Reflectometry techniques for density profile measurements on fusion plasmas

Abstract

Reflectometry applied to the measurement of density profiles on fusion plasmas has been subject to many recent developments. After a brief reminder of the principles of reflectometry, the theoretical accuracy of reflectometry measurements is discussed. The main difficulties limiting the performance, namely the plasma fluctuations and the quality of the transmission lines, are analysed. The different techniques used for reflectometry are then presented grouped into three different categories, depending on the frequency spectrum of the probing wave: single frequency, few discrete frequencies, or broad spectrum. The present status and achievements of actual implementations of these techniques are demonstrated, with an analysis of their respective limitations and merits, as well as foreseen developments. Finally, a discussion of the various reflectometry techniques is made, in particular their ability to cope with plasma fluctuations and complex transmission lines, in view of the application to next step machines and very severe environments.

Particle Flow during Sawtooth Reconnection: Numerical Simulations of Experimental Observations

“…Reflectometry is based on radar principle, measuring the phase of a microwave reflected from a plasma cutoff layer as a function of the probing frequency [7]. Depending on the polarization, the cutoff layer position depends on the local plasma density (Ordinary mode) and local magnetic intensity (eXtraordinary mode).…”

Development and Application of Millimeter-Wave Imaging Radar.

“…Reflectometry is a radar technique that probes the density-dependent cut-off layer by measuring phase delay or group delay between the reflected wave and reference wave [7][8][9][10]. This technique is expected to be one of the key diagnostic methods for measuring density profiles and density/magnetic-field fluctuations in large fusion devices.…”

“…The characteristic frequency and the maximum frequency of an impulse are π/2τ p and 2π/τ p respectively. In this experiment we mainly use a 22 ps impulse, since we can expect high amplitude at the frequency range of [8][9][10][11][12][13][14][15][16][17][18] GHz which corresponds to the electron plasma frequency of ICP. The output of the impulse generator is fed through a WRD-750 waveguide that attenuates the low frequency part of the Fourier component, and stretches into a chirped pulse.…”

Upgrade of reflectometry profile and fluctuation measurements in Alcator C-Mod

“…The AM profile measurement system, as shown in The modulation frequency, Af, must be chosen carefully [7]. It cannot be too large or multiple fringes will have to be followed; but it cannot be so small as to seriously limit spatial resolution.…”

“…An AM reflectometer continuously measures the group delay to the critical surface on a time scale fast compared to changing plasma conditions. The modulation frequency should be chosen so small that less than one fringe needs to be tracked during the time of interest and the two critical layers are in the fluctuation correlation length (See the review article [7]). However, in the existing system, too high a frequency was initially chosen, and there were multiple fringe jumps in the phase measurement.…”