Advanced Thomson scattering techniques as diagnostics for fusion plasmas

The content of this thesis was carried out within the framework of the global magnetic confinement fusion (MCF) effort. The flagship project for MCF is the International Thermonuclear Experimental Reactor (ITER), currently under construction and due for commissioning in 2025. ITER is an unprecedented device that pushes the limits of technology and physics across all sectors, and the field of plasma diagnostics is no exception. Thomson scattering (TS) is one of the most powerful diagnostics available to fusion devices, providing high resolution spatial and temporal profiles of the plasma electron temperature and electron density, which are measurements vital to the understanding of the plasma. Although conventional TS is a well proven method and used in almost all MCF devices in current operation, it too must advance and adapt to the demanding environment of ITER. In this thesis we explore two advanced Thomson scattering techniques which aim to address some of the challenges of ITER TS, through three independent experiments performed on three separate devices. Chapter 1 provides a brief introduction to MCF as part of the solution to the current global energy crisis. Chapter 2 introduces the general theory of Thomson scattering and a description of the advanced techniques under investigation. Chapter 3 describes the dual-laser TS experiment performed in RFX-mod, Padova, during the first year of the PhD. To the author’s knowledge, this was the first successful practical testing of this advanced technique. Chapter 4 describes the polarimetric TS experiment performed in JET, Oxford, during the second year of the PhD. To the author’s knowledge, this was the first successful practical testing of this advanced technique. Chapter 5 describes the dual-laser TS experiment performed in LHD, Japan, during the second year of the PhD. To the author’s knowledge, this was the second successful practical testing of this technique. The experiments performed and described in this thesis demostrate the feasibility of these two previously untested advanced TS techniques, which are both of great interest for application in the next generation of fusion devices.I contenuti di questa tesi sono stati svolti nella cornice della ricerca globale sulla Fusione nucleare a Confinamento Magnetico (FCM). Il progetto guida per la FCM è ITER (International Thermonuclear Experimental Reactor), in costruzione a Cadarache, nel sud della Francia, e con messa in esercizio prevista per il 2025. ITER è una macchina senza precedenti, che spinge all’estremo i limiti della tecnologia e della scienza in tutti i settori, e il campo della diagnostica di plasma non fa eccezione. Lo scattering Thomson (ST) è una delle diagnostiche più potenti tra quelle disponibili su una macchina a fusione, ed è in grado di misurare con elevata risoluzione spaziale e temporale la temperatura e la densità degli elettroni, misure chiave per lo studio dei plasmi. Sebbene lo ST sia una diagnostica già collaudata e in uso su quasi tutte le macchine a fusione esistenti, anch’esso deve essere migliorato e reso adatto agli esigenti requisiti di ITER. In questo lavoro di tesi sono state esplorate due tecniche di scattering Thomson avanzato che hanno lo scopo di risolvere alcune delle sfide dello ST di ITER attraverso tre esperimenti indipendenti svolti su tre macchine separate. Capitolo 1 fornisce una breve introduzione alla FCM vista come parte della soluzione all’attuale crisi globale dell’energia. Capitolo 2 introduce la teoria generale dello scattering di Thomson e una descrizione delle tecniche avanzate trattate in questo lavoro di tesi. Capitolo 3 descrive l’esperimento di ST con laser duale svolto su RFX-mod, Padova, durante il primo anno di dottorato. Questo è stato per l’autore il primo esperimento svolto con successo su questa tecnica avanzata. Capitolo 4 descrive l’esperimento di ST polarimetrico svolto su JET, Oxford, durante il secondo anno di dottorato. Questo è stato per l’autore il primo esperimento svolto con successo su questa tecnica avanzata. Capitolo 5 descrive l’esperimento di ST con laser duale svolto su LHD, Giappone, durante il secondo anno di dottorato. Questo è stato per l’autore il secondo esperimento svolto con successo su questa tecnica avanzata. Gli esperimenti svolti, descritti in questo lavoro di tesi, dimostrano la fattibilità di queste due tecniche di ST avanzato, mai testate prima, e che sono entrambe di grande interesse per l’applicazione sulla prossima generazione di macchine a fusione

Dual-laser, self-calibrating Thomson scattering measurements in RFX-mod

A self-calibrating Thomson scattering technique, based on using two lasers of different wavelength and never implemented before in a fusion experiment, is tested for the first time in RFX-mod. The method employs two laser systems, Nd:YAG (\u3bb = 1064 nm) and Nd:YLF (\u3bb = 1053 nm), fired in sync through the same plasma volume. The combination of the two scattered spectra, individually recorded, is used to simultaneously obtain the measurements of the electron temperature Te and the relative calibration coefficients of the polychromator spectral channels sensitivities Ci. This work is a continuation of the dual-angle technique, a similar alternative method. Due to the small difference in the wavelengths of the two lasers, the sensitivity of this method is low in RFX-mod and reliable measurements of the calibration coefficients could not be obtained on a shot-to-shot basis, but only with a statistical analysis of a reasonable number of pairs of laser shots with good signal-to-noise ratio. The dual-laser method can be applied to most fusion devices and could provide continuous online monitoring of the spectral transmission of the detection system. This may prove essential in larger devices (ITER), where access for traditional calibration is difficult

Robust all-dielectric high Q-factor metasurface for sensing

All-dielectric metasurfaces have seen a recent surge of interest as an alternative to plasmonic devices, due to low losses and desirable optical properties. High Q-factor quasi-bound state in the continuum resonances can be manufactured and manipulated via designed asymmetry in the nanostructures. The presented metasurface design, based on a slotted disk nanostructure, produces strong E-Field enhancement with good surface coverage external to the structure. The design transition from structure-in-air to structure-on-substrate in a water-based sensing medium is presented, along with the robust tunability and multiplexing potential of our fabricated resonances. Our structure maintains a high Q-factor and refractive index sensitivity over a wide wavelength range in the visible and near-IR

Verification of in-situ calibration on spectral transmission of Thomson scattering measurement system under radiological environment in ITER

The ITER radiological environment will not allow operators to calibrate the optics near the vacuum vessel. An in-situ calibration of spectral transmission from the collection optics to the detector by analyzing two Thomson scattering spectra resulting from two laser beams having different wavelengths was investigated experimentally in the large helical device (LHD). We can measure electron temperature accurately and calibrate spectral transmission at the same time even when spectral transmission degrades during the experimental campaign