Doubts on the efficacy of outliers correction methods

While the utilisation of different methods of outliers correction has been shown to counteract the inferential error produced by the presence of contaminating data not belonging to the studied population; the effects produced by their utilisation when samples do not contain contaminating outliers are less clear. Here a simulation approach shows that the most popular methods of outliers correction (2 Sigma, 3 Sigma, MAD, IQR, Grubbs and winsorizing) worsen the inferential evaluation of the studied population in this condition, in particular producing an inflation of Type I error and increasing the error committed in estimating the population mean and STD. We show that those methods that have the highest efficacy in counteract the inflation of Type I and Type II errors in the presence of contaminating outliers also produce the stronger increase of false positive results in their absence, suggesting that the systematic utilisation of methods for outliers correction risk to produce more harmful than beneficial effect on statistical inference. We finally propose that the safest way to deal with the presence of outliers for statistical comparisons is the utilisation of non-parametric test

Numerical studies of a negative ion beam and of a tomographic beam diagnostic

ITER is the first reactor-scale scientific experiment that aims to demonstrate the scientific and the technological feasibility of fusion energy. It is based on the tokamak concept of magnetic confinement, in which the fuel, a mixture of deuterium and tritium heated to temperatures in excess of 150 million degrees Celsius, is contained in a toroidal vacuum chamber. Among the systems used to reach such high temperature range, a fundamental role is played by the injection of intense beams of neutral particles into the plasma, which is consequently heated by collisions. This process is realized by means of two Neutral Beam Injectors (NBIs), capable of delivering to the plasma a power of 16.7 MW each. These devices are mainly composed of a negative deuterium ion source, an electrostatic accelerator where a 40 A beam of negative deuterons will be accelerated to 1 MV and a neutralizer which converts part of the beam into high energy neutrals able to penetrate the high magnetic field confining the ITER plasma. The ITER requirements for these devices have never been simultaneously achieved so far in a full scale, full performance device and therefore a neutral beam test facility is being constructed at Consorzio RFX in Padova. The research activity presented in this thesis work is in the framework of the development of the negative ion source (SPIDER) and full injector (MITICA) prototypes for the ITER neutral beam. In particular, it is focused on two main topics: particle transport studies inside the MITICA accelerator and the development of a tomographic beam diagnostic. A proper modeling of the particle transport inside the MITICA accelerator, considering the main processes that generate secondary particles relevant for the evaluation of the heat loads on the accelerator grids is essential for the thermo-mechanical analysis and the mechanical design of the accelerator. For this reason an upgrade of the relativistic particle tracking code called EAMCC has been undertaken and the simulations performed for evaluating the thermal power deposited on the MITICA accelerator grids are presented in the first part of the present thesis work. For the first time, an entire source called NIO1 installed at RFX and made of nine beamlets has been simulated in EAMCC considering multi-beamlet effects which were neglected earlier and discarding the axis-symmetry hypothesis of the electric fields imposed by the original version of the code. Results obtained, also presented in the first part, will be used for benchmarking the modifications introduced in the code. The second part of the thesis is dedicated to beam tomography, an important diagnostic for the assessment of the density profile of the beam. A tomography code based on algebraic reconstruction techniques has been developed and numerically tested. Beam emissivity profiles considered for testing the code are calculated by the upgraded version of EAMCC. The tomography code has been developed with the aim of realizing a versatile instrument, applicable to linear accelerators as well as to a tokamak and without adding any hypotheses about the beam characteristics or the emissivity in a particular region of the tomography plane, not to limit the capability of the code of detecting irregularities in the beam profiles. The effects of the instrumental noise on tomography reconstructions have also been studied and, in order to reduce its impact, different filtering techniques have been considered both in the frequency and in the spatial domain, demonstrating the feasibility to filter out the effect of the noise by post-processing the reconstructed image of the bea

Preliminary Assessment of Radiolysis for the Cooling Water System in the Rotating Target of {SORGENTINA}-{RF}

The SORGENTINA-RF project aims at developing a 14 MeV fusion neutron source featuring an emission rate in the order of 5-7 x 10(13) s(-1). The plant relies on a metallic water-cooled rotating target and a deuterium (50%) and tritium (50%) ion beam. Beyond the main focus of medical radioisotope production, the source may represent a multi-purpose neutron facility by implementing a series of neutron-based techniques. Among the different engineering and technological issues to be addressed, the production of incondensable gases and corrosion product into the rotating target deserves a dedicated investigation. In this study, a preliminary analysis is carried out, considering the general layout of the target and the present choice of the target material

Numerical studies of a negative ion beam and of a tomographic beam diagnostic

ITER is the first reactor-scale scientific experiment that aims to demonstrate the scientific and the technological feasibility of fusion energy. It is based on the tokamak concept of magnetic confinement, in which the fuel, a mixture of deuterium and tritium heated to temperatures in excess of 150 million degrees Celsius, is contained in a toroidal vacuum chamber. Among the systems used to reach such high temperature range, a fundamental role is played by the injection of intense beams of neutral particles into the plasma, which is consequently heated by collisions. This process is realized by means of two Neutral Beam Injectors (NBIs), capable of delivering to the plasma a power of 16.7 MW each. These devices are mainly composed of a negative deuterium ion source, an electrostatic accelerator where a 40 A beam of negative deuterons will be accelerated to 1 MV and a neutralizer which converts part of the beam into high energy neutrals able to penetrate the high magnetic field confining the ITER plasma. The ITER requirements for these devices have never been simultaneously achieved so far in a full scale, full performance device and therefore a neutral beam test facility is being constructed at Consorzio RFX in Padova. The research activity presented in this thesis work is in the framework of the development of the negative ion source (SPIDER) and full injector (MITICA) prototypes for the ITER neutral beam. In particular, it is focused on two main topics: particle transport studies inside the MITICA accelerator and the development of a tomographic beam diagnostic. A proper modeling of the particle transport inside the MITICA accelerator, considering the main processes that generate secondary particles relevant for the evaluation of the heat loads on the accelerator grids is essential for the thermo-mechanical analysis and the mechanical design of the accelerator. For this reason an upgrade of the relativistic particle tracking code called EAMCC has been undertaken and the simulations performed for evaluating the thermal power deposited on the MITICA accelerator grids are presented in the first part of the present thesis work. For the first time, an entire source called NIO1 installed at RFX and made of nine beamlets has been simulated in EAMCC considering multi-beamlet effects which were neglected earlier and discarding the axis-symmetry hypothesis of the electric fields imposed by the original version of the code. Results obtained, also presented in the first part, will be used for benchmarking the modifications introduced in the code. The second part of the thesis is dedicated to beam tomography, an important diagnostic for the assessment of the density profile of the beam. A tomography code based on algebraic reconstruction techniques has been developed and numerically tested. Beam emissivity profiles considered for testing the code are calculated by the upgraded version of EAMCC. The tomography code has been developed with the aim of realizing a versatile instrument, applicable to linear accelerators as well as to a tokamak and without adding any hypotheses about the beam characteristics or the emissivity in a particular region of the tomography plane, not to limit the capability of the code of detecting irregularities in the beam profiles. The effects of the instrumental noise on tomography reconstructions have also been studied and, in order to reduce its impact, different filtering techniques have been considered both in the frequency and in the spatial domain, demonstrating the feasibility to filter out the effect of the noise by post-processing the reconstructed image of the beamITER (International Thermonuclear Experimental Reactor) è un reattore sperimentale a fusione termonucleare basato sulla configurazione magnetica tokamak e volto a dimostrare la possibilità di sfruttare l’energia da fusione per la generazione di elettricità. Il combustibile nucleare costituito da una miscela di deuterio e trizio, portato a temperature eccedenti i 150 milioni di gradi centigradi, è confinato in una camera di forma toroidale per mezzo di campi magnetici. Tra i sistemi usati per riscaldare il combustibile nucleare, l’iniezione di neutri riveste un ruolo fondamentale. Essa consiste nella iniezione di nuclei di deuterio ad alta energia (1 MeV) che scaldano il combustibile gassoso altamente ionizzato (denominato plasma) a seguito delle collisioni con lo stesso. In ITER sono previsti due iniettori di neutri (NBIs), ciascuno in grado di immettere nel plasma una potenza di 16,7 MW. Tali iniettori sono essenzialmente costituiti da una sorgente di ioni negativi di deuterio, un acceleratore elettrostatico dove un fascio di 40 A di tali ioni viene accelerato fino a raggiungere l’energia di 1 MeV e un neutralizzatore nel quale una parte del fascio viene convertita in particelle neutre ad alta energia che possono penetrare gli intensi campi magnetici usati per confinare il plasma: il sistema dovrà operare continuativamente per un’ora. Le prestazioni richieste per tali iniettori di neutri non sono mai state raggiunte fino ad ora simultaneamente in un unico esperimento e su tale scala. Si è reso pertanto necessario lo studio e lo sviluppo di un prototipo di iniettore, affidato al Consorzio RFX di Padova. Il progetto prevede lo studio e la realizzazione della sorgente di ioni dell’ITER NBI (SPIDER) e successivamente la costruzione del prototipo dell’intero iniettore (MITICA). La mia attività di ricerca, presentata in questa tesi, si inserisce in tale contesto e più in particolare è incentrata sullo studio del trasporto di particelle all’interno di acceleratori lineari finalizzato al calcolo della potenza termica depositata nelle griglie dell’acceleratore di MITICA e sullo sviluppo di una diagnostica tomografica per fasci di particelle. Un appropriato modello fisico, il più realistico possibile, dei fascetti di particelle che compongono il fascio di MITICA è fondamentale per l’analisi termo-meccanica e per il progetto meccanico dell’acceleratore. A tale scopo, sono state eseguite delle modifiche al codice di calcolo EAMCC usato per simulare i processi di creazione di cariche secondarie che generano notevoli carichi termici sulle griglie dell’acceleratore. La versione modificata del codice è stata utilizzata per lo studio del trasporto di cariche nell’acceleratore di MITICA e per il calcolo dei carichi termici, come illustrato nella prima parte della tesi. Inoltre, per la prima volta, l’intera sorgente chiamata NIO1 installata a RFX e costituita da nove fascetti di ioni negativi di idrogeno è stata simulata con EAMCC, considerando effetti fino ad ora non simulati, come l’interazione tra fascetti vicini, ed eliminando l’ipotesi semplificativa di campi elettrici assial-simmetrici. I risultati delle simulazioni su NIO1 verranno usati in futuro per la validazione sperimentale delle modifiche introdotte in EAMCC e sono sintetizzati sempre nella prima parte del presente lavoro di tesi. La seconda parte è invece dedicata alla tomografia del fascio che rappresenta una diagnostica importante per la misura del profilo di densità delle particelle e consente di valutare il grado di uniformità dello stesso, un requisito fondamentale per l’iniettore di neutri. In tale ambito è stato sviluppato un codice tomografico basato su diverse tecniche di ricostruzione algebriche, più indicate rispetto a tecniche basate sulla trasformata di Radon nel caso in cui il numero di rivelatori disponibile sia molto inferiore rispetto al numero di pixel del profilo ricostruito. Tale codice è stato testato su NIO1 e su MITICA con risultati promettenti. Non essendo disponibile alcuna misura sperimentale dell’emissione dei fasci di particelle, grazie alle modifiche introdotte in EAMCC è stato possibile calcolare il profilo di emissività di fotoni del fascio usato poi per il test del codice tomografico. E’ stato inoltre studiato il ruolo del rumore strumentale e il suo impatto sulle ricostruzioni tomograficche. Sono state considerate tecniche di filtraggio sia nel dominio delle frequenze spaziali sia in quello spaziale e in particolare, una tecnica usata per filtrare le immagini radar è stata adattata al caso tomografico e implementata nel codice dimostrando la possibilità di limitare fortemente l’effetto negativo del rumore sulla tomografia del fasci

Doubts on the efficacy of outliers correction methods

While the utilisation of different methods of outliers correction has been shown to counteract the inferential error produced by the presence of contaminating data not belonging to the studied population; the effects produced by their utilisation when samples do not contain contaminating outliers are less clear. Here a simulation approach shows that the most popular methods of outliers correction (2 Sigma, 3 Sigma, MAD, IQR, Grubbs and winsorizing) worsen the inferential evaluation of the studied population in this condition, in particular producing an inflation of Type I error and increasing the error committed in estimating the population mean and STD. We show that those methods that have the highest efficacy in counteract the inflation of Type I and Type II errors in the presence of contaminating outliers also produce the stronger increase of false positive results in their absence, suggesting that the systematic utilisation of methods for outliers correction risk to produce more harmful than beneficial effect on statistical inference. We finally propose that the safest way to deal with the presence of outliers for statistical comparisons is the utilisation of non-parametric tests

Physics design of the injector source for ITER neutral beam injector (invited)

Two Neutral Beam Injectors (NBI) are foreseen to provide a substantial fraction of the heating power necessary to ignite thermonuclear fusion reactions in ITER. The development of the NBI system at unprecedented parameters (40 A of negative ion current accelerated up to 1 MV) requires the real- ization of a full scale prototype, to be tested and optimized at the Test Facility under construction in Padova (Italy). The beam source is the key component of the system and the design of the multi-grid accelerator is the goal of a multi-national collaborative effort. In particular, beam steering is a chal- lenging aspect, being a tradeoff between requirements of the optics and real grids with finite thickness and thermo-mechanical constraints due to the cooling needs and the presence of permanent magnets. In the paper, a review of the accelerator physics and an overview of the whole R&D physics program aimed to the development of the injector source are presented

SORGENTINA-RF project: fusion neutrons for 99Mo medical radioisotope

International audienc

Detailed design optimization of the MITICA negative ion accelerator in view of the ITER NBI

ITERの加熱電流駆動を担う中性粒子入射装置（HNB）を確実にするため、NB実機試験施設（NBTF）をイタリアに建設中である。その中の試験装置MITICAにおいて、ITER HNBに必要とされる１MeVの負イオンビーム源の性能を実証する。本論文ではMITICAで試験する１MeV負イオンビーム源の最新設計を報告するものである