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Fusion Fission Hybrid Systems (FFHS) could have a potential role in the management of fission reactor wastes, with (in principle) some advantage over the comparable Accelerator Driven systems, devoted to the same objective. The validation of the concept poses major challenges in the area of fusion “source” development. However, the physics of the multiplying and transmutation blanket has also to be carefully understood and validated. Simple experiments can be envisaged for that purpose. Some of these experiments have already been performed in a different research framework (i.e. validation of the ADS concepts) and could constitute a very first database. Moreover, a new series of similar experiments could be planned.

JT-60SA is the large Tokamak device that is being built in Japan under the Broader Approach Satellite Tokamak Programme and the Japanese National Programme and will operate as a satellite machine for ITER. The main goal of the JT-60SA Programme is to provide valuable information for the ITER steady-state scenario and for the design of DEMO, where the real-time control of the safety factor profile is very important, in connection with both MHD stability and plasma confinement. It has been demonstrated in this work that to this end polarimetry measurements are necessary, in particular in order to reconstruct the safety factor profile in reversed shear scenarios. In this paper we present the main steps of a conceptual feasibility study of a multi-channel polarimeter diagnostic and the resulting optimised geometry. In this study, magnetic scenario modelling, a realistic CAD-driven design and long-term operation requirements, rarely even considered at this stage, have been considered. It is shown that a far infrared polarimeter system, with a laser operating at a wavelength of 194.7 ?m and up to twelve channels can be envisaged for JT-60SA. The top requirements can be attained, i.e., that the polarimeter, together with other diagnostic measurements, should provide q-profile reconstruction with an accuracy of 10% for the entire plasma cycle and suitable time resolution for real-time applications, in particular in high density and ITER-relevant plasma scenarios.

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The JT-60SA tokamak, being built under the Broader Approach agreement jointly by Europe and Japan, is due to start operation in 2020 and is expected to give substantial contributions to both ITER and DEMO scenario optimisation. A broad set of preparation activities for an efficient start of the experiments on JT-60SA is being carried out, involving elaboration of the Research Plan, advanced modelling in various domains, feasibility and conception studies of diagnostics and other sub-systems in connection with the priorities of the scientific programme, development and validation of operation tools. The logic and coherence of this approach, as well as the most significant results of the main activities undertaken are presented and summarised. EURATOM 633053

Polarimetry measurement results are used in tokamak plasma equilibrium codes as a constraint, decreasing the measurement uncertainty in determination plasma current, safety factor and density profile. In numerical codes, instead of solving rigorous equation of polarimetric state evolution, approximate analytical expression could be used to increase computing efficiency. In this paper is analyzed the precision of three different models compared with exact solution of polarization state evolution equation. Calculations are made for circular plasma model, with plasma parameters representative for operating domain of tokamak device. The obtained results define the limits of applicability of the separate proposed approximated methods in equilibrium codes for plasma in configuration similar to considered in the paper.

International audience; This paper reports numerical investigations on the identification of the electron temperature profile T e from interferometry and polarimetry Stokes vector measurements with the equilibrium code Newton direct and inverse computation for equilibrium. This latter enables the consistent resolution of the inverse equilibrium reconstruction problem in the framework of nonlinear free-boundary equilibrium coupled to the Stokes model equation for polarimetry. We find that for ITER plasma with high I p , N e and T e the identification from noisy measurements is possible.

The study is dedicated to the evaluation of the effects of the input polarisation angle with respect to the toroidal magnetic field on the values of the Faraday Rotation, Cotton-Mouton phase shift angle (and then ellipticity). The need of this evaluation is connected to the optimisation of the polarimetry quantities in view of having a complete information related to magnetic fields, plasma density and electron temperature. A polarimeter with vertical as well as horizontal lines of sights at various angle with respect to the equatorial axis is considered. The equilibrium used is a circular analytic model which considers a tokamak in the slab approximation. The plasma parameters considered are B0=3.1 T (toroidal magnetic field), I0=3.5 MA (plasma current), N0=2⋅ 1020 m−3 (plasma density), R0=3 m (major radius) and a=1 m (plasma radius), so R0/a=3 (aspect ratio). The input polarisation angle is varied between −90o and 90o. It is found that the polarimetry signals of horizontal lines of sights are very sensible to the polarisation input angle leading to strengthening the capability of measuring the plasma density and electron temperature in these lines. Numerical calculations show that the largest change in the polarization state, leading to the reduction of the measurement errors, is for the beam with initial Stokes vector 0=(0,± 1,0) (polarization angle ψ0=± 45o).

Previous work on overcoated dielectric mirrors identified several radiation problems; UV–visible reflectivity degradation, unreliability of manufacturers coating specifications, and conversion of SiO coatings to SiO2 leading to swelling and cracking, and hence possible corrosion during LOCAs (loss of coolant accident). We report the results obtained for gamma and neutron irradiation of prototype mirrors with KU1 and KS-4V high purity silicas replacing the standard Pyrex substrate to overcome the problem of activation due to high impurity content. To avoid the SiO to SiO2 conversion problem, the Al layer has been overcoated with SiO2. The mirrors show high reflectivity (>80%) down to 200 nm. After gamma irradiation at 170 °C in a dry nitrogen atmosphere to 10 MGy, no degradation in reflectivity was observed. They were then neutron irradiated in vacuum at 35 °C to 6.8 × 1016 n/cm2 (fast fluence) and reflectivity examined with a specially designed spectrometer system with a remote integrating sphere for hot cell use. Again no degradation in reflectivity was observed.

The present paper is devoted to a first assessment of the DEMO diagnostics systems and controls in the context of pulsed and steady state reactor design under study in Europe. In particular, the main arguments treated are: (i) The quantities to be measured in DEMO and the requirements for the measurements; (ii) the present capability of the diagnostic and control technology, determining the most urgent gaps, and (iii) the program and strategy of the research and development (R&D) needed to fill the gaps. Burn control, magnetohydrodynamic stability, and basic machine protection require improvements to the ITER technology, and moderated efforts in R&D can be dedicated to infrared diagnostics (reflectometry, electron cyclotron emission, polarimetry) and neutron diagnostics. Metallic Hall sensors appear to be a promising candidate for magnetic measurements in the high neutron fluence and long/steady state discharges of DEMO.

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.

In order to support the operation of ITER and the planned experimental programme an extensive set of plasma and first wall measurements will be required. The number and type of required measurements will be similar to those made on the present-day large tokamaks while the specification of the measurements—time and spatial resolutions, etc—will in some cases be more stringent. Many of the measurements will be used in the real time control of the plasma driving a requirement for very high reliability in the systems (diagnostics) that provide the measurements. The implementation of diagnostic systems on ITER is a substantial challenge. Because of the harsh environment (high levels of neutron and gamma fluxes, neutron heating, particle bombardment) diagnostic system selection and design has to cope with a range of phenomena not previously encountered in diagnostic design. Extensive design and R&D is needed to prepare the systems. In some cases the environmental difficulties are so severe that new diagnostic techniques are required. a Author to whom any correspondence should be addressed.

Numerous experiments were performed on JET to clarify the link between rational q, magnetic shear and internal transport barriers (ITBs) by varying the q-profile from a monotonic one to one with a central reversed shear (Challis et al 2002 Plasma Phys. Control. Fusion 44 1031). The q-profile was found to be crucial in two cases: for ITB formation in the presence of a strong negative magnetic shear and for ITBs in the vicinity of low order rational q surfaces with a small magnetic shear. The ITBs of these two types have been modelled and analysed in this work using transport codes (TRANSP, JETTO, TRB). Transport coefficients were calculated using several widely used theories of micro-turbulence and compared with data deduced from the experiment to verify the applicability of the theories. Different underlying physical mechanisms were found to be responsible for ITB formation in a negative magnetic shear and in the vicinity of a rational minimum q.

The radial electric field gradient or the E×B flow shear has been pointed out as the underlying mechanism for turbulence suppression, responsible for an internal transport barrier formation in advanced tokamak scenarios. A comprehensive study on these subjects requires a direct measurement of the plasma radial electric field Er. The poloidal component of the magnetic field is assessed by the motional Stark effect (MSE) polarimeter, which is currently a standard diagnostic in fusion devices, allowing a local and nonperturbative measurement of the magnetic pitch angle. A precise measure to the state of polarization of the Stark components gives the information on the direction of the magnetic field. Due to the particular orientation of the Lorentz component, that is nearly perpendicular to Er, the MSE diagnostic is very sensitive to the plasma intrinsic radial electric field. This article describes a technique to measure Er involving the change of the polarization angle of the MSE emission, by using two bea...

The current state of investigations of the problem of providing first mirrors (FMs) for diagnostic systems in a reactor-grade fusion device is summarized. Results obtained in simulation experiments that have been conducted during recent years in several laboratories are presented. Attention is concentrated on two processes that can have an opposite effect but both can lead to degradation of mirror optical properties, namely: sputtering by charge exchange atoms which leads to erosion, and deposition which leads to surface contamination. It is shown in the analysis that when sputtering dominates, mirrors of monocrystalline refractory metals (Mo, W) can have a sufficiently long lifetime even for FMs that have to be located close to the first wall. Similarly, films of low sputtering yield metals on high thermal conductivity substrates (e.g., Rh on Cu) can be used for FMs in locations where the charge exchange flux is reduced to about a tenth of that at the first wall. However, deposition poses a serious threa...

The general requirements for a plasma facing mirror (PFM) of a Thomson scattering system (TS) for a burning plasma experiment are (i) high and approximately constant reflectivity in the wavelength spectral range 400–800 nm; (ii) low sputtering yield and low erosion; (iii) high power damage threshold; (iv) good thermo-mechanical properties to preserve quality imaging. Rhodium-coated mirrors are chosen because they meet these requirements. Rhodium coated mirror were realized with substrates of copper and vanadium. The detailed optical characterization of these mirrors is presented: i.e., surface planarity measurements as well as roughness and reflectivity figures are presented. These data can be used for the choice of the PFM of a TS system for international thermonuclear experimental reactor.

In recent years the necessity of understanding the physics concerning new tokamak results has clearly shown the limits of the actual diagnostic capability of existing devices, in terms of reliability, precision, spatial localization, and temporal resolution of measurements. This appears even more evident when next-step tokamak devices are considered. For example, the formation and evolution of the plasma internal transport barriers and the regulation of its positions by a detailed tailoring of the current spatial profile and plasma heating gives demanding requests to a complex set of diagnostics including motional Stark effect as well as incoherent Thomson scattering (ITS). The measurement of detachment of plasma in a divertor puts some requirements on a dedicated ITS. The rotational stabilization of ideal magnetohydrodynamic modes puts some important accent to the plasma rotation measurements. The coherent (or collective) Thomson scattering giving ion temperature can be useful in a prototype fusion react...

The possibility of determining the plasma electron temperature by measuring the depolarization of the radiation scattered by the high temperature fusion reactor plasma is explored in this article. A numerical calculation based on the theory of the article [Sov. J. Plasma Phys. 7, 669 (1981)] in the case of 90° scattering, with the incident radiation linearly polarized and the polarization plane at 45° with respect to the scattering plane, is carried out. The result is that there is a one to one relationship between plasma electron temperature and depolarization, being independent from the detection wavelength. As an example electron density and temperature of 1020 m−3 and 20 keV, respectively, are taken, a wavelength and energy of the incident radiation pulse of 1064 nm and 8 J, respectively, and for a wavelength detection range of 1050–660 nm, a depolarization value of 0.19 and an estimated error on it of 1.7% are found, if the scattering volume is 2 cm long and the collection solid angle is 7.85×10−3 sr...

The possibility of diagnosing the alpha particle distribution in ITER by means of collective Thomson scattering (CTS) using microwaves below the electron cyclotron frequency is investigated. With respect to other schemes, using much shorter wavelengths (e.g., 10 to 200 mu m), this method combines some important advantages, i.e., the availability of the appropriate wave sources, the simplicity of the scattering geometry and the possibility of space resolved measurements. The useful frequency range for this method is limited by refraction effects and by the noise due to spontaneous electron cyclotron emission (ECE). These constraints are evaluated by means of a ray tracing/radiation code, over a wide range of typical ITER parameters. As a result of this general study, a proposal is made for a relatively simple CTS system for ITER, using backscattering of extraordinary waves in the frequency range 60 to 80 GHz. This method allows, in principle, spatially resolved measurements, requiring partial occupation of an equatorial port only

Measured figures (linearity, avalanche gain, frequency response, and noise) of the avalanche photodiodes (APD) detectors used on the FTU Thomson scattering system (FTU-TS) are reported. This investigation on the APD figures is motivated by the necessity of knowing their values over a wide range of input signals, and also of checking the validity of McIntyre model of APD noise, since using this model the experimental signal-to-noise ratio is calculated. From the noise analysis, a method of optimization of the detection system sensitivity is derived, leading to the optimization of the signal-to-noise ratio over the entire FTU-TS system.

An important effect observed in ILW discharges is the change of q = 1 MHD activity in hybrid discharges. The q = 1 activity with the JET C-wall had usually shown no effect on plasma confinement, except for NTM triggering, its main experimental signature being a rotating kink activity interleaving sawteeth crashes. In the C-Wall hybrid scenario sawteeth had low amplitude and frequency, and the rotating precursor had often fishbone characteristic. The radial profile of the mode fluctuation has been calculated using a coherence technique between one pick up coil and 96 ECE measurements along the red L.o.S in Figure 1. As explained in ref.[1], the coherence phase radial profile shows a -jump whenever an island is crossed moving along the plasma radius, this effect is due to the radial displacement of magnetic field lines produced by the rotating island, which has opposite directions on the outer side of the island x-point than on the inner side of the x-point. In the C-Wall hybrid pulses the ECE fluctuation at the q = 1 magnetic frequency did not show any radial phase jump, as can be seen in Figure 2, and therefore had kink characteristic.

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.

The Frascati tokamak upgrade (FTU) Thomson scattering system is employed for the measurement of the electron temperature and density spatial profiles along the vertical torus diameter in 19 spatial points up to ten times in a single plasma discharge with a spatial resolution ranging from 2 cm in the central region to 4 cm in the plasma edge. The radiation source is a Nd:YLF laser at 1053 nm. The scattered radiation is collected by two objectives: the first looks at the plasma center, the second at the plasma edge. Bundles of optical fibers in the focal plane of the objectives carry the scattered light from the tokamak hall to a set of 19 interference filter polychromators, whose transmission is 70% and the rejection of the stray light at the laser wavelength is 1/107. The detectors are avalanche photodiodes with a NEP of the order of 10−13 W/(Hz)1/2 at 1053 nm. The absolute calibration for the electron density measurement has been carried out by Raman scattering on hydrogen and deuterium. Examples of temp...

Polarimetric Thomson scattering (TS) is a novel diagnostic technique proposed as an alternative to conventional (spectral) TS, for the measurement of the electron temperature Te and density ne in very hot fusion plasmas. Contrary to spectral TS, which is based on the reconstruction of the Doppler broadened frequency spectrum, in polarimetric TS Te is determined from the depolarization of the scattered radiation. The technique is suitable for ITER, where it is expected to be competitive with conventional spectral TS for measurements in the highest Te range, specially in backward-like conditions with the scattering angle 90° ? ?

The so called Hybrid (Fusion-Fission) Reactors seem to be a realistic alternative to the drawbacks of both a pure Fusion and/or a pure Fission reactor; this alternative could also guarantee a "substantial" destruction of the transuranic nuclear waste or production of nuclear combustible. So far, several different hypothesis have been proposed for this type of reactor, all of them based on very advanced and hypothetic fusion scenarios. In the present paper we propose two possible compact (R = 2.5-2.9 m; a = 0.73-0.8 m) high toroidal field (BT = 8.5-7.0 T; Ip = 8-6.8 MA) Tokamaks (based or not on superconductors) to perform a robust and "economic" experiment, propaedeutic to the definitive design of the reactor, where the problems connected with the integration of a Tokamak device with a Fission device can be tested on a realistic base. On both solutions the total fusion power will be of the order of 100 MW and the14 MeV neutron density power of the order of 0.5-1 MW/m2.

An equivalent model of JET polarimeter is presented, which overcomes the drawbacks of previous versions of the fitting procedures used to provide calibrated results. First of all the signal processing electronics has been simulated, to confirm that it is still working within the original specifications. Then the effective optical path of both the vertical and lateral chords has been implemented to produce the calibration curves. The principle approach to the model has allowed obtaining a unique procedure which can be applied to any manual calibration and remains constant until the following one. The optical model of the chords is then applied to derive the plasma measurements. The results are in good agreement with the estimates of the most advanced full wave propagation code available and have been benchmarked with other diagnostics. The devised procedure has proved to work properly also for the most recent campaigns and high current experiments.

An overall view of the Frascati Tokamak Upgrade (F.T.U.) Thomson scattering system is illustrated and particular attention is paid to the optical and spectral analysis equipments. The system includes the laser source, the collection optics, a set of filter polychromators, all the electronic equipments necessary to collect the data relevant to the scattered light and to send them to the F.T.U. data acquisition system. The features of this system can be summarized as follows: rejection to laser wavelength: ≥107, laser energy: ≥5 J, overall optical efficiency: ≥40%, and capability of multiple time and space measurements on the Thomson scattered light.

We present the issue raised by electron cyclotron resonance heating for the present development of Fusion Reactors. The possibility of realizing a Free Electron Laser operating in the range 220–450 GHz is discussed also in view of heating in high magnetic field Tokamak

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.

Measurements of plasma radiation in the infrared (IR) BREM_IR range (λ=978 nm, Δλ=38 nm), at 16 horizontal lines of sight intersecting the central vertical chord of the [Frascati Tokamak Upgrade (FTU) R=0.935 m, a=0.3 m, B=4–8 T], from z=−0.22 m below, to z=0.08 m above the equatorial plane, are performed using the detection system of the Thomson scattering apparatus [F. Orsitto et al., Appl. Opt. 34, 2712 (1995)] (TS). The measured BREM_IR intensity is equal I(z)=〈ZeffGne2/Te1/2〉 where 〈〉 means line average. For determining the Zeff, the Gaunt factor (G) is needed, and the Born–Elwert formula is used. The Zeff spatial profiles are determined using the plasma temperature (Te) and density (ne) measured by the TS, DCN interferometer, and Abel inverted intensity profiles. Zeff(r) versus time behavior in FTU discharges is presented for krypton injection experiments aimed to investigate the particle confinement behavior during strong injection of impurities.

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.

Measurement of plasma radiation (i.e., breusstrahlung) in the infrared (IR) range (λ=933, 978 nm), at six lines of sight from z=−20 cm to z=8 cm above the equatorial plane, using the detection system of the Frascati Tokamak Upgrade (FTU) Thomson scattering system (TSS) are reported. The agreement of IR with visible (λ=540 nm) bremsstrahlung intensity S, [S=photons/(m2 sr nm s)] measurements is within 20%–30% and depends upon the absolute calibration of both systems. The intensity is equal S(z)=〈ZeffGn2/Te1/2〉, where 〈⋅〉 means average on a line of sight. For determining the Zeff the Gaunt factor(G) is needed, and analysis the Born–Elwert formula is used. The Zeff spatial profiles (i.e., Zeff(r)), are determined using the plasma temperature (Te) and density (ne) measured by the TSS and the Abel inverted intensity profiles, determined using the plasma radiation S(z) measured from six horizontal chords. Zeff(r) behavior in a variety of FTU discharges is presented.

The density of Fe atoms produced by bombardment of a stainless steel target by Ar+ ions was measured by laser induced fluorescence (LIF) using a tunable dye laser. The excitation transition involved was a 5D4→y 5D04 (λ=302.064 nm); measurements of the fluorescence signal, from the y 5D04→a 5F5 decay (λ=382.043 nm), were made by varying the laser power incident on Fe atoms by means of grey filters. An absolute calibration of the detection system allowed the determination of the number of the emitting atoms, versus the incident power. So the experimental curve of the fraction of total pumped atoms versus the saturation parameter was obtained. On the other hand, that fraction can be calculated by using standard rate equations for LIF process involving three level atoms. The result is strongly dependent on the model for the pumping laser pulse, particularly (as expected) at low incident power level. Assuming a realistic Gaussian temporal dependence for the radiation field energy flux at frequency v, i.e., U ...

The recent development of real-time measurements and control tools in JET has enhanced the reliability and reproducibility of the relevant ITER scenarios. Diagnostics such as charge exchange, interfero-polarimetry, electron cyclotron emission have been upgraded for real-time measurements. In addition, real-time processes like magnetic equilibrium and q profile reconstruction have been developed and applied successfully in real-time q profile control experiments using model based control techniques. Plasma operation and control against magnetohydrodynamic instabilities are also benefiting from these new systems. The experience gained at JET in the field of real-time measurement and control experiments operation constitutes a very useful basis for the future operation of ITER scenarios.

From discussions with the EFDA-JET Task Forces in early 2000, a range of requirements emerged for higher spatial resolution Te and ne Thomson scattering measurements in various specific regions of JET plasmas. These requirements are summarized in table 1 and seek significantly better spatial resolution than is achieved or is achievable with the two LIDAR Thomson scattering systems1,2 currently working on JET (4Hz core LIDAR ∼12cm; 1Hz Edge LIDAR 2–3cm, outer ∼10cm only). Two High Resolution Thomson Scattering (HRTS) proposals were received from the Associations, one from C J Barth, (FOM)3 and one from P Nielsen and R Pasqualotto, (RFX)4. These, in their final form, used almost identical “90 degree” scattering geometry (see figure 1).

A collective Thomson scattering diagnostic is being implemented at JET to diagnose the velocity distribution and density of alpha particles and energetic ions. This diagnostic consists of a number of major subsystems including: a high‐power 140‐GHz gyrotron, a 32‐channel 12‐GHz bandwidth heterodyne receiver, efficient corrugated waveguide systems, variable polarizers, and in vacuum beam steering mirrors. Proper calibration of these subsystems for signal level losses, positioning, and polarization is imperative for the successful operation of the diagnostic and to facilitate the interpretation of the scattered data. A low‐power Gunn oscillator was used to verify the polarization rotation induced by the seven miter bends in the receiver transmission line. The full bandwidth receiver front end losses, including the sapphire tokamak vacuum window, were initially calibrated by using the hot tokamak vessel wall as a blackbody source at approximately 310 °C. Background ECE measurements will be presented.

For physics interpretations as well as for control of advanced operational scenarios the knowledge of electron temperature and density profiles of the ITER plasma is absolutely necessary. The specifications for the core measurements are 10% accuracy for the electron temperature at a spatial resolution of 10 cm over the temperature range 0.5 – 30 keV, 5% accuracy for the density at a spatial resolution of 30 cm over the range 0.1 – 3.0* 1020 m-3. The repetition rate of the measurements should be 100 Hz.

The choice of the laser wavelength for ITER LIDAR THOMSON SCATTERING SYSTEM for the core plasma is linked to the extended range of plasma temperatures1 to be measured, i.e. 0.5–30keV. The additional requirements1 are: temperature relative error ΔTe/Te=0.1 with spatial resolution of 0.1m, plasma density relative error Δne/ne=0.05 and spatial resolution 0.3m, at a temporal resolution of 10ms.These last parameters determine the laser energy, the pulsewidth and the repetition rate. It has been found2 that a single laser working at a wavelength of 790nm solves the problem of measuring the ITER temperature with a relative error less than or equal to 20%. A Titanium-Sapphire laser (Ti:Al2O3) is the natural candidate for such a source since the fluorescence peak is at 800nm, with a very large tunability range between 700 and 1000nm. Other important characteristics of the material are the large gain cross section, and relatively short fluorescence lifetime, its hardness and good thermal conductivity properties inherent to the sapphire host, and the availability in large size, good quality crystals. High power short pulse Ti:Sapphire lasers can be built. The paper is organized as follows: the required laser figures are summarized, the main Ti:Sapphire(TiS) material parameters are listed in comparison to other laser materials, the scheme of a 10Hz TiS laser is presented, the possibilities of improving the previous scheme to 100Hz source are discussed and a comparison with other proposed schemes4 is cited, finally the conclusions are drawn about the possibility of building these sources with present day technology.

The requirements for the plasma facing mirror for the ITER LIDAR THOMSON SCATTERING (ILTS) SYSTEM1 are: high and as uniform as possible reflectivity in the spectral range 400–800 nm, low sputtering yield, and high laser damage threshold. Thermal deformation due to nuclear heating is also one of the effects to be taken into account and it must be compatible with acceptable optical imaging quality. Rhodium coated mirrors with a copper substrate are considered as possible candidates meeting these requirements. The paper describes the techniques used for building prototype mirrors with thick (30–80 nm) Rhodium coatings on copper substrates and their optical characterization.

Measured figures (linearity, avalanche gain, frequency response, and noise) of the avalanche photodiodes (APD) detectors used on the FTU Thomson scattering system (FTU-TS) are reported. This investigation on the APD figures is motivated by the necessity of knowing their values over a wide range of input signals, and also of checking the validity of McIntyre model of APD noise, since using this model the experimental signal-to-noise ratio is calculated. From the noise analysis, a method of optimization of the detection system sensitivity is derived, leading to the optimization of the signal-to-noise ratio over the entire FTU-TS system. © 1997, American Institute of Physics. All rights reserved.

Plasma behaviour in the high density regime has been investigated on the Frascati Tokamak Upgrade (FTU). The items particularly addressed are density limit, MARFE characteristics and fuelling efficiency. With gas puffing, a maximum line average density of 3.2*1020 m-3 has been obtained in the ohmic regime at q=5.1, corresponding to 1.7 times the Greenwald limit; while with pellet injection, a value of 3.7*1020 m-3 has been reached at q=3.5. The density limit appears to be connected with the impurity content and edge parameters, so the best results are obtained with very clean plasmas and peaked electron density profiles. The MARFE phenomenon always appears beyond a critical density that depends on the total input power and the effective charge: emissivities in the range of 2 to 12 MW/m3 have been measured in this highly radiating region. The fuelling efficiency, starting from a value of about 50% at low density, progressively deteriorates, falling to 10% near the density limit: this behaviour is interpreted as being due to a decrease of the scrape-off layer transparency to incoming neutrals

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