[Show abstract][Hide abstract] ABSTRACT: Results from a Cherenkov probe recently installed in FTU are presented on non-thermal electron losses. A range of scenarios are investigated to prove the versatility of the diagnostics by correlation with several other diagnostics, including electron cyclotron emission (ECE), neutron and gamma ray detectors, Mirnov coils and soft x-ray cameras. The data analysed provide useful insights into the dynamics of runaway electron (RE) losses in the presence of magnetic islands, demonstrating the distinct and broad potential of this relatively new diagnostic system. The analysis focuses on the sensitivity of the Cherenkov probe to RE losses in connection with magnetohydrodynamic activity and, generally, with magnetic perturbations and reconnection events. In those cases, the Cherenkov probe signals show that the RE expulsion mechanisms are due to the magnetic perturbation of a magnetic island and its amplitude fluctuations. Importantly, the microsecond resolution of the Cherenkov diagnostics reveals an internal structure of the signal peaks, permitting, for the first time with non-magnetic diagnostics, the detection of high frequency signals linked to perturbations of the magnetic island width, known as beta-induced Alfvèn eigenmodes.
[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.
[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.
Full-text · Article · Dec 2014 · Voprosy atomnoj nauki i techniki = Pytannja atomnoï nauky i techniky = Problems of atomic science and technology
[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.
Full-text · Article · Dec 2014 · Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment
[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.
[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.
[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.
Full-text · Article · Feb 2013 · The Review of scientific instruments
[Show abstract][Hide abstract] ABSTRACT: This invited lecture presents results of research on charged particles emitted from high-temperature plasmas, which was performed by means of corpuscular-diagnostic techniques at NCBJ (former IPJ) during recent years. The studies were performed by joint teams at different facilities, e.g. RPI-IBIS and PF-360 machines at NCBJ, and PF- 1000 facility at IFPiLM. To study spatial and energetic structure of fast ion streams emitted from RPI- and PF-type discharges the use was made of nuclear track detectors (NTDs), ion pinhole cameras and Thomson-type analyzers. The appearance of many ion micro-beams was confirmed and detailed mass- and energy-analysis of them was carried out. For time-resolved ion measurements the use was made of miniature scintillation detectors coupled with fast photomultipliers. Fast electrons from RPI- and PF-facilities were investigated with a magnetic analyzer, and energy spectra of them were determined. Time-resolved measurements of fast electrons were also performed in some tokamaks by means of Cherenkov-type detectors. Recently, new probes have been designed for ion studies in MCF experiments, e.g. COMPASS-U tokamak in Prague. Some research on passive corpuscular diagnostics of plasma was realized in a frame of the Polish-Ukrainian scientific collaboration.
No preview · Article · Jan 2013 · Voprosy atomnoj nauki i techniki = Pytannja atomnoï nauky i techniky = Problems of atomic science and technology
[Show abstract][Hide abstract] ABSTRACT: This invited lecture presents the most important results of research on emission of the visible radiation (VR) from intense pulsed plasma-ion streams generated by high-current discharges within RPI- or PF-type experimental facilities. The optical emission spectroscopy measurements were performed during a free-propagation of such plasma-ion streams, as well as during interactions of such streams with solid targets made of different materials, e.g. CFC and W. Optical studies were performed within RPI-IBIS and PF-360 facilities at NCBJ, as well as PF-1000 facility at IFPiLM. Time-integrated VR pictures were taken by means of a CCD camera and color filters. Spectroscopic measurements were performed with a Mechelle®900 spectrometer, that recorded VR spectra at chosen instants and expositions ranging from 100 ns to several μs. Analyses of the selected spectral lines gave averaged values of plasma concentration and electron temperature. Analogous measurements were performed for plasmas produced at surfaces of targets, those were located at chosen distances from the electrodes. Behaviour of targets made of CFC and W was studied. Attention was paid to erosion of W-targets irradiated by intense plasma streams. Spectral lines of W- and C-ions were identified, and parameters of plasma were estimated from the Dα line. Some measurements were performed for W-targets within the QSPA 50-Kh in Kharkov.
[Show abstract][Hide abstract] ABSTRACT: The paper describes progress in studies of ion beams generated and accelerated within RPI (Rod Plasma Injector) and PF (Plasma-Focus) devices. To perform mass- and energy-analysis of such ion beams the use was made of different mass-spectrometers of the Thomson type. Ion measurements in RPI-IBIS facility were first performed with a big Thomson analyzer placed outside the vacuum chamber. To perform ion measurements near the electrodes two other Thomson analyzers were constructed at IPJ (now NCBJ). A smaller analyzer was applied in a large PF-1000 facility at IFPiLM, and time-integrated ion parabolas were recorded on nuclear track detectors. That analyzer was also adopted for time-resolved measurements by means of miniature scintillation detectors located along the deuteron parabola and it was used in the RPI-IBIS facility. Recently the analyzer construction was modernized to ensure differential pumping of the inlet system, and it was used for measurements within PF-360 and RPI-IBIS facilities.
[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.
Full-text · Article · Aug 2012 · The Review of scientific instruments
[Show abstract][Hide abstract] ABSTRACT: The paper reports on progress in design and use of novel detectors for experimental studies of fast (run-away and ripple-born) electrons in various experiments of the tokamak type. The idea of the use of a Cherenkov effect for direct on-line measurements of the fast electrons within tokamaks was presented by scientists from the NCBJ (former IPJ) several years ago. Successive efforts led to the development of prototype detector heads equipped with diamond or aluminium nitrate (AlN) crystals, which were shielded with very thin metal filters in order to eliminate the visible light from plasma and to enable a rough energy analysis of electrons. Those Cherenkov radiators were coupled through optical-fibre cables with fast photomultipliers. Those prototypes were applied for test measurements within the CASTOR experiment in Prague, and later in the ISSTOK device in Lisbon, but the main aim remained to develop the Cherenkov detectors for the TORE-SUPRA experiment in Cadarache.
[Show abstract][Hide abstract] ABSTRACT: The paper describes spectroscopic measurements of the optical emission from dense plasma streams during their free propagation and interactions with carbon-fiber-composite (CFC) targets within the PF-360 and RPI-IBIS facilities operated at NCBJ in Swierk, Poland. A dependence of the emission of the main CFC-components (i.e., carbon ions) on the crystallographic orientation of carbon filaments in the target was investigated. An influence of plasma parameters on the emission of carbon ions was also analyzed.
[Show abstract][Hide abstract] ABSTRACT: In this paper we report on measurements of the energy spectra and other properties of the electron and fast ion beams emitted in the upstream and downstream direction along the z-axis of a large plasma focus device PF-1000, that was operated at 21-27 kV, delivering 290-480 kJ pulse. Measurements of the electron beam (EB) properties were performed using a magnetic analyzer. Properties of the ion beams were measured by means of pinhole cameras equipped with PM-355 nuclear track detectors and placed at various angles (0, 60 and 180°) to the discharge axis. Measurements revealed a complex spatial structure of the fast ion beams. The ion measurements behind the PF-1000 collector proved that some fast deuterons are emitted also in the upstream direction. Measurements of the EBs emitted in the upstream and downstream direction revealed electron energies in the range from approximately 40 keV to approximately 800 keV. These spectra confirm that in localized regions within the PF-1000 plasma column there appear strong fields accelerating charged particles in different directions along the discharge axis.
[Show abstract][Hide abstract] ABSTRACT: The paper presents advance in a new method developed in the Institute for Nuclear Studies (IPJ) for direct detection of high-energy (super-thermal, runaway) electrons generated in tokamaks. The technique in question is based on registration of the Cherenkov radiation, emitted by energetic electrons, moving through a transparent medium (radiator body) with a velocity higher than the velocity of light in this material. The main aim of the presented studies was to develop a diagnostic technique applicable for investigation of fast electron beams within magnetic confinement fusion (MCF) facilities.
[Show abstract][Hide abstract] ABSTRACT: The paper presents experimental studies of the spatial-microstructure and temporal-characteristics of fast ion beams, which are emitted from high-current plasma-focus (PF) discharges performed within the PF-360 facility at National Centre for Nuclear Research (NCBJ) at Otwock/Świerk, Poland. The spatial structure of the ion beams was investigated by means of pinhole cameras equipped with solid-state nuclear track detectors shielded by absorption filters made of Al-foils of different thickness. In order to perform time-resolved measurements there were applied miniature scintillation detectors placed at different points of the ion-image plane.
[Show abstract][Hide abstract] ABSTRACT: Experimental results are presented on the spatial structure and energetic characteristics of the fast ion beams produced by high-current discharges in a PF-360 device operated at the National Centre for Nuclear Research (NCBJ, Otwock/Świerk, Poland). The 105 kJ discharges were initiated at the initial deuterium pressure around p 0 = 6 hPa and were powered from a 234 μF capacitor bank charged up to 30 kV. The spatial structure of the ion beams was recorded using pinhole cameras equipped with the solid-state nuclear track detectors of the PM-355 type, placed at two different angles (0°, 45°) relative to the discharge axis. The detectors were shielded by thin absorption filters made of pure Al foils of various thickness, which made it possible to record only ions with energies exceeding a chosen threshold value. Similarly as in other plasma focus (PF) experiments, energies of the emitted ions ranged from about 30 keV to about 3 MeV, i.e. they were much higher than the voltage applied to the electrodes. The recorded ion images showed a complex spatial structure of the fast ion beams, which consisted of many micro-beams of different energies. It is possible that these beams were emitted by various local micro-sources (e.g. plasma micro-diodes) the were formed inside the PF pinch column.
Full-text · Article · Jan 2011 · Nukleonika -Original Edition-