Search

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

Determining plasma parameters in low-density, low-temperature, weakly ionized, magnetised plasmas can be challenging, especially when using in-house-built Langmuir probes that may perturb the plasma. Radial profiles along the plasma column in such conditions present further challenges, not only because the densities evanesce at the edges, but also because of the presence of magnetic fields that may significantly affect the measurements. The analysis is presented here of radial profiles in pure argon and pure helium plasmas obtained using in-house-built Langmuir probes. The study was done in detail not only to obtain plasma parameter profiles, but also to gain an insight into the relevant physical mechanisms in the operating conditions of interest. Results from a plasma model were used to complement the analysis together with qualitative observations from resulting electron energy distribution functions. The main conclusions are: profiles obtained with the plasma model closely represent those obtained from Langmuir probe data, with irregularities qualitatively explained using electron density profiles obtained with Druyvesteyn and first derivative methods; plasma densities and electron currents are sufficiently small that no diamagnetic effects are evident; the strong magnetisation of the electrons is evident from the resulting characteristic Bessel-type density profiles.

Plasma position reflectometry for ITER requires interfaces between in-vessel and ex-vessel waveguides. An ultra broadband interface (15-75 GHz) was designed between moderately oversized rectangular waveguide (20 x 12 mm), operated in TE01 (i.e., tall waveguide mode), and circular corrugated waveguide, with 88.9-mm internal diameter, propagating HE11. The interface was designed both as a sequence of waveguide components and as a quasi-optical confocal telescope. The design and the simulated performance are described for both concepts. The latter one requires more space but has better performance, and shall be prototyped.

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.

Atmospheric pressure plasmas have achieved great scientific and technological advances for a wide range of applications including the field of antimicrobial treatment. In this paper, a home-made atmospheric pressure plasma jet device was built and the effects of air plasma treatments on the inactivation of pure bacterial culture (Escherichia coli ATCC 8739) deposited onto the surface of agar plates were investigated. Plasmas were generated using high voltage discharge for 30, 60 and 90 s. The number of viable microorganisms was determined using a plate count method. Microbial Log10 reduction depended on the time of exposure and feed gas flow. A significant reduction of about 2.5 Log10 for E. coli was achieved within 60 s of plasma treatment. This result can be related to the presence of reactive species in the plasma volume, in particular, O radicals.

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.

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.

In this work results of the first D 2 /N 2 experiments in GyM, a linear device able to produce plasmas of interest for the ITER divertor ( n e 5 ⋅ 10 10 cm −3 , Te 5 eV, ion flux 3–5 ⋅ 10 20 m −2 s −1 ) are presented. Plasmas simulating a N-seeding scenario have been performed to evaluate ammonia formation and its effect on exposed W coatings. The presence of ND emission lines in the plasma can be correlated with the formation of ammonia, further directly detected and quantified by chromatography analysis of the exhaust. Four different W specimens were exposed in GyM to a plasma fluence of 8.78 ⋅ 10 23 m −2 . XPS analysis evidenced the formation of W x N y layers with nitrogen concentration in the range of 1–10% depending on the initial morphology and structure of the W samples. In all analyzed cases, nitrogen was bound and retained within the first 6 nm below the surface and no further diffusion of N into the bulk was observed.

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.

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.

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.

The use of turbomolecular pumps in plasma experiments with magnetic field confinement presents troublesome problems because of the presence of the static magnetic fields. The utmost consequence of this interaction can result in failure of the turbomolecular pump. In order to protect these pumps from the applied magnetic field, different magnetic shield layouts have been analyzed and tested: simple cylindrical tube, tube with partial enclosures, and tube with an enclosure plus a reinforced collar. All these shields have been made by cheap and easy to find material such as iron Fe-37. This work presents the study of the expected shielding performance of the three shielding configurations carried out by finite element analysis. Measurements to evaluate the effectiveness of the shielding structure have also been made. The principal aim of the present work is to indicate a possible approach to the problem and how to obtain a practical and inexpensive solution. The general guidelines described in this work can be applied also for vacuum devices exposed to the stray magnetic field of tokamaks such as ITER.

In the framework of thermonuclear fusion research, increasing interest is focussed on high power, high frequency millimeter-wave generators like gyrotrons. Measurements of gyrotron power and energy allow an accurate energy balance in ECRH (Electron Cyclotron Resonance Heating) to be carried out during plasma heating. The expected increase of the power, up to 2 MW, and time, up to 10 s (or continuous wave), calls for new technological solutions in manufacturing bolometric loads, particularly in making use of absorbing layer ceramics very resistant to thermal shocks. In this particular case, also a good absorption capability at 140 GHz is requested.In a previous work [N. Spinicchia, et al., Proc. of 16th Congress Italian Association of Vacuum, Catania (1), ed. Compositori, Bologna, Italy, 2002, p. 259], we studied alumina based coatings with various techniques-ESEM (Environmental Scanning Electron Microscopy), micro-analysis with X-ray EDS, Differential Scanning Calorimeter (DSC), X-ray diffraction analysis (XRD). It has been shown that repeated exposures to high energy fluxes causes the degradation of the coating.The present work is devoted to studying and selecting an absorbing ceramic coating to be deposited on the inner surface of the load. The aim is to characterize the material with respect to absorbing capability at a frequency of 140 GHz and thermal shock and fatigue resistance caused by long-term exposure to high power millimeter-wave radiation. Bolometric loads are planned to resist to 1-2 MW power continuously.To this purpose APS (Air Plasma Spray) and VPS (Vacuum Plasma Spray) ceramic coatings have been studied to search for a convenient substitute to the actual Alumina-Titania coating (Al2O3-13TiO(2)). A number of physical parameters such as thermal conductivity, thermal expansion coefficient, coating porosity and melting point were also considered to select among the candidate coatings. (c) 2005 Elsevier B.V. All rights reserved.

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.

___

IFP Report FP 02/1, February 2002

__

Iron-tungsten (Fe-W) mixed films were exposed to the low flux deuterium plasma of GyM in order to study the behavior of the sputtering yield with the ion fluence and temperature of the samples. From literature, it is known that an increase of the former lowers the Fe-W layers' sputtering yield as a consequence of the preferential sputtering of Fe leading to an enrichment in W of the outermost layers. An opposite trend was instead found for the latter probably due to the inter-diffusion of Fe and W (effective from 200 degrees C) resulting in the suppression of the W enrichment. Moreover, from 500 degrees C, also W segregation to the surface occurs. What is missing from literature is a systematic investigation of the role of temperature on W enrichment. In this work, dedicated experiments in GyM were carried out to fill this gap. After exposure, W enrichment was evaluated by Rutherford Backscattering Spectrometry (RBS) and inferred from measuring the eroded thickness of the samples using RBS and profilometer. Concerning the Fe-W sputtering yield as a function of fluence, it decreases by a factor of similar to 3 between the lowest (3.0 x 10(22) D+ m(-2)) and the highest fluence (9.0 x 10(23) D+ m(-2)) values considered. The other main result is that, at the lowest fluence, the exposure at room temperature leads to an erosion of the Fe-W samples more pronounced than that associated to the exposure at 500 degrees C. (C) 2017 The Authors. Published by Elsevier Ltd.