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Since the 2018 IAEA FEC Conference, FTU operations have been devoted to several experiments covering a large range of topics, from the investigation of the behaviour of a liquid tin limiter to the runaway electrons mitigation and control and to the stabilization of tearing modes by electron cyclotron heating and by pellet injection. Other experiments have involved the spectroscopy of heavy metal ions, the electron density peaking in helium doped plasmas, the electron cyclotron assisted start-up and the electron temperature measurements in high temperature plasmas. The effectiveness of the laser induced breakdown spectroscopy system has been demonstrated and the new capabilities of the runaway electron imaging spectrometry system for in-flight runaways studies have been explored. Finally, a high resolution saddle coil array for MHD analysis and UV and SXR diamond detectors have been successfully tested on different plasma scenarios.

Recently, during Collective Thomson Scattering (CTS) measurements at mm-waves aimed at studying the ion dynamics in fusion plasmas, strong signals of scattering of the injected beam with non-CTS origin have been detected. A possible explanation of these signals in terms of parametric decay instabilities (PDIs) of the injected wave with power threshold much lower than previously envisaged by theory was proposed [1, 2]. The experimental activity with CTS diagnostic at FTU is aimed at two purposes: the characterization of the thermal ion distribution function and the investigation of the possible low power PDIs processes foreseen by the recent models. In order to ease the data analysis, a set of data processing tools has been integrated on purpose, with an activity started in 2014. Here we present the last implementation of an integrated data analysis tool, aimed at the investigation of the signals detected with the CTS diagnostic. The last version of the software integrates information included in the raw spectra of scattered radiation with the modeled ECE emission, with the aim of providing calibrated spectra improved with respect to the ones provided up to now. The correct calibration of the signals on the real line of sight of the beams is helpful to better distinguish anomalous emissions from less powerful CTS radiation. In addition, the analysis tool compares the calibrated spectra with the ones predicted considering real scattering parameters evaluated by means of realistic beam-trajectories, changing during the pulse, allowing also to extract information on ion dynamics and plasma composition. The last version of the software, which takes into account also multi-reflection beam-tracing simulations in both polarization modes in support to the scattering experiments, is presented.

The evidence of Parametric Decay Instabilities (PDI) excited by the ECH power injected in O-Mode has been explored in FTU Tokamak, using the Collective Thomson Scattering (CTS) diagnostic. The experiments show evidences to support the hypothesis of low-threshold excitation of waves generated by PDI mechanisms, formerly proposed in the case of 2nd harmonic X-mode injection in TEXTOR and ASDEX-U. Theoretical analysis predicts low-threshold parametric decay also for O-mode pump-wave injection, which can be injected in FTU at frequencies close to the first Harmonic EC resonance. Experiments were made at different magnetic fields, injecting the 140 GHz probe and observing the emission from the second antenna of the EC launcher in poloidally symmetric and asymmetric configurations, in presence of MHD islands. The signal is detected by the CTS radiometers, with a fast digitizer allowing the spectral reconstruction at very fine time and frequency scales. Different types of emissions are studied in detail, comparing them with the magnetic island rotation frequency in different plasma conditions. In order to locate the plasma volume originating the emissions, a new antenna and receiving line has been installed.

The recent implementation of data processing tools and the code activity, aimed at the interpretation of spectra measured in the tokamak FTU with the renewed Collective Thomson Scattering (CTS) system, are presented in this paper. Such instrumental renovation allowed investigating anomalous emissions originating from parametric decay instability (PDI) of an electron cyclotron (EC) pump wave in presence of magnetohydrodynamics (MHD) rotating islands, as foreseen by recent theoretical models. Aim of these experiments is to study the possible effects of these anomalous phenomena on the EC power absorption and on the CTS used as standard thermal diagnostic. The codes here presented have been implemented for the calibration of the CTS spectra and for the proper visualization of signals, for direct comparison with the MHD spectrograms and with the plasma parameters. The software also allows analysing data acquired simultaneously with two radiometric systems as is possible in FTU since the beginning of 2016. The calibration of the spectra is necessary for comparing the actual power of the signals, that can span orders of magnitude, from the lowest compatible with thermal (CTS) radiation and providing information on the ions dynamic, to the strong anomalous emission theoretically expected from PDIs. The Thermal Collective Scattering (TCS) code, able either to predict or to interpret thermal ion spectra, has been optimized and integrated in the overall data analysis tools. The analysis of the data and the interpretation of the scenarios achieved in the experiments are presently underway using the tools here presented. © 2018 Istituto di Fisica del Plasma - CNR. Published by IOP Publishing Ltd on behalf of Sissa Medialab.

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During the last years the Collective Thomson Scattering (CTS) system installed on the Frascati Tokamak Upgrade (FTU), operating at 140 GHz, has been exploited to investigate different kinds of phenomena: high-frequency daughter waves by Parametric Decay Instabilities, which, under certain conditions, are recently presumed to be conceivable even under routine ECRH scenarios, and bulk ions thermal emissions stimulated by a 140 GHz gyrotron beam. A new receiving line, giving rise to a second further line of sight from the plasma during the scattering experiments and integrated in the CTS antenna, has been recently completed and installed. Such new line will be useful to recognise signals originating in the volume of beams cross from the ones coupled outside or also to monitor the gyrotron's spectrum in real-time during the shot. The present status of this new installation is shown in the paper.

The Collective Thomson Scattering (CTS) diagnostic on FTU tokamak was renewed for investigations on the excitation of Parametric Decay Instabilities (PDI) by Electron Cyclotron (EC) beams in presence of magnetic islands and their effects on the EC absorption. Experiments were performed launching a gyroton probe beam (140 GHz, 400 kW) and receiving the CTS beam in symmetric and asymmetric configurations with respect to the equatorial plane, in different conditions of plasma density and magnetic field (with or without the EC resonance in the plasma), and with magnetic islands generated by Neon injection. The acquisition with a fast digitizer allowed observing spectral features with very high time and frequency resolution. In the shots performed at 7.2 T, with the fundamental EC resonance out of the plasma region, a sequence of faint lines emitted with a fast temporal evolution have been observed in a range 0.5-1.1 GHz from the gyrotron frequency while at 4.7 T, with the resonance on the high field side of the plasma column, asynchronous "bursts" of continuous emissions were observed at a microsecond time scale. In 2015 and 2016 experiments were performed at 4.7 T and 3.6 T, in this last case with the plasma between the first and the second EC harmonics both lying outside the plasma volume. Different types of spectral features with a fast evolution were observed. Their correlation with magnetic probes and other fast signals from the plasma has been investigated, to characterize the observations and exclude parasitic effects, as well as breakdown phenomena in front of the antennas. The variation in the stray radiation distribution in the vessel has been studied with the aid of a diffusive model, to characterize variations on the probe beam absorption associated to the observed phenomena. The latest experiments, with a diagnostic improved both in frequency band (up to ±4.2 GHz from the probe) and with the addition of a second radiometer, are allowing a clearer interpretation of the emissions.

The new collective Thomson scattering diagnostic installed on the Frascati Tokamak Upgrade device started its first operations in 2014. The ongoing experiments investigate the presence of signals synchronous with rotating tearing mode islands, possibly due to parametric decay processes, and phenomena affecting electron cyclotron beam absorption or scattering measurements. The radiometric system, diagnostic layout, and data acquisition system were improved accordingly. The present status and near-term developments of the diagnostic are presented. © 2016 EURATOM.

Experiments on runaway electrons have been performed for the determination of the critical electric field for runaways generation. A large database of post-disruption generated runaway beams has been analysed in order to identify linear dynamical models for new position and current runaway beam controllers, and experiments of Electron Cyclotron assisted plasma start-up have shown the presence of runaway electrons also below the expected electric field threshold, indicating that the radio-frequency power acts as seeding for fast electrons. A linear micro-stability analysis of Neon doped pulses has been carried out to investigate the mechanisms leading to the observed density peaking. A study of the ion drift effects on the MARFE instability has been performed and the peaking of density profile in the high density regime has been well reproduced using a thermo-diffusive pinch in the particle transport equation. The analysis of the linear stability of the 2/1 tearing mode observed in high density plasmas has highlighted a destabilization with increasing peaking of the current profile during the density ramp-up, while the final phase of the mode temporal evolution is characterized by limit cycles on the amplitude/frequency plane. A Liquid Lithium Limiter with thermal load capability up to 10 MW/m2 has been tested. The pulse duration has been extended up to 4.5 s and elongated configurations have been obtained for 3.5 s, with the X-point just outside the plasma chamber. W/Fe samples have been exposed in the SOL in order to study the sputtering of Fe and the W enrichment of the surface layer. Dusts have been collected and analyzed, showing that the metallic population exhibits a high fraction of magnetic grains. A new diagnostic for in-flight RE studies has allowed to provide simultaneously the image and the visible/infrared spectrum of the forward and backward radiation. A fast infrared camera for thermo-graphic analysis has provided the pattern of the toroidal limiter heating by disruption heat loads, and a triple-GEM detector has been mounted on one equatorial port and tested for soft-X rays diagnostic. The Collective Thomson Scattering diagnostic has been upgraded and used for investigations on Parametric Decay Instability excitation by Electron Cyclotron beams correlated with magnetic islands, and new capabilities of the Cherenkov probe have been explored in the presence of Beta-induced Alfvén Eigenmodes associated to high-amplitude magnetic islands.

During the last years anomalous signals have been found in the spectra analyzed with the Collective Thomson Scattering (CTS) diagnostics in tokamak devices like TEXTOR, ASDEX and FTU. The signal frequency, down-shifted with respect to the probing one, suggested the explanation of their origin in terms of Parametric Decay Instability (PDI) processes correlated with the presence of magnetic islands and taking place even for values of the pumping wave power below the threshold predicted by conventionalmodels. A threshold lower or comparable with the power levels routinely used could pose limitations to the use of the ECRH in fusion devices. An accurate characterization of the conditions for the occurrence of this phenomenon and of its consequences is thus of primary importance. The CTS diagnostics of FTU has been recently improved for this aim with a new fast data acquisition, allowing spectra reconstruction by direct FFT of the intermediate frequency signal, added in parallel to the pre-existing multichannel spectrum analyzer. A section with polarizing mirrors has been introduced in the quasi-optical transmission line to control the polarization of the received signals. Exploiting the front-steering configuration available with the real-Time launcher of FTU, the setup allows studying the anomalous emissions in conditions of density and wave injection similar to ITER. Given the possibility of the CTS in FTU to operate in both O-and X-mode, a second radiometer is being installed, to detect simultaneously signals scattered in both polarizations. The upgrades mentioned above are presented in this work as well as the first data obtained in 2014, during the experimental activity carried out under an EUROfusion Enabling Research project.

The design of diagnostics for the Frascati Tokamak Upgrade (FTU) is challenging because of the compactness of the machine (8-cm-wide ports) and the low operating temperatures requiring the presence of a cryostat. Nevertheless, a rather complete diagnostic system has been progressively installed. The basic systems include a set of magnetic probes, various visible and ultraviolet spectrometers, electron cyclotron emission (ECE) for electron temperature profiles measurements and electron tails monitoring, far-infrared and CO{sub 2} interferometry, X-ray (soft and hard) measurements, a multichord neutron diagnostics (with different type detectors), and a Thomson scattering system. Some diagnostics specific to the FTU physics program have been used such as microwave reflectometry for turbulence studies, edge-scanning Langmuir probes for radio-frequency coupling assessment, oblique ECE, and a fast electron bremsstrahlung (FEB) camera for lower hybrid current drive-induced fast electron tails.These systems are briefly reviewed in this paper. Further developments including a scanning CO{sub 2} laser two-color interferometer, two FEB cameras for tomographic analysis, a motional Stark effect system, and a collective Thomson scattering system are also described.

Anomalous emissions were found over the last few years in spectra of Collective Thomson Scattering (CTS) diagnostics in tokamak devices such as TEXTOR, ASDEX and FTU, in addition to real CTS signals. The signal frequency, down-shifted with respect to the probing one, suggested a possible origin in Parametric Decay Instability (PDI) processes correlated with the presence of magnetic islands and occurring for pumping wave power levels well below the threshold predicted by conventional models. A threshold below or close to the Electron Cyclotron Resonance Heating (ECRH) power levels could limit, under certain circumstances, the use of the ECRH in fusion devices. An accurate characterization of the conditions for the occurrence of this phenomenon and of its consequences is thus of primary importance. Exploiting the front-steering configuration available with the real-time launcher, the implementation of a new CTS setup now allows studying these anomalous emission phenomena in FTU under conditions of density and wave injection geometry that are more similar to those envisaged for CTS in ITER. The upgrades of the diagnostic are presented as well as a few preliminary spectra detected with the new system during the very first operations in 2014. The present work has been carried out under an EUROfusion Enabling Research project. A shorter version of this contribution is due to be published in PoS at: 1st EPS conference on Plasma Diagnostics © 2015 Istituto di Fisica del Plasma - CNR.

A collective Thomson scattering (CTS) experiment is now installed on FTU aimed to measure the plasma ion temperature and test the theory of CTS. A high power 140 GHz gyrotron beam is focalized into the plasma center by means of a circular corrugated transmission line and an in-vessel optical system to a radius of 0.025 m. The scattered radiation is collected at 90 degrees scattering angle by a symmetric in-vessel optical system and a low-power quasioptical transmission line which collects the scattered radiation on a superheterodyne receiver. Both in-vessel optical systems are placed in a vertical port. The receiver is based on 32 channels, 1.2 GHz global bandwidth, with a measured noise temperature T-rec = 1 eV and minimum integration time of 1 ms. The antenna decoupling measurement resulted about 60 dB, while the attenuation introduced by the collecting system is evaluated in 5 dB. The angle of the scattering wavevector and the magnetic field directions is 25 degrees. The experiment is run at a magnetic field B = 7.4-7.7 T, i.e., the gyrotron frequency is well below the first electron cyclotron emission (ECE) harmonic, so the ECE plasma background temperature is low and it is measured T-ECE = 70-50 eV, respectively. The detected polarization is the ordinary mode. In this experiment the calculated signal is of the order of the ECE noise level. Indeed in a series of dedicated FTU runs, CTS signals with signal-to-noise consistent with calculation (made using standard formulas! were found. Some features of the detected scattered spectra are reported, as well as plasma radiation measurements in various plasma conditions including lower hybrid rf heating. (C) 1999 American Institute of Physics. [S0034-6748(99)68201-6].

The required rejection ratio of the stray light in a CTS, respect to the launching power is investigated, for obtaining useful signal‐to‐noise ratio. This problem is faced considering the central gyrotron frequency f0, as well as the in‐band stray light raising from the spurious mode of the gyrotron and amplified cyclotron emission (the so‐called gyrotron noise), falling into the scattered light spectrum. The statistical nature of the gyrotron noise in the scattering band is important because the required decoupling is obtained using the S/N formula where the fluctuation of the plasma radiation and the gyrotron noise fluctuation appears. It turns out that the total decoupling required at f0 is of the order of or greater than 80 dB, while in the scattering band could be less than 40 dB.

Different conditions make the CTS diagnostic experiment presently installed at FTU a unique test bed for a proof-of-principle demonstration for a fast ions diagnostic in ITER. The most relevant one is wave propagation at frequencies below the electron cyclotron (EC) resonance where the extraordinary (X) mode can propagate at higher plasma densities. CTS measurements performed in the past with an up-to-down scattering geometry, showed damages on the launching mirror close to the EC resonance located in the port interior. Therefore a different and more flexible CTS geometry was required to achieve a better insight into this difficult problem (found also in the ITER CTS geometry) and to single out a convenient remedy. This opportunity is now offered by the new ECRH launcher that will be installed at FTU. The system has been designed for both ECRH and CTS. It includes two symmetrical lines with front-steerable mirrors and will provide extended focusing capability, independent toroidal and poloidal scans of the probing and the receiving beams, and the possibility of radially scanning, or even changing, the scattering volume during a single gyrotron pulse.

A diagnostic experiment of collective Thomson scattering (CTS) on thermal density fluctuations designed to provide spatially-resolved measurements of the ion temperature is since long being carried out in the high-field tokamak FTU using a high-power 140 GHz gyrotron as a source of probing radiation. A peculiar feature of CTS experiment on FTU is that the propagation at 140 GHz is below the fundamental electron cyclotron (EC) harmonic (195 GHz-220 GHz). With the only exception of the low-power experiment attempted in TFTR, this is the first proof-of-principle experiment of CTS below the EC resonance, hence with accessibility from the low-field side of both ordinary and extraordinary plasma modes. Thus, this is the only experiment capable for an exhaustive demonstration of CTS in the configuration relevant for alpha CTS in ITER, in which the extraordinary mode at 50-60 GHz propagating below the EC resonance is assumed as most liable candidate. In spite of long lasting efforts to detect CTS from thermal density fluctuations, strong anomalous spectra including spectral lines were systematically detected both in aligned- and in misaligned-antenna conditions at spectral power densities orders-of-magnitude higher than those predicted for the thermal CTS spectra. In the present communication, we report on the interpretation of the anomalous spectra observed at FTU following the results of two experimental campaigns of 2004 and 2005 expressly performed to investigate the anomalous spectra.

We first describe the improved receiving system of the diagnostic experiment of millimeter-wave collective Thomson scattering being run on the Frascati Tokamak Upgrade (FTU), and then discuss some peculiar problems and new operating procedures related to the investigation of strong anomalous spectra of nonthermal origin, many-orders-of-magnitude stronger than the ion thermal feature merged in them, systematically observed in the experimentation, and finally ascribed to a perturbation of the gyrotron that generates the probing beam. Arguments in favor of a more general valence of the solutions actuated for the specific case of FTU are finally given. (c) 2007 American Institute of Physics.

Strong anomalous spectra were systematically observed in collective Thomson scattering (CTS) at 140 GHz in the high-field tokamak FTU, a proof-of-principle experiment of CTS on thermal density fluctuations performed with propagation below electron cyclotron (EC) resonance with the final aim of demonstrating high-density CTS in the extraordinary mode. Following the results of two experimental campaigns expressly performed to investigate them, these spectra are ascribed to a gyrotron perturbation caused by a back-reflected signal originating in the beam injection port, where the electron cyclotron layer, the upper-hybrid layer and the right-handed cutoff layer unavoidably crossed by the probing beam are activated by a breakdown plasma sustained by the beam itself. The degree of generality ascribable to our results and the constraints they set on present and future CTS experiments with propagation below electron cyclotron resonance are discussed. A viable solution in terms of a transmit antenna robust against the risk of gyrotron perturbation is suggested.

The design of diagnostics for the Frascati Tokamak Upgrade (FTU) is challenging because of the compactness of the machine (8-cm-wide ports) and the low operating temperatures requiring the presence of a cryostat. Nevertheless, a rather complete diagnostic system has been progressively installed. The basic systems include a set of magnetic probes, various visible and ultraviolet spectrometers, electron cyclotron emission (ECE) for electron temperature profiles measurements and electron tails monitoring, far-infrared and CO2 interferometry, X-ray (soft and hard) measurements, a multichord neutron diagnostics (with different type detectors), and a Thomson scattering system. Some diagnostics specific to the FTU physics program have been used such as microwave reflectometry for turbulence studies, edge-scanning Langmuir probes for radio-frequency coupling assessment, oblique ECE, and a fast electron bremsstrahlung (FEB) camera for lower hybrid current drive-induced fast electron tails. These systems are briefly reviewed in this paper. Further developments including a scanning CO2 laser two-color interferometer, two FEB cameras for tomographic analysis, a motional Stark effect system, and a collective Thomson scattering system are also described.

We investigate performance degradation caused in mm-wave collective Thomson scattering on the high-field tokamak FTU by the joint effects of refraction and polarization. It is shown that optimized performances can be recovered, in different parts of the FTU density range, by selecting the most suitable of the four modal combinations potentially contributing to the scattered signal. This can be achieved by matching the limiting polarization of the selected mode at both the transmitting and the receiving side. A full polarization-control capability is required at this purpose. (C) 1999 American Institute of Physics. [S0034-6748(99)59201-0].

The Fusion Advanced Study Torus (FAST) has been proposed as a possible European satellite facility to study fast-ion physics in deuterium plasmas under conditions relevant to a burning plasma. Energetic minority ions (H or 3He) accelerated by ion cyclotron resonance heating (ICRH), with dimensionless parameters close to those of fusion-born alphas in ITER, can be produced in FAST via 30 MW power ICRH minority heating. This work provides a first assessment of the extent to which the 3He fast-ion population can be diagnosed in FAST with a set of dedicated diagnostics for confined fast particles. Neutron emission spectroscopy (NES), gamma-ray spectroscopy (GRS) and collective Thomson scattering (CTS) diagnostics have been reviewed with a description of the state-of-the-art hardware and a preliminary analysis of the required lines of sight. The results of the analysis, based on numerical simulations of the spatial and energetic particle distribution function of the ICRH-accelerated ions for the standard FAST H-mode scenario, suggest that NES and GRS measurements can provide information on the fast 3He population effective tail temperature, with time resolutions in the range 20–100 ms. The proposed CTS diagnostic can measure the fast-ion parallel and perpendicular temperature with a spatial resolution of 5–10 cm and a time resolution of 10 ms. The paper provides a scientific basis for the prediction of the production and diagnosis of energetic ions in FAST.

We present a numerical and analytical study of the modifications induced in the collective scattering spectra by the ohmic current governing the equilibrium magnetic configuration in toroidal plasmas. The spectral density function is calculated assuming equilibrium distributions for the (bulk and impurity) ion species and a Spitzer-like distribution to describe the response of the electrons to the applied DC electric field. As expected, the spectral asymmetries can be non-negligibly enhanced in the region of the ion-acoustic frequency. They reach their maxima for tangential scattering geometries, where the magnetic effects on the spectra are negligible. This justifies the assumption of the non-magnetized spectra. The role of the spectral asymmetries in affecting the ion temperature measurements is investigated, and the possibility of inferring the local ohmic current density from the analysis of the low-frequency part of the spectra is verified. The additional effects due to the presence of impurities affecting the spectral shapes are also discussed under realistic conditions. Reference is made to the widely complementary FTU and JET tokamak plasmas, taken as examples of high- and medium-current density regimes, respectively. As a general result it is found that, mainly due to the absence of a true ion acoustic resonance in tokamak plasmas (related to a low electron to ion temperature ratio), the peculiarities of the high-current density regimes (high density, low impurity content, etc) must be met for the current-induced asymmetry in the spectra to become measurable. The ion temperature measurements can be made unaffected by the asymmetries using a simple symmetrization procedure which applies in all cases. In conclusion, a theoretically motivated potential is shown to exist for a more detailed experimental investigation of the feasibility of current-density measurements in ohmic plasmas, based on collective scattering.

Collective Thomson scattering (CTS), a promising diagnostic for fusion plasmas now made feasible by the availability of high-power gyrotrons in the millimeter wavelength range (COSTLEY et al., 1988, JET Report R(88)08, Abingdon, Oxon), is known to be particularly sensitive to the presence of fast-ion populations in the plasma. In this paper we investigate the effects on CTS spectra of the anisotropies often involved in the production, heating, confinement and slowing down of these energetic populations in a fusion device. Two kinds of velocity-space anisotropies are considered: (a) that associated with the transfer of energy from an external source to a reduced number of degrees of freedom (e.g. perpendicular acceleration of ions during ICRH), and (b) that associated with transverse particle losses due either to orbits intersecting the walls or to magnetic field ripples. Numerical computations of the CTS spectra have been performed for the relevant cases and a number of features characterizing the plasma behaviour in the presence of suprathermal-ion populations, typically the enhancement of the fluctuation level at the lower hybrid resonance, are quantified analytically.

The empirical analysis of the electron density evolution during ECRH is given. It is shown that when recycling from the walls is low and the neutral gas injection rate is kept constant, during ECW injection the line density drops mainly as a consequence of a net decrease in the total particle number, whereas profile reshaping occurs only for some conditions. It is also shown that the phenomenon is of resonant nature but not a by-product of plasma heating. By considering the fast time scale on which the particle balance changes and by comparing the profile density evolution during ECRH to that following a step-wise reduction of the gas injection rate, it is inferred that particle confinement is degraded in the presence of EC waves. The consistency of this conclusion with all the experimental evidence is positively verified.

The density fluctuation spectrum, resulting from a collective (k lambda De

The feasibility of a tangential laser scattering experiment in a Tokamak discharge is analysed. The electron distribution function is simulated by three different models. In each model the shape factor and the signal-to-noise ratio are studied. The results of the analysis are applied to the case of the Tokamak Thor.

By the joint use of a coherent light source and of photon counting statistics, we have verified the statistical properties of Brownian motion. The light source is a helium-neon laser emitting at 6328 \AA{}. The scattering media are dilute solutions of monodisperse polystyrene spherical particles with diameters ranging from 0.088 to 1.06 \ensuremath{\mu}. Three types of results are presented. First, by working at a density of scattering centers such that neither particle correlations nor multiple scattering processes have any influence, we measure the angular distributions of the field scattered by single particles, and verify the Mie theory. Second, by frequency measurements, we show the Lorentzian character of the scattered light spectrum, as one would expect from the diffusion approximation. Third, by use of photocount distributions, we measure the ensemble distribution of the scattered field within a coherence area and time, and show that the field distribution is Gaussian. The last two results imply that the field scattered by Brownian particles is accurately described as a stochastic Markov process. The techniques here used open the way to similar measurements on more complicated scattering media.

ECRH experiments on the Thor Tokamak aimed at the investigation of r.f. absorption by suprathermal electrons are discussed. At the lowest densities (ne=3*1012 cm-3) the damping of the injected wave is shown to be dominated by a r.f. enhanced suprathermal population in the presence of a bulk with very low optical thickness, so that a rather pure suprathermal interaction regime occurs. At higher density, localization of heating is lost due to refractive effects and multiple reflections. No evidence is found of conversion into electrostatic electron Bernstein waves of the extraordinary mode propagating quasiperpendicularly from the inner side of the torus.