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Register a new userInfrared spectroscopy of surface charges in plasma-facing dielectrics
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We propose to measure the surface charge accumulating at the interface between a plasma and a dielectric by infrared spectroscopy using the dielectric as a multi-internal reflection element. The surplus charge leads to an attenuation of the transmitted signal from which the magnitude of the charge can be inferred. Calculating the optical response perturbatively in first order from the Boltzmann equation for the electron-hole plasma inside the solid, we can show that in the parameter range of interest a classical Drude term results. Only the integrated surface charge enters, opening up thereby a very efficient analysis of measured data.
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We propose a setup enabling electron energy loss spectroscopy to determine the density of the electrons accumulated by an electro-positive dielectric in contact with a plasma. It is based on a two-layer structure inserted into a recess of the wall. Consisting of a plasma-facing film made out of the dielectric of interest and a substrate layer the structure is designed to confine the plasma-induced surplus electrons to the region of the film. The charge fluctuations they give rise to can then be read out from the backside of the substrate by near specular electron reflection. To obtain in this scattering geometry a strong charge-sensitive reflection maximum due to the surplus electrons the film has to be most probably pre-n-doped and sufficiently thin with the mechanical stability maintained by the substrate. We demonstrate the feasibility of the proposal by calculating the loss spectrum for an sapphire film on top of a CaO layer. We find a reflection maximum strongly shifting with the density of the surplus electrons and suggest to use it for its diagnostics.
Here we show that, despite a massive incident flux of energetic species, plasmas can induce transient cooling of a material surface. Using time-resolved optical thermometry in-situ with this plasma excitation, we reveal the novel underlying physics that drive this `plasma cooling that is driven by the diverse chemical and energetic species that comprise this fourth state of matter. We show that the photons and massive particles in the plasma impart energy to different chemical species on a surface, leading to local and temporally changing temperatures that lead to both increases and decreases in temperature on the surface of the sample, even though energy is being imparted to the material. This balance comes from the interplay between chemical reactions, momentum transfer, and energy exchange which occur on different time scales over the course of picoseconds to milliseconds. Thus, we show that through energetically exciting a material with a plasma, we can induce cooling, which can lead to revolutionary advances in refrigeration and thermal mitigation with this new process that is not inhibited by the same limitations in the current state-of-the-art systems.
We show that the charge accumulated by a dielectric plasma-facing solid can be measured by infrared spectroscopy. The approach utilizes a stack of materials supporting a surface plasmon resonance in the infrared. For frequencies near the Berreman resonance of the layer facing the plasma the reflectivity dip--measured from the back of the stack, not in contact with the plasma--depends strongly on the angle of incidence making it an ideal sensor for the changes of the layers dielectric function due to the polarizability of the trapped surplus charges. The charge-induced shifts of the dip, both as a function of the angle and the frequency of the incident infrared light, are large enough to be measurable by attenuated total reflection setups.
Long pulse operation of present and future magnetic fusion devices requires sophisticated methods for protection of plasma facing components from overheating. Typically, thermographic systems are being used to fulfill this task. Steady state operation requires, however, autonomous operation of the system and fully automatic detection of abnormal events. At Wendelstein 7-X (W7-X), a large advanced stellarator, which aims at demonstrating the capabilities of the stellarator line as a future fusion power plant, significant efforts are being undertaken to develop a fully automatic system based on thermographic diagnostics. In October 2018, the first divertor-based experimental campaign has been finished. One of the goals of this operation phase (named OP1.2) was to study the capabilities of the island divertor concept using an uncooled test divertor made of fine-grain graphite tiles. Throughout this campaign, it was possible to test the infrared imaging diagnostic system, which will be used to protect the actively water-cooled plasma facing components (PFCs) during the steady-state operation in the next experimental campaign. An overview of the most relevant thermal events on the PFCs that were detected in OP1.2 using this system are presented. This includes events that limited operation during the campaign, like baffe hot spots and divertor overloads, events that are potentially critical in steady state operation like leading edges, events caused by the ECRH and NBI heating systems and other events which are a common source of false alarms like surface layers. The detected thermal events are now part of an important and extensive image database which will be used to further automate the system by means of computer vision and machine learning techniques in preparation for steady-state operation, when the system must be able to detect dangerous events and protect the machine in real-time.
We present combined experimental and numerical work on light-matter interactions at nanometer length scales. We report novel numerical simulations of near-field infrared nanospectroscopy that consider, for the first time, detailed tip geometry and have no free parameters. Our results match published spectral shapes of amplitude and phase measurements even for strongly resonant surface phonon-polariton (SPhP) modes. They also verify published absolute scattering amplitudes for the first time. A novel, ultrabroadband light source enables near-field amplitude and phase acquisition into the far-infrared spectral range. This allowed us to discover a strong SPhP resonance in the polar dielectric SrTiO3 (STO) at approximately 24 micrometer wavelength of incident light.
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