Design status of the in-vessel subsystem of the ITER Plasma Position Reflectometry system

Abstract

The aim of the Plasma Position Reflectometry (PPR) diagnostic is to provide the ITER Plasma Control System with real-time measurements of the edge density profile at four locations, known as gaps 3, 4, 5 and 6, which can be used to supplement the magnetic measurements of the plasma-wall gaps as well as to correct potential drifts of the magnetics during long pulses. The systems of gaps 4 and 6 will have some components inside the vacuum vessel. These components make up the PPR in-vessel subsystem and consist of: (i) the antennas, installed in small apertures between blanket modules; and (ii) waveguide components connecting the antennas to the primary confinement barriers, which form the boundary between the in-vessel and ex-vessel part of the systems. Herein, we describe the design status of the in-vessel subsystem, namely of the components already prototyped, including the most relevant results of the tests conducted to assess the EM performance of the components as well as the feasibility and quality of the manufacturing techniques. With the exception of one of the waveguide bends of gap 4, the tests show that the waveguide prototypes exhibit adequate performance, which indicates that it is possible to manufacture these components according to their technical specifications. Concerning the antennas, the tests show that both gaps should be able to achieve the target error margin of ±1 cm except in the K band, where they might face some difficulties. K: Microwave Antennas; Nuclear instruments and methods for hot plasma diagnostics; Waveguides; Instrument optimisation 1Corresponding author.

Advances, Challenges, and Future Perspectives of Microwave Reflectometry for Plasma Position and Shape Control on Future Nuclear Fusion Devices

“…The transmission line of gap 6 was then routed along the inner wall of the vacuum vessel between the bend and one of the feed-outs on the top of upper port 14. Further details about the design of the PPR system are provided in [19,20].…”

“…If the above design changes were applied simultaneously, the effects of the structures surrounding the antenna were restricted to the K band (15-26.5 GHz) [20], and the measurement performance of the system was significantly improved with respect to the baseline design. Furthermore, the tests have shown that these modifications also helped in mitigating the impact of the blanket modules if, after installation, the antenna does not end up exactly centred between the blanket surfaces due to the tolerances of the different components involved.…”

On frequency-independent horn antenna design for plasma positioning reflectometers, from simulation to prototype testing

“…At other frequencies the offsets were limited to less than 1°. Care must be taken in the design of the transmission line to the PPR antennas to avoid these higher-order modes, which is challenging considering that the oversized waveguides leading to the PPR antennas in ITER will be very long and involve a lot of bends [34]. If higher-order modes at the antenna input are inevitable, another option would be to make a new antenna optimization which uses the real antenna input.…”

“…Additional PROFUSION simulations with the optimally-shaped antenna showed that also small fractions of higher-order modes at the antenna feed result in asymmetry in the radiation pattern. Considering the length and many bends of the ITER waveguide connecting the antenna [34], care should be taken in the final antenna and waveguide design to minimize these effects.…”

“…Additional PROFUSION simulations with the optimally-shaped antenna showed that also small fractions of higher-order modes at the antenna feed result in asymmetry in the radiation pattern. Considering the length and many bends [34] of the ITER waveguide connecting the antenna, care should be taken in the final antenna and waveguide design to minimize these effects. Building further on the research presented here, a next step is to find an optimal shape of the antenna, so that it has frequency-independent behaviour taking into consideration the blanket environment.…”