Lithium PFC Characterization and Plasma Performance for LTX-beta

Lithium coatings on high-Z plasma facing components (PFCs) in the Lithium Tokamak eXperiment (LTX) 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. To investigate these questions, 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 Li$_2$O and LiOH are prime surface constituents of Li 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 Li$_2$O improves plasma performance in comparison to LiOH by both sequestering oxygen and increasing hydrogen retention

Introduction to gyrokinetic theory with applications in magnetic confinement research in plasma physics

The present lecture provides an introduction to the subject of gyrokinetic theory with applications in the area of magnetic confinement research in plasma physics--the research arena from which this formalism was originally developed. It was presented as a component of the ''Short Course in Kinetic Theory within the Thematic Program in Partial Differential Equations'' held at the Fields Institute for Research in Mathematical Science (24 March 2004). This lecture also discusses the connection between the gyrokinetic formalism and powerful modern numerical simulations. Indeed, simulation, which provides a natural bridge between theory and experiment, is an essential modern tool for understanding complex plasma behavior. Progress has been stimulated in particular by the exponential growth of computer speed along with significant improvements in computer technology. The advances in both particle and fluid simulations of fine-scale turbulence and large-scale dynamics have produced increasingly good agreement between experimental observations and computational modeling. This was enabled by two key factors: (i) innovative advances in analytic and computational methods for developing reduced descriptions of physics phenomena spanning widely disparate temporal and spatial scales and (ii) access to powerful new computational resources

Uncertainty explicit assessment of off-the-shelf software: A Bayesian approach

Assessment of software COTS components is an essential part of component-based software development. Poorly chosen components may lead to solutions of low quality and that are difficult to maintain. The assessment may be based on incomplete knowledge about the COTS component itself and other aspects (e.g. vendor’s credentials, etc.), which may affect the decision of selecting COTS component(s). We argue in favor of assessment methods in which uncertainty is explicitly represented (‘uncertainty explicit’ methods) using probability distributions. We provide details of a Bayesian model, which can be used to capture the uncertainties in the simultaneous assessment of two attributes, thus, also capturing the dependencies that might exist between them. We also provide empirical data from the use of this method for the assessment of off-the-shelf database servers which illustrate the advantages of ‘uncertainty explicit’ methods over conventional methods of COTS component assessment which assume that at the end of the assessment the values of the attributes become known with certainty

Development of Smart, Compact Fusion Diagnostics using Field-Programmable Gate Arrays

Abstract: Fusion research requires high quality diagnostics to understand the complex physical processes involved. Traditional analogue systems are complex, large and expensive, and expansion of diagnostic capabilities is often impossible without building a completely new system at considerable expense. Field-programmable gate array (FPGA) technology can provide a solution to this problem. By implementing complex functionality and digital signal processing on an FPGA chip, diagnostic hardware can be greatly simplified and compacted. In this thesis we describe the enhancements of two diagnostics for the MAST-Upgrade tokamak using FPGA technology. Firstly, the design of the back end electronics for the new divertor bolometer is described. Results of tests of the new electronics at a number of sites, including lab-based testing and tokamak installations, are also presented. We demonstrate the correct functionality of the electronics and illustrate a number of important effects which must be taken into account when interpreting bolometer data on MAST-U. Secondly, we describe the new control and acquisition electronics developed for the MAST-U divertor Langmuir probe diagnostic. Much of the analogue control circuitry of the previous system has been upgraded to a digital implementation on an FPGA, which results in a significantly more compact and cost effective design. Given that MAST-Upgrade will feature around 850 Langmuir probes, these improvements are extremely important to keep the diagnostic manageable. Again, results are presented from the testing of the system at several sites, which both demonstrate the correct functionality of the new system and provide information on the diagnostic behaviour which needs to be accounted for when interpreting the probe data during MAST-U experiments