P. Varela

Status and prospects for mm-wave reflectometry in ITER

Reflectometry with wavelengths in the centimetre to millimetre-wave range will be used in ITER to measure the density profile in the main plasma and divertor regions and to measure the plasma position and shape in order to provide a reference for the magnetic diagnostics in long pulses. In addition, it is expected to provide key information for the measurement of density fluctuations. A set of reflectometers to meet the relevant ITER measurement requirements has been included in its present outline as part of the ITER design since 2001 and is being adapted to the present ITER baseline and to accommodate progress with reflectometry techniques and measurement capabilities. It comprises low and high field side (HFS and LFS, respectively) ordinary (O-) mode systems for the measurement of the density profile in the gradient regions, a LFS extraordinary (X-) mode system for the detailed study of the edge profile, an HFS X-mode system operating in the left hand cutoff to measure the core profile, a dedicated O-mode system for plasma-wall gap measurement and a multi-band, multiple line of sight O-mode system to measure divertor density profiles. This paper describes the evolution of the design, in particular some recent improvements in the engineering implementation and improvements aimed at enhancing the measurement capability. It concludes with a brief assessment of the likely measurement performance against the ITER measurement requirements for the parameters of interest and the overall confidence that the technique will be implanted on ITER.

Density profile measurements with X-mode lower cut-off reflectometry in ASDEX Upgrade

Despite the fact that density profile measurements using X-mode lower cut-off reflectometry are foreseen to be used on ITER, little or no experience is available within the reflectometry community and to our knowledge no results on this subject have been published so far. In ASDEX Upgrade the multichannel broadband reflectometer is equipped with both O- and X-mode channels. While X-mode operation was designed for upper cut-off reflection, it is observed that for both high magnetic field and high density discharges the lower cut-off becomes accessible. Here we present reflectometry measurements obtained in ASDEX Upgrade using X-mode lower cut-off and compare both the resulting group delay and density profile with O-mode measurements performed simultaneously over the same plasma region. The possible use of this comparison to provide estimates of the magnetic field is briefly discussed.

Improved time-frequency analysis of ASDEX Upgrade reflectometry data using the reassigned spectrogram technique.

The spectrogram is one of the best-known time-frequency distributions suitable to analyze signals whose energy varies both in time and frequency. In reflectometry, it has been used to obtain the frequency content of FM-CW signals for density profile inversion and also to study plasma density fluctuations from swept and fixed frequency data. Being implemented via the short-time Fourier transform, the spectrogram is limited in resolution, and for that reason several methods have been developed to overcome this problem. Among those, we focus on the reassigned spectrogram technique that is both easily automated and computationally efficient requiring only the calculation of two additional spectrograms. In each time-frequency window, the technique reallocates the spectrogram coordinates to the region that most contributes to the signal energy. The application to ASDEX Upgrade reflectometry data results in better energy concentration and improved localization of the spectral content of the reflected signals. When combined with the automatic (data driven) window length spectrogram, this technique provides improved profile accuracy, in particular, in regions where frequency content varies most rapidly such as the edge pedestal shoulder.

Novel mono-static arrangement of the ASDEX Upgrade high field side reflectometers compatible with electron cyclotron resonance heating stray radiation

The ASDEX Upgrade frequency modulated continuous wave broadband reflectometer system uses a mono-static antenna configuration with in-vessel hog-horns and 3 dB directional couplers. The operation of the new electron cyclotron resonance heating (ECRH) launcher and the start of collective Thomson scattering experiments caused several events where the fragile dummy loads inside the high field side directional couplers were damaged, due to excessive power resulting from the ECRH stray fields. In this paper, we present a non-conventional application of the existing three-port directional coupler that hardens the system to the ECRH stray fields and at the same time generates the necessary reference signal. Electromagnetic simulations and laboratory tests were performed to validate the proposed solution and are compared with the in-vessel calibration tests.

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

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.