October 7th, 2021
Integrated modelling of impurity transport on WEST, by David Sosa (FUSION-EP 2019)
Abstract: The development and validation of integrated modelling tools that can be applied to the initial current ramp-up phase in Tokamaks is necessary to design a suitable current ramp for fusion scenarios, that lead to peaked and current density profiles, maximizing core heating and current diffusion in order to avoid radiative collapse and MHD instabilities. Since a good understanding of current diffusion physics is needed, in this work we first develop a simple current diffusion model to highlight how different parameters affect this process. Then we solve the complete resistive current diffusion equation using METIS to estimate the changes in the effective charge, Zeff , induced experimentally by Nitrogen seeding in plasmas of the WEST Tokamak. This study indicates that the increment is only weakely dependent on the Nitrogen puffing rate and suggests that the profile is slightly peaked. Transport analysis is then performed using QuaLiKiz to explore more in detail how Nitrogen affects turbulent transport and micro-stability. It is found that Nitrogen injection enhances TEM modes and stabilises ETG near the plasma edge and increases the particle and electron heat fluxes.
September 30th, 2021
Pellet dynamics in Stellarator TJ-II from fast camera recordings to 3D pellet trajectory, by Daniel Medina (FUSION-EP 2019)
Abstract: Currently, cryogenic pellet injection is the leading core fueling technique, since it allows deeper fuel penetrations than conventional gas puffing and does not introduce an energy source in the plasma core. Despite this, some involved mechanisms, particularly for stellarators, remain unclear. For instance, pellet deflections have been observed in both stellarators and tokamaks and several models have been developed to explain this process. In tokamaks, a radial acceleration is usually observed, but predictions tend to overestimate the experimental observations. In contrast, in stellarators the situation is more complex, since pellets are also deflected in the vertical direction. In this work, pellet acceleration experiments are carried out in on-axis electron cyclotron resonance heated hydrogen plasmas in the TJ-II stellarator. The three-dimensional pellet trajectory and the different components of the pellet acceleration, assumed to be constant, are studied using the TJ-II double-bundle fast camera system. This allows estimating the dependency of the different components of the pellet acceleration on the plasma current, electron density and temperature, as well as pellet mass and injection velocity. In addition to the above, experimental results are compared with predictions from the stellarator version of the HPI2 code, including pellet acceler- ation. This is done by using the 1st Principles and Semi-empirical models in HPI2, recently extended to stellarators.
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May 21st, 2021
The promise of a low recycling boundary, by Anurag Maan
Abstract: The Lithium Tokamak eXperiment-beta (LTX-beta) is attempting to answer key questions regarding the feasibility of operating a tokamak with a lithium boundary. Lithium coatings on high-Z plasma facing components (PFCs) in the LTX (LTX-beta predecessor) led to flat temperature profiles. The flat temperature profiles were observed along with a hot low density edge, implying a broad, collisionless Scrape-Off Layer (SOL). Additionally, in-vacuo analysis of PFCs indicated that evaporatively deposited lithium coatings appeared to be oxidized, while the ability to achieve good plasma performance was retained. Theory attributes flat temperature profiles to low recycling walls, which was assumed to be due to hydrogen binding with elemental lithium to form lithium hydride. The presence of oxidized lithium, however, raises questions regarding the exact mechanism of hydrogen retention in LTX. These questions were investigated using a new Sample Exposure Probe (SEP) for detailed in-vacuo analysis of PFC samples was designed and commissioned for LTX-beta. The SEP is equipped with a vacuum suitcase capable of transporting samples representative of LTX-beta outer mid-plane PFCs under high vacuum to a stand-alone high resolution XPS system. Surface analysis using the SEP was performed with sufficient energy resolution to identify for the first time, the compounds that grow on evaporative lithium coatings inside a tokamak. This was the first demonstration that a vacuum suitcase can afford a solution that is simpler in design and affords more flexibility than building material characterization test stands for installation on a tokamak. The results indicate that Lithium Oxide and Lithium Hydroxide are prime surface constituents of Lithium PFCs. Their presence substantiates the hypothesis that lithium oxide grows on elemental lithium before the growth transitions to lithium hydroxide for LTX-beta like vacuum conditions. It is further indicated that Lithium Oxide improves plasma performance in comparison to Lithium Hydroxide by both sequestering oxygen and increasing hydrogen retention. The talk will also outline current efforts that aim at estimating recycling in LTX-beta.
Join the talk via the Zoom link fusion.yt/as on 21/05/2021 at 15:00 (Prague). Password: E71AF0A7
April 27th, 2021
Energy balance during disruptions, by Nicola Isernia.
Abstract: In the present talk we shall study the global energy transfer for a fusion plasma undergoing a disruption, by the means of first principles and evolutionary MHD equilibrium models. The key role of the conducting structures surrounding the device will be highlighted, giving insight in the time constants which are relevant to the global energy transfer. Reference: N. Isernia et al 2020 Plasma Phys. Control. Fusion 62 095024
Join the talk via the Zoom link fusion.yt/ar on 27/04/2021 at 18:00 (Prague). Password: 05049B6A
April 21st, 2021
Sneezing tokamaks : what to do about the boogers?, by William Gracias.
Join the talk via the Zoom link fusion.yt/ar on 21/04/2021 at 16:00 (CET)
Join the talk via the Zoom link fusion.yt/as on 27/04/2021 at 18:00 (CET)
April 7th, 2021
High-Temperature Superconducting Magnets. Open an accelerated path to fusion energy, by Erica Salazar.
Abstract: High temperature superconductors (HTS) have the ability to revolutionize the fusion energy industry by enabling tokamak fusion devices, such as the SPARC fusion device, to be smaller, lower cost, and faster to build. All fusion-class devices built to date have used either low temperature superconductors (LTS) or copper to generate magnetic fields to control and drive the plasma in a fusion device. In contrast to LTS, HTS magnets can withstand and generate significantly higher magnetic fields--leading to smaller and lower cost tokamak devices. However, introducing a new HTS material will bring new scientific challenges and risks. This presentation will highlight the risk reduction strategies and test results of the VIPER cable (a HTS cable designed by MIT and Commonwealth Fusion Systems) such as fabrication feasibility testing, load cycling, strain testing, and quench testing in SPARC-relevant conditions.
Join the talk via the Zoom link fusion.yt/aq on 07/04/2021 at 19:00 (Prague). Password: 9485E377
March 31st, 2021
Why ITER after 30 years? A case study of US deciding to stay, by Emmanouil Maragkoudakis.
Abstract: Research on burning plasmas aims to deliver a carbon free, virtually unlimited and safe power source. After more than 50 years of investigation, a nuclear fusion power plant is still a future objective. For this reason, the disposal of funds and resources on the field is often subject to criticism. By studying the 2019 Final Report of the Committee on a Strategic Plan for US Burning Plasma Research, we will try to understand the motivation and the benefits of investing on nuclear fusion research.
Join the talk via the Zoom link fusion.yt/ao on 31/03/2021 at 18:00 (CET). Password: 93112F18
March 23rd, 2021
Effects of oxide layer on Tungsten, by Mykola Ialovega (FUSION-EP 2015)
Abstract: Investigations of hydrogen isotopes and helium retention in plasma facing components (PFC) that are exposed to various plasma conditions are important for future fusion devices such as ITER and DEMO. Due to its favorable physical properties, in particular its high melting point, tungsten (W) has been chosen as the plasma-facing material of the ITER divetor. In the deuterium/tritium (D/T) phase of ITER, W PFC will be subjected to intense fluxes composed of hydrogen isotopes (HI), helium, impurities and neutrons.The presence of impurities in the edge plasma may cause redeposition or codeposition of mixed layers on the surface of the PFC W, and in the presence of residual oxygen, surface oxidation is possible due to the high temperature of the ITER divertor. Such structural modifications of W PFC may modify the properties of the material, and therefore its life expectancy, as well as its hydrogen retention, which raises safety concerns as tritium is radioactive.In this work, we used laboratory experiments involving ion implantation and thermal desorption spectrometry (TDS) technique to investigate the fundamental retention properties of HI in W PFC due to the presence of an oxide layer formed on the surface of polycrystalline W (PCW) in ITER relevant conditions. The TDS measurements were coupled with microscopy observations in order to characterize the modifications occurring on the surface and in the bulk of the material at different scales: scanning electron and confocal laser scanning microscopy techniques were used for surface observations from micrometer to nanometer scale; transmission electron microscopy was used for cross-sectional observations. Raman and X-ray spectroscopy techniques were used to characterize the structure and chemical composition of the samples.
Join the talk via the Zoom link fusion.yt/an on 18/03/2021 at 16:00 (CET). Password : 269187EB
March 18th, 2021
Tritium permeation in fusion reactors. Experiments with the hypertomate setup, by Floriane Leblond
Abstract:The presentation will start with a general introduction to the challenges faced by first wall materials with a focus on tritium retention and permeation. In a second part, the need for permeation and TDS experiments will be explained, and these experimental techniques will be further explicited. Finally, the remaining challenges (e.g. interfaces modelling) will be discussed.
Join the talk via the Zoom link fusion.yt/am on 18/03/2021 at 17:00 (CET). Password : 864D7135
Info : https://fusionep-talks.egyplasma.com/events/?e=47
March 9th, 2021
A tutorial on automatic differentiation for scientific design: practical, elegant and powerful, by Nick Greivy
Abstract:Automatic differentiation (AD) is a numerical technique for computing the derivative of a function specified as a computer program. Although AD was invented decades ago, it wasnâ€™t until the recent interest in machine learning and the associated development of high-quality automatic differentiation frameworks that the benefits of AD in physics were more widely recognized. In this tutorial, I introduce AD. By the end of the tutorial, you will hopefully understand the fundamentals of how AD works in theory and how it is used in practice. For a short, 5-minute introduction to AD, feel free to read this https://twitter.com/NMcgreivy/status/1351706692317138945?s=20 and this https://twitter.com/NMcgreivy/status/1286057985987563525?s=20
Join the talk via the Zoom link fusion.yt/al on 9/03/2021 at 18:00 (CET).
March 1st, 2021
MASTER CLASS : Energy for our future: a synergetic approach between short-term nuclear fission and long-term nuclear fusion, by Jean-Marie Noterdaeme
Abstract:Ever wondered how fusion power plants could contribute to keeping our climate change in check? Will they not come too late? We need to provide sufficient energy for the increasing needs of the world population, and at the same time, limit the global temperature rise below 1.5 °C for a livable world. This requires a major effort to de-carbonize our energy supply. Time is running short. A first step would be to require that all new plants need to be carbon-free. Renewables alone (windmills and solar panels) cannot be installed in sufficient quantities on the required time scale to cover the expected additional needs. Small modular nuclear reactors could be the solution. Enough could be built and installed if they are standardized, factory build and licensed - though it will still require a major program. Acceptance for nuclear energy could be increased since these reactors can be made passively safe. The questions of fuel supply and waste management for fission reactors can also be put to rest if nuclear fission energy is seen as a transitional measure on the way to long term fusion energy.
Join the talk via the Zoom link fusion.yt/ai on 01/03/2021 at 15:00 (CET). Password 50D09934
February 22nd, 2021
Capturing collisions with neutral nets. Faster collision operators with semantic segmentation, by Andrés Miller
Abstract: Machine learning, specifically convolutional neural networks, is used to examine the possibility of accelerating the solution to a partial integro-differential equation, the Fokker-Planck-Landau collision operator. This is part of the governing equation in the particle-in-cell code, XGC, which is used to study turbulence in fusion energy devices. In particular, training has involved borrowing network architectures used for the computer vision task of semantic segmentation. The neural network emphasizes physics-inspired learning, where it is taught to respect physical conservation constraints of the collision operator by including them in the training loss. It uses a penalization method that enforces the constraints of the system and integrates error in the conservation properties into the loss function. During training, quantities representing the density, momentum, and energy for all species of the system are calculated, mirroring the procedure in XGC. This simple training has produced a median relative loss the order of 10-4, which shows promise. The run time for the current iterative solver of the operator in XGC is O(n2), where n is the number of plasma species. As the code begins to attack problems including a larger number of species, the collision operator will become expensive computationally, making the neural network solver even more important, especially since the neural network training only scales as O(n). A wide enough range of collisionality has been considered in the training data to ensure the full domain of collision physics is captured. Further work includes expansion of the network to include multiple plasma species and the use of more sophisticated optimization techniques.
Join the talk via the Zoom link fusion.yt/ai on 22/02/2021 at 15:00 (CET). Password 067A94A1
February 18th, 2021
Capturing Coulomb Collisions A Metriplectic Bracket Approach, by Riccardo Niccolo Lorio
Abstract: Since the 1980s much effort has been devoted to the study of magnetized plasmas to better comprehend the mechanisms that undergo the confinement of particles and transport phenomena inside fusion devices. The complexity of the particles trajectories in relation to the wide gap of time scales spanning from the electron cyclotron motion to the macroscopic phenomena that occur urged the development of perturbative time-scale reduction techniques to allow us to step over the computational limitations set by the gyromotion of particles. When dealing with the description of a plasma through the Vlasov-Maxwell-Landau system, both gyrokinetic and guiding-center theory are often applied to investigate solely the Vlasov-Maxwell part and the collision operator neglected or heavily approximated. In this talk it will be provided another indication of the possible existence of a metriplectic reduction theory which sheds light over the development of a collision operator for electromagnetic reduced plasma theories. An energy and momentum conserving collisional bracket is constructed for the so-called guiding-center Vlasov-Maxwell model while discussing why extensions to drift-kinetic and gyrokinetic electromagnetic theories are so difficult. Join us for an interactive discussion :-) Info: https://fusionep-talks.egyplasma.com/
Join the talk via the Zoom link fusion.yt/ai on 18/02/2021 at 18:00 (CET). Password A8B8D30F
February 12th, 2021
Isotopic effects on confinement, by Pedro Molina Cabrera (FUSION EP 2012)
Abstract: Join me for an informal discussion about turbulence and transport in tokamaks. How good a thermos is the tokamak after all? What happens if we fill the thermos with tea or coffee: which liquid remains hotter for longer? This talk will present how local temperature fluctuations change in the core of the ASDEX-Upgrade tokamak when going from hydrogen to deuterium plasmas in an effort to understand the basic physics towards deuterium-tritium operation in future fusion reactors such as ITER and SPARC. Join us for an interactive discussion :-) Info: https://fusionep-talks.egyplasma.com/
Join the talk via the Zoom link fusion.yt/ah on 12/02/2021 at 13:30 (CET). Password 2E785CD8
February 3rd, 2021
Anticipating disruptions: insights from COPASS, JET, AUG and DIII-D, by Veronika Klevarova (FUSION EP 2013)
Abstract: Disruptions present a major challenge for high-performance discharges in ITER and tokamak-based power plants. These events are accompanied by a sudden loss of magnetic confinement and, as such, pose a considerable threat to the integrity of the fusion machine. Over the course of my PhD, I collected an extensive database of disruptive discharges from COMPASS, AUG, JET and DIII-D to study the signs of a catastrophic end in fusion plasmas. In this upcoming FusionEPTalks, I will introduce an analytical model of the rotating magnetohydrodynamic modes that appear during braking and wall locking ahead of a disruption. We will look at the steps which were taken to validate this model. You will thereby understand how several factors can influence the mode duration, a key figure for disruption prevention. The corresponding scaling law, derived from this work, yields predictions for ITER from hundreds to thousands of milliseconds. This bodes well for the timely deployment of a mitigation strategy. Info: https://fusionep-talks.egyplasma.com/
Join the talk via the Zoom link fusion.yt/ag on 3/02/2021 at 14:00 (CET). Password FC9C9C21