Electrostatic turbulent structures in TORPEX plasmas

Experimental data from many Tokamaks suggest that an important fraction of radial particle and heat transport in the Scrape-Off-Layer is due to poloidally-localized propagating structures of increased plasma density (blobs). A better understanding of the mechanism leading to blobs might help to lower the radial transport and control damages to plasma facing components. Such structures are experimentally investigated in the basic toroidal device TORPEX. Ion saturation current measurements from a two-dimensional imaging probe are analyzed using conditional sampling (CS). This technique is extensively studied and a large variety of trigger conditions is used to detect coherent structures in the turbulent plasma. Two scenarios are observed which lead to the ejection of blobs. The role of the density gradient in the blob ejection mechanism is investigated. In a first step, the evolution of the gradients leading to blobs is studied by triggering on such events. Their magnitude increases with the size of the blob. In a second step, these gradients are used directly as reference signals for CS. Strong gradients are found to have an enhanced probability to give rise to a blob. The location of the source before a possible blob ejection is also determined. It is concluded that the effect of the source is to steepen up the gradient in a uniform way along the vertical direction

Basic Investigation of Turbulent Structures and Blobs of Relevance for Magnetic Fusion Plasmas

Similarly to neutral fluids, plasmas often exhibit turbulent behavior. Turbulence in plasmas is usually more complex than in neutral fluids due to long range interactions via electric and magnetic fields, and kinetic effects. It gives rise to many interesting phenomena such as self-generated magnetic fields (dynamos), zonal-flows, transport barriers, or particle pinches. Plasma turbulence plays a crucial role for the success of nuclear fusion as a potentially clean, safe, and long-term source for electric power production. Turbulent processes in the edge and scrape-off layer (SOL) of magnetic fusion plasmas determine, to a large extent, the overall confinement properties. They also influence the life time of plasma facing components, impurity production and influx, main chamber recycling, tritium retention, and helium ash removal. Edge turbulence is often dominated by blobs or filaments, magnetic-field-aligned plasma structures observed in the edge of virtually all magnetized plasmas. This thesis investigates basic aspects of edge turbulence and blobs in simple magnetized toroidal TORPEX plasmas. TORPEX includes important ingredients of SOL physics, such as pressure gradients, "∇B" and curvature of the magnetic field, together with open field lines. A relatively simple magnetic geometry, full diagnostics access and the possibility of controlled parameter scans allow isolating and studying instabilities and turbulence effects that occur in more complicated forms in fusion and astrophysical plasmas. Using a number of optimized probe diagnostic methods, the mechanisms for the generation of blobs from ideal interchange waves and for their subsequent propagation are elucidated. A blob velocity scaling law is introduced that takes into account several damping effects of blob cross-field velocity. This scaling law is in good agreement both with blob simulations and experiments on TORPEX. Studies on blob parallel dynamics shed light on blob induced parallel currents and the transport of parallel momentum. Based on this understanding of blob motion, several tools to influence blobs and turbulence as a whole are developed. A methodology for plasma turbulence code validation is established. Using a large set of observables, the agreement between experiments and both 2D and global 3D two-fluid simulations is quantified

Existence of subsonic plasma sheaths

The location of the plasma sheath edge, where quasineutrality is broken, is rigorously derived by using a kinetic description of the plasma. It is shown that sheaths can exist with arbitrarily small ion velocity at the sheath edge, thus violating the Bohm criterion, Vi = cs at the sheath edge. Bohm’s criterion is recovered in the case of large enough ion current through the wall, and it is found to be a reasonable approximation in floating potential conditions. However, in the case of a predominant electron current through the wall, Bohm’s criterion is not able to describe the sheath-edge transition. The analytical results are supported by numerical simulations performed with a fully kinetic particle-in-cell code modeling a source-driven, weakly collisional plasma, bound between two absorbing walls

Assessment of Human Factors After Advanced Life Support Courses Comparing Simulated Team and Real Team Assessment: A Randomized Controlled Cohort Trial.

Aim Human factors are essential for high-quality resuscitation team collaboration and are, therefore, taught in international advanced life support courses, but their assessment differs widely. In Europe, the summative life support course assessment tests mainly adhere to guidelines but few human factors. This randomized controlled simulation trial investigated instructors' and course participants' perceptions of human factors assessment after two different summative assessments. Methods All 5th/6th-year medical students who attended 19 advanced life support courses according to the 2015 European Resuscitation Council guidelines during one study year were invited to participate. Each course was randomized to either: (1) Simulated team assessment (one instructor simulates a team, and the assessed person leads this "team" through a cardiac-arrest scenario test); (2) Real team assessment (4 students form a team, one of them is assessed as the team leader; team members are not assessed and act only on team leader's commands). After the summative assessments, instructors, and students rated the tests' ability to assess human factors using a visual analog scale (VAS, 0 = no agreement, 10 = total agreement). Results A total of 227 students participated in the 1-day Immediate Life Support courses, 196 students in the 2-day Advanced Life Support courses, additionally 54 instructors were included. Instructors judged all human factors significantly better in real team assessments; students rated leadership and situational awareness comparable between both assessments. Assessment pass rates were comparable between groups. Conclusion Summative assessment in real teams was perceived significantly better to assess human factors. These results might influence current summative assessment practices in advanced life support courses

Boundary conditions for plasma fluid models at the magnetic presheath entrance

For the first time, a rigorous definition of the magnetic presheath entrance (MPSE) is provided, as the location where the drift-reduced approximation breaks down. We consider a weakly collisional electrostatic plasma with cold ions in contact with an absorbing wall in the presence of ExB drifts. We provide expressions at the MPSE for the parallel ion and electron velocities, the gradients of plasma density and potential, and the vorticity. In particular, we show that the plasma potential with respect to the wall increases when the angle of incidence of the magnetic field is smaller. A fully kinetic PIC code simulating the plasma wall transition has been developed to validate these local relations, showing an excellent agreement with the theory. This work represents a first step towards a complete formulation of the boundary conditions for fluid codes used to simulate the edge of magnetic confinement devices

PeroxiBase: a database with new tools for peroxidase family classification

Peroxidases (EC 1.11.1.x), which are encoded by small or large multigenic families, are involved in several important physiological and developmental processes. They use various peroxides as electron acceptors to catalyse a number of oxidative reactions and are present in almost all living organisms. We have created a peroxidase database (http://peroxibase.isb-sib.ch) that contains all identified peroxidase-encoding sequences (about 6000 sequences in 940 organisms). They are distributed between 11 superfamilies and about 60 subfamilies. All the sequences have been individually annotated and checked. PeroxiBase can be consulted using six major interlink sections ‘Classes', ‘Organisms', ‘Cellular localisations', ‘Inducers', ‘Repressors' and ‘Tissue types'. General documentation on peroxidases and PeroxiBase is accessible in the ‘Documents' section containing ‘Introduction', ‘Class description', ‘Publications' and ‘Links'. In addition to the database, we have developed a tool to classify peroxidases based on the PROSITE profile methodology. To improve their specificity and to prevent overlaps between closely related subfamilies the profiles were built using a new strategy based on the silencing of residues. This new profile construction method and its discriminatory capacity have been tested and validated using the different peroxidase families and subfamilies present in the database. The peroxidase classification tool called PeroxiScan is accessible at the following address: http://peroxibase.isb-sib.ch/peroxiscan.ph

Convective cells and blob control in a simple magnetized plasma

Blob control by creating convective cells using biased electrodes is demonstrated in simple magnetized toroidal plasmas. A two-dimensional array of electrodes is installed on a metal limiter to obtain different biasing schemes. Detailed two-dimensional measurements across the magnetic field reveal the formation of a convective cell, which shows a high degree of uniformity along the magnetic field. Depending on the biasing scheme, radial and vertical blob velocities can be varied significantly. A high level of cross-field currents limits the achievable potential variations to values well below the applied bias voltage. Furthermore, the strongest potential variations are not induced along the biased flux tube, but at a position shifted in the direction of plasma flows

Convective cells and blob control in a simple magnetized torus

In view of controlling wall and divertor heat loads in magnetic fusion devices, we investigate the possibility of creating convective cells by means of biased electrodes for turbulence and blob control in the simple magnetized toroidal plasmas of TORPEX. A two-dimensional array of 24 electrodes is installed on a metal limiter to test different biasing schemes. This allows influencing significantly the frequency of the dominant mode as well as radial and vertical velocities of blobs. Detailed measurements along and across the magnetic field provide a rather clear picture of the effect of the biasing. The biased electrodes produce perturbations of the plasma potential and density profiles that are fairly uniform along the magnetic field. Background flows influence the location where potential variations are induced. The magnitude of the achievable potential variations in the plasma is strongly limited by cross-field currents. A quantitative discussion on the origin of these currents is presented

Sheath boundary conditions for plasma fluid models

A new definition of the sheath edge is rigorously derived taking into account the kinetic properties of the plasma, and a consistent set of local sheath edge conditions is presented for the case of a magnetic field perpendicular to the wall. These local boundary conditions give explicit expressions for the ion velocity, the electron velocity, and the electron heat flux at the sheath edge, which can be easily implemented in a fluid code. It is shown that in the case of positive current to the wall, the commonly used Bohm's relations well aproximate the proposed boundary conditions, while large discrepancies are observed for negative currents. A fully kinetic PIC code simulating the plasma wall transition has been developed to validate these local relations, showing an excellent agreement with the theory. This work represents a first step towards a complete formulation of the sheath edge local boundary conditions for a general magnetic geometry

Investigating the impact of the molecular charge-exchange rate on detached SOLPS-ITER simulations

Plasma-molecular interactions generate molecular ions which react with the plasma and contribute to detachment through molecular activated recombination (MAR), reducing the ion target flux, and molecular activated dissociation (MAD), both of which create excited atoms. Hydrogenic emission from these atoms have been detected experimentally in detached TCV, JET and MAST-U deuterium plasmas. The TCV findings, however, were in disagreement with SOLPS-ITER simulations for deuterium indicating a molecular ion density ($D_2^+$) that was insufficient to lead to significant hydrogenic emission, which was attributed to underestimates of the molecular charge exchange rate ($D_2 + D^+ \rightarrow D_2^+ + D$) for deuterium (obtained by rescaling the hydrogen rates by their isotope mass). In this work, we have performed new SOLPS-ITER simulations with the default rate setup and a modified rate setup where ion isotope mass rescaling was disabled. This increased the $D_2^+$ content by $> \times 100$. By disabling ion isotope mass rescaling: 1) the total ion sinks are more than doubled due to the inclusion of MAR; 2) the additional MAR causes the ion target flux to roll-over during detachment; 3) the total $D\alpha$ emission in the divertor increases during deep detachment by roughly a factor four; 4) the neutral atom density in the divertor is doubled due to MAD, leading to a 50\% increase in neutral pressure; 5) total hydrogenic power loss is increased by up to 60\% due to MAD. These differences result in an improved agreement between the experiment and the simulations in terms of spectroscopic measurements, ion source/sink inferences and the occurrence of an ion target flux roll-over