The growth of plasma instabilities can cause a sudden loss of thermal and magnetic energy. In this disruptive event, electrons can be accelerated to relativistic energies and gain a significant fraction of the energy stored in the tokamak magnetic field. At these velocities, Coulomb collisions with background plasma become negligible and the acceleration of the runaway electrons is only limited by relativistic effects and radiative losses. When the post-disruption magnetic field is lost, the energetic runaway electron beam can collide with the in-vessel plasma-facing components causing severe and localized damage. Unmitigated runaway electron events can hinder operation by forcing long shutdown periods of several months to allow repairs. The avoidance of these extreme scenarios is paramount to the success of large-scale tokamaks. The threat posed by runaway electrons is a primary focus of the fusion community. Extensive modelling and experimental campaigns are currently ongoing in most large and medium-scale tokamaks. During disruptions, runaway electrons can be accelerated up to energies in the order of several MeVs. One of the mechanisms that limit this acceleration is the emission of bremsstrahlung radiation caused by the interaction of the relativistic particles with the background plasma. Due to the extreme energy these electrons can reach, the bremsstrahlung radiation spectrum extends to several MeVs, in hard X-ray energy range. This work illustrates how information on the runaway electron velocity space can be extracted from the measured bremsstrahlung X-ray emission. In the first half of this work, the development, characterization and implementation of novel hard x-ray spectrometers optimized for runaway electron bremsstrahlung measurement are discussed. A new compact HXR spectrometer with high counting rate capabilities in excess of 1 MCps was developed for the array configuration of the tokamak DIII-D Gamma-Ray Imager system. This detector is based on a YAP:Ce scintillator crystal coupled with a silicon photomultiplier. The detector energy has a wide dynamic range in excess of 20 MeV and an energy resolution of approximately 9% at 661.7 keV. The design of this device was informed by the experimental results collected at DIII-D with a previous prototype based on a LYSO:Ce scintillator crystal coupled with a silicon photomultiplier. In this section, the development of the Runaway Electron GAmma-Ray Detection System (REGARDS) is also presented. REGARDS is a novel portable hard X-ray spectrometer designed for RE bremsstrahlung measurement. The detector is based on a LaBr3:Ce scintillator crystal coupled with a photomultiplier tube. The system has a wide dynamic range for HXR spectroscopy with an upper energy bound in excess of 20 MeV and an energy resolution of approximately 3% at 661.7 keV. REGARDS HXR detector gain is monitored by an external gain control system. REGARDS was deployed at the tokamaks AUG and COMPASS. In the second half of this thesis, analysis of the runaway electron experiments performed at the tokamaks AUG and JET is discussed. A full model of the bremsstrahlung emission is created using the GENESIS code and the HXR spectrometers response function is generated using MCNP. Tikhonov regularization is used to reconstruct the runaway energy distribution function from the measurements. The reconstructed runaway electron energy distribution functions allow for a quantitative description of the runaway electron beam throughout the discharge. The information collected with these techniques is crucial to understand runaway electron formation, to validate first-principle models and to evaluate the effectiveness of different runaway electron mitigation techniques such as massive gas injection (MGI), shattered pellet injection (SPI) and magnetic resonant perturbation (RMP).

La crescita delle instabilità del plasma può causare un'improvvisa perdita di energia termica e magnetica. In questi eventi disruttivi, gli elettroni possono venire accelerati a energie relativistiche e ottenere una frazione significativa dell'energia immagazzinata nel campo magnetico del tokamak. A queste velocità, le collisioni Coulombiane con il plasma di background diventano trascurabili e l'accelerazione dei runaway electrons è limitata solamente da effetti relativistici e perdite radiative. Quando il confinamento viene perso, il fascio energetico di runaway electrons può collidere con le componenti all'interno della camera da vuoto causando gravi danni. Gli eventi non mitigati di runaway electrons possono forzare lunghi periodi di arresto della durata di diversi mesi per consentire le riparazioni. Evitare questi scenari estremi è fondamentale per il successo di tokamak come ITER. Durante le disruzioni, i runaway electrons possono essere accelerati fino a energie nell'ordine di diversi MeV. Uno dei meccanismi che limitano questa accelerazione è l'emissione di radiazione di bremsstrahlung, causata dall'interazione delle particelle relativistiche con il plasma di background. A causa dell’elevata energia di questi elettroni, lo spettro della radiazione bremsstrahlung si estende fino a diversi MeV, nell’ intervallo di energia dei raggi X duri. Questo lavoro illustra come si possa ricostruire lo spazio di velocità dei runaway electrons dall'emissione di bremsstrahlung misurata. Nella prima metà di questo lavoro vengono discussi lo sviluppo, la caratterizzazione e l'implementazione di nuovi spettrometri a raggi X duri ottimizzati per la misura di bremsstrahlung da runaway electrons. Un nuovo spettrometro HXR compatto, con capacità di conteggio superiori a 1 Mcps, è stato sviluppato per il sistema Gamma-Ray Imager del tokamak DIII-D. Questo rivelatore si basa su un cristallo scintillatore YAP: Ce accoppiato con un fotomoltiplicatore di silicio. L'energia del rivelatore ha un ampio intervallo dinamico superiore a 20 MeV e una risoluzione energetica di circa il 9% a 661,7 keV. Il design di questo dispositivo è stato guidato dai risultati sperimentali raccolti a DIII-D con un precedente prototipo, basato su un cristallo scintillatore LYSO: Ce accoppiato con un fotomoltiplicatore di silicio. In questa sezione della tesi viene inoltre presentato lo sviluppo del Runaway Electron GAmma-Ray Detection System (REGARDS). REGARDS è un nuovo spettrometro HXR portatile a progettato per la misurazione della bremsstrahlung dei runaway electrons. Il rivelatore è basato su un cristallo scintillatore LaBr3: Ce accoppiato a un tubo fotomoltiplicatore. Il sistema ha un ampio intervallo dinamica per la spettroscopia HXR con un limite in energia superiore superiore a 20 MeV e una risoluzione energetica di circa il 3% a 661,7 keV. Il guadagno del rivelatore HXR di REGARDS è monitorato da un sistema di controllo esterno. REGARDS è stato utilizzato presso i tokamaks AUG e COMPASS. Nella seconda metà di questa tesi viene discussa l'analisi degli esperimenti di runaway electrons eseguiti presso i tokamaks AUG e JET. Un modello completo dell'emissione di bremsstrahlung è stato creato utilizzando il codice GENESIS e la funzione di risposta degli spettrometri HXR è stata generata utilizzando MCNP. La regolarizzazione di Tikhonov viene utilizzata per ricostruire la funzione di distribuzione dell'energia dei runaway electrons dalle misurazioni. Le funzioni di distribuzione di energia dei runaway electrons ricostruite consentono una descrizione quantitativa del fascio durante la scarica. Le informazioni raccolte con queste tecniche sono cruciali per comprendere la formazione di runaway electrons, per validare i modelli da principi primi e per valutare l'efficacia di diverse tecniche di mitigazione degli runaway electrons come la massive gas injection, la shattered pellet injection e la resonant magnetic perturbation.

(2021). Reconstruction of the velocity space of runaway electrons by spectral measurements of the hard x-ray emission in tokamaks. (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2021).

Reconstruction of the velocity space of runaway electrons by spectral measurements of the hard x-ray emission in tokamaks

DAL MOLIN, ANDREA
2021

Abstract

The growth of plasma instabilities can cause a sudden loss of thermal and magnetic energy. In this disruptive event, electrons can be accelerated to relativistic energies and gain a significant fraction of the energy stored in the tokamak magnetic field. At these velocities, Coulomb collisions with background plasma become negligible and the acceleration of the runaway electrons is only limited by relativistic effects and radiative losses. When the post-disruption magnetic field is lost, the energetic runaway electron beam can collide with the in-vessel plasma-facing components causing severe and localized damage. Unmitigated runaway electron events can hinder operation by forcing long shutdown periods of several months to allow repairs. The avoidance of these extreme scenarios is paramount to the success of large-scale tokamaks. The threat posed by runaway electrons is a primary focus of the fusion community. Extensive modelling and experimental campaigns are currently ongoing in most large and medium-scale tokamaks. During disruptions, runaway electrons can be accelerated up to energies in the order of several MeVs. One of the mechanisms that limit this acceleration is the emission of bremsstrahlung radiation caused by the interaction of the relativistic particles with the background plasma. Due to the extreme energy these electrons can reach, the bremsstrahlung radiation spectrum extends to several MeVs, in hard X-ray energy range. This work illustrates how information on the runaway electron velocity space can be extracted from the measured bremsstrahlung X-ray emission. In the first half of this work, the development, characterization and implementation of novel hard x-ray spectrometers optimized for runaway electron bremsstrahlung measurement are discussed. A new compact HXR spectrometer with high counting rate capabilities in excess of 1 MCps was developed for the array configuration of the tokamak DIII-D Gamma-Ray Imager system. This detector is based on a YAP:Ce scintillator crystal coupled with a silicon photomultiplier. The detector energy has a wide dynamic range in excess of 20 MeV and an energy resolution of approximately 9% at 661.7 keV. The design of this device was informed by the experimental results collected at DIII-D with a previous prototype based on a LYSO:Ce scintillator crystal coupled with a silicon photomultiplier. In this section, the development of the Runaway Electron GAmma-Ray Detection System (REGARDS) is also presented. REGARDS is a novel portable hard X-ray spectrometer designed for RE bremsstrahlung measurement. The detector is based on a LaBr3:Ce scintillator crystal coupled with a photomultiplier tube. The system has a wide dynamic range for HXR spectroscopy with an upper energy bound in excess of 20 MeV and an energy resolution of approximately 3% at 661.7 keV. REGARDS HXR detector gain is monitored by an external gain control system. REGARDS was deployed at the tokamaks AUG and COMPASS. In the second half of this thesis, analysis of the runaway electron experiments performed at the tokamaks AUG and JET is discussed. A full model of the bremsstrahlung emission is created using the GENESIS code and the HXR spectrometers response function is generated using MCNP. Tikhonov regularization is used to reconstruct the runaway energy distribution function from the measurements. The reconstructed runaway electron energy distribution functions allow for a quantitative description of the runaway electron beam throughout the discharge. The information collected with these techniques is crucial to understand runaway electron formation, to validate first-principle models and to evaluate the effectiveness of different runaway electron mitigation techniques such as massive gas injection (MGI), shattered pellet injection (SPI) and magnetic resonant perturbation (RMP).
NOCENTE, MASSIMO
TARDOCCHI, MARCO
Fusione; Runaway Electrons; Spettroscopia; Raggi-X Duri; Tikhonov
Fusion; Runaway Electrons; Spectroscopy; Hard X-Ray; Tikhonov
FIS/01 - FISICA SPERIMENTALE
English
15-feb-2021
FISICA E ASTRONOMIA
33
2019/2020
open
(2021). Reconstruction of the velocity space of runaway electrons by spectral measurements of the hard x-ray emission in tokamaks. (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2021).
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/10281/304289
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