Testing of the ITER plasma position reflectometry high-field side in-vessel antenna assembly prototype

Abstract

The ITER plasma position reflectometry diagnostic aims to provide measurements of the edge plasma to correct or supplement the magnetics for plasma position control. It consists of five reflectometers, two of which have components installed inside the vessel. One of these systems probes the plasma from the high-field side using a bistatic array of small pyramidal horns located in the gap between two blankets. Electromagnetic simulations have shown that the blankets shape the radiation pattern and need to be considered as part of the antenna. Full-wave plasma simulations have confirmed these results and have also shown that the first-wall geometry may induce measurement errors above the required margin. To further address these issues, we manufactured a prototype of the high-field side antenna, which includes a mock-up of the blanket modules. Here, we present the results of the prototype tests, with and without the blankets, using a metallic mirror as a target. The signals reflected from the mirror are used to derive the mirror distance and assess the precision of the measurements under different arrangements. The sensitivity to the blankets’ installation tolerances is also assessed by changing the antennas’ position with respect to the blankets’ surfaces and cut-outs.

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

“…by P. Valera, J.H. Belo and A. Silva [19,20] performs very well. From the simulated patterns, the antenna with beam waist/wavelength relation 0 = 0.588 and diameter = 14 mm (G14L) has the best performance when aiming for frequency-independent antenna characteristics while respecting the spatial constraints.…”

“…Because the PROFUSION optimization is done on the antenna aperture fields, the cylindrical antenna optimization tool does not allow external structures such as blankets to be modelled during the optimization. This limitation means that the optimized antenna might suffer from similar effects as described by Valera et al for square aperture antennas [19]: apart from introducing multi-reflections, the blankets can also change antenna radiation patterns. To assess the performance of the antenna in different surroundings, a prototype of the optimized antenna, and a conical reference antenna with identical dimensions (length = 52.678 mm, input and output diameter in = 8 mm and = 19.598 mm) were built and radiation pattern measurements were made for both antennas with and without mock-up blankets.…”

“…The geometry of the simulation environment (poloidal view) based on the ITER gap 6 setup [19,20], with the antenna aperture, the plasma minor radius and the cut-off layer.…”

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

“…Due to the severe design restrictions and adverse operating environment, it was decided to prototype and test the antennas to assess their EM characteristics as well as the measurement performance of each gap. After characterisation in an anechoic chamber, the prototype antennas were tested [9] with mock-ups of the blanket modules corresponding to the configurations of gaps 4 and 6. These tests were carried out with a metallic mirror to assess the accuracy of the system in recovering the distance to the mirror.…”

“…Concerning gap 6, the tests show that it is compliant with the target error margin for frequencies above 26.5 GHz. The performance of this system is primarily limited by the presence of the first-wall cut-outs just in front of the antennas' apertures that, combined with the very broad radiation pattern of the small-flare pyramidal horns at the lower frequencies, are major source of reflections that negatively impact the accuracy of the measurements [9]. Measurements performed with the mock-ups of gaps 4 and 6.…”