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In this work, an overview of the neutral gas pressures and particle exhaust in the subdivertor of the stellarator Wendelstein 7-X during the last two experimental campaigns is presented for different magnetic field configurations.
The particle exhaust, which depends on the neutral gas pressure as well as the available pumping capacity was analyzed regarding the newly installed cryo-vacuum pumping system after characterizing its pumping speed for different gases in the current neutral gas pressure regime of Wendelstein 7-X. The analysis of the neutral gas pressures shows that the pressures currently reached in the subdivertor of Wendelstein 7-X correspond to the molecular flow regime, in which the cryo-vacuum pumps are not operated efficiently.
A variety of tools has been developed to simulate the neutral gas pressure and particle exhaust in the current divertor geometry and in the future, modified divertor geometries in order to improve the divertor geometry with respect to its particle exhaust properties. Direct Simulation Monte Carlo calculations as well as a ray-tracing approach used to simulate the neutral gas pressure in the molecular flow regime confirm the neutral gas pressures measured during plasma operation. A multi-chamber model of the subdivertor based on the conductance of the subdivertor space is presented as an analytical method to assess the neutral gas pressures in different parts of the subdivertor region without the need of a complete computer-aided design of the subdivertor geometry.
Apart from modifications to the divertor geometry, the extended two-point model for
stellarators has been used to explore how varying upstream plasma parameters affect
the plasma density at the divertor targets—and consequently, the neutral gas pressure and particle exhaust in the subdivertor. These findings can be utilized to develop plasma scenarios that facilitate particle exhaust.

This work presents an exploration into the use of the near-axis expansion
(NAE) method, a first-order expansion of the magnetohydrodynamic
equations, to construct quasi-isodynamic (QI) stellarators with
low neoclassical confinement, at finite aspect ratios.
QI stellarators are attractive fusion reactor candidates due to their
inherent steady-state operation, low toroidal currents and favorable
turbulence properties. However, the plasma boundary optimization
method traditionally used to find QI fields is highly dependent on
the initial guess used, often results in complicated geometries and
does not offer any physical insight into the solution space structure.
Extensions to the NAE theory for the case of QI, stellarator symmetric
fields are presented. This formalism allows the direct and efficient
construction of configurations with low neoclassical transport
and simple boundary shapes, far from the axis, by carefully choosing
the initial NAE parameters. A discussion about the influence of such
parameters into the physical properties of the resulting configuration
is included.
Equilibria constructed using the NAE formalism is defined by a
set of physically intuitive input parameters. This parametric nature
is exploited to shed light on the QI solution space structure. The role
the helicity of the magnetic axis plays in dividing the space in regions
with different confinement properties is described and it is used to
construct NA QI solutions with up-to five field periods.
The use of optimization procedures within the space defined by the
parameters of the NAE is described, providing a systematic method
to generate QI configurations with specific properties. Finally, the
main strengths of the NAE are discussed, namely its suitability to
provide a variety of initial points for traditional optimization, and
the ability to perform systematic and exhaustive search of the QI solution
space, aiding in the search of the next generation stellarator
designs.

Broadband Alfvénic excitation & mode characterization in the Wendelstein 7-X stellarator plasmas
(2024)

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.

At the interface between biology and physics, cell mechanics has been proven to be a sensitive tool essential to understand the functional roles of the cells. In addition, it has been employed to detect pathological conditions and understand the predominant contribution of the cytoskeleton and cell signaling in the onset of diseases. While multitude of reports are available that tried to understand the differences in cell mechanics as a response to changes in biological stimuli, very few studies tried to unveil the effects of physical parameters on cell mechanics. In this thesis, I particularly question the relevance of physical parameters, which are known to be implicated in biological phenomena, for cell mechanics.

Physics-regularized Machine Learning To Approximate 3D Ideal-MHD Equilibria At Wendelstein 7-X
(2024)

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.

In course of the recent results from Wendelstein 7-X, stellarators are on the brink for assessing their maturity as a fusion reactor. To this end, stellarator speciﬁc transport regimes need detailed exploration both with appropriate systematic experimental investigations and models. A way to enhance the efﬁciency of this process is seen in an systematic evaluation of existing experimental data. We propose appropriate tools developed in information theory for examining large datasets. Information entropy calculations, that have proven to assist the systematic assessment of datasets in many other scientiﬁc ﬁelds, are used for novelty detection.
Potentially, as a ﬁrst use-case of this holistic process, this thesis attempts to link and to develop approaches to examine the stellarator speciﬁc core-electron-root-conﬁnement (CERC) regime. The speciﬁc interest for CERC emerges from the behavior of the radial electric ﬁeld. While ion-root conditions exhibit negative radial electric ﬁelds, CERC’s positive ﬁeld in the very core of fusion grade plasmas adds an outward thermodynamic force to high-Z impurities and could add to potential actuators to control impurity inﬂux as to be examined for full-metal wall operation in large stellarators. Recently, this feature received revived intent for reactor scale stellarators.
Also, in this work, parameter regions close to the transition from ion-root to CERC are
examined. At lower rotational transform (a characteristic feature of the magnetic ﬁeld conﬁning fusion grade plasmas), transitions were detected when the plasma current evolved. As in smaller stellarators, it is concluded that low-order rationals and magnetic islands are related to the transitions. This is widely supported by extensive MHD simulations which ﬁnally provide indications for the role of zonal ﬂow oscillations. As one of the outcomes, gyrokinetic instabilities are seen interacting for the ﬁrst time with the neoclassical mechanisms in experiments.
In order to cope with the vast number of highly sampled spatio-temporal plasma data, new
techniques for novelty detection are required. Fundamental prerequisites for the detailed
physics investigations were the feasibility study of entropy-based data analysis techniques, and their adaptation to detect previously unrevealed transition mechanisms. These tools were applied to multivariate bulk plasma emissivity data, which allowed the exploration of large parameter spaces and provided insights in the spatio-temporal dynamics of CERC transitions.
In this manner, this research highlights the feasibility of information ﬂow measure analysis in fusion studies. Applications of different entropy-based complexity measures are explored and this work sheds light on the capabilities, added value and limitations of these techniques. This investigation presents the integration of information ﬂow measures to gain deeper understanding of plasma transport phenomena, by providing an approach to fast systematic data mining suited for real-time analysis. This work paves the way for further development and implementation of information-theoretic methods for plasma data analysis.
In summary, this research highlights the gained insight on CERC transitions, while showcasing the feasibility, added values and limitations of information ﬂow measure analysis for fusion studies, to induce theory based analysis revealing new insights in fundamental, stellarator-speciﬁc transport mechanisms.

Research into nuclear physics has enjoyed a long and rich history since the earliest experiments began investigating atomic constituents. The discovery of the atomic nucleus in the early 20th century started a complex field of research that has undergone many transformations with the advancements of modern technology. Today, atomic nuclei are not only studied to advance our understanding of the strong force but also to gain more information on the synthesis of elements in the universe, to exploit nuclear decay to investigate the weak interaction, and to search for physics beyond the standard model.
In this work, we will study the strong force in atomic nuclei, i.e. the way nucleons (protons and neutrons) arrange themselves in a many-body system governed by the repulsive Coulomb interaction and the attractive strong interaction. In particular, we will focus on nuclear structure near nuclei with a "magic number" of Z protons and N neutrons, so-called doubly-magic nuclei, exhibiting a particularly stable configuration with respect to neighboring nuclei.
Within the nuclear shell model, similar to the atomic shells, the magic numbers indicate shell closures accompanied by energy gaps. Nuclei at double-shell closures and their direct vicinity provide an important playground to benchmark nuclear theories and models that aim to predict the intricate interplay of the nucleons that lead to enhanced nuclear binding energies, significant changes in charge radii and transition strengths, etc.
Of particular interest are nuclear isomers, long-lived excited states, in which the nucleon configuration with respect to its ground state is altered, resulting in a modification of their properties despite having the same number of protons and neutrons.
The main part of this work consists of three publications, which report on nuclear structure investigations through mass measurements and laser spectroscopy near the doubly magic nuclei nickel-78, tin-100, and lead-208.
The nuclides investigated in this work include neutron-deficient indium isotopes, neutron-rich zinc isotopes, and neutron-rich mercury isotopes.

This thesis presents the production of polyanionic clusters within two ion storage devices:
Considering a Penning trap, the accessible range of polyanionic aluminium clusters has been expanded up to the 10th charge state. In particular, abundance curves for clusters with 5 to 9 excess electrons have been measured for the first time and analysed with respect to their lifetime-dependent appearance sizes. These sizes reveal a nearly quadratic dependency on the charge state for experimentally accessible lifetimes.
Additionally, the production of polyanionic clusters has been enabled in a radiofrequency ion trap. Therefore, the transition from a harmonic to a digital 2- and 3-state guiding signal has been investigated with respect to the ion storage. The passing of electrons through the trap during field-free periods of the guiding signal led to the first production of polyanionic clusters within a radiofrequency ion trap.

In this thesis, I was able to provide answers to transport processes in lipid monolayers, which are ultimately, all of biological relevance. In particular, I was interested in lipid oxidation and dynamic compression/expansion processes of surfactant monolayers at the air-water interface:
Lipid oxidation was shown to be a consequence of the formation of a high concentration of reactive oxygen species (ROS) during cell respiration, which finally can lead to severe cell damage. It is not yet understood clearly, which part of the lipid molecules is especially prone to a ROS attack. I was particularly interested in the role of the double bonds of the acyl chains of the lipid molecules during oxidation. Further, I wanted to know the time scales of lipid interaction with the ROS.
Compared to lipid vesicles, lipid monolayers have the advantage that many parameters of the system can be adjusted easily. In our system, I made use of this by setting the lateral pressure to low values during H2O2 treatment, which facilitated the ROS to reach the double bonds in the acyl chains.
A prime example of biological systems out of thermal equilibrium was given in the alveolus surface, which is covered with a surfactant monolayer. During breathing, these monolayers undergo such a highly dynamic compression and expansion. Arising flows from breathing could disrupt a film and consequently, it would lose its protective role. One of my goals was to understand flows and their influence on domain shape. Dependent on the strength of the flows, I expected different growth regimes, with differing prevailing transport processes. Once understanding the underlying mechanisms in domain shaping would allow me to draw conclusions on biological systems.
In order to address these questions, I established two systems, both based on the compression of lipid monolayers. I used isotherms to study the phase behavior of the lipids:9 During compression, the lipids can undergo phase transitions from the gaseous phase to the liquid expanded phase (LE-phase) and further from the LE-phase to the liquid condensed phase (LC-phase). A coexistence regime is observed in between the LE-phase and the LC-phase, characterized by a flat increase of lateral pressure with decreasing molecular area. Some lipids exhibited LC-phase domains. These were further investigated with Brewster angle microscopy (BAM). The used BAM was equipped with an integrated Scheimpflug optics, enabling an overall focused image plane. Furthermore, time-resolved observation of the growth of the domains was possible by recording videos (20 frames per seconds).
The first system enabled the investigation of lipid peroxidation, when the lipids were exposed to ROS. I chose DMPC, POPC, DOPC and PLPC, since these are phospholipids differing in the number and position of double bonds in acyl chains, but not in the head group. I used a H2O2 enriched phosphate buffered saline (PBS) solution, which served as a precursor for more reactive ROS, like hydroxyls (.OH). PBS was chosen, since it resembles the cell environment best. During defined waiting times of H2O2 treatment, the ROS diffused vertically from the subphase towards the monolayer. The lipid molecules were in the LE-phase, which facilitated the ROS molecules to reach also the double bonds of the acyl chains. The oxidized monolayers were then compressed at constant compression speed. Since the corresponding isotherms could be measured with high precision, the relative area increase δA/A between oxidized and non-oxidized monolayer along the isotherm proved to be a good measure for lipid peroxidation. The area increase δA in the molecular area of the oxidized molecules was explained by the eventually added, more hydrophilic −OOH group at the position of a carbon atom adjacent to a double bond in the unsaturated acyl chain. The −OOH group is drawn to the hydrophilic head group of the lipid. This leads to a kink in the acyl chain, which increases the molecular area A by δA. A model, which explained this peroxidation process in lipid vesicles, could be adopted to monolayers.
I compared the oxidation of phospholipids, differing in the number and position of the double bonds of their acyl chains. I found that δA/A increased with the growing number of double bonds in one acyl chain. However, a comparison of DOPC with POPC also showed the importance of the position of the acyl chain. I determined a slow reaction kinetic. It could be estimated by a √t dependence of the number density N_surface, which denominates the ROS sticking on the monolayer. The transport of ROS towards the monolayer was found to be diffusive, because it was the slowest process in the reaction. This interpretation was reinforced by a comparison of the temperature dependence of the relative area increase δA/A with the Stokes-Einstein diffusion coefficient of water molecules. The initial ROS concentration c_0 in the trough could be traced back (c_0~ 50 nM), which is indeed a realistic value found in human cells.
Concluding, our results can be understood as a feasibility study. The complexity of the monolayer can be arbitrarily increased, for example by the addition of proteins, allowing the investigation of other oxidative processes occurring in the cell membrane.
The second system allowed the investigation of growth of LC domains during fast compression processes of monolayers. I chose erucic acid monolayers, due to its low line tension and a continuous nucleation phase, enabling the formation of fractal domains. The monolayers were investigated with isotherms and BAM videos. Since v_C (compression speed of the monolayer) was continuous over the whole compression time, I had a system with well-defined hydrodynamic conditions. This allowed me a complete analysis of the system, starting with descriptive features of the observed domains to a classification of the observed growth regimes by means of hydrodynamic theory, through to the distinction and quantification of different kind of flows and supersaturations, involving Ivantsov theory:
Dependent on the compression speed v_C, I observed seaweed or dendritic domains. The LE/LC phase transition pressure pi_t was slightly increased compared to pi_inf of the equilibrium isotherm. A high compression speed v_C induced a supersaturation Δc. I introduced the excess lateral pressure Δpi=pi-pi_inf as an appropriate quantity to describe the supersaturation Δc. I showed a linear behavior of Δc on Δpi. Δc is a macroscopic quantity since it is averaged over the whole monolayer area. I characterized the domains of the seaweed and dendritic regime with respect to tip radii, branch lengths, side branch separations and fractal dimensions. I calculated the growth speed of the main branches. A roughly doubling of the growth speed of dendritic domains, compared to seaweed domains was observed. This was an evidence of adjunctive (Marangoni) flow in the subphase.
For each monolayer, I observed drifts during domain growth, which I explained by an anisotropy in the LE-phase, caused by the continuous nucleation of the domains. These kind of surface flows were superimposed to bulk flows in the subphase. Since I had a well established system, I could analyze the influence of these surface flows on domain shape, in terms of magnitude, direction and duration of the surface flows. I therefore used FFT spectra and directionality histograms. At low flows, the FFT showed six-fold symmetry. Higher drifts exhibited incisions in the FFT, eventually leading to dumbbell shaped FFTs at very high drifts. The domains grew preferentially in the direction parallel to the incision.
I used directionality histograms to analyze the angular distribution of the growing domains. They showed that the drift direction always correlated with a minimum in the histogram. In order to analyze drift duration, I split the domain in downstream and upstream side. I could show that for small drift durations, downstream growth was preferred. However, for longer drift durations, the flows got more isotropic and consequently growth was more balanced then.
I could observe only a weak correlation between drift velocity v_D and compression speed v_C. However, dendrites were formed when the compression speed v_C was high, while seaweed domains were formed when v_C was small. Domain distortion occurred in the same way, independent if seaweed or dendritic domains were considered. I further showed that hydrodynamic flows in the subphase and surface flows are superimposed and scale differently. Consequently, they have different impact on domain shape: hydrodynamic flows act on μm scale and influence the domain morphology (distance between side branches, and tip radius) and the growth speed of the main branches. Surface flows act on the mm to cm scale, cause an anisotropic flow in the LE phase surrounding the domain, and thus affect the overall domain shape.
The anisotropy in the LE-phase led to a locally different degree of supersaturation. To take this into account, I introduced a local normalized supersaturation Δ, based on the Ivantsov solution. Therefore, I calculated Péclet numbers p of measured quantities of the system. I obtained values of 0.88 ≤Δ≤0.90 for the seaweed regime (p<5) and 0.93 ≤Δ≤0.96 for the dendritic regime (p>6). Since the Ivantsov solution can only be applied for purely diffusive processes, I applied a modified Ivantsov solution Δ_mod, which calculates Δ at a distance 𝛿 ahead of the dendrite tip. I was able to determine the progression of the diffusive layer 𝛿, however a quantitative determination failed.
Applying hydrodynamic theory allowed me to classify the two growth regimes with respect to the Boussinesq number Bq. Since for both growth regimes, I achieved values of Bq<1, bulk viscous losses dominated over surface viscous losses. Further, a cross-over length 𝜉 was calculated, from which one can distinguish, whether advective transport dominates over diffusion.
I further connected the two defined supersaturations Δ and Δc via the excess lateral pressure Δpi. From this, I saw differences in the seaweed and dendritic growth regimes: The local normalized supersaturation Δ of seaweed growth seemed to be quite stable for a further increase of the lateral excess pressure Δpi, whereas it reacted quite sensitive in the dendritic regime. This was found to be an indication of a non-equilibrium regime, caused by the strong coupling of the monolayer to the subphase. It reinforces therefore the theory of Marangoni-flow.
The findings of this thesis emphasize the importance of understanding highly dynamic compression/expansion processes arising in surfactant monolayers. Using the example of the compression of the alveolus surface, it can be seen that a more realistic model of the pulmonary alveolus is not only enabled by increasing the complexity of the surfactant monolayer (e.g. by adding specific proteins or lipid mixtures to the monolayer). Equally important is the understanding in transport processes and the consequences for the monolayer structure. By the analysis of domain shapes, I presented a method, which is suitable for such a study.