Overview on electrical issues faced during the SPIDER experimental campaigns

SPIDER is the full-scale prototype of the ion source of the ITER Heating Neutral Beam Injector, where negative ions of Hydrogen or Deuterium are produced by a RF generated plasma and accelerated with a set of grids up to ~100 keV. The Power Supply System is composed of high voltage dc power supplies capable of handling frequent grid breakdowns, high current dc generators for the magnetic filter field and RF generators for the plasma generation. During the first 3 years of SPIDER operation different electrical issues were discovered, understood and addressed thanks to deep analyses of the experimental results supported by modelling activities. The paper gives an overview on the observed phenomena and relevant analyses to understand them, on the effectiveness of the short-term modifications provided to SPIDER to face the encountered issues and on the design principle of long-term solutions to be introduced during the currently ongoing long shutdown.Comment: 8 pages, 12 figures. Presented at SOFT 202

Model for screening of resonant magnetic perturbations by plasma in a realistic tokamak geometry and its impact on divertor strike points

This work addresses the question of the relation between strike-point splitting and magnetic stochasticity at the edge of a poloidally diverted tokamak in the presence of externally imposed magnetic perturbations. More specifically, ad-hoc helical current sheets are introduced in order to mimic a hypothetical screening of the external resonant magnetic perturbations by the plasma. These current sheets, which suppress magnetic islands, are found to reduce the amount of splitting expected at the target, which suggests that screening effects should be observable experimentally. Multiple screening current sheets reinforce each other, i.e. less current relative to the case of only one current sheet is required to screen the perturbation.Comment: Accepted in the Proceedings of the 19th International Conference on Plasma Surface Interactions, to be published in Journal of Nuclear Materials. Version 2: minor formatting and text improvements, more results mentioned in the conclusion and abstrac

Velocity-space sensitivity of the time-of-flight neutron spectrometer at JET

The velocity-space sensitivities of fast-ion diagnostics are often described by so-called weight functions. Recently, we formulated weight functions showing the velocity-space sensitivity of the often dominant beam-target part of neutron energy spectra. These weight functions for neutron emission spectrometry (NES) are independent of the particular NES diagnostic. Here we apply these NES weight functions to the time-of-flight spectrometer TOFOR at JET. By taking the instrumental response function of TOFOR into account, we calculate time-of-flight NES weight functions that enable us to directly determine the velocity-space sensitivity of a given part of a measured time-of-flight spectrum from TOFOR

Effect of the relative shift between the electron density and temperature pedestal position on the pedestal stability in JET-ILW and comparison with JET-C

The electron temperature and density pedestals tend to vary in their relative radial positions, as observed in DIII-D (Beurskens et al 2011 Phys. Plasmas 18 056120) and ASDEX Upgrade (Dunne et al 2017 Plasma Phys. Control. Fusion 59 14017). This so-called relative shift has an impact on the pedestal magnetohydrodynamic (MHD) stability and hence on the pedestal height (Osborne et al 2015 Nucl. Fusion 55 063018). The present work studies the effect of the relative shift on pedestal stability of JET ITER-like wall (JET-ILW) baseline low triangularity (\u3b4) unseeded plasmas, and similar JET-C discharges. As shown in this paper, the increase of the pedestal relative shift is correlated with the reduction of the normalized pressure gradient, therefore playing a strong role in pedestal stability. Furthermore, JET-ILW tends to have a larger relative shift compared to JET carbon wall (JET-C), suggesting a possible role of the plasma facing materials in affecting the density profile location. Experimental results are then compared with stability analysis performed in terms of the peeling-ballooning model and with pedestal predictive model EUROPED (Saarelma et al 2017 Plasma Phys. Control. Fusion). Stability analysis is consistent with the experimental findings, showing an improvement of the pedestal stability, when the relative shift is reduced. This has been ascribed mainly to the increase of the edge bootstrap current, and to minor effects related to the increase of the pedestal pressure gradient and narrowing of the pedestal pressure width. Pedestal predictive model EUROPED shows a qualitative agreement with experiment, especially for low values of the relative shift

Relationship of edge localized mode burst times with divertor flux loop signal phase in JET

A phase relationship is identified between sequential edge localized modes (ELMs) occurrence times in a set of H-mode tokamak plasmas to the voltage measured in full flux azimuthal loops in the divertor region. We focus on plasmas in the Joint European Torus where a steady H-mode is sustained over several seconds, during which ELMs are observed in the Be II emission at the divertor. The ELMs analysed arise from intrinsic ELMing, in that there is no deliberate intent to control the ELMing process by external means. We use ELM timings derived from the Be II signal to perform direct time domain analysis of the full flux loop VLD2 and VLD3 signals, which provide a high cadence global measurement proportional to the voltage induced by changes in poloidal magnetic flux. Specifically, we examine how the time interval between pairs of successive ELMs is linked to the time-evolving phase of the full flux loop signals. Each ELM produces a clear early pulse in the full flux loop signals, whose peak time is used to condition our analysis. The arrival time of the following ELM, relative to this pulse, is found to fall into one of two categories: (i) prompt ELMs, which are directly paced by the initial response seen in the flux loop signals; and (ii) all other ELMs, which occur after the initial response of the full flux loop signals has decayed in amplitude. The times at which ELMs in category (ii) occur, relative to the first ELM of the pair, are clustered at times when the instantaneous phase of the full flux loop signal is close to its value at the time of the first ELM

FAR-INFRARED POLARIMETRY IN FUSION PLASMAS: INSTRUMENTS AND DATA ANALYSIS IN RFX-MOD AND JET

This thesis reports the work performed during the three years of my PH.D. course at the Electrical Engineering Department of the University of Padova. Most of my research activity has been carried out at Consorzio RFX (Padova) where the RFX-mod experiment is located. RFX-mod (Reversed Field eXperiment-MODified) is the largest toroidal device built to study magnetically confined plasmas of thermonuclear interest in the so-called Reversed Field Pinch (RFP) configuration. The RFP, together with the Tokamak and the Stellarator, is one of the main configurations used to confine plasmas in toroidal devices with the purpose of studying controlled thermonuclear fusion as an energy source. The energy production by fusion in magnetically confined plasmas is an ambitious and important goal, which could contribute to solve the problem of a sustainable energy source for mankind. To be an efficient energy source, a sufficiently dense and hot plasma must be confined for a time long enough. In order to fulfill this goal, energy and particle losses, heating properties, transport and energy confinement must be understood and eventually controlled. One of the most interesting characteristics of RFP plasmas, discovered in the last years in RFX-mod, concerned particular self-organized regimes (SHAx) where transport barriers are observed and the confinement is improved with respect to the traditional RFP configurations. These plasma configurations are characterized by a spectrum of the MHD modes leaded by few but well defined components and by the presence of high temperature and high X emissivity plasma regions. The most recent works suggest that the structure of the modes is strongly related to the profiles of the magnetic field. The measurement of the magnetic fields in the inner regions of the plasma is very important for understanding these plasma configurations. My research activity aimed to experimentally investigate the profiles of the magnetic fields by Far Infrared (FIR) polarimetry in order to characterize such particular plasma regimes and to compare them with the traditional ones. The measurement of the profiles of the magnetic fields in RFX-mod is performed by a Far Infrared polarimeter. This diagnostic is based on the measurement of Faraday rotation angle. In fact a radiation propagating through a magnetically confined plasma suffers a rotation of its polarization plane that is proportional to the line integrated product of the electron density multiplied by the component of the magnetic field parallel to the propagation direction. My research activity concentrated in the measurements and analyses of the polarimetric effects of the plasma column on a radiation probing it, in order to obtain detailed information about the distribution of the magnetic fields inside the plasma. In particular I have been involved into laboratory activities and also in the installation, operation and optimization of the FIR polarimeter in RFX-mod machine. The analysis of the data obtained by this diagnostic allowed me to characterize the plasma column and to get information about the evolution of the magnetic fields inside the plasma during the discharge. In fact, the polarimetric measurements together with the electron density measurements, allow the reconstruction of the profile of the poloidal magnetic field and thus the reconstruction of the position of the magnetic inner surfaces, in particular of the magnetic axis. Given these information it is possible to estimate the plasma current and therefore the safety factor q. My research activity carried out at Consorzio RFX, concerned the installation of the polarimeter in the RFX-mod machine. After several tests in laboratory, the diagnostic has been fully mounted and the entire optical line has been aligned by a visible laser previously put on the FIR beam. The additional activities done to make the polarimeter operative are reported in detail. Innovative acquisition modules have been adopted allowing to acquire many samples with a sampling frequency up to 25kHz. Moreover I contributed to the development and implementation of a software which processes the raw data coming from the detectors and it gives out the Faraday rotation angle in real time. All these activities allowed me to obtain the first polarimetric measurements along five channels. The international context of fusion science and the cooperation with other European Associations involved in the European fusion program, i.e. CEA (F) and CCFE (UK), allowed me to be in contact with many fusion experts and to personally contribute to some advanced aspects of the European and international scientific research, recently reached. In particular, I spent four months at the Culham Research Centre (UK), where the JET machine is sited. During that period I studied in depth the interferometer/polarimeter operating at JET, analyzing its optical characteristics and testing some models useful to obtain measurements of the electron density from polarimetric data. I contributed to the implementation of a code for the raw data processing and I tested some procedures which starting from raw data evaluate all the polarimetric effects of the plasmas accounting of the properties of the optical beam path. This thesis focuses on the main results obtained at RFX-mod and JET machines. The first measurements performed with the polarimeter installed in RFX-mod during my PhD, are reported. Regarding the activity at JET, this thesis describes the results that confirm the polarimeter can provide reliable information of the electron density, very useful for the real time control. The thesis is organized as it follows: -In Chapter 1 the polarization of light is introduced, describing some important historical stages which leaded to the discovery of this fundamental property of the light. In this chapter the light is represented as an electromagnetic wave and its components are mathematically expressed as sinusoids. At the end the concept of polarimetry, as technique to measure the polarization state of a radiation, in introduced. -The Chapter 2 introduces the issues of energy production, highlighting the main characteristics that a new energy source needs; the thermonuclear magnetic fusion is presented as an effective actor in the future energetic outline. A description of the physics of magnetic confinement equilibria and, with more detail of the RFP and Tokamak, is given. -In Chapter 3 the basic theory of polarimetry and interferometry is introduced. The Stokes and Mueller formalisms for the description of the polarization state of a radiation are described. Some polarimetric and interferometric techniques are discussed. -The Chapter 4 is focused in the activities carried out at the RFX-mod machine. In this chapter RFX-mod is briefly described with details on the set of diagnostics. The entire setup of the polarimeter is accurately described and the first results are presented and compared with the theoretical values evaluated according to models. -The Chapter 5 is focused in the activities carried out at JET. In this chapter the JET experiment is briefly introduced and an overview of diagnostics is given. The entire polarimeter/interferometer system is described and the measurements of the electron density obtained by Cotton Mouton values are presented and discussed. The preliminary results of an investigation performed to understand the source of an anomaly behaviour of the calibration data are reported. Finally the polarimetric results evaluated by a new raw data processing code are shown.In questa tesi vengono riassunte le attività scientifiche svolte nei tre anni della Scuola di Dottorato in Ingegneria Industriale, indirizzo Energetica. La mia attività si è svolta principalmente presso il Consorzio RFX a Padova, dove ha sede il più grande esperimento toroidale per lo studio dei plasmi confinati magneticamente nella cosiddetta configurazione reversed field pinch (RFP). La configurazione RFP, insieme con i tokamak e gli stellarator, risulta una delle maggiori configurazioni per confinare i plasmi in esperimenti toroidali, con lo scopo di studiare la fusione termonucleare come sorgente di energia. La produzione di energia attraverso la fusione in plasmi confinati magneticamente è un ambizioso e importante obiettivo, che potrebbe contribuire a risolvere il problema di una sorgente sostenibile di energia per tutta l'umanita. Per essere una sorgente di energia efficiente, un plasma sufficientemente denso e caldo deve essere confinato per un tempo sufficientemente lungo. A questo scopo, in relazione alle proprietà di riscaldamento, del trasporto e del confinamento dell'energia del plasma, il mio progetto di ricerca ha riguardato lo studio sperimentale delle configurazioni magnetiche nei plasmi di tipo Reversed Field Pinch, mediante tecniche di polarimetria nel lontano infarosso. Una delle caratteristiche più interessanti dei plasmi RFP emerse negli ultimi anni sia nell'esperimento RFX-mod di Padova che in altre macchine RFP, è stata la scoperta di configurazioni magnetiche che si caratterizzano sperimentalmente per proprietà di riscaldamento, trasporto e confinamento dell'energia superiori a quelle delle configurazioni RFP tradizionali. Queste configurazioni di plasma sono caratterizzate da uno spettro dei modi MHD dominato da poche e ben definite componenti e dalla presenza di regioni di plasma ad alta temperatura e emissività X. Gli studi più recenti suggeriscono che la struttura dei modi sia fortemente correlata ai profili di campo megnetico, la cui misura sperimentale nelle regioni interne del plasma riveste una importanza decisiva per lo studio di queste configurazioni. Il progetto di ricerca proposto prevede appunto lo studio sperimentale del campo magnetico di queste configurazioni e il confronto con quelle tradizionali. Dal punto di vista sperimentale, per la misura dei profili di campo magnetico nell'esperimento RFX-mod si utilizza una diagnostica polarimetrica. Questa tecnica si basa sulla misura della rotazione del piano di polarizzazione (rotazione di Faraday) di un fascio laser nel lontano infrarosso (FIR), dovuta alla propagazione nel plasma magnetizzato. Tale rotazione è proporzionale all'integrale di linea del prodotto tra la densità elettronica locale e la componente del campo magnetico parallela alla direzione di propagazione della radiazione. Al fine di ottenere informazioni sulla distribuzione dei campi magnetici all'interno della colonna di plasma, la mia attività di ricerca è stata focalizzata sulla misura e sull'analisi degli effetti polarimetrici del plasma stesso su una radiazione che lo attraversa. In particolare, sono stato coinvolto in esperimenti e attività di laboratorio, nell'allestimento, nell'operazione e ottimizzazione del polarimetro FIR presso la macchina RFX-mod. L'analisi dei dati raccolti con questa diagnostica mi ha consentito di caratterizzare la colonna di plasma e di fornire informazioni sull'evoluzione dei campi magnetici al suo interno. Come detto sopra, le misure polarimetriche associate a misure della densità elettronica, permettono la ricostruzione del profilo di campo poloidale e il monitoraggio della posizione delle superfici magnetiche interne, in particolare dell'asse magnetico. Da queste informazioni è possibile risalire alla misura della corrente di plasma e quindi alla misura del fattore di sicurezza q. La mia attività di ricerca svolta sull'esperimento RFX-mod ha riguardato la messa in funzione del nuovo polarimetro come diagnostica di RFX-mod. Dopo aver completato tutti i test di funzionamento in laboratorio la diagnostica è stata installata sulla macchina e ivi ultimati gli allineamenti con la sorgente laser FIR. Le attività accessorie svolte per la messa in funzione della diagnostica sono descritte nel dattaglio in questa tesi. Sono stati installati dei moduli di acquisizione che permettono di acquisire più segnali con una frequenza di campionamento fino a 25kHz. Ho contribuito allo sviluppo di un software che elabora i segnali grezzi provenienti dai rivelatori e fornisce in uscita la misura dell'angolo di Faraday in tempo reale. Tutte queste attività mi hanno permesso di ottenere le prime misure di effetto Faraday su cinque canali, grazie a cui, come detto prima, è possibile caratterizzare il plasma. Il contesto internazionale delle ricerche e la collaborazione con altre Associazioni Europee coinvolte nel programma fusione Europeo, CEA (F) e CCFE (UK), hanno favorito numerosi contatti con esperti internazionali e la possibilità di contribuire in prima persona agli aspetti più avanzati della ricerca scientifica in sede Europea e internazionale maturata negli anni più recenti. In questo ambito ho trascorso un periodo di tempo presso l'esperimento JET (Joint European Torus) del Culham Science Centre (UK). Durante queste visite ho approfondito lo studio della diagnostica Interferometro/Polarimetro operante presso la macchina JET, analizzandone le caratteristiche ottiche e mettendo a punto dei modelli per ricavare la misura di densità dalle informazioni polarimetriche. Ho contribuito in maniera significativa alla scrittura del codice per l'elaborazione dei dati forniti dalla diagnostica mettendo a punto procedimenti di calcolo che a partire dai dati grezzi, calcolano tutti i parametri polarimetrici del plasma tenendo conto delle caratteristiche del percorso ottico compiuto dal fascio laser di misura. Questa tesi descrive i risultati ottenuti presso i due esperimenti RFX-mod e JET. Nel caso di RFX-mod si riportano i primi risultati ottenuti con il polarimetro installato durante il periodo del dottorato. Nel caso di JET si riportano dei risultati che confermano la possibilità della diagnostica di fornire misure polarimetriche tali da poter ricavare informazioni sulla densità elettronica utili per il controllo in tempo reale della scarica. La tesi è strutturata come segue: -Nel Capitolo 1 viene introdotto il concetto di polarizzazione della luce, attraverso la descrizione di alcune tappe di interesso storico scientifico che hanno condotto alla scoperta di questa importante caratteristica della luce. In questo capitolo la luce è rappresentata in termini di onda elettromagnetica e le sue componenti sono espresse matematicamente come sinusoidi. Infine viene introdotto il concetto di polarimetria come strumento per la misura e la determinazione dello stato di polarizzazione di una radiazione. -Il Capitolo 2 introduce il problema della produzione di energia, evidenziando le principali caratteristiche che una nuova fonte di energia deve avere: la fusione termonucleare a confinamento magnetico è presentata come un attore importante nel futuro panorama energetico. Viene fornita la descrizione della fisica degli equilibri magnetici per il confinamento nel caso di macchina di tipo Reversed Field Pinch e Tokamak. -Nel Capitolo 3 vengono presentati i concetti base della polarimetria e dell' interferometria. Vengono descritti i formalismi di Stokes e Mueller per la descrizione dello stato di polarizzazione di un'onda. Inoltre vengono discusse alcune tecniche interferometriche. -Il Capitolo 4 è dedicato all'attività svolta presso la macchina RFX-mod. In questo capitolo viene descritto brevemente l'esperimento RFX-mod con un dettaglio sulle diagnostiche installate. Si passa poi alla descrizione puntuale del set-up del polarimetro per arrivare ai primi risultati ottenuti. I risultati vengono confrontati con i valori attesi secondo alcuni modelli teorici. -Il Capitolo 5 è dedicato all'attività svolta presso la macchina JET. In questo capitolo viene descritto brevemente l'esperimento JET con un dettaglio sulle diagnostiche installate. Si passa poi alla descrizione puntuale del set-up del polarimetro-interferometro per arrivare ai risultati ottenuti nella ricostruzione del profilo di densità elettronica dalle misure di effetto Cotton Mouton. Si presentano i risultati di un lavoro svolto a capire un comportamento anomalo delle misure del polarimetro in fase di calibrazione. Infine si mostrano i primi risultati ottenuti con un nuovo codice di rielaborazione dei dati grezzi

Data analysis for a rotating quarter-wave, far-infrared Stokes polarimeter

Data analysis techniques are reviewed and extended for the measurement of the Stokes vector of partially or completely polarized radiation by the rotating quarter-wave method. It is shown that the conventional technique, based on the Fourier analysis of the recorded signal, can be efficiently replaced by a weighted least-squares best fit, so that the different accuracy of the measured data can be taken into account to calculate the measurement errors of the Stokes vector elements. Measurement errors for the polarization index P and for the azimuth and ellipticity angles \u3c8 and \u3c7 of the radiation are also calculated by propagation error theory. For those cases in which the above technique gives a nonphysical Stokes vector (i.e., with a polarization degree of P > 1) a constrained least-squares best fit is introduced, and it is shown that in this way a Stokes vector with P = 1 (rather than P 64 1) is always obtained. In addition an analysis technique useful to remove from the measured data systematic errors due to initial misalignment of the rotating quarter-wave axis is described. Examples of experimental Stokes vectors obtained by the above tech- niques during the characterization of components for a far-infrared polarimeter at \u3bb = 118.8 \u3bcm for applications in plasma diagnostics are presented and discussed. Finally the problem of the experimental determination of physically consistent Mueller matrices (i.e., of Mueller matrices for which the transformed Stokes vector has always P 64 1) is discussed, and it is shown that for simple Mueller matrices of the ABCD type, whose elements can be determined by the measurement of a single Stokes vector, the imposed P 64 1 constraint gives a sufficient condition for physical consistency. On the other hand, the same constraint, when imposed to the set of four basic Stokes vectors conventionally measured for the determi- nation of a full 16-element Mueller matrix, gives only a necessary but not a sufficient condition

Feasibility study of a flux-gate magnetic field sensor suitable for ITER neutral beam injectors

ITER Neutral Beam Injectors (NBIs) need to be shielded from the relatively strong stray magnetic field generated by the Poloidal Field Coils of the Tokamak. For this reason both the Heating Neutral Beams (HNB) and the Diagnostic Neutral Beam (DNB) will be provided with a Passive Magnetic Shield and with a system of Active Correction and Compensation Coils (ACCC). The ACCC will operate in feedback control and thus require the measurement of magnetic field inside the NBI vessel, i.e. in an environment subjected to the neutron flux coming from the Tokamak. To this purpose, magnetic sensors which are robust, radiation hard, drift-immune and remote-handling compatible are required. Flux-gate magnetic sensors are a good candidate for this task, but commercial sensors of this kind have typically a limited measured range (below 0.1 mT). The feasibility of a flux-gate sensor for the ITER NBI has been studied by developing a numerical model which includes magnetic core hysteresis, and which demonstrated that, by suitable choice of the core magnetic properties and geometry, it is possible to increase the measurement range by at least 2 orders of magnitude. On this basis, a flux-gate sensor prototype has been realized at Consorzio RFX. Experimental tests carried out so far have demonstrated that the measurement range can be increased to 3c10 mT with acceptable accuracy and frequency bandwidth

Design and development of an Allison type emittance scanner for the SPIDER ion source

Low divergence negative ion beams are crucial for the development of ITER-like fusion reactors. SPIDER is the prototype beam source of the ITER heating neutral beam injector, and it recently started beam acceleration, up to a voltage of 30 kV. The main diagnostics used to measure beamlet divergence are a movable diagnostic calorimeter (STRIKE), which gives the thermal footprint of the beamlets; beam emission spectroscopy; and visible imaging. These systems do not allow a direct measurement of single beamlet phase-space distribution, which is useful for comparison with numerical simulations and to estimate accelerator performances. To this purpose, a movable Allison type emittance scanner for the SPIDER negative ion beam was developed and proposed for the installation on the STRIKE supporting structure. This paper describes the numerical analyses performed to dimension the mechanical and electrical components, such as the Faraday cup and the slits. An analytical approach based on the integration of an arbitrary phase-space distribution was adopted in order to simulate the device performances. The constraints due to the operation in a high heat load environment are discussed

Investigation on an Anomalous Behavior of the Polarimetric Measurements at JET

The far-infrared polarimeter at JET is affected by an anomaly that makes difficult the interpretation of both Faraday and Cotton\u2013Mouton effect measurements. The anomaly is clearly displayed during calibration operations in the absence of plasma: As the polarization of the probing beam is rotated, the phase shift of the polarimetric signal with respect to the interferometric signal is not constant, as expected, and changes significantly. It affects all the polarimetric measurement channels and has so far been removed by an empirical preprocessing of the raw data. It can be ascribed to a nonideal behavior of some optical components. Looking for a possible explanation of the anomaly, in this paper, we analyze the optical setup of the JET polari\u2013interferometer according to the laws of classical polarization optics. At first, the optical characteristics of the recombination plates are analyzed in detail. Although they produce ellipticity in the transmitted and reflected beams, the results show that the recombination plates should not be responsible of the anomaly of the polarimeter. Then, the dielectric waveguides used to transfer the recombined beams from the torus hall to the detectors are, for the first time, considered as a possible origin of the anomaly. The anomalous behavior is expected to be mainly originated by reflections on metal mirrors, which may produce rotations of the polarization of the beams. A calculation has been performed in order to analyze the effects of a rotation of the polarization of the recombined beam on the detector signals. As a result, a rotation of the polarization along the line could explain the anomaly. We also suggest some simple and feasible tests, which are useful to give an experimental support to this conclusion, and discuss possible modifications of the optical setup to remove or greatly reduce the anomaly in future measurements