arXiv subjects

F. J. Casson

Publications and source records attributed to F. J. Casson.

13 recordsLinked to original sources

arXiv2025

A quasi-linear model of electromagnetic turbulent transport and its application to flux-driven transport predictions for STEP

A quasi-linear reduced transport model is developed from a database of high-$β$ electromagnetic nonlinear gyrokinetic simulations performed with Spherical Tokamak for Energy Production (STEP) relevant parameters. The quasi-linear model is fully electromagnetic and accounts for the effect of equilibrium flow shear using a novel approach. Its flux predictions are shown to agree quantitatively with predictions from local nonlinear gyrokinetic simulations across a broad range of STEP-relevant local equilibria. This reduced transport model is implemented in the T3D transport solver that is used to perform the first flux-driven simulations for STEP to account for transport from hybrid-KBM turbulence, which dominates over a wide region of the core plasma. Nonlinear gyrokinetic simulations of the final transport steady state from T3D return turbulent fluxes that are consistent with the reduced model, indicating that the quasi-linear model may also be appropriate for describing the transport steady state. Within the assumption considered here, our simulations support the existence of a transport steady state in STEP with a fusion power comparable to that in the burning flat-top of the conceptual design, but do not demonstrate how this state can be accessed.

physics.plasm-ph↗

arXiv2024

Flat-top plasma operational space of the STEP power plant

STEP is a spherical tokamak prototype power plant that is being designed to demonstrate net electric power. The design phase involves the exploitation of plasma models to optimise fusion performance subject to satisfying various physics and engineering constraints. A modelling workflow, including integrated core plasma modelling, MHD stability analysis, SOL and pedestal modelling, coil set and free boundary equilibrium solvers, and whole plant design, has been developed to specify the design parameters and to develop viable scenarios. The integrated core plasma model JETTO is used to develop individual flat-top operating points that satisfy imposed criteria for fusion power performance within operational constraints. Key plasma parameters such as normalised beta, Greenwald density fraction, auxiliary power and radiated power have been scanned to scope the operational space and to derive a collection of candidate non-inductive flat-top points. The assumed auxiliary heating and current drive is either from electron cyclotron systems only or a combination of electron cyclotron and electron Bernstein waves. At present stages of transport modelling, there is a large uncertainty in overall confinement for relevant parameter regimes. For each of the two auxiliary heating and current drive systems scenarios, two candidate flat-top points have been developed based on different confinement assumptions, totalling to four operating points. A lower confinement assumption generally suggests operating points in high-density, high auxiliary power regimes, whereas higher confinement would allow access to a broader parameter regime in density and power while maintaining target fusion power performance.

physics.plasm-ph↗

arXiv2021

First-principles-based multiple-isotope particle transport modelling at JET

Core turbulent particle transport with multiple isotopes can display observable differences in behaviour between the electron and ion particle channels. Experimental observations at JET with mixed H-D plasmas and varying NBI and gas-puff sources [M. Maslov et al., Nucl. Fusion 7 076022 (2018)] inferred source dominated electron peaking, but transport dominated isotope peaking. In this work, we apply the QuaLiKiz quasilinear gyrokinetic transport model within JINTRAC flux-driven integrated modelling, for core transport validation in this multiple-isotope regime. The experiments are successfully reproduced, predicting self consistently $ j $, $ n_{e} $, $ n_{Be} $, $ T_{e} $, $ T_{i} $, $ω_{tor}$ and the isotope composition. As seen in the experiments, both H and D profiles are predicted to be peaked regardless of the core isotope source. An extensive sensitivity study confirmed that this result does not depend on the specific choices made for the boundary conditions and physics settings. While kinetic profiles and electron density peaking did vary depending on the simulation parameters, the isotope ratio remained nearly invariant, and tied to the electron density profile. These findings have positive ramifications for multiple-isotope fuelling, burn control, and helium ash removal.

physics.plasm-ph↗

arXiv2021

Multiple-isotope pellet cycles captured by turbulent transport modelling in the JET tokamak

For the first time the pellet cycle of a multiple-isotope plasma is successfully reproduced with reduced turbulent transport modelling, within an integrated simulation framework. Future nuclear fusion reactors are likely to be fuelled by cryogenic pellet injection, due to higher penetration and faster response times. Accurate pellet cycle modelling is crucial to assess fuelling efficiency and burn control. In recent JET tokamak experiments, deuterium pellets with reactor-relevant deposition characteristics were injected into a pure hydrogen plasma. Measurements of the isotope ratio profile inferred a Deuterium penetration time comparable to the energy confinement time. The modelling successfully reproduces the plasma thermodynamic profiles and the fast deuterium penetration timescale. The predictions of the reduced turbulence model QuaLiKiz in the presence of a negative density gradient following pellet deposition are compared with GENE linear and nonlinear higher fidelity modelling. The results are encouraging with regard to reactor fuelling capability and burn control.

physics.plasm-ph↗

arXiv2020

Flux-driven integrated modelling of main ion pressure and trace tungsten transport in ASDEX Upgrade

Neoclassical and turbulent heavy impurity transport in tokamak core plasmas are determined by main ion temperature, density and toroidal rotation profiles. Thus, in order to understand and prevent experimental behaviour of W accumulation, flux-driven integrated modelling of main ion heat and particle transport over multiple confinement times is a vital prerequisite. For the first time, the quasilinear gyrokinetic code QuaLiKiz has been applied for successful predictions of core kinetic profiles in an ASDEX Upgrade H-mode discharge in the turbulence dominated region within the integrated modelling suite JETTO. Neoclassical contributions are calculated by NCLASS; auxiliary heat and particle deposition profiles due to NBI and ECRH prescribed from previous analysis with TRANSP. Turbulent and neoclassical contributions are insufficient in explaining main ion heat and particle transport inside the $q=1$ surface, necessitating the prescription of further transport coefficients to mimic the impact of MHD activity on central transport. The ion to electron temperature ratio at the simulation boundary at $ρ_\mathrm{tor} = 0.85$ stabilizes ion scale modes while destabilizing ETG modes when significantly exceeding unity. Careful analysis of experimental measurements using Gaussian process regression techniques is carried out to explore reasonable uncertainties. In following trace W impurity transport simulations performed with additionally NEO, neoclassical transport under consideration of poloidal asymmetries alone is found to be insufficient to establish hollow central W density profiles. Reproduction of these conditions measured experimentally is found possible only when assuming the direct impact of a saturated $(m,n)=(1,1)$ MHD mode on heavy impurity transport.

physics.plasm-ph↗

arXiv2015

The role of MHD in causing impurity peaking in JET Hybrid plasmas

In Hybrid plasma operation in JET with its ITER-like wall (JET-ILW) it is found that n>1 tearing activity can significantly enhance the rate of on-axis peaking of tungsten impurities, which in turn significantly degrades discharge performance. Core n=1 instabilities can be beneficial in removing tungsten impurities from the plasma core (e.g. sawteeth or fishbones), but can conversely also degrade core confinement (particularly in combination with simultaneous n=3 activity). The nature of MHD instabilities in JET Hybrid discharges, with both its previous Carbon wall and subsequent JET-ILW, is surveyed statistically and the character of the instabilities is examined. Possible qualitative models for how the n>1 islands can enhance on-axis tungsten transport accumulation processes are presented.

physics.plasm-ph↗

arXiv2015

Charge dependence of neoclassical and turbulent transport of light impurities on MAST

Carbon and nitrogen impurity transport coefficients are determined from gas puff experiments carried out during repeat L-mode discharges on the Mega-Amp Spherical Tokamak (MAST) and compared against a previous analysis of helium impurity transport on MAST. The impurity density profiles are measured on the low-field side of the plasma, therefore this paper focuses on light impurities where the impact of poloidal asymmetries on impurity transport is predicted to be negligible. A weak screening of carbon and nitrogen is found in the plasma core, whereas the helium density profile is peaked over the entire plasma radius.

physics.plasm-ph↗

arXiv2014

Neoclassical and gyrokinetic analysis of time-dependent helium transport experiments on MAST

Time-dependent helium gas puff experiments have been performed on the Mega Ampere Spherical Tokamak (MAST) during a two point plasma current scan in L-mode and a confinement scan at 900 kA. An evaluation of the He II spectrum line induced by charge exchange suggests anomalous rates of diffusion and inward convection in the outer regions of both L-mode plasmas. Similar rates of diffusion are found in the H-mode plasma, however these rates are consistent with neoclassical predictions. The anomalous inward pinch found in the core of L-mode plasmas is also not apparent in the H-mode core. Linear gyrokinetic simulations of one flux surface in L-mode using the gs2 and gkw codes find that equilibrium flow shear is sufficient to stabilise ITG modes, consistent with BES observations, and suggest that collisionless TEMs may dominate the anomalous helium particle transport. A quasilinear estimate of the dimensionless peaking factor associated with TEMs is in good agreement with experiment. Collisionless TEMs are more stable in H-mode because the electron density gradient is flatter. The steepness of this gradient is therefore pivotal in determining the inward neoclassical particle pinch and the particle flux associated with TEM turbulence.

physics.plasm-ph↗

arXiv2014

Effect of turbulence on electron cyclotron current drive and heating in ITER

Non-linear local electromagnetic gyrokinetic turbulence simulations of the ITER standard scenario H-mode are presented for the q=3/2 and q=2 surfaces. The turbulent transport is examined in regions of velocity space characteristic of electrons heated by electron cyclotron waves. Electromagnetic fluctuations and sub-dominant micro-tearing modes are found to contribute significantly to the transport of the accelerated electrons, even though they have only a small impact on the transport of the bulk species. The particle diffusivity for resonant passing electrons is found to be less than 0.15 m^2/s, and their heat conductivity is found to be less than 2 m^2/s. Implications for the broadening of the current drive and energy deposition in ITER are discussed.

physics.plasm-ph↗

arXiv2014

Theoretical description of heavy impurity transport and its application to the modelling of tungsten in JET and ASDEX Upgrade

Recent developments in theory-based modelling of core heavy impurity transport are presented, and shown to be necessary for quantitative description of present experiments in JET and ASDEX Upgrade. The treatment of heavy impurities is complicated by their large mass and charge, which result in a strong response to plasma rotation or any small background electrostatic field in the plasma, such as that generated by anisotropic external heating. These forces lead to strong poloidal asymmetries of impurity density, which have recently been added to numerical tools describing both neoclassical and turbulent transport. Modelling predictions of the steady-state two-dimensional tungsten impurity distribution are compared with experimental densities interpreted from soft X-ray diagnostics. The modelling identifies neoclassical transport enhanced by poloidal asymmetries as the dominant mechanism responsible for tungsten accumulation in the central core of the plasma. Depending on the bulk plasma profiles, neoclassical temperature screening can prevent accumulation, and can be enhanced by externally heated species, demonstrated here in ICRH plasmas.

physics.plasm-ph↗

arXiv2014

Angular momentum transport modeling: achievements of a gyrokinetic quasi-linear approach

QuaLiKiz, a model based on a local gyrokinetic eigenvalue solver is expanded to include momentum flux modeling in addition to heat and particle fluxes. Essential for accurate momentum flux predictions, the parallel asymmetrization of the eigenfunctions is successfully recovered by an analytical fluid model. This is tested against self-consistent gyrokinetic calculations and allows for a correct prediction of the ExB shear impact on the saturated potential amplitude by means of a mixing length rule. Hence, the effect of the ExB shear is recovered on all the transport channels including the induced residual stress. Including these additions, QuaLiKiz remains ~10 000 faster than non-linear gyrokinetic codes allowing for comparisons with experiments without resorting to high performance computing. The example is given of momentum pinch calculations in NBI modulation experiments.

physics.plasm-ph↗

arXiv2013

Analysis of Lithium Driven Electron Density Peaking in FTU Liquid Lithium Limiter Experiments

The impact of lithium impurities on the microstability and turbulent transport characteristics in the core of a typical FTU Liquid Lithium Limiter (LLL)(Mazzitelli et al., Nucl. Fusion, 2011) discharge during the density ramp-up phase is studied. A non-linear gyrokinetic analysis performed with GKW (Peeters et al.,Comp. Phys. Comm., 2009) accompanied by a quasi-linear fluid analysis is presented. We show that a centrally peaked, high concentration lithium profile contributes to the electron peaking by reducing the outward electron flux, and that it leads to inward turbulent deuterium transport through ion flux separation.

physics.plasm-ph↗

arXiv2013

Validation of gyrokinetic modelling of light impurity transport including rotation in ASDEX Upgrade

Upgraded spectroscopic hardware and an improved impurity concentration calculation allow accurate determination of boron density in the ASDEX Upgrade tokamak. A database of boron measurements is compared to quasilinear and nonlinear gyrokinetic simulations including Coriolis and centrifugal rotational effects over a range of H-mode plasma regimes. The peaking of the measured boron profiles shows a strong anti-correlation with the plasma rotation gradient, via a relationship explained and reproduced by the theory. It is demonstrated that the rotodiffusive impurity flux driven by the rotation gradient is required for the modelling to reproduce the hollow boron profiles at higher rotation gradients. The nonlinear simulations validate the quasilinear approach, and, with the addition of perpendicular flow shear, demonstrate that each symmetry breaking mechanism that causes momentum transport also couples to rotodiffusion. At lower rotation gradients, the parallel compressive convection is required to match the most peaked boron profiles. The sensitivities of both datasets to possible errors is investigated, and quantitative agreement is found within the estimated uncertainties. The approach used can be considered a template for mitigating uncertainty in quantitative comparisons between simulation and experiment.

physics.plasm-ph↗