Abstract: The Wendelstein 7-X (W7-X) stellarator has completed two consecutive experimental campaigns OP 2.2 (Sep.-Dec. 2024) and OP 2.3 (Feb.-May 2025) under a new operational strategy enabling more than one year of uninterrupted device availability. This approach, supported by exceptionally high subsystem reliability, allowed sustained high-efficiency plasma operations with up to 80–100 discharges per day across a broad range of magnetic configurations. Several key technical upgrades-most notably the first operation of a 1.5 MW class steady-state gyrotron, a new steady-state pellet injector, and advanced real-time feedback control systems significantly enhanced heating, fueling, and plasma control capabilities. Together, these improvements enabled major advances in long-pulse performance, high-β operation, and confinement optimization. Long-pulse discharges achieved 1.8 GJ of injected energy under fully detached divertor conditions, while reduced-field scenarios facilitated record volume-averaged β values approaching 3%. High-performance plasmas with centrally peaked density profiles, created via neutral beam injection (NBI) or sustained pellet fueling, demonstrated strongly reduced turbulent transport and stellarator-record fusion triple products. Complementary studies of power exhaust and divertor heat loads revealed the role of scrape-off-layer drift physics in shaping strike-line patterns under attached conditions. Together, the results from OP 2.2 and OP 2.3 significantly expand the operational space of W7-X and strengthen its role as a leading platform for steady-state stellarator research and reactor-relevant plasma scenarios.

Abstract: The forced generation of zonal flows by microinstability-driven turbulence is investigated within the framework of local gyrokinetic theory far from marginality. We use a numerically and physically informed three-wave truncation scheme, which allows the prediction of the zonal-flow kx-spectrum during the early phase of nonlinear gyrokinetic simulations. The model reproduces the known 2-ɣ growth rate resulting from nonlinear beating of linearly unstable primary modes, in line with previous results, without any marginal stability point. The phase-space structure of such zonal flow is strongly constrained by that of the driving fluctuations, which is essential to understand its behaviour in the region of validity. It is shown that this leads to an enhanced residual spectrum compared to the classic Rosenbluth-Hinton calculation.

Abstract: This thesis investigates microturbulence in three-dimensional magnetised plasma geometries, with a specific focus on the impact of turbulence to plasma stability. Microturbulence is the small-scale turbulent fluctuations in a plasma caused by instabilities at or near the ion and electron gyroradius scales. Microturbulence remains a critical challenge within the field of plasma physics, and in order to control the effects of turbulence, we first need to accurately model it. This task is particularly complex in non-axisymmetric geometries, where mode-coupling occurs across non-identical field lines, leading to potential inconsistencies in the common representation of zonal flows.

The first part of the thesis focuses on the development and validation of a novel δf-gyrokinetic code, designed to model microturbulence across a full flux-annulus in non-axisymmetric magnetic configurations. This is especially important when considering turbulent dynamics in stellarators and tokamaks with 3D magnetic perturbations, as capturing the coupling of modes across different field lines is necessary to accurately resolve 'zonal' modes, that are constant across a flux surface. The code is benchmarked against the current flux-tube implementation of the stella code, along with other existing gyrokinetic codes. Novel results incorporating kinetic electrons are also presented.

The second part of the thesis addresses microstability, which poses unique optimisation challenges due to the large number of tunable parameters in magnetic confinement devices. To tackle this problem, an efficient method for calculating the derivative of the linear growth rate with respect to multiple externally-controllable parameters, has been developed using an adjoint method. The theoretical framework is derived in the limit of no field-line coupling, including electromagnetic effects and collisions. Numerical simulations then demonstrate the efficacy of the approach in the electrostatic, collisionless regime.