A new technique for tokamak edge density measurement based on microwave interferometer

Abstract

A novel approach for density measurements at the edge of a hot plasma device is presented-Microwave Interferometer in the Limiter Shadow (MILS). The diagnostic technique is based on measuring the change in phase and power of a microwave beam passing tangentially through the edge plasma, perpendicular to the background magnetic field. The wave propagation involves varying combinations of refraction, phase change, and further interference of the beam fractions. A 3D model is constructed as a synthetic diagnostic for MILS and allows exploring this broad range of wave propagation regimes. The diagnostic parameters, such as its dimensions, frequency, and configuration of the emitter and receiver antennas, should be balanced to meet the target range and location of measurements. It can be therefore adjusted for various conditions, and here, the diagnostic concept is evaluated on a chosen example, which was taken as suitable to cover densities of ∼10 15 to 10 19 m −3 on the edge of the ASDEX Upgrade tokamak. Based on a density profile with a fixed ra dial sh ape, ap propriate fo r experimental density approximation, a database of synthetic diagnostic measurements is built. The developed genetic algorithm genMILS of density profile reconstruction using the constructed database has quite low errors. It is estimated as ∼5% to 15% for density ≥10 17 m −3 . Therefore, the new diagnostic technique (with a dedicated data processing algorithm) has a large potential in practical applications in a wide range of densities, with low errors in the numerical model and in the method of density reconstruction, so the total error and the density estimation accuracy are expected to be defined mostly by experimental uncertainties. FIG. 15. Dependence of the refractive index on density. Gray dashed line shows the cut-off density, nc = 2.74 × 10 19 m −3 .

Sensitivity of Microwave Interferometer in the Limiter Shadow to filaments in ASDEX upgrade

“…The axis of the interferometer is oriented perpendicularly to the background magnetic field and the wave at f = 47 GHz is sent in the O-mode polarization, making the measured signal sensitive to the density only. [4] In the synthetic diagnostic, [3] the wave propagation is simulated in a limited region between the horns, where it matters for the received signal. The full-wave 3D modelling is done in COMSOL by using RAPLICASOL approach of wave propagation simulation in cold plasma [5] (cold plasma approximation is valid for the SOL plasma conditions, where electron temperature is ≤100 eV), where absorbing material surrounds the plasma domain and ensures a single-pass absorption.…”

“…Figure 1b shows the radial‐poloidal cross‐section of the simulation domain with an example of wave propagation in plasma with density exponentially increasing towards the core, as described in details in Ref. [3]. The core plasma is on the left, with the flux surfaces approximated by circles concentric to the circle that defines the limiter.…”

“…The previous study of the measurement region of MILS shows that the part of the radial density profile (for monotonic profiles), which influences the MILS measurements, has a length of ∼5 cm and is located near the limiter edge, more towards the core plasma for lower densities and towards outside for higher densities. [ 3 ]…”

“…Properties of filamentary transport have been a subject of an extensive research. [1,2] Microwave Interferometer in the Limiter Shadow (MILS) [3] is a new diagnostic, installed on ASDEX Upgrade for electron density measurements in the far SOL. At the chosen frequency of 47 GHz, the radial span of measurements lies within several centimetres before and after the limiter.…”

“…Forward modelling is used for the reconstruction of the plasma density profile in far SOL from measured signals. [ 3 ] This method is applicable to a monotonic inter‐ELM‐like density profile without large perturbations. In this work, we aim at studying the influence of the filament presence on the signals of MILS.…”