[Show abstract][Hide abstract] ABSTRACT: Since the 2012 IAEA-FEC Conference, FTU operations have been largely devoted to runaway electrons generation and control, to the exploitation of the 140 GHz 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 140 GHz 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.
[Show abstract][Hide abstract] ABSTRACT: Uncontrolled rapid loss of runaway current represents one of the most critical issues in tokamak
reactors. Significant research efforts are necessary to study and understand efficient ways
to control Runaway Electrons (REs) and to implement adequate control schemes to prevent disruptions.
In this context, the aim of the study presented here is to identify the potential of a
Cherenkov optical diagnostics in detecting runaway electron losses in the presence of magnetic
islands, in a scenario that is more controllable than a disruption. The Cherenkov probe was recently
installed and tested in FTU in collaboration with the Polish National Centre for Nuclear
Research (NCBJ). Data interpretation and correlation with several other diagnostics operating in
FTU, brought out the high level of detail provided by the Cerenkov probe in detecting RE losses
driven by magnetic islands. The development of the diagnostic system was designed to enable
RE energy-discrimination and simultaneous detection at two toroidal/poloidal positions to evaluate
the potential of this diagnostic system for implementation in next-generation fusion reactors.
The demonstration and testing of the improved diagnostic system is planned for the upcoming
FTU experimental campaign.
1st EPS Conf on Plasma Diagnostics, Frascati; 04/2015
[Show abstract][Hide abstract] ABSTRACT: The paper presents feasibility and design studies of the Cherenkov-type detectors designed for measurements of energetic electrons within tokamak devices. The technique in question enables the identification of electron beams, the determination of their spatial distribution, as well as the measurements of their temporal characteristics. On the basis of the presented analyses, i.e. heat transfer studies, a prototype of the Cherenkov measuring head has been designed, constructed and tested within CASTOR tokamak. Obtained experimental results demonstrated that relatively intense Cherenkov signals appear particularly during the final phase of the discharge, when the expanding plasma column reaches the detector.
[Show abstract][Hide abstract] ABSTRACT: The paper describes an influence of a Cherenkov-type probe, which is used for measurements of fast electron streams inside the ISTTOK chamber, on other probes and behaviour of a plasma ring. The reported study shows that such a probe situated near the plasma column has a strong influence on signals from another Cherenkov probe, and can cause a considerable reduction of electron-induced signals. This effect does not depend on positions of the probes in relation to the limiter. Measurements of hard X-ray (HXR) emission show that the deeply immersed Cherenkov probe can also influence on the limiter . Under specific experimental conditions such a Cherenkov probe can play the role of a new limiter and change the plasma configuration.
Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 12/2014; 767:61–66. DOI:10.1016/j.nima.2014.08.013 · 1.22 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This paper presents a summary of the most important results of fast electron measurements performed so far within different tokamaks by means of Cherenkov-type detectors. In the ISTTOK tokamak (IPFN, IST, Lisboa, Portugal), two measuring heads were applied, each equipped with four radiators made of different types of alumina-nitrate poly-crystals. A two-channel measuring head equipped with diamond radiators was also used. Within the COMPASS tokamak (IPP AS CR, Prague, Czech Republic) some preliminary measurements have recently been performed by means of a new single-channel Cherenkov-type detector. The experimental data from the TORE SUPRA tokamak (CEA, IFRM, Cadarache, France), which were collected by means of a DENEPR-2 probe during two recent experimental campaigns, have been briefly analyzed. A new Cherenkov probe (the so-called DENEPR-3) has been mounted within the TORE SUPRA machine, but the electron measurements could not be performed because of the failure of this facility. Some conclusions concerning the fast electron emission are presented.
Physica Scripta 05/2014; T161:014011. DOI:10.1088/0031-8949/2014/T161/014011 · 1.13 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The paper presents the results of the recent fast electron measurements performed by means of a new measuring head, which was equipped with two Cherenkov-type detectors made of diamonds coated with appropriate absorption filters. The high light efficiency of those detectors enabled a high temporal resolution and a good signal-to-noise ratio to be achieved. The obtained results made it possible to determine the conditions of the runaway electrons' appearance and to study their correlations with hard x-rays.
Physica Scripta 05/2014; T161:014012. DOI:10.1088/0031-8949/2014/T161/014012 · 1.13 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The Note reports on experimental studies of ripple born fast electrons within the TORE-SUPRA facility, which were performed by means of a modified measuring head equipped with diamond detectors designed especially for recording the electron-induced Cherenkov radiation. There are presented signals produced by fast electrons in the TORE-SUPRA machine, which were recorded during two experimental campaigns performed in 2010. Shapes of these electron-induced signals are considerably different from those observed during the first measurements carried out by the prototype Cherenkov probe in 2008. An explanation of the observed differences is given.
The Review of scientific instruments 02/2013; 84(1):016107. DOI:10.1063/1.4776190 · 1.61 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A diagnostic technique based on the Cherenkov effect is proposed for detection and characterization of fast (super-thermal and runaway) electrons in fusion devices. The detectors of Cherenkov radiation have been specially designed for measurements in the ISTTOK tokamak. Properties of several materials have been studied to determine the most appropriate one to be used as a radiator of Cherenkov emission in the detector. This technique has enabled the detection of energetic electrons (70 keV and higher) and the determination of their spatial and temporal variations in the ISTTOK discharges. Measurement of hard x-ray emission has also been carried out in experiments for validation of the measuring capabilities of the Cherenkov-type detector and a high correlation was found between the data of both diagnostics. A reasonable agreement was found between experimental data and the results of numerical modeling of the runaway electron generation in ISTTOK.
The Review of scientific instruments 08/2012; 83(8):083505. DOI:10.1063/1.4740512 · 1.61 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The paper describes experimental studies of electron beams emitted from a plasma torus within the ISTTOK tokamak, which were performed by means of a new four-channel detector of the Cherenkov type. A range of electron energy was estimated. There were also measured hard X-rays, and their correlation with the fast run-away electron beams was investigated experimentally.
Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 11/2010; 623(2):686-689. DOI:10.1016/j.nima.2010.02.133 · 1.22 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The paper concerns detectors of the Cherenkov radiation which can be used to measure high-energy electrons escaping from short-living plasma. Such detectors have high temporal (about 1 ns) and spatial (about 1 mm) resolution. The paper describes a Cherenkov-type detector which was designed, manufactured and installed in the ISTTOK tokamak in order to measure fast runaway electrons. The radiator of that detector was made of an aluminium nitride (AlN) tablet with a light-tight filter on its front surface. Cherenkov signals from the radiator were transmitted through an optical cable to a fast photomultiplier. It made possible to perform direct measurements of the runaway electrons of energy above 80 keV. The measured energy values and spatial characteristics of the recorded electrons appeared to be consistent with results of numerical modelling of the runaway electron generation process in the ISTTOK tokamak.
[Show abstract][Hide abstract] ABSTRACT: A diagnostics capable of characterizing the runaway and superthermal electrons has been developing on the ISTTOK tokamak. In previous paper, a use of single-channel Cherenkov-type detector with titanium filter for runaway electron studies in ISTTOK was reported. To measure fast electron populations with different energies, a prototype of a four-channel detector with molybdenum filters was designed. Test-stand studies of filters with different thicknesses (1, 3, 7, 10, 20, 50, and 100 μm) have shown that they should allow the detection of electrons with energies higher than 69, 75, 87, 95, 120, 181, and 260 keV, respectively. First results of measurements with the four-channel detector revealed the possibility to measure reliably different fast electrons populations simultaneously.
The Review of scientific instruments 10/2010; 81(10):10D304. DOI:10.1063/1.3478658 · 1.61 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The paper presents a schematic design and tests of a system applicable for measurements of fast electron pulses emitted from high-temperature plasma generated inside magnetic confinement fusion machines, and particularly in the TORE-SUPRA facility. The diagnostic system based on the registration of the Cherenkov radiation induced by fast electrons within selected solid radiators is considered, and electron low-energy thresholds for different radiators are given. There are some estimates of high thermal loads, which might be deposited by intense electron beams upon parts of the diagnostic equipment within the TORE-SUPRA device. There are some proposed measures to overcome this difficulty by the selection of appropriate absorption filters and Cherenkov radiators, and particularly by the application of a fast-moving reciprocating probe. The paper describes the measuring system, its tests, as well as some results of the preliminary measurements of fast electrons within TORE-SUPRA facility.
The Review of scientific instruments 01/2010; 81(1):013504. DOI:10.1063/1.3280221 · 1.61 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The paper describes diagnostics of fast ion beams emitted from a large PF-1000 facility operated at 21…27 kV, 290…480 kJ. Those beams were measured with pinhole cameras and PM-355 nuclear track detectors, placed at different angles to the discharge axis. The measurements showed a complex spatial structure of the fast ion beams. Measurements behind an axial channel in the inner electrode have shown that some ion beams are emitted also in the upstream direction. The ion energy spectra were measured with a miniature Thomson-type spectrometer. Time-resolved measurements of ions were performed with miniature scintillation detectors placed behind a pinhole.
[Show abstract][Hide abstract] ABSTRACT: The paper describes the application of SSNTDs of the PM-355 type to diagnostics of reaction products emitted from high-temperature deuterium plasmas produced in Plasma Focus (PF) facilities. Acceleration processes occurring in plasma lead often to the generation of high-energy ion beams. Such beams induce nuclear reactions and contribute to the emission of fast neutrons, fusion protons and alpha particles from PF discharges with a deuterium gas. Ion measurements are of primary importance for understanding the mechanisms of the physical processes which drive the charged-particle acceleration. The main aim of the present studies was to perform measurements of spatial- and energy-distributions of fusion-reaction protons (about 3MeV) within a PF facility. Results obtained from energy measurements were compared with the proton-energy spectra computed theoretically. The protons were measured by means of a set of ion pinhole cameras equipped with PM-355 detectors, which were placed at different angles relative to the electrode axis of the PF facility.
[Show abstract][Hide abstract] ABSTRACT: The paper presents an improved version of a miniature mass-spectrometer of the Thomson-type, which has been adopted for ion analysis near the dense plasma region inside a vacuum chamber. Problems connected with the separation of ions from plasma streams are considered. Input diaphragms and pumping systems, needed to ensure good vacuum inside the analyzing region, are described. The application of the miniature Thomson-type analyzer is illustrated by ion parabolas recorded in plasma-focus facility and rod plasma injector experiment. A quantitative analysis of the recorded ion parabolas is presented. Factors influencing accuracy of the ion analysis are discussed and methods of the spectrometer calibration are described.
The Review of scientific instruments 06/2009; 80(5):053504. DOI:10.1063/1.3131805 · 1.61 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Summary form only given. Measurements of the energetic particles (electrons and ions) are one of the main scientific tasks for various experimental devices generating high-temperature plasmas. The significance of such studies is proportional to the amount of information about different processes occurring inside plasma, in which such charged particles are involved. For measurements of fast electrons in tokamaks there were developed special Cherenkov-type probes of high spatial- and temporal-resolution. The applicability of the Cherenkov detectors for measurements of fast electrons in high-current plasma discharges has been studied theoretically and experimentally, with particular attention to their capabilities. In all cases the practical application of such detectors requires the selection of appropriate radiators and consideration of geometrical- and thermal-limitations. On the basis of our previous studies the Cherenkov-type detectors have been designed and adapted for the direct observation of fast (runaway) electrons in small-size tokamaks. During experiments with the ISTTOK facility discharges with low plasma currents (3-4 kA) were studied and runaway electrons of energies > 80 keV were successfully recorded. The electron measurements, as performed at different radial positions of the detector head, demonstrated a distinct increase in the radiation signal when the probe approached the bulk plasma. An analysis of the experimental results revealed good agreement between the obtained data and theory of runaway processes. This paper reports on the most important results of the experiment and its analysis.
IEEE International Conference on Plasma Science 01/2009; DOI:10.1109/PLASMA.2009.5227401
[Show abstract][Hide abstract] ABSTRACT: Gas, fluid, or solid Cherenkov-type detectors have been widely used in high-energy physics for determination of parameters of charged particles, which are moving with relativistic velocities. This paper presents experimental results on the detection of runaway electrons using Cherenkov-type detectors in the ISTTOK tokamak discharges. Such detectors have been specially designed for measurements of energetic electrons in tokamak plasma. The technique based on the use of the Cherenkov-type detectors has enabled the detection of energetic electrons (energies higher than 80 keV ) and determination of their spatial and temporal parameters in the ISTTOK discharges. Obtained experimental data were found in adequate agreement to the results of numerical modeling of the runaway electron generation in ISTTOK.