Microstability and turbulence in high-performance tokamak plasmas

Turbulence, driven by microscale instabilities, is known to reduce the performance of fusion devices by exacerbating the radial transport of heat and particles from the high-pressure core where fusion can occur. In this Thesis, we study two ways of reducing this turbulent transport, focusing in particular on microstability and turbulence at high plasma pressure where electromagnetic effects are important. Local δf gyrokinetic simulations are used throughout. We uncover a novel destabilizing effect of increased elongation that is manifest in plasmas with steep pressure gradients. This is explained as the competition between local magnetic shear and finite-Larmor-radius damping as elongation is varied. At high β, we show that this effect can lead to the removal of second stability of the kinetic ballooning mode (KBM) with increased elongation, which could have severe implications for future high-performance tokamaks for which access to second stability is crucial for good performance. In the second half of the Thesis, we study a JET pulse exhibiting an internal transport barrier in the ion temperature. By a combination of linear and nonlinear studies, we determine a mechanism that allows the steepest radial gradients of temperature to coexist with low levels of transport. We propose that electromagnetic effects stabilize the ion-temperature-gradient instabilities that otherwise drive significant transport well above experimental levels. The KBM that is usually destabilized at high plasma β is stabilized by significant negative magnetic shear, and equilibrium flow shear should be avoided as it rapidly destabilizes the KBM via the parallel velocity gradient. In determining this mechanism, we matched experimental fluxes simultaneously in multiple transport channels at multiple radial locations, validating the use of local δf gyrokinetics for the further study of transport barriers

COVID-19 patients share common, corticosteroid-independent features of impaired host immunity to pathogenic molds

Patients suffering from coronavirus disease-2019 (COVID-19) are susceptible to deadly secondary fungal infections such as COVID-19-associated pulmonary aspergillosis and COVID-19-associated mucormycosis. Despite this clinical observation, direct experimental evidence for severe acute respiratory syndrome coronavirus type 2 (SARS-CoV-2)-driven alterations of antifungal immunity is scarce. Using an ex-vivo whole blood stimulation assay, we challenged blood from twelve COVID-19 patients with Aspergillus fumigatus and Rhizopus arrhizus antigens and studied the expression of activation, maturation, and exhaustion markers, as well as cytokine secretion. Compared to healthy controls, T-helper cells from COVID-19 patients displayed increased expression levels of the exhaustion marker PD-1 and weakened A. fumigatus - and R. arrhizus -induced activation. While baseline secretion of proinflammatory cytokines was massively elevated, whole blood from COVID-19 patients elicited diminished release of T-cellular (e.g., IFN-γ, IL-2) and innate immune cell-derived (e.g., CXCL9, CXCL10) cytokines in response to A. fumigatus and R. arrhizus antigens. Additionally, samples from COVID-19 patients showed deficient granulocyte activation by mold antigens and reduced fungal killing capacity of neutrophils. These features of weakened anti-mold immune responses were largely decoupled from COVID-19 severity, the time elapsed since diagnosis of COVID-19, and recent corticosteroid uptake, suggesting that impaired anti-mold defense is a common denominator of the underlying SARS-CoV-2 infection. Taken together, these results expand our understanding of the immune predisposition to post-viral mold infections and could inform future studies of immunotherapeutic strategies to prevent and treat fungal superinfections in COVID-19 patients

Impact of shaping on microstability in high-performance tokamak plasmas

We have used the local-δf gyrokinetic code GS2 to perform studies of the effect of flux-surface shaping on two highly-shaped, low- and high-β JT-60SA-relevant equilibria, including a successful benchmark with the GKV code. We find that for a high-performance plasma, i.e. one with high plasma beta and steep pressure gradients, the turbulent outwards radial fluxes may be reduced by minimizing the elongation. We explain the results as a competition between the local magnetic shear and finite-Larmor-radius (FLR) stabilization. Electromagnetic studies indicate that kinetic ballooning modes are stabilized by increased shaping due to an increased sensitivity to FLR effects, relative to the ion-temperature-gradient instability. Nevertheless, at high enough β, increased elongation degrades the local magnetic shear stabilization that enables access to the region of ballooning second-stability

Advances in the physics studies for the JT-60SA tokamak exploitation and research plan

International audienceJT-60SA, the largest tokamak that will operate before ITER, has been designed and built jointlyby Japan and Europe, and is due to start operation in 2020. Its main missions are to support ITERexploitation and to contribute to the demonstration fusion reactor machine and scenario design.Peculiar properties of JT-60SA are its capability to produce long-pulse, high-β,and highlyshaped plasmas. The preparation of the JT-60SA Research Plan, plasma scenarios, andexploitation are producing physics results that are not only relevant to future JT-60SAexperiments, but often constitute original contributions to plasma physics and fusion research.Results of this kind are presented in this paper, in particular in the areas of fast ion physics, high-beta plasma properties and control, and non-linear edge localised mode stability studies