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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]

Following the improvements of compatibility of ICRF system with the tungsten (W) wall in ASDEX Upgrade (AUG) by using the boron-coated limiters on the 2-strap ICRF antennas and the 3-strap antennas with the W-limiters, the ICRF operating space was further extended in 3 areas: power, phase control between the antennas and frequency range. The available ICRF power in AUG was increased by using an additional RF generator to feed the central straps of the 3-strap antennas which require a 1.5:1 or 2:1 power ratio between the central strap and the two outer straps (combined) to minimize the ICRF-specific W sources. This increases the total experimentally achievable launched ICRF power in the optimized configuration to∼5.7 MW. A new phase control system enabled phase-locked operation of all 4 AUG ICRF antennas. This allows a better control of the launched k|| spectrum which on its turn is modified when both neighboring antennas are active, as well as of the structure of the global RF field distribution within the AUG vessel. Measurements by the RF magnetic field (B-dot) probes show that the RF field distribution can indeed be significantly affected by the variation of the phasing between the ICRF antenna pairs. However, its effect on the core plasma and on the residual ICRF specific plasma-wall interactions is small, for the cases so far limited to the H-modes at medium plasma densities. The available frequency range was extended and now covers four discrete frequencies (f): 30.0, 36.5, 41.8 and 55.1 MHz. In addition to the standard hydrogen (H) minority in deuterium (D) on-axis heating scheme at the magnetic field (Bt) of 2.0, 2.5, 2.8 and 3.1 T, the RF frequency range allows the use of the 3-ion D-(3He)-H and 4He-(3He)-H heating schemes at Bt = 2.5-3.1 T (f = 30 MHz – off-axis and on-axis), on-axis 3ωc D heating at Bt = 1.9 T (f = 41.8 MHz) and at Bt = 2.5 T (f = 55.1 MHz) and 2ωc H heating at Bt = 1.9T (f = 55.1 MHz). [ABSTRACT FROM AUTHOR]

IShTAR (Ion cyclotron Sheath Test ARrangement) is a linear magnetized plasma test facility dedicated to the investigation of RF wave/plasma interaction [1] in the Ion Cyclotron Range of Frequencies (ICRF). It provides a better accessibility for the instrumentation than tokamaks while being representative of the neighboring region of the wave emitter. It is equipped with a magnetized plasma source (1 m long, 0.4 m diameter) powered by a helical antenna up to 3 kW at 11 MHz. We present the results of the first analysis of the plasma characteristics (plasma density, electron temperature) in function of the operating parameters (injected power, neutral pressure and magnetic field) as measured with fixed and movable Langmuir probes, spectrometer and cameras. The plasma is presently produced only by the helical antenna (no ICRF). We show that the plasma exists in three regime depending on the power level: the first two ones are stable and separated by a jump in density; a first spatial profile of the plasma density has been established for these modes; The third mode is unstable, characterized by strong oscillations of the plasma tube position. [ABSTRACT FROM AUTHOR]

This manuscript overviews the latest developments and results from the ASDEX Upgrade ICRF team. The ICRF control system has been upgraded to allow the operation of active antennas with a controlled arbitrary phase difference between them. This upgrade, in combination with an extensive RF probe coverage inside the ASDEX Upgrade torus, makes it possible to study a global wave phenomenon that may result from the two antenna pairs' fields. The first results show that multiple active antennas form a superposition of the launched waves, with limited plasma shielding. A combination of high- and low-field side RF probes makes it possible to study the efficiency of various heating schemes such as the hydrogen minority or the 3-ion species scheme with a He-3 minority. The effect of the hydrogen minority concentration on the ICRF wave absorption and transmission through the core is best shown during conditioning discharges following a torus opening: as the hydrogen fraction is reduced from 80% to below 20%, the amplitude of the RF waves that reaches the high-field side probes is reduced by nearly two orders of magnitude indicating a strong absorption increase. The experimental results are supported by full wave FELICE code showing a similar absorption improvement as a function of the hydrogen fraction. The low-field side probes, on the other hand, do not show a sensitivity to the hydrogen fraction, which is expected as the low-field side wave amplitude is dependent on antenna coupling, not wave absorption. For the case of the 3-ion heating scheme, the high-field side probes detect a minimum in the wave amplitude as the wave absorption layer is radially scanned past the optimal position in the core. The minimum is interpreted as the optimal core absorption condition and matches the observed maximum in the core soft x-ray data. The optimal radial position of the wave absorption layer is also indirectly indicated via the appearance of strong ion cyclotron emission (ICE). The frequency of ICE is matched with the cyclotron frequency of RF-accelerated He-3 ions and the emission origin is placed ∼midway between the magnetic axis and the separatrix. In order to self-consistently assess the global ICRF wave behavior, it is necessary to account for the three dimensional (3D) nature of the toroidal tokamak plasma and the launching antenna structures. The ASDEX Upgrade ICRF team studies multiple aspects of how 3D effects influence launched ICRF wave fields and antenna performance. These include 3D plasma density perturbations in the plasma edge and the scrape off layer, either intrinsically generated by turbulence or externally imposed by resonant magnetic perturbation coils. The experimental study is supported with RAPLICASOL: a computational tool that solves for launched ICRF wave fields in the presence of realistic 3D plasma density profiles and antenna geometry. Additionally, the work of the ASDEX Upgrade ICRF team includes fundamental plasma physics and RF wave studies on the linear test device IShTAR: the areas of interest are RF sheath rectification and arc formation. [ABSTRACT FROM AUTHOR]