M. Moresco

Application of microwave reflectometry for plasma density diagnostics in a reversed-field pinch machine

The implementation of reflectometry as a means of measuring plasma density is examined in relation to the reversed-field pinch machine. The effects of magnetic shear and density fluctuations are studied. Ray-tracing and simulation results are presented and the proposed reflectometer system for the RFX reversed-field pinch machine is described.

Upgrade of the RFX microwave frequency modulated reflectometer to ultrafast sweeping rate

The swept homodyne reflectometer installed on the RFX experiment is composed of five units which cover the frequency range 26.5–110 GHz. This diagnostic system is based on solid state IMPATT sources and can be operated in the O or X mode. During the first operation, at a sweep rate of 0.1 GHz/μs, the typical density fluctuations present in the reversed field pinch configuration usually prevented correct profile reconstruction, which was possible only in some special cases. To increase the sweep speed and overcome this limit, a new IMPATT driver has been built and IF amplifiers, control, and acquisition systems have been modified accordingly. The new configuration has been extensively tested in the 34–38 GHz range with a modulation rate exceeding 4 GHz/μs on many different plasma conditions. Measurements on plasma showed that both the phase and reflected power level can be correctly recovered from the IF signals. These results indicated the criteria necessary to extend the fast sweep capability to the full...

Focused aperture microwave antennas operating in the near field zone

Focusing a microwave antenna is often convenient in order to produce a Fraunhofer-type pattern at short distance from the aperture. The characteristics of the radiation patterns produced by some specimens were experimentally tested and compared with theoretical predictions. Basic criteria are given to design focused antennas, once their radiation pattern characteristics are specified.

Detection of lower hybrid waves by coherent scattering of microwaves: Preliminary results in Wega

The coherent scattering of a microwave beam is considered as a diagnostic tool for monitoring the penetration of the lower hybrid heating waves in a plasma. A set up using a 2 mm wavelength has been installed on the Wega torus. Preliminary experimental data are reported and discussed. The amplitude of the electron density fluctuations has been found lower than the one we expected. The variation of the wave amplitude as the plasma density varies, qualitatively agrees with a cold plasma electromagnetic theory. The feasibility of the measurements was checked and indications of some improvements to be made in the diagnostic method came out. Some important topics (both theoretical and practical) in scattering experiments on L.H. heating waves are reviewed.

A focussing hog‐horn antenna for microwave diagnostics in plasma machines

We describe the design of a type of millimeter wave antenna which is particularly suitable in plasma diagnostics applications. It is an all metal focussing hog‐horn antenna. Its main advantages are high spatial resolution, capability of working at short distance, little space taken, wide band, ruggedness. Moreover it does not produce vacuum or plasma bombardment problems.

First results in reflectometric plasma density measurements on RFX

A five‐band, high‐sweep‐speed reflectometer system for the RFX reverse field pinch experiment has been developed. To control the high‐noise environment we improved the intermediate frequency amplifying and conditioning system. We checked the modified system using a new in situ calibration method that pointed out the improvements. This new method is a suitable one for calibrating the reflectometer, because it allows one to place a mirror in a well‐defined position, thus giving a good radial (<2 mm) and angular resolution (<10−3 rad). Preliminary results on maximum density time evolution are presented.

Extension of lock−in operation to vhf frequencies

A simple dual frequency converter is described which applies the lock−in technique to frequencies much higher than that of commercially available lock−in amplifiers.

Robust measurement of group delay in the presence of density fluctuations by means of ultrafast swept reflectometry.

It is well known that reflectometric measurements can be seriously disturbed by density fluctuations. In this paper the basic mechanisms that critically affect the measurements are outlined with the help of a simplified two-dimensional model which simultaneously mimics the dynamic of the plasma turbulence and the frequency sweep. It turns out that an adequate modulation rate can easily overcome the adverse effect of wave scattering produced by short wavelength fluctuations even when measurements taken at a single frequency would fail. Nevertheless the Doppler effect caused by long wavelength fluctuations can produce a substantial shift in the group delay, which accounts for nonmonotonous density profile reconstructions. In turn this effect can be corrected using a very fast ascending and descending frequency modulation, similarly to what is done in FM radars. The technical feasibility of such a system is demonstrated using the Ka band reflectometer of RFX, where a triangular modulation of 2.5 MHz, with a bidirectional modulation rate up to 4 GHz/200 ns, has been obtained.

Multilayer vacuum window for wide-band microwave plasma diagnostic systems

Microwave diagnostics, e.g., reflectometry and electron cyclotron emission in plasma machines, often require large windows displaying low attenuation and reflection on a wide band, in addition to some basic features such as vacuum tightness and mechanical robustness. Wide-band matching is usually achieved by coating the window with dielectric layers of suitable permittivity and thickness. On the vacuum side the coating must also be vacuum compatible and resistant to the thermal radiation coming from plasma. On the RFX machine, to satisfy these requirements, a boron nitride disk properly machined on the surface facing the plasma has been clamped to the existing fused silica window. The addition of a Teflon layer on the air side allowed to attain power reflection coefficients as low as 0.025 in the frequency ranges of 26.5–40, 40–56, and 56–75GHz and as low as 0.01 in the frequency ranges of 75–92 and 92–110GHz.

About the Effects of the Movements of the Critical Reflecting Layer on Swept Reflectometric Measurements

The impact of density fluctuations on reflectometry measurements has been studied extensively and is still matter of discussion. Detailed numerical models [1–4] (either in WKB or full-wave simulations, I-D and 2-D) gave insight on many effects that the fluctuations may cause to the reflectometric measurements. However some fundamental requirements which have to be taken into account in designing swept reflectometric systems can be recovered from a simple ID model with analytical considerations.