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

A new receiving antenna for collecting signals of the Collective Thomson Scattering (CTS) diagnostics in FTU Tokamak has been recently installed. The squared corrugated section and the precisely defined length make it possible to receive from different directions by remotely steering the receiving mirrors. This type of Remote-Steering (RS) antennas, being studied on FTU for the DEMO Electron Cyclotron Heating (ECH) system launch, is already installed on the W7- X stellarator and will be tested in the next campaign. The transmission of the signal from the antenna in the tokamak hall to the CTS diagnostics hall will be mainly realized by means of oversized circular corrugated waveguides carrying the hybrid HE11 (quasi-gaussian) waveguide mode, with inclusion of a special smooth-waveguide section and a short run of reduced-size square-corrugated waveguide through the tokamak bio-shield. The coupling between different waveguide types is made with ellipsoidal focusing mirrors, using quasi-optical matching formulas between the gaussian-shaped beams in input and output to the waveguides. In this work, after a complete study of feasibility of the overall line, a design for the receiving line will be proposed, in order to realize an executive layout to be used as a guideline for the commissioning phase. © 2018 ENEA. Published by IOP Publishing Ltd on behalf of Sissa Medialab.

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

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 use of quadrupole mass spectrometers in plasma experiments with magnetic field confinement presents troublesome problems because of the presence of static magnetic fields. In mass spectrometers, the Lorentz force associated with the magnetic field deflects the ion trajectory in the analyzer section with a subsequent loss of instrument sensitivity. In order to reduce the effect of the externally applied magnetic field in the quadrupole, different magnetic materials have been considered as a shielding structure iron Fe-37, soft iron Armco(TM), Amumetal(TM) sheets. The present work describes a solution to shielding quadrupole mass spectrometers against strong magnetic fields giving an introduction to magnetic shield design, by means of finite element calculations, taking into account many factors such as the material used, the length and thickness of the shield, and the effects of openings. To prove the efficiency of the shielding system, the hydrogen signal from the quadrupole instrument has been monitored. The intensity and the direction of the magnetic field with respect to the quadrupole head axes have been varied. Measurements of the magnetic field have been made in order to validate the calculations and an example of application of the shield design guidelines to a magnetic confined plasma device is presented and tested in situ. The principal aim of the present work is to indicate a possible approach to the problem and how to obtain a practical solution with a worthy compromise between costs and performances.

The non-axisymmetric magnetic field, naturally present in every tokamak, constraints the machine operational space. A set of dedicated ‘Error Field Correction Coils’ (EFCC) is proposed for JET, which will replace the unreliable existing ‘saddle coils’. The new coils are specified to compensate the main harmonic contribution to error field induced modes. The EFCC structural design comprises a cable coil wound in a metallic casing supported on the JET iron magnetic circuit. Existing power supplies will drive the coils. The EFCC are expected to be installed in 2001 JET shutdown.

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.

A spherical compact matched load, for high vacuum operation suited for short pulses (2 MW, 0.1 s) precise measurement has been designed to test high power gyrotrons Bruschi, Gandini, Muzzini, Spinicchia, Cirant, Gittini, Granucci, Mellera, Nardone, Simonetto, and Sozzi (Fusion Eng. Des. 56-57:649-654, 2001); Bruschi, Cirant, Gandini, Granucci, Mellera, Muzzini, Nardone, Simonetto, Sozzi, and Spinicchia (Nucl. Fusion 43:1513-1519, 2003); Bruschi, Cirant, Gandini, Gittini, Granucci, Mellera, Muzzini, Nardone, Simonetto, Sozzi, Spinicchia, Angella, and Signorelli (Development of CW and short-pulse calorimetric loads for high power millimeter-wave Beams, 23rd Symposium on Fusion Technology, September 20-24, 2004, Venice, Italy). In order to enhance the power handling capability of the load and to reduce the operation problems that may arise from an excessive reflection from the load, a ray tracing code has been written to model the power distribution on the inner surface and the pattern of the reflected radiation. The outcome of this code has been used to select a more convenient profile for the spreading mirror of the load and to optimize a pre-load specially conceived to minimize the power reflected fraction.