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Plasma properties can be useful in a wide spectrum of applications. Experimental projects on controlled nuclear fusion are the most challenging of these applications and, at the same time, the best way to approach plasma science. Since nuclear fusion reactors can ensure a large-scale, safe, environmentally-friendly and virtually inexhaustible source of energy, several fusion-oriented megaprojects and innovative companies are appearing all over the world. PROTO-SPHERA (Spherical Plasma for HElicity Relaxation Assessment) is the first plasma project with a simply connected configuration, namely not requiring additional objects inside the plasma volume. This is obtained by a plasma arc, shaped as a screw pinch, acting as the centerpost of a spherical torus with minimal aspect ratio. Due to its intrinsic physical, engineering and economic advantages, this new approach is attractive also on an industrial scale and with several developments that still needs to be explored. This paper presents the PROTO-SPHERA basic principles, its first encouraging results and its expected and potential evolutions.

A method is presented to determine all the elements of the plasma Mueller matrix with high time-resolution (about 1 µs) using only measurements of phase. This knowledge allows one to derive the pure Cotton–Mouton and Faraday effects even when polarimetric effects are large.

PROTO-SPHERA is the first plasma project with a simply connected configuration, namely not requiring other objects inside the plasma. This is obtained by a hydrogen plasma arc, shaped as a screw pinch, acting as the central column of a spherical torus with minimal aspect ratio. The reduced size and complexity of the configuration can lead to several physical and engineering advantages, providing relevant contributions to the magnetic plasma confinement methods. Moreover, due to the limited costs and requirements, the set-up is attractive for replication and developments also on an industrial scale with many potential applications. This paper describes the initial activities of the PROTO-SPHERA project, with emphasis on the electrical tests and commissioning performed to implement the plasma central column. In particular, an intermediated experiment classified as MULTI-PINCH was designed to investigate the breakdown conditions and the pinch stability. © 2015 IEEE.

A simple exact method is described for the analysis of mode purity produced inside magnetized plasma as a function of input polarization of the injected electromagnetic wave. The method is applied to experimental results obtained for RF heating on the large helical device (Notake T et al 2005 Plasma Phys. Control. Fusion 47 531).

Two new methods are proposed for the simultaneous determination of the Cotton–Mouton and the Faraday effects in a magnetized plasma from direct measurements of phase only, which are preferable to amplitude measurements since they do not require detector calibrations and allow better signal-to-noise ratios. The present methods can achieve frequencies up to hundreds of kHz, much higher than with a previously proposed method. The choice between the two alternative methods is dictated by practical considerations and is briefly discussed.

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.

The effects of a finite electron temperature on the refractive index N and on the evolution of polarization for radiation propagating in a magnetized plasma are considered. Analytic expressions for N and for the Stokes vector evolution equation are found to lowest order in Te/mec2. For interferometric and polarimetric measurements on typical plasma confinement configurations, the thermal effects are found to be significant. Neglecting them can lead to systematic errors in polarimetry greater than 10% for a temperature of 5 keV and to a systematic (positive) error on electron density from interferometry of 3% for Te=15 keV.

The potentiality of exploring the plasma dynamical behavior of Mercury via energetic neutral atom (ENA) analysis is briefly discussed. The analysis of the Hermean ENA would give clues to the global magnetospheric configuration, clarifying crucial issues like the existence of radiation belts. In order to demonstrate the feasibility of ENA imaging in the Hermean environment, an empirical model of the equatorial proton distributions, originally developed for the Earth's magnetosphere, is here modified in order to be applicable to the expected conditions, according to the present knowledge of the planet. Simulations of the ENA radiation are then presented and discussed. It is shown that the simulated radiation is significantly high, especially when pointing the field of view towards the Hermean surface. The way of determining the proton flux distributions and the cross-tail electric field characteristics by using the ENA signal is discussed. Finally, a preliminary design of an ENA detector suitable for the ESA mission to Mercury ‘BepiColombo’ is proposed and discussed.

High density plasmas (n0 ≈ 8 × 1020m-3) featuring steady improved core confinement have been obtained in FTU up to the maximum nominal toroidal field (8 T) by deep multiple pellet injection. These plasmas also feature high purity efficient electron-ion coupling and peaked density profiles sustained for several confinement times. Neutron yields in excess of 1 × 1013 n/s are measured, consistent with the reduction of the ion transport to neoclassical levels.

First the complete change of polarization is obtained for radiation crossing a layer of quasi-transverse propagation in a plasma where the longitudinal component of the magnetic field changes signs along the propagation direction, while the transverse component is different from zero. The analysis is made for arbitrary input polarization. The results are then used for the interpretation of previously published measurements of radio emission from solar active regions. It is found that if a sufficiently narrow detection bandwidth is used, more information can be obtained from measurements than has been considered up to now. The present analysis is also of interest for other astrophysics problems.

The Large Helical Device (LHD) is the largest helical machine with superconducting coils. Key diagnostics issues for LHD are: (a) capability for multidimensional measurements because of the nonaxisymmetric toroidal plasma; (b) measurements of the electric field; (c) cross check of fundamental parameters using different methods; (d) advanced measurements appropriate for steady-state operation; and (e) a satisfactory data acquisition system. The design and research and development of plasma diagnostics were carried out taking these issues into consideration. As a result, the present status of diagnostics is described: diagnostics for LHD operation, fundamental diagnostics for plasma performance, diagnostics for physics subjects, innovative diagnostics and diagnostics for long-pulse operation. The LHD experiment started in March, 1998. Since then, the development of diagnostics has kept pace with the experimental campaigns.

The state of polarization of radiation scattered from hot plasma for any given input polarization is obtained by a relativistic derivation. Approximate analytical expressions are obtained for electron temperatures below about 10 keV. The characteristics of the scattered polarization are presented in detail for fully and linearly polarized incident radiation and their implications for the design of experiments are discussed.

Electron temperatures in excess of 8 keV have been obtained by electron-cyclotron-resonance heating on FTU plasmas at peak densities up to $8\ifmmode\times\else\texttimes\fi{}{10}^{19}\mathrm{m}{}^{\ensuremath{-}3}$. The magnetic shear in the plasma core is low or negative, and the electron heat diffusivity remains at, or below, the Ohmic level $(0.2\mathrm{m}{}^{2}/\mathrm{s})$, in spite of the very large heating power density $(10--20\mathrm{MW}/\mathrm{m}{}^{3})$ which produces extremely high temperature gradients (up to 120 keV/m). The ion heat transport remains at the neoclassical level.

A general purpose data analysis and display program, named SHOW, has been developed over the years as the answer to the needs of the experimentalists working on FTU, a high magnetic field tokamak devoted to the study of plasma behaviour both with ohmic and additional heating. The description of the main characteristics of the SHOW program can be seen as a summary of many of the facilities required by the experimental physicists in their data analysis activity. Some of these facilities rely heavily on the program architecture, that derives from the awareness that only a very flexible structure permits an easy adaptation to the changing requirements of the data analysis activity. The program has been developed in FORTRAN on the IBM mainframe (running MVS-ESA) where the main FTU databases are located. The GDDM package has been used to implement the interactive user interface, based on panels, and the graphic display facilities, that include 2- and 3-D plot sections. The program provides utilities for the evaluation of derived quantities (as integrals, derivatives, smoothing filters) and for the time series analysis. A spread-sheet section permits the analysis of tables of data. A quantitative analysis of the utilisation of the SHOW program is also presented, during a period of >3 years. In the last year a preliminary version of the program has been ported to the UNIX environment, on a DEC Alpha workstation, and the user interface has been realised making use only of libraries available for free. A client/server software has been developed so that the IBM mainframe can still be used as a file server.

When polarimetric effects are large the Cotton–Mouton and Faraday effects do not combine linearly and it is not possible to separate exactly the pure Cotton–Mouton effect W1 and the pure Faraday effect, W3. Four alternative approximate expressions for W1 and W3 in terms of measurable quantities are examined for tokamak configurations. Two of these approximations proposed recently are found to be preferable, some previous statements concerning them are corrected and the errors incurred by their use are evaluated.

Over the last few years, a great deal of effort has been devoted to solving the problem of power and particle handling in divertors, which has been recognized as a critical issue for the operation of a magnetic fusion reactor. In particular, the choice of materials for plasma facing components has been examined with a view to developing heat and erosion resistant materials for divertor target plates. A large database on the behaviour of low-Z (carbon or beryllium) materials in tokamaks is available, while for high-Z materials there is little experience in the present generation of magnetic fusion devices. Frascati Tokamak Upgrade (FTU), a high field compact tokamak, has devoted part of its experimental campaign to studying the plasma characteristics when its limiter material is changed from the usual Inconel (nickel) to molybdenum and tungsten. Siliconization of the machine has also allowed the comparison of plasma performance when a relatively low-Z (silicon) ion is the dominant impurity. In this article, results are reported concerning the plasma operation, the differences in plasma characteristics and radiation losses, the impurity generation mechanisms and the relative impurity concentrations in the core plasma. A simulation of the experimental results, made with a self-consistent edge-core coupled model is presented, in order to provide evidence of the main physics mechanisms responsible for the observed behaviour

Experimental determinations of the radial plasma flow velocity, of Zeff and of the thermal energy content, made on the Frascati Tokamak Upgrade (FTU), have been compared with the predictions of the conventional neoclassical theory, of the extended neoclassical theory and of the pseudoneoclassical theory. In the comparison only the momentum balance and continuity equations were used. The best overall agreement is found for the second of these theories, thus confirming the result of previous analyses

New FTU ohmic discharges with a liquid lithium limiter at I P = 0.7–0.75 MA, B T = 7 T and n e0 ⩾ 5 × 1020 m−3 confirm the spontaneous transition to an enhanced confinement regime, 1.3–1.4 times ITER-97-L, when the density peaking factor is above a threshold value of 1.7–1.8. The improved confinement derives from a reduction of electron thermal conductivity (χe) as density increases, while ion thermal conductivity (χi) remains close to neoclassical values. Linear microstability reveals the importance of lithium in triggering a turbulent inward flux for electrons and deuterium by changing the growth rates and phase of the ion-driven turbulence, while lithium flux is always directed outwards. A particle diffusion coefficient, D ∼ 0.07 m2 s−1, and an inward pinch velocity, V ∼ 0.27 m s−1, in qualitative agreement with Bohm–gyro-Bohm predictions are inferred in pellet fuelled lithized discharges. Radio frequency heated plasmas benefit from cleaner plasmas with edge optimized conditions. Lower hybrid waves penetration and current drive effects are clearly demonstrated at and above ITER densities thanks to a good control of edge parameters obtained by plasma operations with the external poloidal limiter, lithized walls and pellet fuelling. The electron cyclotron (EC) heating system is extensively exploited in FTU for contributing to ITER-relevant issues such as MHD control: sawtooth crash is actively controlled and density limit disruptions are avoided by central and off-axis deposition of 0.3 MW of EC power at 140 GHz. Fourier analysis shows that the density drop and the temperature rise, stimulated by modulated EC power in low collisionality plasmas are synchronous, implying that the heating method is the common cause of both the electron heating and the density drop. Perpendicularly injected electron cyclotron resonance heating is demonstrated to be more efficient than the obliquely injected one, reducing the minimum electric field required at breakdown by a factor of 3. Theoretical activity further develops the model to interpret high-frequency fishbones on FTU and other experiments as well as to characterize beta-induced Alfvén eigenmodes induced by magnetic islands in ohmic discharges. The theoretical framework of the general fishbone-like dispersion relation is used for implementing an extended version of the HMGC hybrid MHD gyrokinetic code. The upgraded version of HMGC will be able to handle fully compressible non-linear gyrokinetic equations and 3D MHD.

It is shown that, within the framework of the extended neoclassical theory, for a tokamak there exists a relationship between the normalized thermalenergy content, β, and the normalized plasma current, 1/q. This relationship, which depends essentially only on plasma fuelling, is a generalization of the usual Bennett relation and it is of importance for tokamak confinement and heating.

This article reports on the design of the motional Stark effect diagnostic (MSE) for the Frascati Tokamak Upgrade tokamak (Rmaj=93 cm, rmin=33 cm, BT

Progress in thermonuclear fusion energy research based on deuterium plasmas magnetically confined in toroidal tokamak devices requires the development of efficient current drive methods. Previous experiments have shown that plasma current can be driven effectively by externally launched radio frequency power coupled to lower hybrid plasma waves. However, at the high plasma densities required for fusion power plants, the coupled radio frequency power does not penetrate into the plasma core, possibly because of strong wave interactions with the plasma edge. Here we show experiments performed on FTU (Frascati Tokamak Upgrade) based on theoretical predictions that nonlinear interactions diminish when the peripheral plasma electron temperature is high, allowing significant wave penetration at high density. The results show that the coupled radio frequency power can penetrate into high-density plasmas due to weaker plasma edge effects, thus extending the effective range of lower hybrid current drive towards the domain relevant for fusion reactors.

The electrical conductivity measurements reported from various tokamaks (D-III, PLT, TEXT, ASDEX, JT-60, TEXTOR, JET, TFTR) and compared with the usual neoclassical theory are here also compared with the extended neoclassical theory where the electron-electron collision rate is anomalous while the electron-ion collision rate remains Coulombian. It is found that, out of the 14 experiments considered, three are consistent with both the neoclassical and the extended neoclassical theories, four are consistent only with the extended neoclassical theory, and four are consistent with the neoclassical theory and also, within the experimental errors, not inconsistent with the extended neoclassical theory; the remaining three experiments appear to be incompatible with both theories. It is concluded that the extended neoclassical theory is in better agreement with conductivity experiments than the conventional neoclassical theory and, indeed, the extended theory is a serious candidate for explaining tokamak behaviour, since it accommodates naturally an anomalous electron thermal transport, which the conventional neoclassical theory is unable to do

Since the 2012 IAEA-FEC Conference, FTU operations have been largely devoted to runaway electrons generation and control, to the exploitation of the 140GHz electron cyclotron (EC) system and to liquid metal limiter elements. Experiments on runaway electrons have shown that the measured threshold electric field for their generation is larger than predicted by collisional theory and can be justified considering synchrotron radiation losses. A new runaway electrons control algorithm was developed and tested in presence of a runaway current plateau, allowing to minimize the interactions with plasma facing components and safely shut down the discharges. The experimental sessions with 140GHz EC system have been mainly devoted to experiments on real-time control of magnetohydrodynamic (MHD) instabilities using the new EC launcher with fast steering capability. Experiments with central EC injection have shown the onset of 3/2 and 2/1 tearing modes, while EC assisted breakdown experiments have been focused on ITER start-up issues, exploring the polarization conversion at reflection from inner wall and the capability to assure plasma start-up even in presence of a large stray magnetic field. A new actively cooled lithium limiter has been installed and tested. The lithium limiter was inserted in the scrape-off layer, without any damage to the limiter surface. First elongated FTU plasmas with EC additional heating were obtained with the new cooled limiter. Density peaking and controlled MHD activity driven by neon injection were investigated at different plasma parameters. A full real-time algorithm for disruption prediction, based on MHD activity signals from Mirnov coils, was developed exploiting a large database of disruptions. Reciprocating Langmuir probes were used to measure the heat flux e-folding length in the scrape-off layer, with the plasma kept to lay on thea internal limiter to resemble the ITER start-up phase. New diagnostics were successfully installed and tested, as a diamond probe to detect Cherenkov radiation produced by fast electrons and a gamma camera for runaway electrons studies. Laser induced breakdown spectroscopy measurements were performed under vacuum and with toroidal magnetic field, so demonstrating their capability to provide useful information on the surface elemental composition and fuel retention in present and future tokamaks, such as ITER. © 2015 EURATOM.

Explicit use of the poloidal Ohm's law, together with the radial plasma velocity (obtained from the distribution of plasma sources), leads to a very general constraint on the possible radial profiles of plasma density and temperature. The constraint does not require any ad hoc assumption; it can place severe restrictions on the allowed profiles and is independent of energy and particle transport; also, it may be the underlying principle of profile consistency. The constraint is discussed in the framework of neoclassical theory, using results from the Frascati tokamak.

High Z materials, as possible plasma facing components for a fusion reactor, are being re-evaluated due to their advantages connected to the low sputtering yield and small erosion. A data base of experimental results related to their use on present Tokamak is missing. FTU has carried out a series of experiments with different medium and high Z limiter materials, Ni, Mo, and W, to improve the knowledge of the plasma behavior in these conditions. The results on plasma operation, core parameters, impurity content, and corresponding radiation losses are presented, and compared with those obtained with a low Z dominant impurity (siliconization). A simple model coupling the SOL and the plasma core is able to reproduce the experimental results, indicating that sputtering and self-sputtering are the main physical mechanisms of impurity production. © 1995 American Institute of Physics.

The power deposition profile in the ion cyclotron range of frequencies (ICRF) has been investigated experimentally in JET by means of a square wave modulated RF perturbation. The study has been conducted in D(H) and D(3He) plasmas for two heating scenarios. In D(3He) plasmas and for central heating in a scenario where mode conversion to Bernstein waves is accessible, the direct power deposition profile on electrons has been derived. It accounts for 15% of the total coupled power and extends over 25% of the minor radius. Outside the RF power deposition zone, the electron thermal diffusivity χe inside the inversion radius surface (ri) can be estimated through observation of the diffusive electronic transport. In discharges without monster sawteeth and for a low central temperature gradient (∇Te(r ≤ ri) ≤ ∇Te(r ≥ ri) ≈ 5 keVm−1) the value obtained is small (≈ 0.24 ± 0.05 m2s−1), typically ten times ower than χe values deduced from heat pulse propagation in similar discharges at radii larger than the inversion radius. For the D(H) minority heating scheme, a large fraction of the ICRF modulated power is absorbed by minority ions, and the minority tail is modulated with a characteristic ion-electron (i–e) slowing-down time. In this scheme, electron heating occurs only through collisions with the minority ion tail and no modulation of the electron temperature is observed in sawtoothing discharges. This is interpreted as a consequence of the long i–e equipartition time, acting as an integrator for the modulated ICRF signal. Finally, a correlation between the time of the sawtooth crash and the periodic turn-off of the ICRF power is found and its consequence for modulation experiments is reviewed.

Neutral hydrogen atoms, which give rise to the prominent so lar Ly-alpha corona, are closely coup led to the emerging solar-wind plasma. The density ratio of neutral hydrogen to protons is minute, ~10-6; therefore, the neutral atoms are tracers in the solar wind. In-situ observations of the neutral atoms, their flight paths (imag ing), density, and velocity distribu tions are a new tool to the understanding of the Ly-alpha corona, i.e. setting limits on the plasma velocity distribution along the solar magnetic field lines. The other goal of the neutral solar- wind instrumentation is the in-situ observation of the interactions between solar wind plasma and dust grains near the Sun. We will discuss the science objectives and the potential "zero charge" solar-wind instrument envelope onboard Solar Orbiter.

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.

An overview of the Frascati tokamak upgrade (FTU) results during the period 2000-2002 is presented. Long duration internal transport barriers (ITBs) have been obtained on FTU with combined injection of lower hybrid (LH) and electron cyclotron (EC) waves in 5 T/0.5 MA discharges. The ITB phase lasts about 10 energy confinement times and is characterized by an energy confinement time up to 1.6 times the ITER97 L-mode scaling. Temperatures up to I I keV are measured at 0.9 x 10(20) m(-3) central density. ITB studies using IBW injection have also been continued up to 8 T/0.8 MA. The LH system has operated at full power allowing us to complete the current drive (CD) studies at ITER-relevant densities. At these density values the electrons and ions are coupled and an increase in the ion temperature is clearly observed. A preliminary sign of enhanced CD efficiency has been obtained in the combined injection of EC and LH waves at magnetic field values lower than the resonant field for thermal EC absorption. Pellet optimization studies have been performed in order to test the conditions under which a quasi-steady-state confinement improvement can be obtained and impurity accumulation can be avoided. The confinement time in Ohmic discharges is generally in agreement with the ITER97 L-mode scaling. Transient confinement improvement is observed for a duration less than one energy confinement time. Radiation improved mode studies have been started thanks to the recently inserted boronization system, which has allowed reduction of the radiated power. Confinement improvement with neon injection has been observed in 6 T/0.9 MA discharges. Transport studies on profile stiffness and magnetohydrodynamic (MHD) studies of fast reconnection and snakes will also be presented.

High field pellet fuelled discharges in FTU have achieved good energy and particle confinement properties in ohmic quasi-steady regime. Peak densities close to 1x1021 m-3 have been obtained, showing a post pellet decay time of several energy confinement times. Experiments aimed at obtaining Radiative Improved Modes (RIM) at high field have also been carried out using Neon injection. We have observed an increase of average density with respect to the reference discharge, which cannot be explained by the contribution of Neon. The neutron yield increases also by a factor 3- 6 and the energy confinement time increases by a factor 1.4

The time-of-flight neutral particle measurement system1 has been installed on the 10-O port at the 2nd campaign. It has the capability of two-dimensional measurement with a larger viewing angle by using the pivot on the rectangular port, which is set inside of the 10-O mother port. The movable stage with the vertical scan mechanism rotates on the fan shaped stainless rails around the pivot. The sight lines of −2 to 32 degrees against the 10-O port surface makes the angle between the sight line and the magnetic axis of the standard configuration (3.6 m) of 95 to 35 in the 4th campaign. Therefore the analyzer can observe high-energy particles with these pitch angles. The higher scan speed of 0.17 degree/second than that in the last campaign (0.05 degree/s) can be obtained in this campaign. Precise spatial distribution of the high-energy particles can be expected because it enables the continuous scanning in the long discharge.

The potentiality of exploring the plasma dynamical behavior of Mercury via Energetic Neutral Atom (ENA) analysis is briefly discussed. Through ENA detection, the Hermean magnetospheric configuration and morphology as well as the existence of radiation belts would be clarified. In order to demonstrate the feasibility of ENA imaging in the Hermean environment, an empirical model of the equatorial proton distributions, originally developed for the Earth’s magnetosphere, is here modified in order to be applicable to the expected conditions, according to the present knowledge of the planet. Simulations of the ENA radiation are then presented and discussed. It is shown that the simulated radiation is significantly high, especially when pointing the field of view towards the Hermean surface. The way of determining the proton flux distributions and the cross-tail electric field characteristics by using the ENA signal is discussed. Finally, e preliminary design of an ENA detector suitable for the ESA mission to Mercury ‘BepiColombo’ is proposed and discussed.

It has been suggested by various authors over the past 15 years, and also recently, that in tokamaks the perpendicular resistivity η⊥ is between 102 and 103 times the parallel (Spitzer) resistivity. Here it is shown that on the contrary the measurements on the FTU tokamak [Proceedings of the 16th Symposium on Fusion Technology, London, 1990 (North‐Holland, Amsterdam, 1991), Vol. 1, p. 218] are consistent with η⊥/η∥=2 when the extended neoclassical theory is used (which is preferable to the conventional neoclassical theory for reasons which are here reviewed).

The EC system at 140 Ghz, 2 MW, being implemented on FTU tokamak for performing electron heating, profile control and current drive, is composed by four gyrotrons with 0.5 s pulse length capability, and four hybrid mirror/waveguide transmission lines. The launching system is capable of poloidal/toroidal beam steering through a set of tiltable in-vacuum mirrors, coupling to the O-mode at the fundamental resonance, with full control of the e.m. field polarization. Experiments have been made with two gyrotrons, by launching 0.8 MW to the plasma. In the high density regime (local n{sub e} from {approx_equal}0.8 up to 2 10{sup 20} m{sup -3}) the e-i energy transfer is significant, and ion heating is observed through the enhancement of the neutron emission in Deuterium plasmas. The highest electron heating is observed when on-axis ECRH is performed in the current ramp-up, sawtooth-free phase. In steady-state conditions, saw-tooth period is increased up to stabilization by off-axis ECRH. Localized ECCD is also performed to assist in the shaping of the current density profile.

About 400 kW of rf power at the frequency of 433 MHz were coupled to the plasma of the FTU tokamak by a waveguide antenna. During the experiment, a peaking of the Te and ne profiles and a Hα drop have been observed during the IBW pulse, only when operating with toroidal magnetic field value which allows the wave penetration to in the plasma bulk. Moreover, the electron temperature increased typically from about 2.5 keV to about 4.5 keV with a coupled power of about 350 kW; the line-averaged central plasma density increased from 3×1019 m−3 to about 5×1019 m−3.

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To operate the International Experimental Thermonuclear Reactor (ITER) in a safe and reliable way, it is of vital importance to have a proper knowledge of the current density distribution in the plasma. Moreover, the fact that very promising results have been achieved with the socalled advanced tokamak scenarios indicates that active feedback on the current density profile would be very desirable.

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Results are reported of the 8 GHz Lower Hybrid experiments on FTU after the installation of the new toroidal limiter. A figure of merit of the Current Drive efficiency eta(CD) approximate to 0.11.10(20) A/Wm(2) is estimated for plasma density = 10(20) m(-3) and no appreciable broadening of the launched frequency is detected. In low density experiments sawteeth are stabilised and m = 1 activity is present in the plasma. Shear reversed discharges with large reversal radius, r(S)/a approximate to 0.5, are obtained at higher density, lower temperature, B-T = 4 T, q(a) approximate to 5.5, by off-axis LH CD. The reversed configurations exhibit high central temperature coexisting with regular m = 2, n = 1 relaxations of large amplitude and are maintained up to LH switch off. At higher magnetic field, B = 5.2 T, q(a) approximate to 7, irregular DTM crashes are present during the whole LH pulse. Confinement time of radiofrequency heated discharges (P-LH = 0.5 divided by 2.P-OH) exhibits the same behaviour of FTU ohmic discharges following the ITER89-P scaling. Preliminary results of central 140 GHz Electron Cyclotron Resonant Heating (ECRH) during the plasma current ramp-up, aimed at obtaining shear reversed configurations are also reported.

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This paper presents a general overview of the Frascati Tokamak Upgrade (FTU) machine. The status of the major systems and the relevant upgrades are discussed together with a few data on the operation status and the operating experience, and the near term improvements and programme.