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Significant advances in modeling and simulations for microwave reflectometry have been conducted in recent years. An example is the integration of improved numerical descriptions of magneto-hydrodynamics (MHD) instabilities, obtained from MHD codes, together with 2D full-wave codes to implement synthetic reflectometry diagnostics in realistic conditions. In particular, the nonlinear MHD code JOREK has been used previously together with the 2D full-wave code REFMUL, to assess the response of conventional O-mode reflectometry along a Type-I edge localized mode (ELM) crash. In this work, a similar framework and synthetic diagnostic set-up are considered but with an extended MHD simulation where a Type-I ELM cycle comprised of several ELM crashes is studied. This includes the self-consistent MHD evolution along inter-ELM periods, which was absent in the previous study. The reflectometer response at the mid-plane of the low magnetic field side is used to provide information about the behavior of density fluctuations along the full ELM cycle. The inter-ELM periods display density fluctuations in agreement with the ELM triggering mechanisms manifesting as reflectometry signatures which are identified with ELM precursors. The precursor signatures are observed a few milliseconds before a transition from linear to non-linear reflectometry regimes occurs, associated with the explosive onset of the ELM. [ABSTRACT FROM AUTHOR]

The measurement of the plasma parameters close to the ICRF antennas is important to understand the wave propagation and improve modelling. For this purpose one of the 3-strap ICRF antennas in ASDEX Upgrade, that were installed in 2014, is equipped with 10 horn antenna pairs; three of these antenna pairs are used for reflectometry in the extended U-Band at frequencies from 40 – 68 GHz. One of the unused horn antenna pairs close to the ICRF antenna limiter was removed; with some additional waveguide sections and new supports for the horn antennas we could set up a new microwave diagnostic that allows density measurements in the limiter shadow. The diagnostic, called Microwave Intensity refractometer in the Limiter Shadow (MILS), operates with 47 GHz amateur radio transmitter and receiver units and allows phase and amplitude measurements at an intermediate frequency of 145 MHz. After the successful installation and commissioning of MILS in 2019 and some modifications in 2020, we plan to upgrade the system to three receiver horn antennas, as the microwave beam from the transmitter horn antenna, which is tangential to the plasma, is refracted by the large density gradient at the plasma edge. The amplitude and phase information from the addition receiver antennas will improve our knowledge about the plasma properties. [ABSTRACT FROM AUTHOR]

Applications of magnetized Radio Frequency (RF) capacitive discharges for mirror cleaning applications of optical diagnostic systems in fusion devices and heat flux measurements have received a large interest in last years. Ensuring the ho-mogeneity of heat flux to the RF electrode requires localized measurements of plasma parameters in front of the RF electrode. Langmuir probe measurements were taken in the ALINE plasma device around the active RF electrode with the help of a 3D manipulator equipped with RF compensation. Highly negative floating potential values were recorded inside the magnetic flux tube connected to the active RF electrode whilst plasma potential remained positive. Since the difference between the plasma po-tential and the floating potential is proportional to the average electron temperature, experimental investigations were carried out in capacitive coupling and direct coupling to study the change in floating potential as a function of input gas pressure, magnetic field strength (B), and the angle between the RF electrode and magnetic field. The observed change in floating potential is attributed to the stochastic heating leading to the generation of hot electrons, which was confirmed using the Langmuir probe current-voltage curves. The experimental results are found to agree with the analytical model based on the flux balance equations. [ABSTRACT FROM AUTHOR]

The simple Maxwellian EEDF (Electron Energy Distribution Function) used in collisional plasmas at thermodynamic equilibrium can be distorted when the plasma is close enough to a RF source especially when electrons are well magnetized. This has been verified in our experimental device Aline. During the experiment, negative floating potentials have been found when a Langmuir probe is magnetically connected to the RF electrode, in both direct and capacitive coupling. Floating potentials as low as -40 to -100 V have been measured while the plasma potential remains positive. The large gap between them can be explained by energetic electron beams accelerated in the RF sheath surrounding the electrode. Such acceleration is known as stochastic heating, which is investigated using PIC (Particle in Cells) simulation to demonstrate the importance of the tilting angle of the electrode with respect to the magnetic field direction in the enhancement of these electron beams. Both measurements and PIC simulations show these beams are much stronger at grazing angle, inducing a bi-Maxwellian EEDF with two characteristic temperatures, the highest value explaining the high negative floating potentials. [ABSTRACT FROM AUTHOR]