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May 11th, 2023
Designing divertors for fusion reactors through shape optimization, by Wouter Dekeyser (Katholieke Universiteit, Leuven)


Abstract: The exhaust of power and particles from the reactor through the divertor remains a key challenge for the development of safe, reliable fusion power plants. To assist divertor design, one usually has to resort to plasma edge codes as SOLPS-ITER to extrapolate the current understanding of divertor exhaust physics towards the operational windows anticipated in reactors. However, due to the complex physical interactions, uncertain parameters and the large number of design variables, this design process is computationally extremely demanding, and often requires tedious manual iterations. 

To facilitate the procedure, in this talk divertor design is reformulated as a shape optimization problem, and solved efficiently using adjoint-based, one shot methods. With this automated approach, divertor designs are achieved that optimally spread the power across the targets, at a cost of only a few plasma edge simulations. Moreover, it is demonstrated how the method is able to compute accurate sensitivities in the presence of statistical Monte Carlo noise. Progress towards integration of the methodology in the widely-used SOLPS-ITER code and robust divertor design is discussed.

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May 11th, 2023
SOL-divertor plasma fluid simulations based on the anisotropic ion pressure with a virtual divertor model, by Satoshi Togo (University of Tsukuba, Japan)


Abstract: Improving scrape-off layer (SOL)-divertor plasma fluid models is highly required to design DEMO divertors. As an alternative to the conventional Braginskii’s plasma fluid model, we have been developing one based on the anisotropic ion pressure which we call AIP model. The parallel viscous flux term, which is an approximation of the anisotropic component of the ion pressure, is excluded in AIP model by directly describing the fluid equations in terms of the parallel and perpendicular ion pressures. That also makes it possible to exclude an explicit use of the Bohm criterion as the sheath boundary condition. Instead, a virtual divertor (VD) model has been incorporated within AIP model which reproduces effects of a sheath region by setting artificial volumetric sink terms in a virtual region beyond a sheath edge. It is demonstrated that the Bohm criterion can be automatically satisfied including some supersonic flow cases. A direct comparison between the Braginskii’s and AIP model shows that there can be a qualitative deviation of solutions between them in a rare collisional plasmas.
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May 4th, 2023
Design of a divertor for HSX, by Dieter Boeyaert (University of Wisconsin, USA)


Abstract: The Helically Symmetric eXperiment (HSX) is a stellarator at the University of Wisconsin-Madison which is optimized for quasi-helical symmetry. As part of a proposed upgraded, it is investigated what is the best way to include a divertor. Starting from a selected magnetic equilibrium, a new vessel wall is proposed. With field-line tracing using FLARE, and plasma edge simulations with EMC3-Lite and EMC3-EIRENE, the divertor location is determined. In a next step, the shape of the divertor will be investigated to optimize the power and neutral particle exhaust. In the talk, the focus is on determining the locations for the divertor.
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April 27th, 2023
My experience with ill-posed problems (tomography, deconvolution), by Jan Mlynář (Czech Technical University in Prague, FUSION-EP Prague coordinator)


Abstract: In tokamak data analyzes we are often dealing with ill-posed problems where sparse or erroneous data may seriously hamper the analysis result and produce artifacts. In my talk I shall briefly define ill-posedness, explain the cases of plasma tomography and spectral deconvolution and outline some basic ideas on how to treat the ill-posed problems using constraints. I will also explain the procedure that allowed me to consider the results of my analyses dependable and reliable contributions to tokamak data analyses.
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April 18th, 2023
Spacecraft heat shields and meteoroids in the DIII-D tokamak, by Dmitri M. Orolov (Center for energy Research, UC San Diego)


Abstract: The development of materials that can withstand extreme heat flux environments is crucial for both the successful exploration of the solar system and the realization of fusion energy. To this end, a study of carbon ablation in high heat plasma relevant to hypervelocity spacecraft entries was conducted in the DIII-D tokamak as part of the Frontiers in Plasma Science campaign. Spacecraft heat shields that can withstand high heat flux are required for exploration missions to the gaseous giants and for hyperbolic re-entries into the Earth's atmosphere. However, testing and modeling material performance in this regime is challenging due to the lack of adequate ground testing facilities. Nonetheless, we showed that conditions in the DIII-D L-mode edge plasma can reproduce the flow velocity and high heat flux experienced during the Galileo probe's entry into the atmosphere of Jupiter.
In this study, three types of experiments were conducted using stationary graphite rods, porous carbon spherical pellets, and glassy carbon spherical pellets. In each case, the mass loss rates as a function of heat flux was determined from an extensive array of spectroscopic measurements and compared against several semi-empirical ablation models obtained from spacecraft flight data. Additionally, the experimental results for the pellet trajectories and mass loss rates of the porous and glassy carbon pellets were confirmed using the UEDGE-DUSTT simulations. The experiments further revealed that the glassy pellets shatter fragments with sharp edges due to thermo-mechanical stress, which is similar to what was observed during the Galileo probe's entry into the Jovian atmosphere. This process, called spallation, is expected to affect the survivability of both spacecrafts and meteors in the Earth's atmosphere. Finally, coefficients learned from experiment and the semi-empirical models were used in simulations to investigate the ablation of different carbonaceous objects as they enter Jupiter's atmosphere.

The results of this study demonstrate that scaling between DIII-D experiments, available flight data, and numerical models can be used to optimize heat shields for future planetary missions. We plan to extend these studies to meteoroid entries and investigate the possibility of organics formation in meteoroid tails during atmospheric entries.

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April 6th, 2023
Integrated core transport modelling of NSTX plasmas using the OMFIT workflow, by Galina Avdeeva (Oak Ridge Associated Universities, General Atomics)


Abstract: A numerical plasma modeling provides the most accurate representation of the experimental reality when various models are integrated in a way that enables the determination of the most consistent solution. The OMFIT framework provides a convenient user-friendly interface to combine various codes into an integrated workflow with opportunities for the device specification, many options of data visualization and modeling/experiment comparison. In this work, such a workflow: from an equilibrium reconstruction to the plasma profiles prediction will be demonstrated in applications to a heat plasma transport study on the low aspect ratio NSTX tokamak. Spherical tokamaks are one of the leading concepts for the design of future fusion power pilot plants and the analysis of NSTX plasma helps to determine the optimal aspect ratio for a next-step fusion facility.

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March 13th, 2023
Using gyrokinetics to inform spherical tokamak power plant design, by Robert Davies


Abstract: Now is an exciting time for magnetic confinement fusion, with a great deal of private and public interest in a variety of reactor concepts. However, a major consideration for the design and operation of commercially viable fusion power plants is plasma turbulence, which constrains the energy confinement, density and temperature in the plasma. In this talk, I describe how plasma turbulence (and the spatially small instabilities which drive it, called "microinstabilities") can be simulated using gyrokinetic codes. These simulations can be used to understand and predict experimental results, but also to assess the viability of hypothetical fusion plasmas. In this way, gyrokinetics can be used to influence reactor design. As a specific example of this, I describe how a particular microinstability (the "kinetic ballooning mode") provides a constraint on the plasma shape for commercially viable spherical tokamak (ST) power plants.


March 7th, 2023
Molecular plasma spectroscopy in the JET tokamak, by Ewa Pawelec


Abstract: Magnetic confinement fusion ordinarily is not connected with molecular spectroscopy, because most of the interest is directed at the hot core and confining pedestal. Nevertheless, the plasma spreads out of those regions and, at certain point touches the walls, so at in this region it is, by every measure, a low-temperature, if certainly very specific plasma. In the regions close enough to the vessel walls, the nearly-total ionization of the core and pedestal regions is not present anymore, and atoms, molecules and molecular ions strongly contribute to the overall mixture. Their presence influences both the overall plasma behavior and the vessel walls erosion, which contributes to the impurities permeating the pedestal and core plasma. Most important molecules in the magnetic confinement fusion are the hydrogen-containing ones, from the hydrogenic species in different isotopic combinations (H2, D2, T2 and mixed) to all kinds of hydride, created where the hydrogenic plasma encounters other elements. Those other elements can be present in the walls, such as beryllium, tungsten, boron or carbon, or be one of the seeded impurities, like nitrogen. In those reactions different hydrides may be created. Most important are the metallic hydrides, especially BeH/D/T, which contribute to the wall erosion by a process called CAPS (Chemical Assisted Physical Sputtering), which is also a process which may be detrimental both to the walls and to the pedestal and core plasma. The hydrogenic molecules are very important e.g. in the behavior of the divertor, where the detachment conditions are strongly affected by molecular processes. Spectroscopic study of molecules is not simple, because the spectra are complex, but on the other hand, it provides data on the creation process of the light-emitting molecular states. In this presentation, examples of the spectra and their analyses will be taken from JET experiments and comprise different isotopologues of hydrogen molecule and beryllium and nitrogen hydride. It will also be shown how those results contribute to better understanding of different processes happening in the low-temperature regions of the magnetic confinement fusion plasma.
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