Understanding the dynamics of Energetic Particles (EPs) is essential for optimizing the perfor- mance of fusion devices. EPs serve as the primary energy carriers in fusion plasmas, and their loss due to transport from the core to the edge results in a significant reduction in efficiency of the fusion device. One of the key phenomena driving the EPs transport is the precessional fishbone...
Disruptions represent a critical challenge to the safe and reliable operation of future fusion devices like ITER, as they impose severe thermal and mechanical loads on the tokamak structure and generate high-energy runaway electrons (REs). This study expands on the work of [Vallhagen et al, Nucl. Fusion 64 (2024)] and presents a significant upgrade of the DREAM [1] disruption simulation...
Plasma disruption is one of the key factors limiting the stable and safe operation of future large tokamaks. It involves a sudden collapse of the plasma confinement and can cause severe heat loads and electromagnetic forces on surrounding structures. Understanding the chain of events leading to unintentional/natural disruptions remaining a critical goal in the pursuit of sustainable fusion...
Turbulent transport represents one of the major topics in plasma physics, especially taking into account its impact on the performance of nuclear fusion devices. However, modelling turbulence requires long-time highly resolved simulations to capture the fine spatial and temporal scales, making it numerically intensive. The use of surrogate models might represent a good compromise between...
Disruptions, i.e. major instabilities in which plasma confinement is lost, are a significant threat to tokamak operation. During a disruption, the resistivity of the plasma increases as the thermal energy is quickly lost, causing the current to decrease. Due to the self-inductance of the plasma this leads to the generation of a strong parallel electric field. As the friction force experienced...
A disrupting plasma in a high-performance device such as ITER and SPARC may generate
large runaway electron (RE) currents that, upon impact with the tokamak wall, can cause
serious damage to the device. To quickly identify regions of safe operation in parameter
space, it is useful to develop reduced models and analytical criteria that predict when a
significant fraction of the Ohmic...
In stellarators, the absence of axisymmetry poses challenges for confinement, as particle orbits are more prone to drift losses. Quasisymmetry [1] offers a pathway to overcome this limitation by optimizing the magnetic field configuration to approximate the favorable confinement of axisymmetric systems. Nevertheless, traditional stellarator designs often involve high aspect ratios and...
Sheath boundary conditions are an unavoidable fact of fluid based Scrape-Off Layer (SOL) modelling. The choice of boundary conditions can dictate the equilibrium that is reached, this choice is usually the Bohm criterion [1]. Recent work by Li et al [2,3,4] proposed a novel boundary condition, in the form of a correction to the Bohm criterion, for modelling based on 1D Particle In Cell (PIC)...
Gyrokinetic codes are currently the most advanced numerical tools for simulating turbulence in to-kamak plasmas. The code Gysela [1], written in Fortran 90 and developed for more than 20 years, is one of the flux-driven gyrokinetic codes available worldwide. However expanding this code to use more complex mathematical methods such as non-uniform points (vital for handling the different...
Neutral Beam Injection (NBI) and Ion Cyclotron Resonance Heating (ICRH) are two key systems that generate a population of Energetic Particles (EPs) in magnetically confined fusion plasmas. These EPs play a crucial role in plasma performance, providing heat, torque and non-inductive current. Furthermore, beam EPs are excellent candidates for studying wave–particle interaction phenomena...
New plasma current ramp up (RU) scenario has been developed recently on TCV to demonstrate for the first time the possibility to raise the plasma current (Ipl) non-inductively (i.e. with zero flux contribution from the central solenoid) by applying ECCD in the early discharge phase after the break down [1]. Similar strategy has been tested previously in the non-inductive (NI) RU scenario fully...
Pellet refuelling will be a key technique for sustaining plasma density in future fusion reactors based on the stellarator concept. Although this method is more mature in tokamak experiments, the departure from axisymmetry in stellarators introduces unique challenges and opportunities that remain poorly understood.
Here, the stellarator extension of the 3D nonlinear MHD code JOREK is used...
Guiding Center (GC) orbits in shaped tokamak equilibria exhibit rich structure, once the magnetic field magnitude develops secondary local minima. Using Negative Triangularity (NT) and small aspect ratio equilibria we show that shaping-induced wells in B introduce additional mirror points and, consequently, multiple families of trapped orbits. Two generic bifurcation scenarios are identified...
Understanding plasma behaviour in the scrape-off layer (SOL) is critical for predicting particle and energy exhaust in magnetically confined fusion devices. A key feature in the SOL is magnetic flux expansion, which reduces peak heat loads on divertor targets by spreading exhaust across a larger surface area. This phenomenon is typically modelled using fluid codes, which incorporate flux...
High confinement mode operation of tokamaks is inevitably linked to edge localized mode (ELM) instabilities. Larger tokamaks like ITER will be severely threatened by their occurrence as they are associated with large transient heat loads that exceed material limits. Applying 3D resonant magnetic perturbations (RMP) is a possible means to suppress ELMs. However, so far RMP ELM suppression was...
Accurate numerical modelling of turbulent transport in edge tokamak plasma remains a significant challenge. Many key experimental features, such as the formation of edge transport barriers, are still difficult to simulate, especially for ITER-sized tokamaks. Predicting the scrape-off layer (SOL) width or the power load imbalance between the inner and outer divertor legs remain an open issue,...
The stability analysis of gyrokinetic slab ITG modes is well-established and results in temperature and density gradient stability thresholds that depend on the parallel and perpendicular wavenumbers.
In a recent PRL [1], using 6D turbulence simulations with a specially optimized code that resolves the Larmor orbits, we found potentially significant non-gyrokinetic instabilities for steep,...
Typically, stellarator optimization relies on the notion of omnigenity [1, 2], which guarantees small radial neoclassical transport at the low collisionalities that are characteristic of fusion-grade plasmas. In omnigenous magnetic fields, collisionless particles do not move radially on average and, in the course of their motion along flux surfaces, they never undergo transitions between...
Good plasma confinement is crucial to harness fusion energy. Experiments on the TCV [1] and DIII-D [2] tokamaks have shown that negative triangularity (NT) reduces the turbulent transport, hence improving confinement. Trapped Electron Modes (TEM) are thought to play an important role in this process. A full understanding of the underlying physics is necessary for assuring NT plasmas to be...
Symmetry‑breaking perturbations in fusion plasmas—whether produced by intrinsically excited magnetohydrodynamic modes such as Alfvénic Eigenmodes and Geodesic Acoustic Modes, or by externally applied fields such as Resonant Magnetic Perturbations and Toroidal‑Field ripples—interact with charged particles through resonances that govern the transport of particles, energy, and momentum. Building...
Solution of the drift kinetic equation [1] is a required step in analyzing and optimizing neoclassical transport in stellarators. A variety of codes [2-6] have been developed to handle the complex geometry and wide range of collisionality regimes present in stellarators. Existing codes are generally either high fidelity codes that are accurate in a wide range of regimes, but are too expensive...
The magnetised plasma sheath is a region that forms in front of a solid target and is composed of a magnetic presheath, where the electrostatic potential varies on the scale of the ion Larmor radius, and a Debye sheath, where the potential varies on the scale of the Debye length. The transmission of electrons and of their energy through the magnetised sheath must be calculated to find the wall...
Evaluation of the bootstrap current is an important part of stellarator optimization procedures aiming at self-consistent equilibria or configurations with strongly reduced bootstrap current. The later is especially relevant for an accurate control of the $\iota$ profile, enabling the operation of an island divertor. For such optimization workflows, a fast method of evaluating the bootstrap...
Burning plasmas in fusion reactors are complex systems where energetic particles (EP) play a fundamental role in cross-scale interactions [1]. This study reviews phase space zonal structures (PSZS) [2-5] and their significance in transport analyses. Using synthetic diagnostics from the HMGC and ORB5 codes [6,7], we illustrate the role of PSZS in capturing transport dynamics in burning plasmas...
We present a novel method for calculating particle and energy flows in the 6D kinetic Vlasov equation with adiabatic
electrons. This approach enables the determination of energy and particle fluxes from lower-order moments of
the distribution function, such as the kinetic energy density and the momentum transfer tensor. In addition to this
decomposition, we derive the residual Poynting flux...
The study presents global 3D simulations of a linear plasma device using the first-principles drift-reduced fluid code SOLEDGE3X. The effects of parallel currents on shaping the turbulent regime are investigated, focusing on the interplay between drift-wave and sheath-driven instabilities. In open field-line regions of a tokamak, sheath losses have traditionally been regarded as the sole...
The stellarator Wendelstein 7-X (W7-X) demonstrated the effectiveness of reducing neoclassical transport through magnetic field optimization [1]. Its confinement is primarily governed by turbulence arising from instabilities at scales comparable to the gyroradius [2,3]. For small plasma beta (the ratio of kinetic to magnetic pressure), these instabilities are predominantly electrostatic and...
This abstract outlines the motivations behind my PhD project and introduces preliminary work whose results will be presented in a poster format. The content consists of a set of turbulence simulations exploring the parameter space.
Accurate prediction of core turbulent transport, along with its coupling to edge transport models or physics-informed edge boundary conditions, is essential for developing operational scenarios for future fusion devices. Nonlinear gyrokinetic models for core transport, though highly accurate, are computationally expensive to use in this type of integrated modeling. Instead, quasilinear models...
The turbulence in the core of future devices will be very different from current devices [1]. Fast ions and electromagnetic (EM) effects have a complex impact on the turbulence both through linear effects, such as linear EM stabilization of electrostatic (ES) modes, and nonlinear effects, such as enhanced coupling to zonal flows. As an initial step towards better understanding this regime we...
In the last decades, the modelling of RF-driven toroidal current advanced importantly at the Lower Hybrid (LH) frequency in tokamaks, especially in non-inductive regimes [1]. Of particular importance is the understanding of spectral broadening in the scrape-off layer (SOL), enabling single-pass wave absorption [2], notably in in high-density, low-temperature plasmas with small aspect ratios....
Evidence from magnetically confined fusion experiments over the past decades shows that X-mode waves used for ECRH (X2) are prone to nonlinear wave interactions. These interactions are known as parametric decay instabilities, and they typically occur in localized regions of the plasma edge. The instabilities originate from thermal upper-hybrid (UH) waves that are trapped by non-monotonic...
We present an iterative algorithm for the self-consistent computation of the electrostatic potential in 3D magnetic fields using the guiding-center tracing code GORILLA. Due to the piecewise linear interpolation of electromagnetic field quantities leading to linear equations of motion within small volume elements, GORILLA has favourable computational costs while still retaining symplectic...
Recent experiments at the JET tokamak with the Be/W wall have led to the development of new H-mode regimes featuring high energy confinement and small edge-localized modes (ELMs). These regimes, achieved with low or no gas injection, reproduce key plasma parameters relevant to the ITER baseline scenario, such as $q_{95} = 3.2$, $\beta_{\text{p}} < 1$, $\beta_{\text{N}} = 1.8$–$2$, $H_{98} =...
Pressure anisotropy due to auxiliary heated fast ions or runaway electrons can significantly
impact the macroscopic magnetic equilibrium. Moreover, since the plasma toroidal
diamagnetism is predominantly impacted by the perpendicular pressure, measurements of the
outside poloidal magnetic field combined with measurements of the toroidal flux allow to
distinguish between both components, as...
In tokamak plasmas, micro-tearing modes (MTMs) are destabilised at high plasma beta and large electron temperature gradient. They are electro-magnetic instabilities particularly prone to be excited in spherical tokamaks, where beta is large, and in improved confinement regimes or in the pedestal for conventional tokamaks.
For MTMs, the transport arising from magnetic flutter scales as the...
The stellarator concept offers advantages for a fusion reactor compared to the tokamak, but stellarator magnetic fields require careful optimization to achieve a confinement quality comparable to that of tokamaks. The numerical optimization for reduced neoclassical transport has already been experimentally validated in W7-X [1] and HSX [2], and great improvements have been made in the last few...
Tokamak plasmas are complex non-equilibrium systems where turbulence plays a critical role in the transport of heat and particles. These turbulent processes span a wide range of spatial and temporal scales, making their observation particularly challenging. Ultra-fast sweeping reflectometry is a diagnostic technique capable of measuring electron density fluctuations with very high spatial and...
Self-organised turbulent processes are common in many non-equilibrium physical and biological systems and lead to pattern formation. Well-known examples in magnetic fusion include zonal flows or magnetic islands. Other processes can occur, often less emphasised, such as turbulence self-advection (spreading), front propagation or blob emission and many other structures, such as staircases, can...
Future nuclear fusion reactors will have to magnetically confine burning plasmas. In such scenarios, even a small fraction of fusion-born energetic particles (EP), which are 100 times hotter than the electrons, will contribute greatly to the kinetic pressure and therefore to the shaping of the MHD equilibrium, mainly via the Shafranov Shift. Nonetheless, many numerical works still prefer to...
Leveraging the results of a series of $3$D flux-driven $2$-fluid simulations in a diverted equilibrium with GBS, it is shown how a regime of high confinement can develop as the power crossing the separatrix exceeds a critical value. As the edge temperature increases, the resistive-ballooning turbulence characteristic of L-mode conditions becomes subdominant, and turbulence is mostly driven by...
Experiments at TCV [1–3], DIII-D [4–6] and AUG [7,8] indicate that Negative Triangularity (NT) plasmas [9] can achieve H-mode-like performance with negligible ELM activity. Therefore, NT is investigated as a possible scenario for fusion reactors [10, 11]. However, the physics of NT is still to be fully understood. An experimental and theoretical effort is ongoing to fill this gap.
A NT...
The runaway electron (RE) beam benign termination observed in JET pulse 95135 has been modelled in [1] using the nonlinear MHD code JOREK [2]. The study demonstrated the role of magnetic stochasticity in causing RE loss. This demonstration was based on particular assumptions regarding the properties of the background plasma, such as resistivity, that might not be accurate due to the...
Integrated modelling of tokamaks combines a host of different codes to self-consistently model plasma discharges with key applications in modelling plasma scenarios and reactor designs as well as analysing experimental discharges. Fast and accurate integrated modeling is crucial to enable rapid iteration and efficient use of limited computational resources. Currently, a major bottleneck in...
Core contamination is a key issue for full Tungsten devices and will be a major concern for the operation of ITER. Today’s experiments on medium size tokamaks such as WEST or AUG are vital to further characterize the mechanisms at play that regulate Tungsten concentration. WEST for instance, is characterized by a near-constant radiated fraction, regardless of input power. To better understand...
