Antonio De Lorenzi - Academia.edu (original) (raw)

Papers by Antonio De Lorenzi

Fusion Engineering and Design

Journal of Applied Physics

Counterintuitive experimental evidences have been observed during High Voltage Direct Current (HV... more Counterintuitive experimental evidences have been observed during High Voltage Direct Current (HVDC) tests of two concave, axial-symmetric, electrodes insulated by large vacuum gaps of 3 and 7 cm with voltages from 150 to 370 kVdc. The dissipation of microdischarge power during the conditioning procedure occurs mostly on the anodic side in a region close to the axis of the system where the electric field is at a minimum, far from the positions where the breakdowns have been observed. The analyses of the phenomena are carried out by comparing the temporal evolution of voltages, currents, pressure, measurements of x-ray energy spectra, and images from infrared and visible light cameras. Numerical simulations, based on ray-tracing algorithm, correctly identify the positions where the power dissipation of microdischarges occurs. A mutual exchange of charged particles in the electrostatic field between electrodes seems a reasonable physical mechanism to interpret the observations. These ...

17th IEEE/NPSS Symposium Fusion Engineering (Cat. No.97CH36131)

The characteristics of the 66 kA test facility built up at the Consorzio RFX are presented and di... more The characteristics of the 66 kA test facility built up at the Consorzio RFX are presented and discussed together with the results of interruption and life tests performed on a new vacuum circuit breaker intended to fulfil possible ITER requirements for the Fast Discharge Units

Nuclear Fusion, 2020

High voltage (HV) insulation across a single gap in vacuum and low-pressure gas is a critical iss... more High voltage (HV) insulation across a single gap in vacuum and low-pressure gas is a critical issue in relation to the development and realization of the electrostatic accelerator for the ITER neutral beam injector (NBI) (Toigo et al 2017 New J. Phys. 19 085004). The present paper describes and analyzes the recent experimental results obtained at the High Voltage Padova Test Facility (HVPTF), the laboratory aimed at supporting the development of the prototype for the ITER NBI (De Lorenzi et al 2011 Fusion Eng. Des. 86 742-5). A voltage up to 800kVDC was achieved in the HVPTF during the experimental campaigns with a sphere-plane configurations having variable gap length (from 30 to 150 mm) and pressure ranging from high vacuum (10-7 mbar) to 10-3 mbar in argon. Such an experimental campaign represents one of the few examples where voltages higher than 500-550 kV DC are sustained by a single vacuum gap between electrodes (Rohrbach 1971 CERN Report 71-5) and therefore constitutes additional experience to improve the knowledge of voltage holding across large vacuum gaps. The results in high vacuum indicate that at the beginning of voltage conditioning the breakdown events occur at the same cathodic electric field irrespective of the electrode geometry. However, after sufficient conditioning time, the breakdown voltage distribution seems to depend also on the electric field at the anode and on the total voltage between electrodes. A benchmark between a numerical tool previously developed to predict the voltage holding in high vacuum (voltage holding prediction model, VHPM (Pilan et al 2011 Fusion Eng. Des. 86 742-5)) and the experimental results is also reported and discussed.

IEEE Transactions on Magnetics, 2018

An innovative approach has been recently proposed for the voltage breakdown prediction in high-vo... more An innovative approach has been recently proposed for the voltage breakdown prediction in high-voltage systems, insulated by large vacuum gaps. This approach is based on complementary geometric formulations for electrostatics coupled to the analytical solution of the equations of motion for charged particles. In this paper, a goal-oriented local mesh refinement technique is introduced, which allows to increase the rate of convergence of the solution, enabling an effective voltage breakdown prediction also in large-scale systems with complex geometries.

Nuclear Fusion, 2019

The ITER Heating Neutral Beam (HNB) injectors, one of the tools necessary both to achieve burning... more The ITER Heating Neutral Beam (HNB) injectors, one of the tools necessary both to achieve burning conditions and to control plasma instabilities, are characterized by such demanding parameters as to require the construction of a Neutral Beam Test Facility (NBTF) dedicated to their development and optimization. The NBTF was realized in Padua (Italy), with the direct contribution of the Italian government, through the Consorzio RFX as the host entity, IO, the in kind contributions of three Domestic Agencies (F4E, JADA, INDA) and the technical and scientific support of various European laboratories and universities. The NBTF hosts two experiments: SPIDER and MITICA. The former is devoted to the optimization of the HNB and DNB ion sources and to the achievement of the required source performances. MITICA is the full size prototype of the ITER HNB, with an ion source identical to the one used in SPIDER.

Proceedings of 1996 International Power Modulator Symposium

The paper summarizes the actions taken to make safer the operation of the ignitron main crowbar s... more The paper summarizes the actions taken to make safer the operation of the ignitron main crowbar system used to protect the winding and the power supplies forming the circuit which produces the toroidal magnetic field for plasma confinement in the RFX machine. Such actions, which comprise a self-triggering system, a monitoring system and a discharge resistors system, are intended to

15th IEEE/NPSS Symposium. Fusion Engineering

ABSTRACT

2013 IEEE 25th Symposium on Fusion Engineering (SOFE), 2013

ABSTRACT MITICA is the complete full-scale prototype of a 17 MW Heating Neutral Beam Injector for... more ABSTRACT MITICA is the complete full-scale prototype of a 17 MW Heating Neutral Beam Injector for ITER. This experimental device, presently under construction in Padova, includes a negative Ion Source (H- or D-), and an electrostatic Accelerator (1 MV, 40 A, 3600 s). Voltage holding is recognized to be one of the most critical issues for the 1 MV accelerator operations, not only due to the complex multi-stage electrostatic accelerator structure, but also for the presence of magnetic field, which is necessary for deflecting the co-extracted and secondary electrons as early as possible, before they are accelerated. The required magnetic field is produced by a combination of several sources, such as permanent magnets and current-carrying conductors. In order to avoid breakdowns, the design of the accelerator shall guarantee that electrostatic configuration and the pressure distribution correspond to an operating point located on the left branch of the Paschen curve, i.e. where the H2 gas pressure multiplied by the largest electrode distance (p.d) is always below a certain limit, which is 0.1-0.3 Pa.m. Actually, the presence of the magnetic field might reduce this limit, moving the left branch of the Paschen curve towards the operating point thus affecting voltage holding even during the conditioning phase. In order to support the design of MITICA at low gas pressure and in the presence of magnetic field, an experimental campaign has been carried out at the High Voltage Padova Test Facility (HVPTF[1])

The results of a measurement campaign regarding the driving point harmonic impedance of one node ... more The results of a measurement campaign regarding the driving point harmonic impedance of one node of the 400 kV Italian transmission network are presented and discussed, using a method suitable for high order harmonics (up to 50th). A model of the network has been then set up and the relevant results compared with the measurement, showing the crucial effect of the equivalent impedance simulating the external network.

Nuclear Fusion, 1999

... R. Bartiromo, A. Buffaa, V. Antonia, L. Apolloni, M. Bagatina, W. Baker, F. Bellina, P. Betti... more ... R. Bartiromo, A. Buffaa, V. Antonia, L. Apolloni, M. Bagatina, W. Baker, F. Bellina, P. Bettini, R. Bilatoa, T. Bolzonellaa, A. Canton, S. Cappello, L. Carraro, R. Cavazzanaa, G. Chitarin, S. Costa, F. D'Angelo, A. De Lorenzi, G. De Pol, D. Desideria, D. Escande, P ... [16] Buffa, A., et al ...

IEEE Transactions on Power Delivery, 1991

ABSTRACT

IEEE Transactions on Dielectrics and Electrical Insulation, 2011

ABSTRACT The voltage holding in the 1 MV ITER Neutral Beam Accelerator is recognized to be one of... more ABSTRACT The voltage holding in the 1 MV ITER Neutral Beam Accelerator is recognized to be one of the most critical issues for long lasting beam operation, due to the complex electrostatic structure formed by differently shaped electrodes biased at different potentials. At present, no model is available to predict the breakdown probability of electrostatic system with a comparable complexity degree. This paper is aimed at proposing an innovative modelling for the voltage breakdown prediction of such complex systems, based on the implementation of the micro particle (clump) induced breakdown Cranberg-Slivkov theory into a statistical approach. After a detailed description, the model is applied to simple geometries and the results are compared to the experiments found in literature; finally, the model is applied to an electrostatic mock-up of the real ITER Neutral Beam accelerator, showing a good agreement with the experiment.

Fusion Engineering and Design, 1995

... the equipment can be performed from the LCCs, while a reduced set of commands and monitoring,... more ... the equipment can be performed from the LCCs, while a reduced set of commands and monitoring, required for ... is the worst inter nal fault, and was verified with the interven tion of only one fuse. ... to be below 100 V. The switch is made of a tripolar vacuum circuit breaker rated for ...

Fusion Engineering and Design, 1999

A movable power electrode has been developed on the RFX experiment to modify the radial electric ... more A movable power electrode has been developed on the RFX experiment to modify the radial electric field at the edge of the plasma configuration. The electrode insertion head is a mushroom shaped limiter made of a carbon–carbon composite, and boron nitride is used as insulating material to be exposed to the plasma. The power electrode is designed to carry a

Fusion Engineering and Design, 2009

The energy stored in the 1 MV ITER Neutral Beam Injector power supply system will exceed by far t... more The energy stored in the 1 MV ITER Neutral Beam Injector power supply system will exceed by far the energy stored in the existing largest NB Injectors; as a consequence, the limitation of the grid breakdown effects-grids damage and Electro Magnetic Interference emission-are critical issues. In the present ITER NBI reference design the mitigation system is based on the concept of the concentrated core snubber which, due to the large amount of stored energy, is a huge component. Furthermore, in the NBI a relatively large part of HV capacitance to ground remains downstream the core snubber, so neither the arc peak current nor the high-frequency oscillations can be effectively limited. Moreover, the concentrated core snubber is ineffective in limiting the voltage reversal caused by internal insulation fault, increasing the risk of cascade failures in components like HV bushing and transmission line. The paper proposes an alternative approach to limit the grid breakdown effects, based on the concepts of Damper Resistorsubstituting the direct connection to ground of the zero-potential accelerating grid-and of Distributed Core Snubber (DCS)-installed along the whole length of the transmission lines. The DCS concept has been subjected also to experimental validation by a small scale setup supported by electrical modelling.

Fusion Engineering and Design, 2007

Aiming at predictive simulation of the plasma magnetic parameters during the RFX-mod discharges, ... more Aiming at predictive simulation of the plasma magnetic parameters during the RFX-mod discharges, a model was designed to simulate the poloidal field power supply system, detailing each individual switching unit and the single ac-dc converter with its local control circuit, together with the magnet system and the complete multi-variable feedback system devoted to the plasma shape and position control. The model was realized by coupling a linear model of the axi-symmetric evolution of the plasma with a detailed implementation of the RFX-mod circuit developed by means of the SimPowerSystem Toolbox in the framework of the Matlab-Simulink TM package. In order to evolve the plasma current, a Bessel Function Model (BFM) is adopted to implement the energy transfer from the toroidal to the poloidal field. The axi-symmetric controller is implemented to close the loop and is the same running on the VME nodes of the real control system. The model was successfully tested against experimental data, showing good agreement between measured and calculated waveforms.

Fusion Engineering and Design, 2011

In the context of the ITER contract "ITER/CT/07/219-200 kV Stored Energy Tests", electrical break... more In the context of the ITER contract "ITER/CT/07/219-200 kV Stored Energy Tests", electrical breakdown tests have been performed in vacuum with a stored energy of up to 425 J. The experiments have been conceived and performed with the collaboration of Consorzio RFX. The tests are being performed in the 1 MV test facility at IRFM, CEA-Cadarache. They should simulate the conditions that will be found in the ITER Neutral Beam accelerator, at 200 kV. This paper presents the setup of the test bed, the choice of critical components, the diagnostic equipments and the results obtained with 200 kV applied on the anode electrode.

IEEE Transactions on Plasma Science, 2009

Fusion Engineering and Design

Journal of Applied Physics

Counterintuitive experimental evidences have been observed during High Voltage Direct Current (HV... more Counterintuitive experimental evidences have been observed during High Voltage Direct Current (HVDC) tests of two concave, axial-symmetric, electrodes insulated by large vacuum gaps of 3 and 7 cm with voltages from 150 to 370 kVdc. The dissipation of microdischarge power during the conditioning procedure occurs mostly on the anodic side in a region close to the axis of the system where the electric field is at a minimum, far from the positions where the breakdowns have been observed. The analyses of the phenomena are carried out by comparing the temporal evolution of voltages, currents, pressure, measurements of x-ray energy spectra, and images from infrared and visible light cameras. Numerical simulations, based on ray-tracing algorithm, correctly identify the positions where the power dissipation of microdischarges occurs. A mutual exchange of charged particles in the electrostatic field between electrodes seems a reasonable physical mechanism to interpret the observations. These ...

17th IEEE/NPSS Symposium Fusion Engineering (Cat. No.97CH36131)

The characteristics of the 66 kA test facility built up at the Consorzio RFX are presented and di... more The characteristics of the 66 kA test facility built up at the Consorzio RFX are presented and discussed together with the results of interruption and life tests performed on a new vacuum circuit breaker intended to fulfil possible ITER requirements for the Fast Discharge Units

Nuclear Fusion, 2020

High voltage (HV) insulation across a single gap in vacuum and low-pressure gas is a critical iss... more High voltage (HV) insulation across a single gap in vacuum and low-pressure gas is a critical issue in relation to the development and realization of the electrostatic accelerator for the ITER neutral beam injector (NBI) (Toigo et al 2017 New J. Phys. 19 085004). The present paper describes and analyzes the recent experimental results obtained at the High Voltage Padova Test Facility (HVPTF), the laboratory aimed at supporting the development of the prototype for the ITER NBI (De Lorenzi et al 2011 Fusion Eng. Des. 86 742-5). A voltage up to 800kVDC was achieved in the HVPTF during the experimental campaigns with a sphere-plane configurations having variable gap length (from 30 to 150 mm) and pressure ranging from high vacuum (10-7 mbar) to 10-3 mbar in argon. Such an experimental campaign represents one of the few examples where voltages higher than 500-550 kV DC are sustained by a single vacuum gap between electrodes (Rohrbach 1971 CERN Report 71-5) and therefore constitutes additional experience to improve the knowledge of voltage holding across large vacuum gaps. The results in high vacuum indicate that at the beginning of voltage conditioning the breakdown events occur at the same cathodic electric field irrespective of the electrode geometry. However, after sufficient conditioning time, the breakdown voltage distribution seems to depend also on the electric field at the anode and on the total voltage between electrodes. A benchmark between a numerical tool previously developed to predict the voltage holding in high vacuum (voltage holding prediction model, VHPM (Pilan et al 2011 Fusion Eng. Des. 86 742-5)) and the experimental results is also reported and discussed.

IEEE Transactions on Magnetics, 2018

An innovative approach has been recently proposed for the voltage breakdown prediction in high-vo... more An innovative approach has been recently proposed for the voltage breakdown prediction in high-voltage systems, insulated by large vacuum gaps. This approach is based on complementary geometric formulations for electrostatics coupled to the analytical solution of the equations of motion for charged particles. In this paper, a goal-oriented local mesh refinement technique is introduced, which allows to increase the rate of convergence of the solution, enabling an effective voltage breakdown prediction also in large-scale systems with complex geometries.

Nuclear Fusion, 2019

The ITER Heating Neutral Beam (HNB) injectors, one of the tools necessary both to achieve burning... more The ITER Heating Neutral Beam (HNB) injectors, one of the tools necessary both to achieve burning conditions and to control plasma instabilities, are characterized by such demanding parameters as to require the construction of a Neutral Beam Test Facility (NBTF) dedicated to their development and optimization. The NBTF was realized in Padua (Italy), with the direct contribution of the Italian government, through the Consorzio RFX as the host entity, IO, the in kind contributions of three Domestic Agencies (F4E, JADA, INDA) and the technical and scientific support of various European laboratories and universities. The NBTF hosts two experiments: SPIDER and MITICA. The former is devoted to the optimization of the HNB and DNB ion sources and to the achievement of the required source performances. MITICA is the full size prototype of the ITER HNB, with an ion source identical to the one used in SPIDER.

Proceedings of 1996 International Power Modulator Symposium

The paper summarizes the actions taken to make safer the operation of the ignitron main crowbar s... more The paper summarizes the actions taken to make safer the operation of the ignitron main crowbar system used to protect the winding and the power supplies forming the circuit which produces the toroidal magnetic field for plasma confinement in the RFX machine. Such actions, which comprise a self-triggering system, a monitoring system and a discharge resistors system, are intended to

15th IEEE/NPSS Symposium. Fusion Engineering

ABSTRACT

2013 IEEE 25th Symposium on Fusion Engineering (SOFE), 2013

ABSTRACT MITICA is the complete full-scale prototype of a 17 MW Heating Neutral Beam Injector for... more ABSTRACT MITICA is the complete full-scale prototype of a 17 MW Heating Neutral Beam Injector for ITER. This experimental device, presently under construction in Padova, includes a negative Ion Source (H- or D-), and an electrostatic Accelerator (1 MV, 40 A, 3600 s). Voltage holding is recognized to be one of the most critical issues for the 1 MV accelerator operations, not only due to the complex multi-stage electrostatic accelerator structure, but also for the presence of magnetic field, which is necessary for deflecting the co-extracted and secondary electrons as early as possible, before they are accelerated. The required magnetic field is produced by a combination of several sources, such as permanent magnets and current-carrying conductors. In order to avoid breakdowns, the design of the accelerator shall guarantee that electrostatic configuration and the pressure distribution correspond to an operating point located on the left branch of the Paschen curve, i.e. where the H2 gas pressure multiplied by the largest electrode distance (p.d) is always below a certain limit, which is 0.1-0.3 Pa.m. Actually, the presence of the magnetic field might reduce this limit, moving the left branch of the Paschen curve towards the operating point thus affecting voltage holding even during the conditioning phase. In order to support the design of MITICA at low gas pressure and in the presence of magnetic field, an experimental campaign has been carried out at the High Voltage Padova Test Facility (HVPTF[1])

The results of a measurement campaign regarding the driving point harmonic impedance of one node ... more The results of a measurement campaign regarding the driving point harmonic impedance of one node of the 400 kV Italian transmission network are presented and discussed, using a method suitable for high order harmonics (up to 50th). A model of the network has been then set up and the relevant results compared with the measurement, showing the crucial effect of the equivalent impedance simulating the external network.

Nuclear Fusion, 1999

... R. Bartiromo, A. Buffaa, V. Antonia, L. Apolloni, M. Bagatina, W. Baker, F. Bellina, P. Betti... more ... R. Bartiromo, A. Buffaa, V. Antonia, L. Apolloni, M. Bagatina, W. Baker, F. Bellina, P. Bettini, R. Bilatoa, T. Bolzonellaa, A. Canton, S. Cappello, L. Carraro, R. Cavazzanaa, G. Chitarin, S. Costa, F. D'Angelo, A. De Lorenzi, G. De Pol, D. Desideria, D. Escande, P ... [16] Buffa, A., et al ...

IEEE Transactions on Power Delivery, 1991

ABSTRACT

IEEE Transactions on Dielectrics and Electrical Insulation, 2011

ABSTRACT The voltage holding in the 1 MV ITER Neutral Beam Accelerator is recognized to be one of... more ABSTRACT The voltage holding in the 1 MV ITER Neutral Beam Accelerator is recognized to be one of the most critical issues for long lasting beam operation, due to the complex electrostatic structure formed by differently shaped electrodes biased at different potentials. At present, no model is available to predict the breakdown probability of electrostatic system with a comparable complexity degree. This paper is aimed at proposing an innovative modelling for the voltage breakdown prediction of such complex systems, based on the implementation of the micro particle (clump) induced breakdown Cranberg-Slivkov theory into a statistical approach. After a detailed description, the model is applied to simple geometries and the results are compared to the experiments found in literature; finally, the model is applied to an electrostatic mock-up of the real ITER Neutral Beam accelerator, showing a good agreement with the experiment.

Fusion Engineering and Design, 1995

... the equipment can be performed from the LCCs, while a reduced set of commands and monitoring,... more ... the equipment can be performed from the LCCs, while a reduced set of commands and monitoring, required for ... is the worst inter nal fault, and was verified with the interven tion of only one fuse. ... to be below 100 V. The switch is made of a tripolar vacuum circuit breaker rated for ...

Fusion Engineering and Design, 1999

A movable power electrode has been developed on the RFX experiment to modify the radial electric ... more A movable power electrode has been developed on the RFX experiment to modify the radial electric field at the edge of the plasma configuration. The electrode insertion head is a mushroom shaped limiter made of a carbon–carbon composite, and boron nitride is used as insulating material to be exposed to the plasma. The power electrode is designed to carry a

Fusion Engineering and Design, 2009

The energy stored in the 1 MV ITER Neutral Beam Injector power supply system will exceed by far t... more The energy stored in the 1 MV ITER Neutral Beam Injector power supply system will exceed by far the energy stored in the existing largest NB Injectors; as a consequence, the limitation of the grid breakdown effects-grids damage and Electro Magnetic Interference emission-are critical issues. In the present ITER NBI reference design the mitigation system is based on the concept of the concentrated core snubber which, due to the large amount of stored energy, is a huge component. Furthermore, in the NBI a relatively large part of HV capacitance to ground remains downstream the core snubber, so neither the arc peak current nor the high-frequency oscillations can be effectively limited. Moreover, the concentrated core snubber is ineffective in limiting the voltage reversal caused by internal insulation fault, increasing the risk of cascade failures in components like HV bushing and transmission line. The paper proposes an alternative approach to limit the grid breakdown effects, based on the concepts of Damper Resistorsubstituting the direct connection to ground of the zero-potential accelerating grid-and of Distributed Core Snubber (DCS)-installed along the whole length of the transmission lines. The DCS concept has been subjected also to experimental validation by a small scale setup supported by electrical modelling.

Fusion Engineering and Design, 2007

Aiming at predictive simulation of the plasma magnetic parameters during the RFX-mod discharges, ... more Aiming at predictive simulation of the plasma magnetic parameters during the RFX-mod discharges, a model was designed to simulate the poloidal field power supply system, detailing each individual switching unit and the single ac-dc converter with its local control circuit, together with the magnet system and the complete multi-variable feedback system devoted to the plasma shape and position control. The model was realized by coupling a linear model of the axi-symmetric evolution of the plasma with a detailed implementation of the RFX-mod circuit developed by means of the SimPowerSystem Toolbox in the framework of the Matlab-Simulink TM package. In order to evolve the plasma current, a Bessel Function Model (BFM) is adopted to implement the energy transfer from the toroidal to the poloidal field. The axi-symmetric controller is implemented to close the loop and is the same running on the VME nodes of the real control system. The model was successfully tested against experimental data, showing good agreement between measured and calculated waveforms.

Fusion Engineering and Design, 2011

In the context of the ITER contract "ITER/CT/07/219-200 kV Stored Energy Tests", electrical break... more In the context of the ITER contract "ITER/CT/07/219-200 kV Stored Energy Tests", electrical breakdown tests have been performed in vacuum with a stored energy of up to 425 J. The experiments have been conceived and performed with the collaboration of Consorzio RFX. The tests are being performed in the 1 MV test facility at IRFM, CEA-Cadarache. They should simulate the conditions that will be found in the ITER Neutral Beam accelerator, at 200 kV. This paper presents the setup of the test bed, the choice of critical components, the diagnostic equipments and the results obtained with 200 kV applied on the anode electrode.

IEEE Transactions on Plasma Science, 2009