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Frequency swept reflectometry has reached the symbolic value of 1 μs sweeping time; this performance has been made possible, thanks to an improved control of the ramp voltage driving the frequency source. In parallel, the memory depth of the acquisition system has been upgraded and can provide up to 200 000 signals during a plasma discharge. Additional improvements regarding the trigger delay determination of the acquisition and the voltage ramp linearity required by this ultra-fast technique have been set. While this diagnostic is traditionally dedicated to the plasma electron density profile measurement, such a fast sweeping rate can provide the study of fast plasma events and turbulence with unprecedented time and radial resolution from the edge to the core. Experimental results obtained on ASDEX Upgrade plasmas are presented to demonstrate the performances of the diagnostic.

Although single sideband modulation offers a simple solution to active heterodyne devices and is common in devices working over 150 GHz, it has been scarcely used for reflectometers. For a few years, large bandwidth (12–20 GHz), high performance single sideband modulators (SSBM) have been available. For higher frequency application, a sideband rejection around −25 dBc seems necessary to ensure that the image sideband amplitude remains low after a frequency multiplier. A SSBM provided by Miteq Company presents high enough rejection levels to be assembled with frequency multipliers. Based on this SSBM, we developed a simple microwave scheme suitable for all Tore-Supra reflectometers—from 50 to 155 GHz, dedicated either to density profile reconstruction or to density fluctuation measurements. Compared to the double sideband modulator previously used, SSBM offers higher signal to noise ratio and enables fixed frequency measurements, paving the way for new measurement techniques.

A method to extract the radial wave-number spectrum of the density fluctuations from the reflectometry phase signal is proposed and the main points to interpret it are described. The method buildup is based on reflectometry studies devoted to phase or amplitude variations induced by coherent density perturbations. These previous analyses show that all the wave numbers up to the Bragg detection limit kf (fluctuation radial wave number)

Significant improvements have been achieved in the performances of the broadband reflectometry for the density profile measurements on Tore Supra. The frequency range of the former 50–75 GHz X-mode reflectometer has been extended with an additional device operating between 75 and 110 GHz. Both setups have ultrafast sweeping capabilities allowing full band measurements in 20 μs. The heterodyne detection and the intermediate frequency system have been upgraded to record beat frequencies up to 100 MHz with a signal to noise ratio of about 40 dB. The in phase and quadrature-type phase detection system (IQ detector) provides sine and cosine components of the received signal for a separate measurement of the phase and the amplitude. A new fast data acquisition system in Versa Module Europa format has been developed. It allows sampling frequency up to 200 MHz with 32 Mo memory capabilities per channel and 10 bit resolution digitizers. Edge profile measurements are now available for a wide range of toroidal magnetic fields (from 3 to 4 T).

A new broadband reflectometer operating in the frequency range 50–75 GHz in extraordinary mode polarization has been developed and tested on Tore Supra to measure edge density profiles. Using solid state source and active frequency multipliers, it performs routine measurements in 20 μs. Modulation of the source frequency is the clue to heterodyne detection in order to ensure a high dynamic sensitivity without any phase locking system. The reflectometer can achieve a repetition rate of 5 μs between sweeps, so the dynamic behavior of fast plasma events can be followed. The profile is reconstructed fully automatically from raw data and initialization is given by detection of the first cutoff. The profiles are part of the public database of Tore Supra. High reliability of the measurements for various plasma conditions makes this diagnostic an ideal tool to study the plasma–surface interaction physics and rf antenna coupling processes.

Tore Supra is equipped with two 20 μs fast sweep X-mode reflectometers operating between 50–110 GHz dedicated to density profile determination and an X-mode fixed frequency reflectometer operating between 105–155 GHz for density fluctuation measurements. Heterodyne and sine–cosine detection provide measurements of the reflected signal with high sensitivity. Operating profile reflectometer in burst mode (5 μs dead time between two consecutive sweeps) allows quasi-simultaneous measurements at fixed frequency over a broad frequency band. Thus, information on plasma fluctuations, such as magnetohydrodynamic (MHD) activity, up to 20 kHz as well as a radial localization of the modes is accessible. The temporal evolution of the q=1 rational surface during sawtooth crash activity has been recorded in the plasma center with high spatial resolution. In addition, a direct comparison between signals associated with a central MHD mode from both profile and fluctuation reflectometers, positioned at different toroidal a...

A new system has been installed on the JET tokamak consisting of six independent fast-sweeping reflectometers covering four bands between 44 and 150 GHz and using orthogonal polarizations. It has been designed to measure density profiles from the plasma edge to the center, launching microwaves through 40 m of oversized corrugated waveguides. It has routinely produced density profiles with a maximum repetition rate of one profile every 15 μs and up to 100,000 profiles per pulse.