To reach ITER’s mission goals, a thorough understanding of the underlying physical processes, plant systems and operational scenarios is mandatory. Preparation for ITER operations is supported by state-of-the-art modelling implemented in the ITER Integrated Modelling & Analysis Suite (IMAS). This modular framework has been developed to standardize the communication between the various codes...
Good confinement of the bulk plasma and fusion-generated alpha particles are two basic design properties of a fusion reactor. First, small radial energy fluxes are necessary for the plasma to achieve fusion-relevant conditions. In turn, fusion-born alpha particles are expected to contribute to heat the plasma, which requires their confinement time to be sufficiently long. In stellarators,...
Three dimensional magnetic equilibria are in general composed of nested flux surfaces, magnetic islands and chaotic field lines, although it is possible to design stellarator coil configurations that produce vacuum fields with nested flux surfaces (Pedersen, S. T. et al. 2016, Nature comm.). At finite $\beta$ however, currents self-generated by the plasma, such as diamagnetic, Pfirsch-Schlüter...
Local gyrokinetic simulations use a field-aligned domain that twists due to the magnetic shear of the background magnetic equilibrium. However, if the magnetic shear is strong and/or the domain is long, the twist can become so extreme that it fails to properly resolve the turbulence. In this work, we derive and implement the non-twisting flux tube, a local simulation domain that remains...
Given the large anisotropy of transport processes in magnetized plasmas, the magnetic field structure can strongly impact the heat diffusion: magnetic surfaces and cantori form barriers to transport while chaotic layers and island structures can degrade confinement. When a small but finite amount of perpendicular diffusion is included, the structure of the magnetic field becomes less...
Direct numerical solution of the nonlinear Boltzmann transport equation remains elusive nearly 150 years after its discovery. Appropriate approximations continue to serve as the foundation of aerodynamics and plasma physics. However, some important problems in fusion don’t lend themselves to such approximations, such as runaway electron avalanche and regimes of dense neutral populations in...
Magnetic fusion is now approaching next-step D-T burning plasma experiments in ITER, which will be mostly self-heated by fusion-born alpha-particles. Burning plasmas will be a highly nonlinear medium, and predicting with confidence the alpha-particle heating and alpha-particle losses in such plasmas is a very challenging task. To advance it, dedicated experimental studies are being performed...
Exploiting mechanisms to actively control and reduce turbulence is essential for maximizing the performance of fusion devices. Already in 2010, it became clear that low levels of turbulent transport observed in several JET experiments could not be explained by the usual turbulence reduction mechanisms such as strong plasma rotation and magnetic shear. This talk summarizes main analysis and...
The hybrid tokamak scenario is characterized by low magnetic shear in the plasma core and a central value of the safety factor close to unity. It represents a hybrid between standard scenarios and advanced scenarios and is a candidate scenario for ITER and DEMO. The hybrid scenario allows for high-performance, sawtooth-free operation with extended discharge lengths and has the advantage that...
The trajectories of collisionless charged particles in static magnetic fields are Hamiltonian flows, hence may be understood using tools of Hamiltonian dynamics. In the presence of three independent constants of the motion, it is known that a trajectory is regular. A low-energy particle in an axis-symmetric device possesses two exact constants of motion: the total energy and the momentum along...
The Guiding Center (GC) theory has been widely used for more than four decades as the basis for the study of single and collective particle dynamics in toroidal magnetic fields utilized in fusion devices. The Hamiltonian formulation of the theory has been originally given in terms of non-canonical variables and then extended to canonical ones. The canonical Hamiltonian description, apart from...
In tokamak the parallel divergence of the parallel current is non-zero due to the time variation of the vorticity. This has an interesting connection with the axial anomaly usually invoked in baryogenesis. In two-dimensions (good approximation for the plasma in strong magnetic field) the field of vorticity can be seen as a discrete set of positive and negative elementary vortices, of fixed...
Relativistic effects in astrophysical objects and fusion plasmas do not necessarily require extremely high temperatures and energies. They appear to be non-negligible even for electron temperatures $T_e$ of the order of tens of keV, i.e. when $T_e\ll m_ec^2$. Relativistic effects in transport physics appear due to macroscopic features of the relativistic thermodynamic equilibrium given by the...
While the curse of high dimensionality associated with the Fokker-Planck equation can be resolved by Monte Carlo solution to the underlying stochastic process, the distribution of discretization points for the resulting random walk-based method follows the evolution of the target distribution function, leading to the weight-spreading phenomenon in the $\delta f$ method, and therefore increase...
The pedestal, and transport barriers in general, play an important role in tokamak performance and thus it is desirable to find a comprehensive model for these regions. In transport barriers, the applicability of standard neoclassical theory is limited because of sharp gradients of temperature, density, and radial electric field. We have developed a new neoclassical approach that sets the...
We provide a gyrofluid model of a collisionless and magnetized plasma, valid for finite βe, finite parallel magnetic perturbations and electron finite Larmor radius effects. This model is used to study the linear and non-linear evolution of magnetic reconection and magnetic islands. Gyrofluid models provide an effective tool, complementary to kinetic models, for studying such effects.
To understand plasma behaviour in the scrape-off layer (SOL), we need to know the boundary conditions for the plasma and electromagnetic fields near a divertor. At the boundary, in the direction perpendicular to the wall, there are four length scales of interest. These are the Debye length $\lambda_D$, the ion gyroradius $\rho_i$, the projection of the collisional mean free path in the...
The transport of particles and energy from the core plasma in a divertor tokamak to the reactor walls, via the scrape-off layer (SOL), occurs largely parallel to the magnetic field lines. Experimental evidence and theoretical considerations suggest that a fluid approach to modelling this transport may miss some important behaviour. In particular, temperatures at the target may be modified by...
Particle-in-cell (PIC) codes are one of the workhorses for numerically exploring plasma dynamics across a vast parameter space. While explicit discretizations of PIC systems allow for a straightforward time integration, stability requirements set strict limitations on both the maximal time-step and maximal grid spacing. Implicit PIC methods on the other hand put laxer restrictions on the...
Numerical codes for electromagnetic wave propagation in fusion plasmas are mainly based on frequency-domain asymptotic methods, which provide a fast solution and are thus valuable for experiment design and control applications. However, in several cases of practical interest (like O-X-B mode conversion, mm-diagnostics) these tools run close to their limits of validity and should be compared to...
Wave-particle interactions are ubiquitous in space and laboratory plasma systems and have been the subject of intense research interest for many decades. From a theoretical point of view, the nonlinear motion of a charged particle with an electrostatic wave has been one of the basic paradigms of complex and chaotic Hamiltonian dynamics. However, although single particle dynamics have been...
The assumption of a small relative deviation from background f_0 under the delta-f scheme often used to simulate the plasma core will not be valid when simulating the plasma edge, characterized by low density and temperature and strong gradients. In order to retain the noise reduction benefit of the delta-f scheme as compared to the full-f approach, a study of a transition scheme by means of a...
The development of the EU-DEMO reactor is at the pre-conceptual design phase. At this stage, close attention is paid to the heating mix necessary to fulfill all the plasma requirements: breakdown, ramp-up, L-H transition, burn control, NTM stabilization, sawteeth pacing, radiative instability control and ramp-down. Integrated modeling is an effective tool to compare the impact of dominant...
The MHD dynamo effect is an intrinsic and fundamental feature of reversed-field pinch (RFP) plasmas. It plays an important role in the tokamak as well (commonly as referred to as the "flux pumping" mechanism) in particular for the hybrid scenario with central safety factor close to one. In this contribution, we review results based on the above-mentioned nonlinear 3D MHD theory and...
JT-60SA is a fully superconducting new tokamak device, designed, built and exploited jointly by Europe and Japan. It is the largest tokamak ever built before ITER and it is now in its commissioning phase. JT-60SA will exploit and extend the legacy both of JET and of the superconducting tokamaks presently in operation (WEST, EAST, KSTAR). It is expected to be at the forefront of the...
This presentation will focus on recent progress towards the validation of 3D non-linear MHD disruption simulations with the JOREK code. Simulations of a disruption triggered by an argon massive gas injection in JET pulse 85943 have been compared in detail to experimental data. Synthetic diagnostics have been used for the purpose, including interferometry, bolometry and saddle loops. A good...
Shattered pellet injection (SPI) is the current concept for the ITER disruption mitigation system (DMS) to prevent disruption-related damage from thermal loads, electromagnetic forces or runaway electron (RE) beams. Compared with impurity or mixed deuterium-impurity pellets that contain large quantity of impurities like neon, pure deuterium (D2) SPI is expected to strongly dilute the plasma...
The resistive tearing mode instability is a well-known phenomenon that is often linked to magnetic reconnection. When magnetic field lines reconnect, this leads to a conversion of magnetic energy into kinetic or thermal energy. In turn, this can result in interesting events such as solar flares in the solar corona or the disruption of plasma confinement in tokamaks. Therefore, understanding...
The ideal-MHD theory of axisymmetric modes with toroidal mode number n = 0 in tokamak plasmas is developed. These modes are resonant at the magnetic X-points of the tokamak divertor separatrix. Consequently, current sheets form along the separatrix, which profoundly affect the stability of vertical plasma displacements. In particular, current sheets at the magnetic separatrix lead to...
There is considerable interest in studying plasma physics problems that will be solvable to error-correcting quantum computers. An interesting class of such problems is the propagation of electromagnetic waves based on Maxwell equations, with the plasma physics determining the dielectric properties of the medium. In order to develop the fundamental concepts for casting a classical wave...
The solar wind carries a broadband of fluctuations in density, velocity and magnetic fields that, at the large scales, have been interpreted in terms of an ongoing magnetohydrodynamic turbulent cascade. Alfvénic fluctuations have indeed been commonly observed in the solar wind since the first in-situ measurements, and they are thought to provide a possible mechanism to heat the solar corona at...
The performance of magnetic confinement devices is determined mainly by turbulent transport inducing particle and energy losses, and limiting plasma confinement. Among the different experimental actuators of turbulence, supra-thermal particles – generated via external heating – are typically considered one of the most efficient in suppressing ion-temperature-gradient (ITG) driven turbulence....
Solar flares are the most powerful explosions in the solar system, and are widely accepted to result from release of stored magnetic energy through magnetic reconnection. Many aspects of flare physics remain poorly understood: in particular, the large numbers of high-energy electrons and ions, forming a non-thermal element of the energy distribution, which can be the predominant energy...
Metallic PFC melting events in contemporary machines as well as future reactors fall under a rather unique regime; (i). Due to the limited wetted area, the liquid pools are surrounded by progressively colder solid surfaces so that once the melt is accelerated out of the pool, under the action of plasma-induced forces, it promptly solidifies. This necessitates modelling of the full coupling...
The suppression of turbulence in fusion plasmas, crucial to the success of next-generation tokamaks such as ITER, depends on a variety of physical mechanisms including the shearing of turbulent eddies via zonal flow and possibly the generation of intrinsic rotation. The turbulence exhibits interesting features such as avalanche structures and self-organisation, and its absence is associated...
A simplified local model of a tokamak plasma is derived in the low-beta limit of gyrokinetics in a slab of constant magnetic field curvature and gradient. The ordering adopted was chosen in order to retain Alfvénic perturbations to the magnetic field, while ordering out compressive perturbations, in a similar manner to previous work. In the electromagnetic regime, we demonstrate the existence...
This communication introduces the concept of plasma-wall self-organization (PWSO) in magnetic fusion. The basic idea is the existence of a time delay in the feedback loop relating radiation and impurity production on divertor plates. Both a zero and a one-dimensional description of PWSO are provided. They lead to an iterative equation whose equilibrium fixed point is unstable above some...
We present the results of 3D, flux-driven, global, two-fluid electrostatic turbulence simulations in a 5-field period stellarator with an island divertor. The numerical simulations are carried out with the GBS code, which solves the two-fluid drift-reduced Braginskii equations and has been extended recently to simulate plasma turbulence in non-axisymmetric magnetic equilibria. The vacuum...
Experimental results in the first campaigns of Wendelstein 7-X (W7-X) have shown that, due to the optimization of the magnetic configuration with respect to neoclassical transport, turbulence is essential to understand and predict the total particle and energy fluxes. This has motivated much work on gyrokinetic modelling in order to interpret the already available experimental results and to...
The excitation of toroidicity induced Alfven eigenmodes (TAEs) using an electromagnetic antenna acting on a confined toroidal plasma is studied. The antenna is described by an electrostatic potential resembling the target TAE mode structure along with its corresponding parallel electromagnetic potential computed from Ohm's law. Stable long-time linear simulations are achieved by integrating...
Axisymmetric modes (i.e. with toroidal mode number n=0) destabilized by fast ions have been observed in recent JET experiments. Motivated by these experimental results, we have reconsidered the dispersion relation of macroscopic n=0 vertical displacements in shaped tokamak plasmas. Vertical displacements are normally stable thanks to a combination of passive and active feedback stabilization....
While the achievement of high confinement regime is ensured through the formation and sustainment of edge transport barriers associated with sheared flows, the effect of non-axisymmetric perturbations of the magnetic field, like non-resonant magnetic perturbations or ripple, on the transition remains an open issue. The underlying loss of axisymmetry is responsible for a toroidal torque,...
In this work, an implicit scheme for electromagnetic particle-in-cell/Fourier simulations is developed using the v_∥ formula and applied to studies of Alfvén waves in one dimension and in tokamak plasmas on structured meshes. While the “particle enslavement” scheme has been introduced for reducing the degree of freedom of particles in the field-particle system, in this work, we focus on the...
Filaments in the boundary of magnetically confined fusion plasmas lead to enhanced erosion of the main chamber walls. These high-density, coherent structures can be thought of as intermittent fluctuations described by the Filtered Poisson Process (FPP) as a superposition of pulses with a fixed shape and a constant duration. Additionally, these fluctuations have large amplitudes compared to the...
In a tokamak, radio frequency (RF) electromagnetic waves that propagate through low density plasma (n_l) enter the strongly turbulent edge region (n_e) before passing into the fusion plasma (n_p). Whether used for diagnostics or for heating and current drive, it is important to quantify the spectral properties of these waves. The magnetized n_l, n_p and n_e (via homogenization) regions are...
Three-dimensional force-free states describing toroidal plasmas with D-shaped cross section, are constructed. The construction is carried out by perturbing two-dimensional axisymmetric single-Beltrami states with translationally symmetric ones. The perturbation and the unperturbed magnetic field have a common Beltrami parameter λ, thus their superposition still satisfies the Beltrami equation....
H-mode plasma scenarios, with enhanced energy and particle confinement at plasma edge, are a viable option for fusion energy production and represent the core operational regimes for present day and next generation fusion experiments such as ITER. Good confinement however comes at the price of potentially large pressure and current density gradients in the edge region, leading to the so-called...
Verification and validation of the internal kink instability in tokamak have been performed for both gyrokinetic (GTC) and kinetic-MHD codes (GAM-solver, M3D-C1-K, NOVA, XTOR-K). Using realistic magnetic geometry and plasma profiles from the same equilibrium reconstruction of the DIII-D shot #141216, these codes exhibit excellent agreements for the growth rate and mode structure of the n=1...
In the Wendelstein 7-X stellarator, the vacuum rotational transform, iota, has a flat radial profile and does not cross any major rational resonance. Nevertheless, during plasma operation the iota profile can be strongly modified by electron cyclotron current drive (ECCD) in such a way that the resulting iota profile passes through low-order rational values, and this can trigger...
Radio frequency waves (RF) are scattered by filamentary structures which exist in the edge region of a tokamak plasma. The waves are reflected, refracted, and diffracted leading to a change in their spectral properties. The spatial profile of the launched power gets fragmented and part of the launched power can be coupled to an unwanted cold plasma wave. Consequently, the efficiency of heating...
Understanding the field excitation and power losses in the plasma edge from an ICRH antenna is of paramount importance to avoid impurity release from the edge in reactor conditions. The semi-analytical code ANTITER IV provides a complete description in plane geometry (z along the total B0 field in front of the antenna, x the radial component) in the cold plasma approximation and with...
In the pedestal, large bootstrap and Pfirsch-Schluter currents, arising from the steep pressure gradient, drive kink instabilities and enhance the drive for the peeling-ballooning modes. The latter triggers periodic plasma eruptions at the edge of a tokamak that significantly influence the pedestal properties, limiting the pedestal pressure gradient and degrading core confinement. The...
The Debye sheath is shown to vanish completely in magnetised plasmas for a sufficiently small electron gyroradius and small angle between the magnetic field and the wall. This angle depends on the current onto the wall. When the Debye sheath vanishes, there is still a potential drop between the wall and the plasma across the magnetic presheath. The magnetic field angle corresponding to sheath...
What if you are interested in performing a particle simulation with kinetic electrons and gyrokinetic ions? In principle, the kinetic motion of electrons is described in terms of electric and magnetic fields, while the standard gyrokinetic motion of ions is described in terms of electric and magnetic potentials. The dependence of standard gyrokinetic theory on perturbed potentials, instead of...
We report our numerical observation of seeded blob dynamics at the SOL region of tokamak. We have used Gkeyll computational plasma framework to perform our 5D gyrokinetic simulation for helical open magnetic field lines. We simulate for plasma parameters similar to ASDEX upgrade experimental shots. Toroidally elongated coherent density structures (known as plasma blobs) are seeded just outside...
