L-mode negative triangularity (NT) operation is a promising alternative to the positive triangularity (PT) H-mode as a high-confinement edge localised mode-free operational regime. In this work, two TCV Ohmic L-mode core density ramps with opposite triangularity are investigated using SOLPS-ITER modelling. This numerical study aims to investigate the power exhaust differences between NT and PT focusing, in particular, on the geometrical effect of triangularity. To disentangle the latter from differences related to cross-field transport, anomalous diffusivities for particle () and energy () transport are fixed to the same values in PT and NT. The simulation results clearly show dissimilar transport and accumulation of neutral particles in the scrape-off layer for the two configurations. This gives rise to different ionization sources in the edge and divertor regions and produces differences in the poloidal and cross-field fluxes, ultimately leading to different power and particle divertor fluxes in the two configurations. Simulations recover the experimental feature of a hotter and attached outer target ( ) in the NT scenario compared to the PT counterpart.
Purpose-led Publishing is a coalition of three not-for-profit publishers in the field of physical sciences: AIP Publishing, the American Physical Society and IOP Publishing.
Together, as publishers that will always put purpose above profit, we have defined a set of industry standards that underpin high-quality, ethical scholarly communications.
We are proudly declaring that science is our only shareholder.
ISSN: 1361-6587
Plasma Physics and Controlled Fusion is a monthly publication dedicated to the dissemination of original results on all aspects of plasma physics and associated science and technology.
Open all abstracts, in this tab
E Tonello et al 2024 Plasma Phys. Control. Fusion 66 065006
T D Arber et al 2015 Plasma Phys. Control. Fusion 57 113001
Particle-in-cell (PIC) methods have a long history in the study of laser-plasma interactions. Early electromagnetic codes used the Yee staggered grid for field variables combined with a leapfrog EM-field update and the Boris algorithm for particle pushing. The general properties of such schemes are well documented. Modern PIC codes tend to add to these high-order shape functions for particles, Poisson preserving field updates, collisions, ionisation, a hybrid scheme for solid density and high-field QED effects. In addition to these physics packages, the increase in computing power now allows simulations with real mass ratios, full 3D dynamics and multi-speckle interaction. This paper presents a review of the core algorithms used in current laser-plasma specific PIC codes. Also reported are estimates of self-heating rates, convergence of collisional routines and test of ionisation models which are not readily available elsewhere. Having reviewed the status of PIC algorithms we present a summary of recent applications of such codes in laser-plasma physics, concentrating on SRS, short-pulse laser-solid interactions, fast-electron transport, and QED effects.
A Pavone et al 2023 Plasma Phys. Control. Fusion 65 053001
This article reviews applications of Bayesian inference and machine learning (ML) in nuclear fusion research. Current and next-generation nuclear fusion experiments require analysis and modelling efforts that integrate different models consistently and exploit information found across heterogeneous data sources in an efficient manner. Model-based Bayesian inference provides a framework well suited for the interpretation of observed data given physics and probabilistic assumptions, also for very complex systems, thanks to its rigorous and straightforward treatment of uncertainties and modelling hypothesis. On the other hand, ML, in particular neural networks and deep learning models, are based on black-box statistical models and allow the handling of large volumes of data and computation very efficiently. For this reason, approaches which make use of ML and Bayesian inference separately and also in conjunction are of particular interest for today's experiments and are the main topic of this review. This article also presents an approach where physics-based Bayesian inference and black-box ML play along, mitigating each other's drawbacks: the former is made more efficient, the latter more interpretable.
Clemente Angioni 2021 Plasma Phys. Control. Fusion 63 073001
In this paper, the theory of collisional and turbulent transport of impurities in tokamak plasmas is reviewed. The results are presented with the aim of providing at the same time a historical reconstruction of the scientific progress and a complete description of the present theoretical knowledge, with a hopefully sufficiently complete reference to the works which have been published in the field in the last decades. After a general introduction on the physics challenges offered by the problem of impurity transport and their relevance for practical nuclear fusion energy, the theory of collisional transport is presented. Here a specific section is also dedicated to the transport parallel to the magnetic field lines. A complete review of the transport mechanisms produced by turbulence follows. The corresponding comparisons between theoretical predictions and experimental observations are also presented, highlighting the influence that the validation activities had in motivating further theoretical investigations. The paper is completed by a section on the direct interactions between collisional and turbulent transport and by a final specific review dedicated to the progress in the theory–based modelling activities. In the writing of this review paper, the main goal has been to combine readability with completeness and scientific rigour, providing a comprehensive list of references for deeper documentation on specific aspects.
D A Kaltsas et al 2024 Plasma Phys. Control. Fusion 66 065016
We derive axisymmetric equilibrium equations in the context of the hybrid Vlasov model with kinetic ions and massless fluid electrons, assuming isothermal electrons and deformed Maxwellian distribution functions for the kinetic ions. The equilibrium system comprises a Grad–Shafranov partial differential equation and an integral equation. These equations can be utilized to calculate the equilibrium magnetic field and ion distribution function, respectively, for given particle density or given ion and electron toroidal current density profiles. The resulting solutions describe states characterized by toroidal plasma rotation and toroidal electric current density. Additionally, due to the presence of fluid electrons, these equilibria also exhibit a poloidal current density component. This is in contrast to the fully kinetic Vlasov model, where axisymmetric Jeans equilibria can only accommodate toroidal currents and flows, given the absence of a third integral of the microscopic motion.
S Guizzo et al 2024 Plasma Phys. Control. Fusion 66 065018
Negative triangularity (NT) tokamak configurations may be more susceptible to magneto-hydrodynamic instability, posing challenges for recent reactor designs centered around their favorable properties, such as improved confinement and operation free of edge-localized modes. In this work, we assess the vertical stability of plasmas with NT shaping and develop potential reactor solutions. When coupled with a conformal wall, NT equilibria are confirmed to be less vertically stable than equivalent positive triangularity (PT) configurations. Unlike PT, their vertical stability is degraded at higher poloidal beta. Furthermore, improvements in vertical stability at low aspect ratio do not translate to the NT geometry. NT equilibria are stabilized in PT vacuum vessels due to the increased proximity of the plasma and the wall on the outboard side, but this scenario is found to be undesirable due to reduced vertical gaps which give less spatial margin for control recovery. Instead, we demonstrate that informed positioning of passively conducting plates can lead to improved vertical stability in NT configurations on par with stability metrics expected in PT scenarios. An optimal setup for passive plates in highly elongated NT devices is presented, where plates on the outboard side of the device reduce vertical instability growth rates to 16% of their baseline value. For lower target elongations, integration of passive stabilizers with divertor concepts can lead to significant improvements in vertical stability. Plates on the inboard side of the device are also uniquely enabled in NT geometries, providing opportunity for spatial separation of vertical stability coils and passive stabilizers.
J Rueda-Rueda et al 2024 Plasma Phys. Control. Fusion 66 065025
In this paper we demonstrate how the inversion, in energy and major radius (E, R) coordinates, of imaging neutral particle analyser (INPA) measurements can be used to obtain the fast-ion distribution. The INPA is most sensitive to passing ions with energies in the range (20–150) keV and pitches near 0.5 in the core and 0.7 near the plasma edge. Inversion of synthetic signals, via 0th-order Tikhonov and Elastic Net regularization, were performed to demonstrate the capability of recovering the ground truth fast-ion 2D phase-space distribution resolved in major radius and energy, even in the presence of moderate noise levels (10%). Finally, we apply our method to measure the 2D phase-space distribution in an MHD quiescent plasma at ASDEX Upgrade and find good agreement with the slowing down fast-ion distribution predicted by TRANSP.
A P L Robinson 2024 Plasma Phys. Control. Fusion 66 065020
It is argued that fusion chain reactions in the D-D system is feasible with supra-thermal deuterons in the MeV regime, with new generations of deuterons being generated either via neutron–deuteron or proton–deuteron collisions. The propagation of supra-thermal deuterons in an infinite, hot, dense deuterium target was studied using a Monte Carlo method that includes multiple nuclear reactions, electron and ion stopping, along with neutron and proton knock-ons. Over a wide range of densities we observed significant, albeit sub-critical chain reactions in the multi-keV temperature regime. At very high densities (over 1000 gcm−3) and temperatures (over 40 keV) we observed chain reactions that reached criticality. These results suggest that there is a case to re-assess the potential of inertial confinement fusion based on deuterium-heavy targets.
O Février et al 2024 Plasma Phys. Control. Fusion 66 065005
In recent years, negative triangularity (NT) has emerged as a potential high-confinement L-mode reactor solution. In this work, detachment is investigated using core density ramps in lower single null Ohmic L-mode plasmas across a wide range of upper, lower, and average triangularity (the mean of upper and lower triangularity: δ) in the TCV tokamak. It is universally found that detachment is more difficult to access for NT shaping. The outer divertor leg of discharges with could not be cooled to below through core density ramps alone. The behavior of the upstream plasma and geometrical divertor effects (e.g. a reduced connection length with negative lower triangularity) do not fully explain the challenges in detaching NT plasmas. Langmuir probe measurements of the target heat flux widths (λq) were constant to within 30% across an upper triangularity scan, while the spreading factor S was lower by up to 50% for NT, indicating a generally lower integral scrape-off layer width, λint. The line-averaged core density was typically higher for NT discharges for a given fuelling rate, possibly linked to higher particle confinement in NT. Conversely, the divertor neutral pressure and integrated particle fluxes to the targets were typically lower for the same line-averaged density, indicating that NT configurations may be closer to the sheath-limited regime than their PT counterparts, which may explain why NT is more challenging to detach.
Y Q Tao et al 2024 Plasma Phys. Control. Fusion 66 075006
Low-frequency drift-wave instabilities play a crucial role in the radial transport of present-day tokamaks, and trapped electron collisions can significantly influence these instabilities. In this paper, the effects of trapped electron collisions on these instabilities are investigated based on linear gyro-kinetic simulations. The basic numerical techniques including dispersion relation integral method and orthogonal basis function expansion are presented in detail with necessary benchmark work. The results demonstrate that in medium gradients, the increase of trapped electron proportions promotes the growth rate and radial heat transport largely for quasi-linear trapped electron modes (TEMs) and ion temperature gradient (ITG) modes. Moreover, trapped electron collisions have strong stabilizing effects, especially for TEMs driven by electron temperature gradients. Two distinctive branches, namely Mode #1 and #2, are investigated in steep gradients. Both behave in a varied instability nature during different ranges of the normalized wave vector . Mode #1 mainly induces radial heat transport during and is significantly suppressed by the collisions. Mode #2 mainly induces radial heat transport during , and is largely enhanced by the collisions. When the collisionality is large enough, Mode #2 has stronger transport capacity than the other. Mode #2 at the medium wave vector, known as dissipative TEM, may provide the mechanism of the edge coherent mode observed in EAST H-mode plasmas, wherein collisionality plays an important role in the mode excitation.
Open all abstracts, in this tab
Marco Brambilla and Roberto Bilato 2024 Plasma Phys. Control. Fusion 66 075009
In this note we describe how Large Larmor Radius corrections can be incorporate in TORIC and other codes which solve the Finite Larmor Radius wave equations in toroidal axisymmetric geometry.
S M Kaye et al 2024 Plasma Phys. Control. Fusion 66 079501
T Barberis and F Porcelli 2024 Plasma Phys. Control. Fusion 66 075007
This study explores the influence of sawtooth oscillations on the velocity space distribution of fast ions in tokamak plasma discharges. The relevant Fokker–Planck equation for fast ions is solved analytically. Two distinct effects arising from the temperature drop associated with a sawtooth crash and their impact on the distribution function of fast ions are considered. The first effect involves the modulation of the fusion alpha particle source on the timescale of the sawtooth period, linked to the drop in fusion yield resulting from the sawtooth temperature relaxations. The second effect is tied to the increase of the slowing-down time during the sawtooth ramp, causing particles born later in the sawtooth cycle to experience reduced slowing down compared to those born right after the crash, creating an accumulation-like mechanism at higher energies. In regimes where the sawtooth period is shorter than the fast ion slowing-down time, the combined influence of these effects gives rise to fast ion distribution functions that transiently exhibit positive slopes in velocity space.
A Miyashita et al 2024 Plasma Phys. Control. Fusion 66 075008
Non-uniform fluctuation characteristics are observed within an edge magnetic island in Heliotron J. The island possesses a long connection length comparable to the confined region. These fluctuations are measured using a Langmuir probe. The island's presence is confirmed through the plasma response, observed in the modulation amplitude of electron temperature and its phase delay relative to the heat source in a heat modulation experiment. Within the island, the electron density is notably high, accompanied by distinct profiles of electron temperature and electric field, likely attributable to the magnetic island. Contrary to expectations, density fluctuations within the edge magnetic island are not locally minimized, despite the reduced gradient of the profile within the island. Statistical analysis shows a suppression of intermittent transport inside the island, while intermittent fluctuations increase towards the exterior. A further analysis to segregate turbulence-driving and spreading factors reveals that both turbulence-driven and spreading contributions are comparably significant inside the island. Additionally, the non-uniform turbulence results in a spatially structured fluctuation-driven particle flux. Overall, the experimental findings indicate that fluctuation characteristics exhibit notable non-uniformity both inside and near the island. This non-uniformity potentially complicates heat transport and may lead to three-dimensional, asymmetric transport within and at the periphery of the islands.
Y Q Tao et al 2024 Plasma Phys. Control. Fusion 66 075006
Low-frequency drift-wave instabilities play a crucial role in the radial transport of present-day tokamaks, and trapped electron collisions can significantly influence these instabilities. In this paper, the effects of trapped electron collisions on these instabilities are investigated based on linear gyro-kinetic simulations. The basic numerical techniques including dispersion relation integral method and orthogonal basis function expansion are presented in detail with necessary benchmark work. The results demonstrate that in medium gradients, the increase of trapped electron proportions promotes the growth rate and radial heat transport largely for quasi-linear trapped electron modes (TEMs) and ion temperature gradient (ITG) modes. Moreover, trapped electron collisions have strong stabilizing effects, especially for TEMs driven by electron temperature gradients. Two distinctive branches, namely Mode #1 and #2, are investigated in steep gradients. Both behave in a varied instability nature during different ranges of the normalized wave vector . Mode #1 mainly induces radial heat transport during and is significantly suppressed by the collisions. Mode #2 mainly induces radial heat transport during , and is largely enhanced by the collisions. When the collisionality is large enough, Mode #2 has stronger transport capacity than the other. Mode #2 at the medium wave vector, known as dissipative TEM, may provide the mechanism of the edge coherent mode observed in EAST H-mode plasmas, wherein collisionality plays an important role in the mode excitation.
Open all abstracts, in this tab
R J Groebner and S Saarelma 2023 Plasma Phys. Control. Fusion 65 073001
This paper reviews current understanding of key physics elements that control the H-mode pedestal structure, which exists at the boundary of magnetically confined plasmas. The structure of interest is the width, height and gradient of temperature, density and pressure profiles in the pedestal. Emphasis is placed on understanding obtained from combined experimental, theoretical and simulation work and on results observed on multiple machines. Pedestal profiles are determined by the self-consistent interaction of sources, transport and magnetohydrodynamic limits. The heat source is primarily from heat deposited in the core and flowing to the pedestal. This source is computed from modeling of experimental data and is generally well understood. Neutrals at the periphery of the plasma provide the dominant particle source in current machines. This source has a complex spatial structure, is very difficult to measure and is poorly understood. For typical H-mode operation, the achievable pedestal pressure is limited by repetitive, transient magnetohydrodynamic instabilities. First principles models of peeling–ballooning modes are generally able to explain the observed limits. In some regimes, instability occurs below the predicted limits and these remain unexplained. Several mechanisms have been identified as plausible sources of heat transport. These include neoclassical processes for ion heat transport and several turbulent processes, driven by the steep pedestal gradients, as sources of electron and ion heat transport. Reduced models have successfully predicted the pedestal or density at the pedestal top. Firming up understanding of heat and particle transport remains a primary challenge for developing more complete predictive pedestal models.
A Pavone et al 2023 Plasma Phys. Control. Fusion 65 053001
This article reviews applications of Bayesian inference and machine learning (ML) in nuclear fusion research. Current and next-generation nuclear fusion experiments require analysis and modelling efforts that integrate different models consistently and exploit information found across heterogeneous data sources in an efficient manner. Model-based Bayesian inference provides a framework well suited for the interpretation of observed data given physics and probabilistic assumptions, also for very complex systems, thanks to its rigorous and straightforward treatment of uncertainties and modelling hypothesis. On the other hand, ML, in particular neural networks and deep learning models, are based on black-box statistical models and allow the handling of large volumes of data and computation very efficiently. For this reason, approaches which make use of ML and Bayesian inference separately and also in conjunction are of particular interest for today's experiments and are the main topic of this review. This article also presents an approach where physics-based Bayesian inference and black-box ML play along, mitigating each other's drawbacks: the former is made more efficient, the latter more interpretable.
Annick Pouquet 2023 Plasma Phys. Control. Fusion 65 033002
Nonlinear phenomena and turbulence are central to our understanding and modeling of the dynamics of fluids and plasmas, and yet they still resist analytical resolution in many instances. However, progress has been made recently, displaying a richness of phenomena, which was somewhat unexpected a few years back, such as double constant-flux cascades of the same invariant for both large and small scales, or the presence of non-Gaussian wings in large-scale fields, for fluids and plasmas. Here, I will concentrate on the direct measurement of the magnitude of dissipation and the evaluation of intermittency in a turbulent plasma using exact laws stemming from invariance principles and involving cross-correlation tensors with both the velocity and the magnetic fields. I will illustrate these points through scaling laws, together with data analysis from existing experiments, observations and numerical simulations. Finally, I will also briefly explore the possible implications for the validity and use of several modeling strategies.
J Citrin and P Mantica 2023 Plasma Phys. Control. Fusion 65 033001
In recent years tokamak experiments and modelling have increasingly indicated that the interaction between suprathermal (fast) ions and thermal plasma can lead to a reduction of turbulence and an improvement of confinement. The regimes in which this stabilization occurs are relevant to burning plasmas, and their understanding will inform reactor scenario optimization. This review summarizes observations, simulations, theoretical understanding, and open questions on this emerging topic.
S M Kaye et al 2021 Plasma Phys. Control. Fusion 63 123001
In this paper, we review the thermal plasma confinement and transport properties observed and predicted in low aspect ratio tokamaks, or spherical tokamaks (STs), which can depart significantly from those observed at higher aspect ratio. In particular, thermal energy confinement scalings show a strong, near linear dependence of energy confinement time on toroidal magnetic field, while the dependence on plasma current is more modest, the opposite of what is seen at higher aspect ratio. STs have revealed a very strong improvement in normalized confinement with decreasing collisionality, much stronger than at higher aspect ratio, which bodes well for an ST-based fusion pilot plant should this trend continue at an even lower collisionality than has already been accessed. These differences arise because of fundamental differences in transport in STs due to the more extreme toroidicity (i.e. reduced region of bad curvature), and to the relatively larger shearing rates, both of which can suppress electrostatic drift wave instabilities at both ion and electron gyroradius scales. In addition, electromagnetic effects are much stronger in STs because they operate at high βT. Gyrokinetic (GK) studies, coupled with low- and high-k turbulence measurements, have shed light on the underlying physics controlling transport. At lower βT, both ion- and electron-scale electrostatic drift turbulence may be responsible for transport. At higher βT, microtearing, kinetic ballooning, and hybrid trapped electron/kinetic ballooning modes increasingly play a role, and they have a much stronger impact in the core of ST plasmas than at higher aspect ratio. Flow shear affects the balance between ion- and electron-scale modes. Non-linear GK simulations find regimes where the electron heat flux decreases with decreasing collisionality, consistent with the experimental global normalized confinement scaling. The ST is unique in that the relatively low toroidal magnetic field allows for localized measurements of electron-scale turbulence, and this coupled with turbulence measurements at ion-scales has facilitated detailed comparisons with GK simulations. These data have provided compelling evidence for the presence of ion temperature gradient and electron temperature gradient turbulence in some plasmas, and direct experimental support for the impact of experimental actuators like rotation shear, density gradient and magnetic shear on turbulence and transport.
Open all abstracts, in this tab
Sabui et al
In this paper we make an attempt to study the effect of a nanosecond prepulse on electron acceleration from microscopic solid particles. While standard scaling laws predict temperatures of $\sim 50keV$, our investigations showed an anomalous enhancement ($\sim 200keV$) in electron temperatures from microscopic solids subjected to intense laser fields. Conventional isotropic expansion of the plasma target as a result of pre-pulse interaction is unable to explain the enhancement, prompting an experimental inquiry into the expansion form. Tentacle-like structures emerging from the spherical plasma are detected as an outcome of the pre-pulse interaction, with the region between the tentacles being targeted by the main laser pulse. We resort to PIC simulations to understand and predict the exact plasma condition that exists in this region. In this paper, we study a series of approximate pre-plasma conditions, particle shapes and electron densities, examining them for their role in electron acceleration from this extended over-critical plasma target
Zeng et al
MHD stabilization of flute and ballooning modes in an axisymmetric mirror trap is studied under the assumption of a strong Finite Larmor Radius effect that suppresses all perturbations with azimuthal numbers m ≥ 2 and makes the m = 1 mode 'rigid'. The rigid mode can be effectively suppressed using a perfectly conducting lateral wall without any additional means of stabilization or combined with end MHD anchors. Numerical calculations were carried out for an anisotropic plasma produced in the course of neutral beam injection into the minimum of the magnetic field at the right angle to the trap axis. The stabilizing effect of the conducting shell made of a straightened cylinder is compared with a proportional chamber, which, on an enlarged scale, repeats the shape of the plasma column.
 It is confirmed that for convincing wall stabilization of the rigid modes, the plasma beta (β, the ratio of the plasma pressure to the magnetic field pressure) must exceed some critical value βcr2. When the conducting lateral wall is combined with conducting end plates imitating MHD end anchors, there are two critical betas and, respectively, two stability zones β < βcr1 and β > βcr2 that can merge, making the entire range 0< β <1 of betas allowable for stable plasma confinement.
 The dependence of the critical betas on the plasma anisotropy, mirror ratio, width of the vacuum gap between the plasma column and the lateral wall, radial pressure profile, and axial magnetic field profile is examined.
Lazerson et al
The effect of magnetic islands in the core region of Wendelstein 7-X (W7-X) on fast ion confinement is explored through simulations with the BEAMS3D code. A magnetic configuration where the n/m = 5/5 island chain is shifted to r/a ∼ 0.7 allows the exploration of core island physics in W7-X. The control coil system on W7-X allows the tuning of the island size either increasing the island width or decreasing it. A coupling of the BEAMS3D code to the FIELDLINES code provides a versatile mechanism for incorporating magnetic islands and stochastic regions into the BEAMS3D code. Collisionless simulations suggest that the presence of core islands degrade the confinement of passing particles in the region of the island chain. Full neutral beam simulations of W7-X show a similar behavior with confinement decreasing as the island width is increased. Comparisons between a vacuum magnetic field and low beta HINT2 simulation are made showing similar fast ion behavior. Measurements of lost fast ions in W7-X confirm this trend with the control coil suppressed island configuration showing lower losses than that with no control coils applied. Simulations of fast ion wall loads are performed suggesting no drastic change in loss pattern and a slight reduction in losses with minimized islands.
Shi et al
The tearing mode instabilities were numerically studied in two distinct models: the finite electron inertial magnetohydrodynamics (MHD) and the electron MHD (EMHD). The finite electron inertial MHD model employed a modified Hall-MHD model that incorporated the electron inertial effects in the generalized Ohm's Law. On the other hand, the electron dynamics were described by the EMHD model. It is found that both electron inertial effects and electron dynamics significantly influence the linear and nonlinear growth of tearing mode instabilities, with electron dynamics playing a more dominant role. The dependence of the linear growth rate of tearing modes on the electron inertial length d_e was investigated. The results show that electron inertial effects enhance the growth rate but resemble the behavior of resistivity η. Whereas, in the EMHD model, electron inertia plays a dominant role in tearing mode instabilities. Additionally, a study on the nonlinear saturation of (2,1) tearing modes was conducted, demonstrating consistency with relevant analytical theories. The study indicates that, in both models, the magnetic island exhibits faster growth and achieves a larger saturated island width as d_e increases.
Qian et al
We performed 3D Particle-In-Cell simulations to study electron acceleration in the electron dissipation region of asymmetrical electron magnetic reconnection driven by ultra-intensity lasers, which is similar to the Earth's magnetosphere reconnection process. Within the electron dissipation region, electrons exhibit a nonthermal distribution, and as the asymmetry increases, the power-law spectrum becomes steeper. Remarkably, the electron spectrum closely resembles a delta distribution, arising from the intense acceleration imparted by the reconnection electric field near the X-line. Both parallel electric field acceleration and the Betatron acceleration mechanism play pivotal roles in this reconnection process. Furthermore, as the magnetic reconnection asymmetry intensifies, the parallel electric acceleration mechanism becomes stronger near the X-point region, whereas the Betatron acceleration mechanism wanes, primarily concentrated in the outflow region.
Open all abstracts, in this tab
Marco Brambilla and Roberto Bilato 2024 Plasma Phys. Control. Fusion 66 075009
In this note we describe how Large Larmor Radius corrections can be incorporate in TORIC and other codes which solve the Finite Larmor Radius wave equations in toroidal axisymmetric geometry.
Samuel A Lazerson et al 2024 Plasma Phys. Control. Fusion
The effect of magnetic islands in the core region of Wendelstein 7-X (W7-X) on fast ion confinement is explored through simulations with the BEAMS3D code. A magnetic configuration where the n/m = 5/5 island chain is shifted to r/a ∼ 0.7 allows the exploration of core island physics in W7-X. The control coil system on W7-X allows the tuning of the island size either increasing the island width or decreasing it. A coupling of the BEAMS3D code to the FIELDLINES code provides a versatile mechanism for incorporating magnetic islands and stochastic regions into the BEAMS3D code. Collisionless simulations suggest that the presence of core islands degrade the confinement of passing particles in the region of the island chain. Full neutral beam simulations of W7-X show a similar behavior with confinement decreasing as the island width is increased. Comparisons between a vacuum magnetic field and low beta HINT2 simulation are made showing similar fast ion behavior. Measurements of lost fast ions in W7-X confirm this trend with the control coil suppressed island configuration showing lower losses than that with no control coils applied. Simulations of fast ion wall loads are performed suggesting no drastic change in loss pattern and a slight reduction in losses with minimized islands.
T Barberis and F Porcelli 2024 Plasma Phys. Control. Fusion 66 075007
This study explores the influence of sawtooth oscillations on the velocity space distribution of fast ions in tokamak plasma discharges. The relevant Fokker–Planck equation for fast ions is solved analytically. Two distinct effects arising from the temperature drop associated with a sawtooth crash and their impact on the distribution function of fast ions are considered. The first effect involves the modulation of the fusion alpha particle source on the timescale of the sawtooth period, linked to the drop in fusion yield resulting from the sawtooth temperature relaxations. The second effect is tied to the increase of the slowing-down time during the sawtooth ramp, causing particles born later in the sawtooth cycle to experience reduced slowing down compared to those born right after the crash, creating an accumulation-like mechanism at higher energies. In regimes where the sawtooth period is shorter than the fast ion slowing-down time, the combined influence of these effects gives rise to fast ion distribution functions that transiently exhibit positive slopes in velocity space.
A Miyashita et al 2024 Plasma Phys. Control. Fusion 66 075008
Non-uniform fluctuation characteristics are observed within an edge magnetic island in Heliotron J. The island possesses a long connection length comparable to the confined region. These fluctuations are measured using a Langmuir probe. The island's presence is confirmed through the plasma response, observed in the modulation amplitude of electron temperature and its phase delay relative to the heat source in a heat modulation experiment. Within the island, the electron density is notably high, accompanied by distinct profiles of electron temperature and electric field, likely attributable to the magnetic island. Contrary to expectations, density fluctuations within the edge magnetic island are not locally minimized, despite the reduced gradient of the profile within the island. Statistical analysis shows a suppression of intermittent transport inside the island, while intermittent fluctuations increase towards the exterior. A further analysis to segregate turbulence-driving and spreading factors reveals that both turbulence-driven and spreading contributions are comparably significant inside the island. Additionally, the non-uniform turbulence results in a spatially structured fluctuation-driven particle flux. Overall, the experimental findings indicate that fluctuation characteristics exhibit notable non-uniformity both inside and near the island. This non-uniformity potentially complicates heat transport and may lead to three-dimensional, asymmetric transport within and at the periphery of the islands.
Jane Johnston and David Perry 2024 Plasma Phys. Control. Fusion
An overview of the preparation and implementation of the JET DTE2 Safety Case is presented. The Safety Case regime, developed by UKAEA for fusion applications to demonstrate compliance with relevant regulations, and its implementation on UKAEA sites is outlined. The hazard assessment process and details of the methodology applied to assess key JET fault scenarios are provided. An outline of key inventories, their composition, and applied factors for consequence and risk assessment are discussed. The consequences resulting from key fault sequences, the designation of appropriate safety measures and the associated risks are summarised and compared against Basic Safety Limits and Objectives. Finally, some of the lessons learned from DTE2 following implementation are discussed.
Lorenzo Aucone et al 2024 Plasma Phys. Control. Fusion
The paper presents experimental and modelling results of a comparison of negative (NT) and positive
(PT) triangularity ASDEX Upgrade (AUG) discharges using the plasma shapes presently foreseen in the
DTT tokamak, under construction in Italy. This work is part of a broader effort of investigation to understand
whether the good properties observed in NT scenarios in DIII-D and TCV may be extrapolated to
the DTT device and more in general to DEMO future operations. The experimental results have shown a
practical gain of running these AUG plasmas with only ECRH and mixed NBI+ECRH phases in negative
triangularity, even if they access the H-mode. Indeed, the NT electron kinetic profiles recover in all cases
the PT electron pressures inside mid-radius due to reduced transport in the region ρtor = 0.7 − 0.9, while
exhibit lower individual ELM (Edge Localised Mode) energy losses. The ion pressure and expected fusion
performance are comparable in the case of similar densities. Integrated modelling has been performed using
the transport solver ASTRA and the quasi-linear turbulent model TGLF, investigating the transport properties
of these discharges. The modelling reproduces the experiments qualitatively with reasonable accuracy.
Nonetheless, the heat transport in NT cases is partially overestimated. This may be because TGLF uses
the Miller equilibrium, which approximates the magnetic flux surfaces as up-down symmetric. In the case
of these asymmetric NT shapes, the simulated outer surfaces lose part of the tilt with respect to the z-axis,
reducing the upper δ < 0 effect. A numerical test to discern the impact of the geometry by symmetrically
flipping the shape has shown a beneficial effect of the negative triangularity on heat transport.
Alessandro Balestri et al 2024 Plasma Phys. Control. Fusion
In this work, we study the impact of aspect ratio A = R0/r (the ratio of major radius R0 to minor radius r) on the confinement benefits of Negative Triangularity (NT) plasma shaping. We use high-fidelity flux tube gyrokinetic GENE simulations and consider several different scenarios: four of them inspired by TCV experimental data, a scenario inspired by DIII-D experimental data and a scenario expected in the new SMART spherical tokamak. The present study reveals a surprising and non-trivial dependence. NT improves confinement at any value of A for ITG turbulence, while for TEM turbulence confinement is improved only in the case of large and conventional aspect ratios. Additionally, through a detailed study of a large aspect ratio case with pure ITG drive, we develop an intuitive physical picture that explains the beneficial effect of NT at large and conventional aspect ratios. This picture does not hold in TEM-dominated regimes, where a complex synergistic effect of many factors is found. Finally, we performed the first linear gyrokinetic simulations of SMART, finding that both NT and PT scenarios are dominated by micro-tearing-mode (MTM) turbulence and that NT is more susceptible to MTMs at tight aspect ratio. However, we found that a regime where ITGs dominate in SMART can be found, and in this regime NT is more linearly stable.
Y Q Tao et al 2024 Plasma Phys. Control. Fusion 66 075006
Low-frequency drift-wave instabilities play a crucial role in the radial transport of present-day tokamaks, and trapped electron collisions can significantly influence these instabilities. In this paper, the effects of trapped electron collisions on these instabilities are investigated based on linear gyro-kinetic simulations. The basic numerical techniques including dispersion relation integral method and orthogonal basis function expansion are presented in detail with necessary benchmark work. The results demonstrate that in medium gradients, the increase of trapped electron proportions promotes the growth rate and radial heat transport largely for quasi-linear trapped electron modes (TEMs) and ion temperature gradient (ITG) modes. Moreover, trapped electron collisions have strong stabilizing effects, especially for TEMs driven by electron temperature gradients. Two distinctive branches, namely Mode #1 and #2, are investigated in steep gradients. Both behave in a varied instability nature during different ranges of the normalized wave vector . Mode #1 mainly induces radial heat transport during and is significantly suppressed by the collisions. Mode #2 mainly induces radial heat transport during , and is largely enhanced by the collisions. When the collisionality is large enough, Mode #2 has stronger transport capacity than the other. Mode #2 at the medium wave vector, known as dissipative TEM, may provide the mechanism of the edge coherent mode observed in EAST H-mode plasmas, wherein collisionality plays an important role in the mode excitation.
N T Mitchell et al 2024 Plasma Phys. Control. Fusion 66 075005
A reduced kinetic method (RKM) with a first-principles collision operator is introduced in a 1D2V planar geometry and implemented in a computationally inexpensive code to investigate non-local ion heat transport in multi-species plasmas. The RKM successfully reproduces local results for multi-species ion systems and the important features expected to arise due to non-local effects on the heat flux are captured. In addition to this, novel features associated with multi-species, as opposed to single species, cases are found. Effects of non-locality on the heat flux are investigated in mass and charge symmetric and asymmetric ion mixtures with temperature, pressure, and concentration gradients. In particular, the enthalpy flux associated with diffusion is found to be insensitive to sharp pressure and concentration gradients, increasing its significance in comparison to the conductive heat flux driven by temperature gradients in non-local scenarios. The RKM code can be used for investigating other kinetic and non-local effects in a broader plasma physics context. Due to its relatively low computational cost it can also serve as a practical non-local ion heat flux closure in hydrodynamic simulations or as a training tool for machine learning surrogates.
L Scotti et al 2024 Plasma Phys. Control. Fusion 66 075004
The detachment cliff is a bifurcative transition to partial detachment recently discovered at the DIII-D tokamak (McLean et al 2015 J. Nucl. Mater.463 533–6). This work presents a database analysis of target parameters in L-mode and H-mode discharges to search for a detachment cliff at ASDEX Upgrade (AUG). Most of the transitions from attached to partially detached divertor conditions observed in H- and L-mode discharges in AUG show bifurcative-like characteristics that are consistent with the properties of the detachment cliff if the drift is directed towards the active X-point. In the operational space of power and density, the bifurcative transitions identified during an L-mode discharge occur at injected power and density higher than a threshold value ( > 0.7 MW and ne > 1.6 × 10m−3, respectively). Furthermore, the temperatures at which the transitions start are found to be insensitive to the injected impurity, the injected power and the value of the upstream density. Finally, the study of the evolution of the target parameters, of the intensity of the line and of specific manometers and bolometer lines of sights shows that the physical process underlying the detachment cliff and the self-sustained divertor oscillations (Heinrich 2020 Nucl. Fusion60 076013) might be the same.