Open Access

Shear Alfvén waves (SAWs) can be important in plasma magnetohydrodynam- ics (MHD) stability. In the Wendelstein 7-X (W7-X) stellarator plasmas, Alfvénic fluctuations have been identified in routine plasma scenarios of the opera- tional phase (OP) known as OP1.2b. Magnetic fluctuations between f = 100 − 200 kHz are measured using a system of Mirnov coils. The work presented in the thesis aims to contribute to the physics understanding of SAWs in fu- sion plasmas without externally driven fast ions. The dynamics of a broad frequency range of SAWs in W7-X plasmas are analyzed. The time variations of frequency, amplitude, and width of frequency bands of the magnetic fluc- tuations’ spectra are determined using a new analysis method, the so-called tracking method. The SAW variations are correlated to global plasma param- eters during W7-X plasmas. The new tracking method enabled determining the role of different plasma parameters in the observed variations of the SAW spectral properties. This thesis furthermore discusses potential couplings be- tween SAWs and ITG-driven turbulent modes to explain amplitude variations of SAWs.

For the creation of a positron-electron (pair) plasma the A Positron Electron eXperiment (APEX) collaboration needs large quantities of positrons. Accumulating many positrons is an experimentally challenging task. To achieve this task, a new prototype multi cell Penning-Malmberg trap (MCT) was designed and constructed. The accumulation of large charged-particle ensembles with one charge sign dominating, so-called non-neutral plasmas, in Penning-Malmberg traps is limited by the plasma space charge. The MCT avoids this limitation by separating the plasma space charge into multiple storage traps in the same magnetic field. This MCT includes a master-cell, and three storage cells (one on-axis, and two off-axis). With this device plasma transfer to the off-axis cells was tested and improved. The goal was to transfer multiple plasmas off-axis while avoiding particle losses during the process. The dissertation introduces the vacuum setup, diagnostics, and the MCT, and explores its operation. The plasma creation process, cyclotron cooling, and the plasma confinement is detailly described. Different schemes for the autoresonant excitation of the diocotron mode are discussed as well as the dynamics during the transport to the off-axis cells. These dynamics are dominated by competing diocotron drift modes that can lead to significant particle losses. New techniques are presented which allow to suppress these modes. These techniques mitigate losses and centre the plasma in the off-axis cells during the transfer process, significantly improving it. In addition, the dissertation demonstrates the first consecutive transfer and confinement in two different off-axis cells. The confinement in multiple off-axis traps is a milestone for the future use of a MCT at the NEPOMUC positron source in Munich.

One type of ion thruster that has gained attention in recent years is the High Efficiency Multistage Plasma (HEMP) thruster. Optimizing the performance of these thrusters can be challenging due to the complexity of the underlying physics. Since the construction of new designs is expensive, cheaper methods for optimization, e.g., numerical optimization, are being sought. This paper presents a fast, analytical approach to finding realistic starting points for the magnetic geometry design of HEMP thrusters. First, a ratio of length to radius is presented, where the magnetic field is especially parallel at the center of the magnetic ring. This result is confirmed with the open-source library magpylib. Its speed and accuracy qualify this tool for further optimization processes. Here, we present some simple performance indicators, which may be beneficial to characterize the magnetic field structure for further optimization.

The magnetohydrodynamic (MHD) equilibrium model is one of the fundamental building blocks in the description of a magnetically confined plasma. The computational cost of constructing solutions to the 3D ideal-MHD equilibrium problem is one of the limiting factors in stellarator research and design; in particular, it limits the extent to which we can perform sample-intensive applications, applications which require many samples to be evaluated to yield meaningful results. Sample-intensive applications in stellarator research and design include, for example, equilibrium reconstruction, stellarator optimization, and flight simulators. In this thesis, we investigate how faithfully artificial neural networks (NNs) can quickly approximate ideal-MHD equilibria in stellarator geometries, starting with Wendelstein 7-X (W7-X), the world’s most advanced stellarator. In particular, we investigate (see section 1.7): RQI: to what extent can NN models approximate the MHD equilibrium solution for different W7-X configurations and plasma profiles? What is the speed-accuracy trade-off offered by NN models? RQII: to what degree the NN model faithfully reproduces equilibrium quantities of interest (e. g., MHD stability)? To what extent can NN models meet the requirements of downstream applications (e. g., Bayesian inference, stellarator optimization) in terms of equilibrium quantities accuracy? RQIII: whether we can exploit the implicit representation of a MHD equilibrium, i. e., the equilibrium solution should satisfy the ideal-MHD force balance equation, to improve the NN approximation’s accuracy; RQIV: the reconstruction of the full posterior istribution of plasma parameters and equilibrium quantities with self-consistent MHD equilibria; moreover, how does the adoption of MHD equilibria approximated by NN models affect the inferred plasma parameters? A deep NN model is developed to learn the ideal-MHD solution operator in W7-X operational subspace, yielding 3D equilibria up to six orders of magnitude faster than currently available MHD equilibrium codes. Physics domain knowledge is embeded into the NN model: equilibrium solution symmetries are satisfied by construction, and the MHD force balance regularizes the NN model to satisfy the ideal-MHD equations. The model accurately predicts the equilibrium solution and it faithfully reproduces global equilibrium quantities and proxy functions used in stellarator optimization. Finally, the developed fast NN equilibrium model has been applied in downstream applications to obtain W7-X configurations with improved fast-particle confinement and to infer plasma parameters with self-consistent MHD equilibria at W7-X.

The multi-cell Penning–Malmberg trap concept has been proposed as a way to accumulate and confine unprecedented numbers of antiparticles, an attractive but challenging goal. We report on the commissioning and first results (using electron plasmas) of the World's second prototype of such a trap, which builds and improves on the findings of its predecessor. Reliable alignment of both ‘master’ and ‘storage’ cells with the axial magnetic field has enabled confinement of plasmas, without use of the ‘rotating wall’ (RW) compression technique, for over an hour in the master cell and tens of seconds in the storage cells. In the master cell, attachment to background neutrals is found to be the main source of charge loss, with an overall charge-confinement time of 8.6 h. Transfer to on-axis and off-axis storage cells has been demonstrated, with an off-axis transfer rate of 50% of the initial particles, and confinement times in the storage cells in the tens of seconds (again, without RW compression). This, in turn, has enabled the first simultaneous plasma confinement in two off-axis cells, a milestone for the multi-cell trap concept.