Core Confinement Research Articles

Nonlinear gyrokinetic modelling of high confinement negative triangularity plasmas

Nonlinear gyrokinetic simulations correctly predict particle as well as ion and electron energy fluxes of high confinement plasmas with a negative triangularity cross sectional shape, showing that core transport in these plasmas is well described by standard gyrokinetic models. Experimentally inferred power balance fluxes are mostly reproduced within one standard deviation across a wide portion of the minor radius. Experimental conditions are reproduced by ion scale simulations, without the need to include density and temperature profile curvature effects. The experimental case is used as baseline to predict that the non-dimensional confinement scaling in negative triangularity plasmas increases strongly with plasma current while slightly degrading at increasing normalized pressure and decreasing collisionality. Recent experiments showed that low toroidal rotation negatively impacts confinement; consistent with the experiment, simulations predict that low rotational shear significantly affects confinement unless the plasma effective charge is maintained above a minimum level. Core confinement is predicted to significantly degrade in low aspect ratio devices.

Assessing core ion thermal confinement in critical-gradient-optimized stellarators

We investigate the core confinement properties of two recently devised quasi-helically symmetric stellarator configurations, HSK and QSTK. Both have been optimized for large critical gradients of the ion temperature gradient mode, which is an important driver of turbulent transport in magnetic confinement fusion devices. To predict the resulting core plasma profiles, assuming a fixed edge temperature, we utilize an advanced theoretical framework based on the gyrokinetic codes GENE and GENE-3D, coupled to the transport code TANGO. Compared to the HSX stellarator, both HSK and QSTK achieve significantly higher core-to-edge temperature ratios, partly thanks to their smaller aspect ratios, with the other part due to more detailed shaping of the magnetic geometry achieved during optimization. The computed core confinement time, however, is less sensitive to core temperature than the fixed edge temperature, simply due to the disproportionate influence, the edge has on stored plasma energy. We, therefore, emphasize the possible benefits of further optimizing turbulence in the outer core region, and the need to include accurate modeling of confinement in the edge region in order to assess overall plasma performance of turbulence optimized stellarators.

Highest fusion performance without harmful edge energy bursts in tokamak

The path of tokamak fusion and International thermonuclear experimental reactor (ITER) is maintaining high-performance plasma to produce sufficient fusion power. This effort is hindered by the transient energy burst arising from the instabilities at the boundary of plasmas. Conventional 3D magnetic perturbations used to suppress these instabilities often degrade fusion performance and increase the risk of other instabilities. This study presents an innovative 3D field optimization approach that leverages machine learning and real-time adaptability to overcome these challenges. Implemented in the DIII-D and KSTAR tokamaks, this method has consistently achieved reactor-relevant core confinement and the highest fusion performance without triggering damaging bursts. This is enabled by advances in the physics understanding of self-organized transport in the plasma edge and machine learning techniques to optimize the 3D field spectrum. The success of automated, real-time adaptive control of such complex systems paves the way for maximizing fusion efficiency in ITER and beyond while minimizing damage to device components.

Experimental Investigation and Numerical Analysis of the Axial Load Capacity of Circular Concrete-Filled Tubular Columns

This paper focuses on the experimental investigation of the axial load capacity of CFT (concrete-filled steel tube) columns under actual construction conditions during building reconstruction. A total of four samples were loaded up to failure. The varied parameters were the column length and absence/presence of the concrete infill within the steel tube. Further, the analysis is extended to developing a numerical model in the finite element-based software ABAQUS version 6.9. This numerical model includes material and geometrical nonlinearities and was validated with the experimental results. The contribution of the concrete core to the column capacity and the concrete core confinement effect are discussed. Finally, the column capacity was calculated according to several design codes: the Eurocode 4 with and without considering the confinement effect, American specifications, Australian standards, the American Institute of Steel Construction, and the Architectural Institute of Japan. The Eurocode 4 considering the confinement effect provides the closest results to those obtained in the tests.

Role of turbulent separatrix tangle in the improvement of the integrated pedestal and heat exhaust issue for stationary-operation tokamak fusion reactors

The magnetic separatrix surface is designed to provide the final and critical confinement to the hot stationary-operation core plasma in modern tokamak reactors in the absence of an external magnetic perturbation (MP) or transient magneto-hydrodynamic perturbation, while diverting the exhaust heat to divertor plates. All the stationary operational boundary plasma studies and reactor designs have been performed under this assumption. However, there has been a long-standing suspicion that a stationary-operation tokamak plasma even without external MPs or edge localized modes (ELMs) activities may not have a stable closed separatrix surface, especially near the magnetic X-point. Here, the first gyrokinetic numerical observation is reported that the divertor separatrix surface, due to homoclinic tangles caused by intrinsic electromagnetic turbulence, is not a stable closed surface in a stationary operation phase even without MPs or ELMs. Unlike the MP- or ELM-driven homoclinic tangles that could cause deleterious effects to core confinement or divertor plates, it is found that the micro-turbulence driven homoclinic tangles could connect the divertor plasma to the pedestal plasma in a constructive way by broadening the divertor heat-exhaust footprint and weakening the pedestal slope to the ELM-safe direction. Micro-turbulent homoclinic tangles can open a new research direction in understanding and controlling these two most troublesome and non-locally connected edge-plasma issues in a tokamak fusion reactor.

Overview of advances in ASDEX Upgrade plasma control to support critical physics research for ITER and beyond

The successful operation of fusion reactors requires plasma scenarios with good core confinement and acceptable first wall heat loads that are stable and robust to external perturbations. This poses both physical and technological challenges. One of the technologies that addresses these challenges is a complex feedback control system that supports advances in physical understanding and helps to ensure stable operating conditions. The operation of marginally stable plasmas often leads to off-normal events (such as disruptions) and feedback control can prevent these to some extent. This contribution gives an overview of the main results of the development and operation of the feedback control algorithms on ASDEX Upgrade (AUG). Fueling actuators, using a combination of gas valves and pellet injection, can simultaneously control neutral density of the divertor and the density of the plasma core above the Greenwald limit. Impurity injection is employed to control the position of the X-point radiator, allowing the creation of an ELM-suppressed H-mode with high radiation fraction. Heating actuators are used to control the plasma energy content, which supports advanced tokamak experiments and enables stable I-mode operation, and the electron temperature control, which supports turbulence studies. In control technology, AUG has pioneered the use of virtual actuators, which allow effective use of the limited number of heating actuators, adaptive control policies, and exception handling. Such technologies will also be used in ITER. Advanced nonlinear state observers (RAPTOR, RAPDENS) and codes to evaluate the power deposition properties (RABBIT, TORBEAM) are available for routine use in the AUG feedback controllers. Extensive use of the AUG discharge control system further enhances the research capabilities of this machine.

β-induced Alfvén eigenmodes and fishbone driven by energetic electrons in EAST electron heating dominant plasmas

β-induced Alfvén eigenmodes (e-BAE) and fishbone (e-fishbone) driven by the energetic electrons have been identified on the experimental advanced superconducting tokamak in electron cyclotron resonance heating (ECRH), low hybrid wave (LHW) together with neutral beam injection plasma and in pure radio frequency heating plasma, respectively. The e-BAE is predominantly located near the pedestal region, and its frequency typically displays a chirping characteristic. The frequency is notably associated with the count of energetic electrons with energy between 30 keV and 80 keV driven by LHW and undergoes three distinct phases. A direct correlation is established between the frequency of e-BAE and the global parameters, including electron temperature, Alfvén velocity and βp . The e-fishbone emerges in the core and becomes noticeable shortly after ECRH initiation, in an electron heated dominant and core confinement enhanced discharge. Experimental analyses suggest that the e-fishbone ties closely with both the population and energy distribution of energetic electrons. These observations can provide valuable insights into the behavior of energetic electrons, which play a crucial role in the formation and sustainability of transport barriers within hybrid plasma, both in the internal and pedestal regions.

Isotope physics of heat and particle transport with tritium in JET-ILW type-I ELMy H-mode plasmas

As part the DTE2 campaign in the JET tokamak, we conducted a parameter scan in T and D-T complementing existing pulses in H and D. For the different main ion masses, type-I ELMy H-modes at fixed plasma current and magnetic field can have the pedestal pressure varying by a factor of 4 and the total pressure changing from βN=1.0 to 3.0. We investigated the pedestal and core isotope mass dependencies using this extensive data set. The pedestal shows a strong mass dependence on the density, which influences the core due to the strong coupling between both plasma regions. To better understand the causes for the observed isotope mass dependence in the pedestal, we analysed the interplay between heat and particle transport and the edge localised mode (ELM) stability. For this purpose, we developed a dynamic ELM cycle model with basic transport assumptions and a realistic neutral penetration. The temporal evolution and resulting ELM frequency introduce an additional experimental constraint that conventional quasi-stationary transport analysis cannot provide. Our model shows that a mass dependence in the ELM stability or in the transport alone cannot explain the observations. One requires a mass dependence in the ELM stability as well as one in the particle sources. The core confinement time increases with pedestal pressure for all isotope masses due to profile stiffness and electromagnetic turbulence stabilisation. Interestingly, T and D-T plasmas show an improved core confinement time compared to H and D plasmas even for matched pedestal pressures. For T, this improvement is largely due to the unique pedestal composition of higher densities and lower temperatures than H and D. With a reduced gyroBohm factor at lower temperatures, more turbulent drive in the form of steeper gradients is required to transport the same amount of heat. This picture is supported by quasilinear flux-driven modelling using TGLF-SAT2 within Astra. With the experimental boundary condition TGLF-SAT2 predicts the core profiles well for gyroBohm heat fluxes >15 , however, overestimates the heat and particle transport closer to the turbulent threshold.

Bending the rules: Strain accommodation in layered crystalline solids through nanoscale buckling over dislocations

Basal dislocations and ripplocations both offer explanations for the deformation of layered crystalline solids in response to compressive strain. Existing work, however, presents no clear definition of a ripplocation and the distinction between these two mechanisms remains unclear. Molecular dynamics simulations in graphite and modified graphitic models reveal that equivalent additional half-planes of material can induce both dislocations and ripplocations. In this work, we find that ripplocations are essentially an elastic buckling phenomenon operating at the atomic scale, rather than a discrete crystallographic defect. Dislocation core confinement produces incommensurate planar elastic strains with sufficient energy to trigger a buckling instability that transfers strain energy to the surrounding lattice. The interplay between incommensurate strain and buckling is a quasi-continuum phenomenon where both mechanisms may accommodate arbitrary disregistry, not only the discrete values accommodated by perfect or partial basal dislocations. The mechanistic transition depends critically on the atomic-scale interfacial energies between layers and the continuum elastic behavior of the bulk material. Buckling is possible, though unfavorable, in the MAX phase Ti3AlC2 and systems of multiple layers buckle at lower strains than only a single layer. These extensions of the ripplocation model in van der Waals layered solids suggest that nanoscale elastic buckling potentially plays a role in the deformation of a wide range of layered crystalline systems in response to common mechanical configurations.

Investigation of helium exhaust dynamics at the ASDEX Upgrade tokamak with full-tungsten wall

An efficient removal of He ash by active pumping in future fusion devices is necessary to avoid fuel dilution and not degrade the core confinement properties. Therefore, a deep understanding of the underlying physics mechanisms is mandatory. Helium exhaust has been experimentally investigated at the ASDEX Upgrade tokamak. This is an ideal test environment, thanks to the ITER-like divertor geometry, an extensive diagnostics coverage and the presence of plasma-facing components made of tungsten. The exhaust efficiency, characterized by the He compression in the divertor, was found to improve with increasing divertor neutral pressure but to degrade with detachment. A multi-reservoir particle balance model was developed to interpret the observed exhaust dynamics, accounting for plasma transport and wall retention. The limited performance of the pumping system and the efficient helium retention capability of the tungsten wall were identified to have the strongest impact on the exhaust dynamics.

Development of compact tokamak fusion reactor use cases to inform future transport studies

The OMFIT STEP (Meneghini et al., Nucl. Fusion, vol. 10, 2020, p. 1088) workflow has been used to develop inductive and steady-state H-mode core plasma scenario use cases for a $B_0 = 8 \, {\rm T}$ , $R_0 = 4 \, {\rm m}$ machine to help guide and inform future higher-fidelity studies of core transport and confinement in compact tokamak reactors. Both use cases are designed to produce 200 MW or more of net electric power in an up-down symmetric plasma with minor radius $a = 1.4 \, {\rm m}$ , elongation $\kappa = 2.0$ , triangularity $\delta = 0.5$ and effective charge $Z_{{\rm eff}} \simeq 2$ . Additional considerations based on the need for compatibility of the core with reactor-relevant power exhaust solutions and external actuators were used to guide and constrain the use case development. An extensive characterization of core transport in both scenarios is presented, the most important feature of which is the extreme sensitivity of the results to the quantitative stiffness level of the transport model used as well as the predicted critical gradients. This sensitivity is shown to arise from different levels of transport stiffness exhibited by the models, combined with the gyroBohm-normalized fluxes of the predictions being an order of magnitude larger than other H-mode plasmas. Additionally, it is shown that although heating in both plasmas is predominantly to the electrons and collisionality is low, the plasmas remain sufficiently well coupled for the ions to carry a significant fraction of the thermal transport. As neoclassical transport is negligible in these conditions, this situation inherently requires long-wavelength ion gyroradius-scale turbulence to be the dominant transport mechanism in both plasmas. These results are combined with other basic considerations to propose a simple heuristic model of transport in reactor-relevant plasmas, along with simple metrics to quantify coupling and core transport properties across burning and non-burning plasmas.

Observation of electron-scale turbulence suppression under weak magnetic shear with neon seeding in EAST plasma

Improved confinement caused by Ne injection is investigated in EAST. The safety factor q profile and electron-scale turbulence are studied with a polarimeter-interferometer system and CO2 laser collective scattering system in a series of Ne injection experiments in EAST. Ne with a suitable injection rate can lead to negative magnetic shear in the plasma core, which promotes electron-scale turbulence suppression. The continuous injection of Ne under the reversed shear condition further reduces the electron-scale turbulence intensity. Core electron temperature increase and confinement improvement are observed. QuaLiKiz is applied to model the influence of an impurity and reversed q on turbulence. The combination of stabilization effects of negative magnetic shear and Ne on turbulence is much stronger than the impurity’s own effect, which could explain the experimentally observed electron temperature (T e) increase and turbulence suppression. Besides, the relationship between pedestal structure induced by Ne injection and lower hybrid wave (LHW)-driven current deposition is discussed. These results provide a new perspective to improve the confinement in the case of Ne injection, i.e. to adjust the Ne injection to modify the ideal plasma equilibrium.

Long-pulse H-mode operation with stored-energy monitoring for detachment feedback control with a new lower tungsten divertor in EAST

One of the key challenges for future fusion research is to mitigate the high steady-state heat load on the divertor target plates, and divertor detachment offers a promising solution. The Experimental Advanced Superconducting Tokamak (EAST) has developed several feedback control methods for divertor detachment. However, when an off-normal event momentarily disturbs the main plasma, impurity seeding may still be conducted by these methods for detachment, which probably drives the main plasma further away from its stable equilibrium or even causes the disruption. These off-normal events include excessive impurity seeding, loss of heating and dust droplets, which are not rare in current tokamak experiments, especially in long-pulse operation. To compensate for the drawbacks of these methods, we propose and develop a module of stored-energy monitoring to ensure stable plasmas in long-pulse operation. The stored energy usually decreases when the main plasma is away from its stable equilibrium, which is suitable for monitoring the state of the main plasma. Once the stored energy falls below a certain threshold, the module actively switches off the impurity seeding system. Without impurity seeding, the main plasma can recover with the increase in the stored energy. Only when the stored energy exceeds another threshold does the module switch on the impurity seeding to continue the detachment operation. The module function has been verified during the EAST radiative divertor experiments in the newly-upgraded lower tungsten divertor. A typical ∼20 s discharge in grassy-ELM H-mode regime with ∼5 MW source heating power is demonstrated with divertor partial detachment and good energy confinement by active impurity seeding (50% neon, 50% D2). The energy confinement factor is maintained at a high level, i.e. The electron temperature in the core region only has a slight change after the impurity seeding, while the electron density has a ∼10% increase. Furthermore, the ion temperature near the axis also has a remarkable increase. These achievements provide an important demonstration of the actively controlled radiative divertor mitigating the heat loads with good core confinement, which is an essential step toward steady-state operation of fusion reactors.

Prompt core confinement improvement across the L–H transition in DIII-D: Profile stiffness, turbulence dynamics, and isotope effect

We elaborate on the nature of the prompt core confinement improvement observed at the L–H transition in DIII-D, which is a long-standing issue unsolved for more than two decades and can impact future fusion reactor performance. Dynamic transport analysis suggests the essential role of the profile stiffness for understanding the mechanism of the prompt core confinement improvement. Beam emission spectroscopy shows that transport reduction at the core cannot be explained only by the ion scale turbulence density fluctuation suppression. Properties of nonlocal confinement improvement across the L–H transition are experimentally assessed in hydrogen (H) and deuterium (D) plasmas. Prompt core confinement improvement is found to be more rapid in the lighter hydrogen isotope.

Power exhaust and core-divertor compatibility of the baffled snowflake divertor in TCV

A baffled snowflake minus low-field side (SF-LFS) is geometrically-optimised in tokamak à configuration variable, increasing divertor neutral pressure, to evaluate the roles of divertor closure (comparing with an unbaffled SF-LFS) and magnetic geometry (comparing with a baffled single null (SN)) in power exhaust and core-divertor compatibility. Ohmically-heated L-mode discharges in deuterium, with a line-averaged core density of approximately m−3, are seeded with nitrogen to approach detached conditions. Baffles in the SF-LFS configuration are found to reduce the peak outer target heat flux by up to , without significantly affecting the location of the inter-null radiation region or the core-divertor compatibility. When compared to the baffled SN, the baffled SF-LFS exhibits a reduction in the outer target heat flux by up to and the ability to balance the strike-point distribution of heat flux. These benefits are less significant with N2 seeding, with similar peak target quantities (such as heat flux, electron temperature and ion flux) and divertor radiated power. Despite a radiating region located farther from the confined plasma for the SF-LFS than the baffled SN, no change in core confinement is observed. Core effective charge even indicates an increase in core impurity penetration for the SF-LFS. These experiments constitute a good reference for detailed model validations and extrapolations, exploring important physics such as core impurity shielding and the dependence of divertor cross-field transport on magnetic geometry.

The effect of impurity seeding into the closed divertor on plasma detachment in the HL-2A tokamak

Recent progress towards an increased understanding of detached divertor physics has been made with the highly closed divertor geometry in HL-2A. Non-intrinsic impurities were injected into the outer divertor chamber, and increased divertor neutral pressure and enhanced volumetric energy loss in the divertor were observed. Meanwhile the neutral pressure in the main chamber decreased slightly, and neutral compression between the divertor and main chamber increased greatly. This led to divertor detachment with a low upstream plasma line-averaged electron density (). In the H-mode, slight degradation of the core confinement, characterized by a decrease in plasma stored energy and pedestal pressure and an increase in edge-localized mode frequency, was observed, but the H-mode was still sustained well with H98 > 1. Pedestal density fluctuation was increased during detachment, implying that the enhanced pedestal transport might be responsible for the degradation. During the divertor detachment phase, the impurities were well controlled in the divertor without strong radiation near the X-point region, and the main plasma density did not increase but decreased slightly; this could be a benefit of the highly closed divertor geometry. The experimental results suggest that a closed divertor geometry has the advantages of volumetric energy loss, gas pumping and impurity control in the divertor without significant effects on the plasma confinement, thus giving a wider operating window for divertor detachment.

Experimental investigation of partially confined concrete-filled steel tubular square columns under lateral cyclic loading

This paper presents an experimental investigation on partially confined concrete-filled steel tubular (PCCFST) square columns (i.e., short inner concrete core confinement near the support), using circular hollow section (CHS) configuration as partial inner confinement, under combined axial and cyclic lateral loadings. Hysteretic behavior of load, ultimate lateral strength, energy dissipation capacity, and ductility of the PCCFST columns were studied as a function of (a) the ratio of the height of the inner tube to the diameter of the outer tube (0–2.5) and (b) diameter/width ratio of inner tube to outer tube (0.44, 0.53). Based on the investigation, it has been seen that the height and diameter of the inner CHS have an evident influence on the cyclic performance of the column. Ultimate lateral strength, energy dissipation capacity, and ductility were enhanced by ∼15.3%, 11.4%, and 9.6%, respectively, up to a height of 1.5D of inner CHS. Enhancement of the cyclic performance of PCCFST column with the height of inner CHS beyond 1.5D is not very significant. Based on the limited experimental results herein and code provisions (Eurocode 4), a design rule has been proposed to predict the ultimate lateral strength of the PCCFST columns.

Influence of Tie Configuration on Seismic Behaviour of Ultra High Performance Concrete Columns

Ultra-High-Performance Concrete (UHPC) is one of the latest advances in concrete technology as it overcomes the shortcomings of conventional concrete such as low strength to weight ratio, high permeability, low tensile strength, low ductility, and volume instability. Confinement is capable of increasing capacity of the concrete structure to sustain large deformation without a substantial strength loss. The present study discusses the influence of tie configuration, percentage of longitudinal reinforcement and tie spacing in improving the confinement and behaviour of UHPC columns. In order to determine the behaviour of UHPC column, half scaled specimens of original short column with four different tie configurations were used. The configurations considered for the study included a double legged ties (A), welded ties with bars running centrally across the length and width (B), ties with inclined bars connecting two adjacent mid-points (C) and ties with bars connecting two adjacent sides at one-third distances (D). For the same tie configuration, the transverse reinforcement ratios are varied by changing the diameter of the ties. The specimens were tested under quasi static reverse cyclic loading under a constant axial load. The test results were represented and compared in terms of load-deflection envelope, ductility and cracking patterns. Tie configuration C exhibited better performance in stiffness and ductility, when compared with other configurations. For same parameters, “B” and “D” configuration displayed better behaviour than “A” configuration. This variation in performance was due to increase in transverse reinforcement percentage and the consequent effectiveness of core confinement. Transverse reinforcement percentage of “B” and “C” configurations is less than “D” configuration. The enhancements of properties of “B” and “C” configurations are better than “D” configuration. It was due to the effective core confinement of “B” and “C” configuration. For the same tie configuration, ultimate load, initial stiffness and relative ductility increases with increase in transverse reinforcement percentage. UHPC columns with “C” type tie configuration can effectively use in moderate and high seismic areas and its performance can be improved by increasing the transverse reinforcement ratio.

Improved heat and particle flux mitigation in high core confinement, baffled, alternative divertor configurations in the TCV tokamak

Nitrogen seeded detachment has been achieved in the tokamak a configuration variable (TCV) in alternative divertor configurations (ADCs), namely X-divertor and X-point target, with and without baffles in H-mode plasmas with high core confinement. Both ADCs show a remarkable reduction in the inter-ELM particle and heat fluxes to the target compared to the standard divertor configuration. 95%–98% of the inter-ELM peak heat flux to the target is mitigated as a synergetic effect of ADCs, baffling, and nitrogen seeded detachment. The effect of divertor geometry and baffles on core-divertor compatibility is investigated in detail. The power balance in these experiments is also investigated to explore the physics behind the observed reduction in heat fluxes in the ADCs.

Divertor detachment of high β N H-mode discharges with the HL-2A closed divertor geometry

Divertor detachment of high H-mode discharges has been achieved by the injection of radiating impurities in the very closed divertor geometry of HL-2A. Non-intrinsic impurity is injected into the outer divertor chamber, and enhanced volumetric energy loss in the divertor and decreased divertor heat flux are then observed. Divertor detachment leads to a little degradation of the core confinement, characterized by slight decrease of plasma stored energy and pedestal pressure as well as increase of edge localized mode (ELM) frequency, but the high H-mode is still sustained with > 2 and H98 > 1. After impurity injection, the pedestal density fluctuation is increased, implying that the enhanced pedestal transport might be responsible for the degradation. During divertor detachment phase, the impurities are well controlled in the divertor without strong radiation losses near X-point region, and the main plasma density does not increase, but decreases slightly, which could be contributed by the very closed divertor geometry. These results are encouraging to achieve better heat load control and high plasma performance simultaneously for tokamak operations and to evaluate the effect of the divertor geometry for future tokamak devices.