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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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assword: F400E03A

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