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A new ECRH launcher mainly aimed at experiments on MHD stabilization has been recently installed in FTU and integrated with a Real Time controller. The new plug-in antenna is capable of injecting two beams over wide angular ranges, ±40 in toroidal and ±15 < θ < 44 in poloidal direction with fast mirror speed. The power is delivered to the launcher with two additional transmission lines (TL) connected to the existing ECRH system with remotely controlled switches. After the optical alignment of the TLs, a campaign of tests on the steering antenna was made with the launcher ex- and in-vessel. A procedure to calibrate the absolute mirror angular position was developed and successfully used. First high power pulses were carried out on plasma target delivering 2 × 0.3 MW EC power for 200 ms. © 2013 CNR-IFP Instituto di Fisica Plasma.

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.

The development of high power gyrotrons for plasma physics research needs properly matched calorimetric loads able to absorb and measure the power, which nowadays is foreseen to be as high as 2 MW during CW operations. To this end IFP/CNR has developed a family of matched loads useful in the mm-wave frequency band for applications ranging from a few ms to CW in pulse length. The different loads in the family, made of an integrating sphere with a partially reflecting coating on the inner wall, are characterized by having the same absorbing geometry for the incoming beam and a different heat removal system for the specific application. Some important advances have been recently achieved from the point of view of the uniformity of power distribution on the absorbing wall and of the load construction. With the high precision achieved in the coating thickness a better control of the heating power distribution is possible by proper shaping of the local reflectivity, in addition to the shaping of the mirror dispersing the input beam. A more sophisticated model describing the power distribution has been developed, taking into account a variable thickness of the absorbing coating, the proper shape of the spreading mirror, the frequency of the incoming radiation and the shape of the input beam. Lower coating thickness is shown to be preferable, for a given local reflectivity, from the point of view of a lower peak temperature and thermal stress. The paper describes a load with variable coating thickness along the meridian of the sphere, showing a uniform power deposition on the inner walls. The cooling pipe is completely electroformed on the spherical copper shell, ensuring the maintenance of the correct curvature of the inner surface and fast heat conduction from the absorbing coating to the water through the thin copper body. For CW use all heated parts of the load must be cooled and this is achieved with 16 electroformed spiral channels. Both short pulse loads (0.1–1 s) and the CW version at 2 MW, 170 GHz, are described in the paper. High power tests on short-pulse loads have been done using a double frequency gyrotron, 105 GHz/600 kW for 0.5 s and 140 GHz/800 kW for 1 s. Also a method for emulating 2 MW conditions while using 1 MW gyrotron has been applied to test the load to be used for the European 2 MW coaxial cavity gyrotron development programme.

An increased power handling of rf-beam loads is required by the development of high power gyrotrons for fusion research, as well as the achievement of full capability to work under vacuum conditions, which is imposed by the use of evacuated transmission lines. New techniques were developed at IFP/CNR, leading to two prototypes. One is capable of 1 MW CW operation, while the second one is designed for precise measurements at short-pulses (0.1 s) at higher peak power (2 MW). Preliminary tests at 140 GHz, high power and long pulse (500 kW, 10 s) were performed. (c) 2005 Published by Elsevier B.V.

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.

For the electron cyclotron resonance heating (ECRH) system on the frascati tokamak upgrade (FTU), composed of four 0.5 MW gyrotrons pulsed for 0.5 s, two different calorimetric loads were designed to measure the mm-wave power at 140 GHz propagating either as a gaussian beam or as an HE(11) mode carried by an oversized corrugated waveguide. The first one, designed for low reflectivity and I MW power handling capability, is now routinely used for gyrotron testing, conditioning and optimization on the generator side of the transmission line at 500 kW, 0.5 s. The basic design of the load, with a substantial upgrading of the cooling system for adequate heat removal, has full CW operation capability. The other one, whose cylindrical shape allows positioning as a waveguide replacement, is occasionally used when tests on the transmitted power along the 40 m of waveguide are needed. The loads are described together with low power measurements of reflected radiation. (C) 2001 Elsevier Science BN. All rights reserved.

Barrier windows are critical components in electron cyclotron plasma heating/current drive (ECRH/ECCD) systems for fusion plasma experiments in which high frequency and high power microwave beams are used as additional heating or plasma profiles control. Even though material characteristics and thickness are chosen in order to reduce the absorbed power to a minimum, the window temperature is of major concern in high power, long pulse or CW operation. An infrared diagnostic device has been implemented on one of the boron nitride windows of the 140 GHz, 0.5 s, 0.5 MW per line ECRH system of the FTU tokamak. The evolution of the temperature of the barrier window can be monitored by acquisition of several IR emission profiles during the gyrotron pulse. Moreover, information on the beam pattern quality can be inferred. (C) 2001 Elsevier Science BN. All rights reserved.

The quasi-optical beam launching antenna for the Electron Cyclotron Resonance Heating (ECRH) experiment at 140 GHz, 1.6 MW to the plasma, on the Frascati Tokamak Upgrade (FTU) has been successfully exploited in experimental operations. It provides four beams 400 kW each, with independent poloidal and toroidal steering capability and a maximum power density of 60 kW/cm2 at the plasma edge. The beam radius in the plasma is ≈20 mm, allowing a very high localisation of the absorbed power. The main characteristics of the antenna are: • Four movable launching mirrors under vacuum, far from the plasma edge (no movable parts near the plasma). • Oblique toroidal injection capability at fixed angles obtained with reflections on two fixed stainless steel plates, gold plated, inserted at the sides of the port. • Vacuum gate separating the main vacuum from the appendix containing the two matching mirrors and the barrier window (for safety and maintenance). • All movements are transferred outside the vacuum chamber through bellows and linear displacement. Actuators and encoders are in air. • Full capability to adjust small movements of the machine during cool-down and warm-up, preserving the alignment. Pattern measurements at low power before installation were performed to characterise the effects of the real system on the ideal shape and the polarisation of the launched beams, with particular attention to diffraction effects. Final control of the shape of the beams and the alignment of the mirrors in the system is performed after installation, under vacuum and with the tokamak at liquid nitrogen temperature, with the aid of a retractable probe and using a thermopile as the RF detector.

The Electron Cyclotron (EC) fast launcher for real time control experiments has been installed on FTU and characterized to be fully integrated in a real-time MHD control module under development. The launcher scheme is based on a two module sys- tem, symmetric with respect to the equatorial plane of FTU, with a front steering concept and the launched beams are real-time controllable both in poloidal and toroidal directions. Specific design parameters, defined by the FTU MHD dynamics (typically island size and q profile changes), are the beam dimensions with zooming capabilities, the steering range and mirror speed with the most demanding requirement on poloidal speed of the Steering Mirror (SM) = 1 in 10 ms. A set of tests has been done to verify the system perfor- mance. High power tests of the launcher have been done on a 500 kA, 0.5 10 20 m -3 and 5.3 T plasma with 2 300 kW of EC power delivered to the plasma. The steering mechanism was tested under the automatic control system and showed a dynamic response in line with the requirements. Results of these tests will be presented in the paper.

A new electron cyclotron (EC) antenna for real time control experiments on FTU tokamak has been tested at IFP-CNR laboratory. mm-Wave characterization was performed at the operating frequency of 140 GHz with low power measurements, primarily to test the internal optics of the launcher in terms of focusing properties and beam directivity. The antenna is devoted to the injection of high power EC waves in different working configurations using a front steering setup. The beam direction is controlled with a fast mirror (steerable both toroidally and poloidally) and the available launching angles have been determined to fix the steering mirror working space. Beam dimension can also be controlled, using a sliding structure in the plug-in system, equipped with a focusing mirror that provides zooming capabilities. This paper contains results of low power tests performed on the antenna in order to characterize the launcher in its quasi-optical design. Beams are found to be slightly astigmatic (maximum ratio w(y)/w(x) = 1.1) and with an expected beam radius in the plasma ranging from 18.6 to 26.0 mm due to cited zooming effects. Additional beam patterns and linear scans of the launched mm-wave power for different injection angles have been measured, in some cases in realistic conditions (using a mock-up of the FTU port provided with metallic side-walls) to evaluate diffraction and beam truncation effects. (C) 2011 EURATOM ENEA - CNR - Association/Instituto di Fisica del Plasma. Published by Elsevier B.V. All rights reserved.

A compact matched load for high power gyrotron testing in ECRH plants for fusion research applications is in development, with the main goal of providing reflection-free absorption and fast calorimetric measurement of the millimeter-wave power, also at long pulse duration. It is based on the results of the tests at full power and pulse length (140 GHz, 0.5 Mw, 0.5 s) on similar loads installed on the ECRH plant of the FTU Tokamak in Frascati. Basically, the load is made of an integrating sphere in copper, with the inner wall covered by plasma spray with a mixture of lossy ceramics, providing an average wall reflectivity in the order of 40%. Absorbing material degradation, small-scale and large-scale damage patterns, caused by fatigue and long-term exposure to high power mm-wave, have been analyzed in detail, with the aim of improving the performances in terms of power handling capability and energy extraction efficiency. Physical-chemical analyses, performed on the original and on the degraded absorbing material, showed the effects of the mm-wave exposure. Damage patterns were recognized as interference of multiple reflections inside the load, with radiation accumulation close to the entrance port, as confirmed by a detailed numerical analysis. Increase in power and energy deposition capabilities require improvements of the present design in different areas:. - strategies for the mitigation of the different causes of non-homogeneous deposition;. - use of different plasma-sprayed materials as mm-wave absorbers. - improved cooling channel geometry with increased heat transfer rate to the cooling medium, for achieving 1MW-CW power capability.

High power gyrotron testing at long pulses is a necessary step in the development of mm-wave systems for fusion research. For this purpose a compact calorimetric load is being developed, with low overall reflectivity (< 5%) and aiming at 1 MW-CW capabilities, after the good performances of the loads (0.5 MW, 0.5 s per pulse) installed on the 140 GHz ECRH plant of the FTU Tokamak in Frascati. Tests at the same frequency but at higher power (0.7 MW, 1 s) and longer pulses (up to 2 s at 0.5 MW) on the ASDEX-Upgrade ECRH plant showed, after repeated exposures, damages on the absorbing layer, recognized as thermal effects due to the local enhancement of incident power: the effects visible near the entrance port are explained studying the superposition (with interference) of waves reflected inside the sphere. Samples of degraded and non-degraded coating have been analyzed with various techniques, revealing changes of the substrate's physical characteristics. Measurements of absorber temperature during and after the pulse, performed with an infrared detector looking directly into the load, allowed estimates of the peak temperature and thermal properties of the coating in the real working conditions. Improvements on power deposition, geometry and cooling are proposed, to be integrated in a new load design. (C) 2003 Elsevier Science B.V. All rights reserved.

The design of a compact matched load for high-power measurements and testing of gyrotrons and transmission lines in ECRH plants for fusion research applications is currently in an advanced phase. The aim is to provide more than 95% absorption and precise calorimetric measurement of the input power in CW. This work is based on the results of tests at high power and short pulse length (140 GHz, 0.5 MW, 0.5 s) on loads installed on the ECRH plant of the FTU Tokamak in Frascati. The loads consist basically of hollow spheres of copper with the inner wall covered by plasma-sprayed lossy ceramics. Tests at higher power and longer pulses on the ASDEX-Upgrade ECRH plant showed, after a number of successful pulses, progressive damage on the absorbing layer, marked by the appearance of electrical arcs. The absorber degradation, showing specific damage patterns, due to exposure to high-power millimetre waves, has been analysed in detail and strategies are proposed, in order to improve the power-handling capabilities and the energy extraction rate. New measurements of millimetric absorption and thermal conductivity have been performed on samples of different ceramics, for choosing the best absorbing layer.A modified expander mirror surface with a better deposition profile, numerically computed with a multi-reflection model of the sphere, is designed to avoid radiation accumulation close to the entrance port. Improved cooling channels, which in principle can exploit the increased heat transfer rate due to surface boiling, as used in high-performance cooling circuits such as plasma-facing components, will provide 1-MW CW power capability. In this paper, some technical solutions for the construction and the constraints on the allowable deformation during pulses are given.

Fusion-oriented devices using powerful source of electron cyclotron (EC) waves are subjected to high levels of RF-stray radiation. This radiation is potentially harmful for any object made of RF-absorbing material exposed to it. Special material or/and suitable techniques to scrambling the radiation could be needed, depending on the effective expected level of such radiation. Therefore arises the need for a reliable method to foresee the distribution and the level of the RF-stray radiation. A multi-modal antenna, with sensitivity approximately constant for any incoming linear polarization, with great radiating surface and low directivity has been developed. Its purpose is collecting 140 GHz stray radiation from FTU tokamak and Wendelstein 7-AS stellarator plasmas during ECRH and current drive experiments. Theoretical predictions for stray radiation levels are obtained by two different methods: a multiple ray tracing code developed for FTU and a multi-cell code, originally written for W7-X. Both codes have been applied to FTU geometry, resulting in a good agreement with the experimental data from the multi-modal antenna. Descriptions of the antenna design and of the results from both codes are presented. (C) 2001 Elsevier Science B.V. All rights reserved.

The 140GHz, 2MW, 0.5s ECRH system on FTU tokamak integrates closed waveguide transmission lines (≈30 m) with quasi optical systems at both ends for efficient coupling from the 4 gyrotrons to the 4 waveguides and from these to the plasma through a single access port. Poloidal and toroidal control of the beam’s launching angles and polarization is performed without movable components close to the plasma. Most of the components of each generation and transmission system were designed to operate at a power level higher than 0.5 MW, and a possible up-grade to a full 1 MW, 0.5 s capability is discussed.

A family of compact matched loads for high power millimeter wavelength beams absorption, good for atmospheric and vacuum operation, suited for CW (1MW) or short pulses (2MW, 0.ls) precise measurement has been designed to test high power gyrotrons. The common origin for members of this family is a toad developed for the ECRH FTU plant (140 GHz, 0.5 s, 0.5 MW) and successfully exploited. A CW version of the load has been built and used as absorber and bolometer for the high-power test of a remote steering launcher mock-up at 140 GHz. The high-power tests have been performed with powers up to 700 kW and pulse lengths of up to 10 seconds without limitation due to the load. An investigation of the gas emission properties of the load coating material at high temperature and under vacuum has been accomplished by means of high power deposition from an electron gun. Results from these tests are presented.

Beam absorbers play an important role both in electron cyclotron heating systems at high power and in millimeter-wave diagnostics that need a low level of stray or reflected power. In the first case short- and long-pulse loads are used, whose back-reflection can be kept within a few percent with proper techniques. In the second case, absorbers or scramblers are envisaged, to be put in hostile environments. At Istituto di Fisica del Plasma in Milan, a number of calorimetric loads have been developed, adopting several techniques for overall reflectivity reduction, which are suitable for beam sinking with calorimetric capability. They achieve a low overall reflectivity and high-power capability by a properly chosen power distribution in the absorbing wall provided by a dispersing mirror, by a smooth geometrical shape, by heat-resistant absorbing coatings of optimized thickness, and by accurate trapping of most of the escaping radiation with preload structures. Fundamental, when it becomes impossible to diffuse the incoming beam by the mirror alone, mostly because of side lobes at large angles, is the use of a newly developed phase-scrambling surface presented in this paper. It provides the necessary spreading, complementing all the other techniques into a set that can be helpful in designing millimeter-wave systems and diagnostics, in order to reduce spurious or unwanted signals.