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X-ray optics is the branch of optics that manipulates X-rays instead of visible light. It deals with focusing and other ways of manipulating the X-ray beams for research techniques such as X-ray crystallography, X-ray fluorescence, small-angle X-ray scattering, X-ray microscopy, X-ray phase-contrast imaging, and X-ray astronomy. Since X-rays and visible light are both electromagnetic waves they propagate in space in the same way, but because of the much higher frequency and photon energy of X-rays they interact with matter very differently. Visible light is easily redirected using lenses and mirrors, but because the real part of the complex refractive index of all materials is very close to 1 for X-rays, they instead tend to initially penetrate and eventually get absorbed in most materials without changing direction much. There are many different techniques used to redirect X-rays, most of them changing the directions by only minute angles. The most common principle used is reflection at grazing incidence angles, either using total external reflection at very small angles or multilayer coatings. Other principles used include diffraction and interference in the form of zone plates, refraction in compound refractive lenses that use many small X-ray lenses in series to compensate by their number for the minute index of refraction, Bragg reflection from a crystal plane in flat or bent crystals. X-ray beams are often collimated or reduced in size using pinholes or movable slits typically made of tungsten or some other high-Z material. Narrow parts of an X-ray spectrum can be selected with monochromators based on one or multiple Bragg reflections by crystals. X-ray spectra can also be manipulated by having the X-rays pass through a filter (optics). This will typically reduce the low-energy part of the spectrum, and possibly parts above absorption edges of the elements used for the filter.

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This thesis focuses on the physics of suprathermal electrons generated by electron cyclotron (EC) waves in tokamak plasmas, which play an important role in the physics of current drive and energetic particle-driven instabilities. The suprathermal electron dynamics and their effect on the plasma stability have been experimentally studied utilizing high power EC waves, in the TCV tokamak of the Swiss Plasma Center at EPFL, Switzerland. A hard X-ray diagnostic, which measures the bremsstrahlung radiation of the suprathermal electrons in radial and energy spaces, has been mainly used for the analysis, and the measurement has been compared to an estimation made by Fokker-Planck modeling coupled with a hard X-ray synthetic diagnostic. In order to study the response of the suprathermal electrons to ECCD, ECCD modulation discharges have been developed. The time evolution of the hard X-ray profiles has been measured using coherent averaging techniques in order to observe the creation and relaxation of suprathermal electrons. Time-dependent Fokker-Planck modeling coupled with the hard X-ray synthetic diagnostic has been used to compare the experimental and simulation results, with various suprathermal electron transport models. A dependency of the radial transport of suprathermal electrons on the EC wave power has been demonstrated and a possibility of EC wave scattering has been addressed. The effect of the suprathermal electron population on the plasma stability has been studied, and in particular the destabilization and dynamics of the electron fishbone mode. The response of hard X-ray profiles to the internal kink mode has been observed directly by the hard X-ray diagnostic for the first time, at the frequency of the mode. The experimental evidence and a solution of a linear fishbone dispersion relation coupled to the Fokker-Planck modeling demonstrate the role of suprathermal electrons in destabilizing the fishbone mode and in particular the interaction of trapped electrons with the mode. This work provides the framework for a comprehensive understanding of the physics of suprathermal electrons related to ECCD, and explains how ECCD-generated suprathermal electrons behave in real and velocity spaces, and how they interact with and are redistributed by the MHD mode.

EPFL2020

X-ray optics

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