smooth development of the EAST experiment. Because of fatigue degradation of internal components of the molecular pump, vacuum leakage may occur during long-term operation, causing secondary hazards to the device. In order to improve the accuracy of molecular pump fault prediction, based on the long short-term memory network (LSTM), the deep long short-term memory network (DE-LSTM) and the bidirectional long short-term memory network (Bi-LSTM) are combined. The deep bidirectional long short-term memory network (DE-Bi-LSTM) algorithm is proposed, and the piecewise linear degradation model is introduced to predict fault of the molecular pump. By collecting the vibration signals leaked in the atmosphere and running to the fault time series on the destructive test platform simulating molecular pump fault, data were extracted in the time domain. Finally, the obtained feature vector set was used as the input of the DE-Bi-LSTM algorithm through data standardization to train the model and realize the prediction of molecular pump fault. The experimental results show that the proposed method is optimal to LSTM, DE-LSTM, and Bi-LSTM in predicting performance. [ABSTRACT FROM AUTHOR]

Runaway electrons (REs) generated during disruptions pose a significant safety threat to tokamaks, as they can melt and damage the plasma-facing components (PFCs). Therefore, studying RE behavior is crucial for fusion devices. The interaction between REs and the first wall/PFCs results in the emission of high-energy X-rays, known as bremsstrahlung. To investigate RE behavior, it is necessary to quantitatively evaluate the emission of hard X-rays. A real-time hard X-ray spectrometer, utilizing a LaBr3 detector, has been successfully developed for studying REs on the HL-2M tokamak. This spectrometer has a counting rate capability reaching 3 MHz, with an energy resolution of 3.3% at 662 keV (137Cs). The time resolution for energy spectrums is as short as 1 ms. During the HL-2M discharge, observations were made on the hard X-ray energy spectrum, and by analyzing the spectrum within the energy range of 250 keV to 750 keV, the temperature of the corresponding REs was deduced. [ABSTRACT FROM AUTHOR]

Abstract This study explores the application of machine learning techniques for detecting and tracking plasma filaments around the boundary of magnetically confined tokamak plasmas. Plasma filaments, also called blobs, are responsible for enhanced turbulent transport across magnetic field lines, and their accurate characterization is crucial for optimizing the performance of magnetic fusion devices. We present a novel approach that combines machine learning methods applied to data obtained from ultra-fast cameras, including YOLO (You Only Look Once) for object detection, semantic segmentation, and specific tracking methods. This approach enables fast and accurate detection and tracking of filaments while overcoming the limitations of conventional methods, which are time-consuming and prone to human subjectivity. A significant advance in our study lies in the development of a method for automatically labeling a large batch of data, which greatly facilitates the training of supervised machine learning algorithms. Using these techniques, we obtained promising results demonstrating a significant improvement over conventional tracking methods, achieving a detection accuracy of up to 98.8%, while reducing the inference time per frame by 15% to 31% compared to conventional Kalman filter tracking. These results open up new perspectives for investigating turbulent phenomena in tokamaks, and could have important implications for the development of controlled nuclear fusion.

As it turns out, both isotope scaling and density limits are phenomena closely linked to fluid closure. The necessity to include ion viscosity arises for both phenomena. Thus, we have added ion viscosity to our model. The experimental isotope scaling has been successfully recovered in our fluid model through parameter scans. Although ion viscosity typically exerts a small effect, the density limit is manifested by increasing the density by approximately tenfold from the typical experimental density. In our case, this increase originates from the density in the Cyclone base case. Notably, these phenomena would not manifest with a gyro-Landau fluid closure. The isotope scaling is nullified by the addition of a gyro-Landau term, while the density limit results from permitting ion viscosity to become comparable to the gyro-Landau term. The mechanism of zonal flows, demonstrated analytically for the Dimits upshift, yields insights into the isotope scaling observed in experiments. In our approach, ion viscosity is introduced in place of the Landau fluid resonances found in some fluid models. This implies that the mechanism of isotope scaling operates at the level of fluid closure in connection with the generation of zonal flows. The strength of zonal flows in our model has been verified, particularly in connection with the successful simulation of the nonlinear Dimits shift. Consequently, a role is played by our approach in the temperature perturbation part of the Reynolds stress.

The characteristics of ion-scale turbulence in the presence of a magnetic island are numerically investigated using a gyrokinetic model in fusion plasma. We observe that in the absence of the usual ion temperature gradient (ITG) drive gradient, a magnetic island and its flatten effect could drive ITG instability. The magnetic island (MI) not only drives high-n modes of ITG instability but also induces low-n modes of vortex flow. Moreover, as the magnetic island width increases, the width of the vortex flow also increases. This implies that wider islands may more easily induce vortex flows. The study further indicates that the saturated amplitude and transport level of MI-induced ITG turbulence vary with different magnetic island widths. In general, larger magnetic islands enhance both particle and heat transport. When the magnetic island width reaches to 21ρi, the turbulence-driven transport becomes the same level with the cases that ITG is driven by pressure gradients. Our findings indicate the presence of intricate nonlinear effects in the modulation of plasma turbulence by MIs. These effects are of significant importance for comprehending the phenomenon of nonlinear coupling in forthcoming tokamaks such as ITER. [ABSTRACT FROM AUTHOR]

The architecture of the Spherical Tokamak for Energy Production (STEP) has been developed to enable a hybrid maintenance approach using ports in the vacuum vessel for a limited list of tasks that must be performed shortly after shutdown, and larger openings to simplify and speed up major refits. Robotic handling systems in zero-human entry facilities will prevent workers from being exposed to the most hazardous environments. While the approach is largely grounded in existing technologies, the scale and environment of STEP will require significant technology development. Notably, programmes have been established to develop service connections and in-vessel robotic technologies. The engineering integration of the maintenance strategy into the tokamak remains a priority, as does ongoing work to simplify and reduce the cost of the buildings required to facilitate maintenance. Remountable magnet joints are critical to ensuring life-limited magnet components can be replaced during the STEP lifetime and realizing the STEP maintenance strategy. It is a high-risk endeavour owing to the low technology maturity of the potential solutions and owing to the tough and intertwined technical challenges and constraints imposed by both the fundamental physics and the STEP requirements and architecture. An integrated design approach has been taken to balance many competing factors and integrate with interfacing systems, and a multi-faceted technology development programme has been established to address technical risk and to inform, verify and validate the STEP remountable magnet design. This article is part of the theme issue 'Delivering Fusion Energy – The Spherical Tokamak for Energy Production (STEP)'. [ABSTRACT FROM AUTHOR]

This theme issue collects together papers summarising the conceptual design of the Spherical Tokamak for Energy Production (STEP). In 2019, the UK government funded the first design stages of a prototype fusion powerplant based on a compact toroidal geometry, called STEP. The primary technical aims of STEP are to produce net energy, to be self-sufficient in tritium fuel and to demonstrate a maintenance regime that would extrapolate to appropriate availability for commercial powerplants. After 5 years and over 1000 person-years of detailed scientific and engineering conceptual design, this theme issue acts as a compendium of the current design basis for STEP, noting that this is a snapshot in time and that the design will continue to evolve. This article is part of the theme issue 'Delivering Fusion Energy – The Spherical Tokamak for Energy Production (STEP)'. [ABSTRACT FROM AUTHOR]