Papers by Roberto Pasqualotto
IEEE Transactions on Plasma Science, 2012
The paper describes a system for digitization and optical transmission of thermal measurements on... more The paper describes a system for digitization and optical transmission of thermal measurements on high voltage devices in high vacuum environmental conditions and presents the tests conducted on a prototype. The system has been designed in particular to satisfy such technical requirements as to be mounted on the grounded grid of the SPIDER facility (a 100-keV/60-A particle accelerator) and to withstand frequent fault conditions in which the voltage of the grid transiently rises up to some tens of kV. The system is based on a circuit which samples and transmits the signals to the central acquisition system while preserving the signals and avoiding any electrical links between the high voltage device and the vacuum vessel. Moreover, the system has to be designed so as to minimize the electromagnetic noise affecting the low amplitude signals from the thermocouples (TCs). The circuit design is presented, describing the layout and the electronic components for the acquisition of the TC signals and for the data transmission via optical fiber. When SPIDER is operational, with up to 1-h pulse duration, the circuit is powered by a battery, which is in turn recharged by the energy coming from a photovoltaic cell when SPIDER is not operational and the circuit is not acquiring. Data are digitally transmitted according to RS-232 protocol for easy interfacing to the central data acquisition system. The circuit has been tested to check its proper operation, with particular care devoted to the data transmission and the recharging phase. The results are reported and discussed.
Review of Scientific Instruments
Giant negative ion sources for neutral beam injectors deliver huge negative ion currents, thanks ... more Giant negative ion sources for neutral beam injectors deliver huge negative ion currents, thanks to their multi-beamlet configuration. As the single-beamlet optics defines the transmission losses along the beamline, the extraction of a similar current for all beamlets is extremely desirable, in order to facilitate the beam source operation (i.e., around perveance match). This Review investigates the correlation between the vertical profile of beam intensity and the vertical profiles of plasma properties at the extraction region of the source, focusing on the influence of increasing cesium injection. Only by the combined use of all available source diagnostics, described in this Review, can beam features on the scale of the non-uniformities be investigated with a sufficient space resolution. At RF power of 50 kW/driver, with intermediate bias currents and a filter field of 2.4 mT, it is found that the central part of the four vertical beam segments exhibits comparable plasma density ...
HAL (Le Centre pour la Communication Scientifique Directe), Sep 22, 2019
Review of Scientific Instruments, 2021
An incoherent Thomson scattering diagnostic will be installed in the JT-60SA tokamak to measure e... more An incoherent Thomson scattering diagnostic will be installed in the JT-60SA tokamak to measure electron temperature and electron density profiles. The target radial spatial resolution is 25 mm with 46 spatial channels. The accuracy in electron temperature and density is a few percent at ne = 7.5 × 1019 m−3, which is the expected value in the plasma core. This paper presents the designs of collection optics, fibers with their alignment system, and polychromators. The collection optics overcomes unique issues for superconducting fusion devices, i.e., limited design space, high-temperature measurements, and harsh radiation condition. When in several years the more performing plasma will generate intense nuclear radiation, the lens materials of the optics can be replaced by radiation resistant glasses without major changes in the lens holder. It will prevent transmission degradation and keep stable measurement accuracy.
Fusion Engineering and Design, 2021
Fusion Engineering and Design, 2021
Abstract The SPIDER ion source has been designed to generate a current of H- ions up to 60 A and ... more Abstract The SPIDER ion source has been designed to generate a current of H- ions up to 60 A and accelerate them up to 100 keV. In addition, the ratio between the co-extracted electrons and negative ions has to be below 1, and a beam inhomogeneity within 10 % has to be attained. A magnetic filter field is produced by a current flowing through the plasma electrode in the vertical direction with the aim of reducing electron temperature and density in the extraction region, so as to enhance the survival probability of H- ions while reducing the co-extracted electrons. Another strategy to reduce the co-extracted electrons is to apply a small bias voltage to the plasma grid with respect to the ion source body. In addition to this, a so-called bias plate is installed in the vicinity of the plasma grid, which can also be biased with respect to source body. However, the application of a bias voltage affects the beam inhomogeneity by modifying the ExB plasma drift within the ion source. Recently, the filter field configuration of SPIDER was modified so as to improve the plasma diffusion towards the extraction region. In addition, the voltage ratings of the bias power supplies were increased. Both these modifications permit to efficiently lower the electron co-extracted current. The present contribution describes such improvements while focusing on the effects they have on the beam inhomogeneity.
Fusion Engineering and Design, 2021
To reach fusion conditions and control the plasma configuration in ITER, the next step in tokamak... more To reach fusion conditions and control the plasma configuration in ITER, the next step in tokamak fusion research, two neutral beam injectors (NBIs) will supply 16.5MW each, by neutralizing accelerated negative hydrogen or deuterium ions. The requirements of ITER NBIs (40A/1MeV D-ions for ≤1h, 46A/870keV H-ions for ≤1000s) have never been simultaneously attained. So in the Neutral Beam Test Facility (NBTF, Consorzio RFX, Italy) the operation of the full-scale ITER NBI prototype (MITICA) will be tested and optimised up to full performances, focussing on accelerator (including voltage holding), beam optics, neutralisation, residual ion removal. The NBTF includes also the full-scale prototype of the ITER NBI source with 100keV particle energy (SPIDER), for early investigation of: negative ion production and extraction, source uniformity, negative ion current density and beam optics. This paper will describe the main results of the first two years of SPIDER operation, devoted to characterizing plasma and beam parameters, including investigation of RF-plasma coupling efficiency and magnetic filter field effectiveness in reducing co-extracted electrons. SPIDER is progressing towards the first caesium injection, which aims at increasing the negative ion density. A major shutdown, planned for 2021, to solve the issues identified during the operation and to carry out programmed modifications, will be outlined. The installation of each MITICA power supply and auxiliary system is completed; in-vessel mechanical components are under procurement by Fusion for Energy (F4E). Integration, commissioning and test of the power supplies, procured by F4E and QST, as the Japanese Domestic Agency (JADA), will be presented. In particular, 1.0MV insulating tests were carried out step-by-step and successfully completed. In 2020 integrated tests of the power supplies on the accelerator dummy load started, including the assessment of their resilience to accelerator grid breakdowns using a short-circuit device located in vacuum. The aggressive programme, to validate the NBI design at NBTF and to meet ITER schedule (requiring NBIs in operation in 2032), will be outlined. Unfortunately, in 2020 the coronavirus disease infection affected the NBTF activities. A solution to proceed with integrated power tests despite the coronavirus is presented.
Review of Scientific Instruments, 2020
The full-size ITER ion source prototype SPIDER (Source for the Production of Ions of Deuterium Ex... more The full-size ITER ion source prototype SPIDER (Source for the Production of Ions of Deuterium Extracted from a Radio frequency plasma) has recently started beam operation, whose objective is to produce 100 keV, 60 A hydrogen negative ions for 1 h. The source is presently operated in the volume regime, and the beam power is consequently limited. In such a configuration, the high resolution calorimeter STRIKE (Short-Time Retractable Instrumented Kalorimeter Experiment), even though uncooled, may be used instead of the SPIDER beam dump without limiting the beam-on time. STRIKE is formed by unidirectional carbon fiber–carbon matrix (CFC) composite tiles that are exposed to the beam while their temperature is recorded by using two infra-red cameras. This setup, thanks to the moderate broadening of the temperature profile guaranteed by the anisotropy of CFC, allows for the determination of detailed features of the beam current distribution (spatial resolution is about 2 mm). Furthermore,...
Journal of Instrumentation, 2020
In the Divertor Tokamak Test (DTT) facility two Thomson scattering (TS) systems are under design ... more In the Divertor Tokamak Test (DTT) facility two Thomson scattering (TS) systems are under design for the measurements of Te and ne in the core plasma region and in the divertor respectively. The divertor TS system under study is a conventional TS system based on a Nd:YAG laser source, a fiber optic based light collection system and a set of filter polychromators equipped with Si APD detectors. The laser beam and the collection optics share an aperture between adjacent cassettes of the lower divertor and the scattering signal is collected from a set of scattering volumes close to one of the divertor legs by a collection optics system located under the divertor dome and is carried to the polychromators by fiber optic bundles. The filter polychromators are designed to measure Te as low as 1 eV . Measurements with a spatial resolution of 10 mm are possible, with accuracy limited by the plasma ne and the background light. For the core TS system, two options are under consideration: a conventional system, similar to that designed for the ITER core TS, in which Te and ne are measured along a large fraction of a laser beam crossing the plasma near the equatorial plane and the detection system is again based on fiber optic coupled filter polychromators. The spatial resolution is 5 cm in the central region and 1 cm at the plasma edge. Alternatively a TS system based on the LIDAR concept, previously implemented in JET, is under consideration. Recent advancements in laser and detector technology allow achieving a spatial resolution similar to that of a conventional system, but with a simpler and reliable experimental set-up and possibly at a lower cost.
Plasma and Fusion Research, 2018
High power neutral beam injectors, like those for ITER, must satisfy very demanding parameters (4... more High power neutral beam injectors, like those for ITER, must satisfy very demanding parameters (40 A of negative ion current accelerated up to 1 MV for one hour). They are made of various components, which influence each other, so that the global performances eventually require the simultaneous control of several interacting parameters: thus the NBI is an example of a complex system. In the present paper, complex network theory is applied to verify the controllability conditions of the NIO1 experiment, a particle beam source operating at Consorzio RFX (Padova, Italy). Previous work on the subject is adapted to NIO1, the controllability conditions are assessed and the driver nodes are identified; first comparison between theoretical predictions and experimental data is also discussed.
Fusion Engineering and Design, 2019
The ITER neutral beam test facility under construction in Padova will host two experimental devic... more The ITER neutral beam test facility under construction in Padova will host two experimental devices: SPIDER, a 100 kV negative H/D RF source, and MITICA, a full scale, 1MeV deuterium beam injector. A detection system called Close-contact Neutron Emission Surface Mapping (CNESM) is under development with the aim to resolve the horizontal beam intensity profile in MITICA and one of the eight beamlet groups in SPIDER, with a spatial resolution of 3 and 5 cm2 respectively. This is achieved by the evaluation of the map of the neutron emission due to interaction of the deuterium beam with the deuterons implanted in the beam dump surface. CNESM uses nGEM detectors, i.e. GEM detectors equipped with a cathode that also serves as neutron-proton converter foil. The diagnostic will be placed right behind the SPIDER and MITICA beam dump, i.e. in an UHV environment, but the nGEM detectors need to operate at atmospheric pressure, so to contain the detector a vacuum sealed box has been designed to be installed inside the vacuum vessel and at atmospheric pressure inside. The box design was driven by the need to minimize the neutron attenuation and the distance between the beam dump surface and the detector active area. This paper presents the status of the CNSESM diagnostics. It describes the detector box and the different phases followed during the installation of the diagnostic on the SPIDER beam dump. Also the general layout of the diagnostic as part of the SPIDER experiment will be discussed. Finally the preliminary design of MITICA CNESM diagnostic will be introduced. This work was set up in collaboration and financial support of Fusion for Energy.
AIP Conference Proceedings, 2018
The use of caesium vapour is mandatory in plasma sources for negative ion beams to achieve a suff... more The use of caesium vapour is mandatory in plasma sources for negative ion beams to achieve a sufficient negative ion density. In the SPIDER ion source, three caesium ovens will provide a sufficiently high caesium flux to the walls of the ion source, but also a relatively even caesium distribution in the ion source. In order to ensure the caesium ovens are operating properly in terms of repeatability of the evaporated caesium flow, and to characterize the caesium flux emitted from its nozzles, a CAesium Test Stand (CATS) was equipped with a set of diagnostics. This paper reports the present configuration of the diagnostic setup. Surface ionization probes are installed in front of the nozzle apertures. Movable surface ionization detectors are installed to reconstruct the angular distribution of the emission. A quartz microbalance is installed at a fixed position, to measure the cumulated caesium flux. Finally, the laser absorption spectroscopy measures the line-integrated caesium density. An example of these combined measurements is presented.
Fusion Engineering and Design, 2017
A dedicated experiment on beam-target neutron emission has been performed at the ELISE neutral be... more A dedicated experiment on beam-target neutron emission has been performed at the ELISE neutral beam test facility to investigate the accuracy of Local Mixing Model based calculations of neutron emission of relevance for the design of neutron diagnostics at the SPIDER neutral beam prototype. Compared to a previous experiment, we make use of fully developed infra-red and calorimetry diagnostics to include accurate data on beam profile and current in the calculations. Results show that variations of the neutron emission can be predicted with an accuracy better than 10%, which compares to 30% in our previous investigation. At the same time, convincing evidence of diffusion effects beyond the Local Mixing Model is identified in the data. Implications of these findings for neutron emission at SPIDER are discussed.
Plasma Physics and Controlled Fusion, 2015
Polarimetric Thomson scattering (TS) is an alternative method for the analysis of Thomson scatter... more Polarimetric Thomson scattering (TS) is an alternative method for the analysis of Thomson scattering spectra in which the plasma temperature T e is determined from the depolarization of the TS radiation. This is a relativistic effect and therefore the technique is suitable only for very hot plasmas (T e > 10 keV) such as those of ITER. The practical implementation of polarimetric TS requires a method to calibrate the polarimetric response of the collection optics carrying the TS light to the detection system, and in particular to measure the additional depolarization of the TS radiation introduced by the plasma-exposed first mirror. Rotational Raman scattering of laser light from diatomic gases such as H2, D2, N2 and O2 can provide a radiation source of predictable intensity and polarization state from a well-defined volume inside the vacuum vessel and is therefore suitable for these calibrations. In this paper we discuss Raman polarimetry as a technique for the calibration of a hypothetical polarimetric TS system operating in the same conditions of the ITER core TS system and suggest two calibration methods for the measurement of the additional depolarization introduced by the plasma-exposed first mirror, and in general for calibrating the polarimetric response of the detection system.
Nuclear Fusion, 2014
ABSTRACT This paper describes the challenges of Thomson Scattering implementation in the ITER div... more ABSTRACT This paper describes the challenges of Thomson Scattering implementation in the ITER divertor and evaluates the capability to satisfy project requirements related to the range of the measured electron temperature and density. A number of aspects of data interpretation are also discussed. Although this assessment and the proposed solutions are considered in terms of ITER compatibility, they may also be of some use in currently operating magnetic confinement devices.
In ELMy H modes in JET, high triangularity (δ≅0.47) is found necessary to achieve H 98 =1 at n e ... more In ELMy H modes in JET, high triangularity (δ≅0.47) is found necessary to achieve H 98 =1 at n e / n G ≥0.85, the ITER requirements. This paper reports on experiments in JET to study the scaling with plasma current and edge safety factor of the global and pedestal confinement at high triangularity (δ≥0.40). It is shown that high confinement quality (H 98 =0.9-1) at high density (n e / n G ≥1) is linked with access to the mixed Type I/II ELMy regime. This regime is characterized by higher pedestal pressure at high density than with Type I ELMs, and, so far, has been observed up to 3MA. The variation in behaviour in the mixed I/II regime with δ and q 95 is described. At the ITER q 95 of 3, the ρ* dependence of the global confinement scaling is confirmed up to 3.5MA (where ρ*≅1.7 ρ* ITER , ν*≅5.3 ν* ITER at n e /n G ≅0.8). In the entire range of I p and q 95 explored (I p from 1 to 3.5MA, q 95 from 3 to 5), the pedestal pressure is found to scale with I p 2 or slightly weaker, as expected from ideal ballooning stability. The ratio between the thermal stored energy (W th) and pedestal energy (W ped) is similar with both Type I and mixed Type I/II ELMs, such that variations in pedestal with density, plasma current and edge safety factor are reflected in the core thermal energy.
Journal of Instrumentation, 2012
We describe the use of the M1TeV transport code to interpret the strong heating which is observed... more We describe the use of the M1TeV transport code to interpret the strong heating which is observed inside magnetic islands in the core of a reversed-field pinch during the quasi-single helicity (QSH) state. M1TeV describes the evolution of an internal kink mode in a Tokamak, using helical flux coordinates ootnotetextF.Porcelli et al., Phys. Rev. Lett. 82, 1458 (1999).. We adapted
Physical Review Special Topics - Accelerators and Beams, 2011
In this paper the tomographic diagnostic developed to characterize the 2D density distribution of... more In this paper the tomographic diagnostic developed to characterize the 2D density distribution of a particle beam from a negative ion source is described. In particular, the reliability of this diagnostic has been tested by considering the geometry of the source for the production of ions of deuterium extracted from an rf plasma (SPIDER). SPIDER is a low energy prototype negative ion source for the international thermonuclear experimental reactor (ITER) neutral beam injector, aimed at demonstrating the capability to create and extract a current of D À (H À) ions up to 50 A (60 A) accelerated at 100 kV. The ions are extracted over a wide surface (1:52 Â 0:56 m 2) with a uniform plasma density which is prescribed to remain within 10% of the mean value. The main target of the tomographic diagnostic is the measurement of the beam uniformity with sufficient spatial resolution and of its evolution throughout the pulse duration. To reach this target, a tomographic algorithm based on the simultaneous algebraic reconstruction technique is developed and the geometry of the lines of sight is optimized so as to cover the whole area of the beam. Phantoms that reproduce different experimental beam configurations are simulated and reconstructed, and the role of the noise in the signals is studied. The simulated phantoms are correctly reconstructed and their two-dimensional spatial nonuniformity is correctly estimated, up to a noise level of 10% with respect to the signal.
Advanced Diagnostics for Magnetic and Inertial Fusion, 2002
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Papers by Roberto Pasqualotto