Collective Thomson scattering measurements of fast-ion dynamics at ASDEX Upgrade

Fast ions will play a primary role in plasma heating and confinement in future fusion reactors, making the accurate measurement and prediction of their behavior essential. In current fusion experiments, collective Thomson scattering (CTS) is capable of resolving fast-ion properties in time, space, and projected velocity. This active microwave diagnostic is thus instrumental in validating fast-ion transport models and understanding deviations from neoclassical predictions. As CTS has been selected as the primary diagnostic for measuring fusion-born alpha particles across their full energy range in ITER, it is crucial to refine its capabilities to diagnose fast ions in reactor-relevant conditions. To support these applications, this thesis investigates the development and operation of the CTS diagnostic at ASDEX Upgrade (AUG) for measuring fast-ion dynamics. The work presented here focuses on extending the diagnostic’s operational capabilities through advancements in data acquisition and analysis, offering insights broadly relevant to microwave diagnostics and CTS operation in general.

As fusion experiments approach the burning plasma regime, higher fusion performance is generally accompanied by increased plasma temperatures and densities. These conditions lead to elevated electron cyclotron emission (ECE) levels, which can generate CTS diagnostic background noise several times stronger than the CTS signals of interest. Additionally, parametric decay instabilities (PDI) may generate anomalous radiation that can distort CTS spectra or even pose a risk to diagnostic equipment. Accurately accounting for this dynamic background is essential for reliable CTS measurements, particularly for quantifying fast-ion contributions, which are typically an order of magnitude weaker than the bulk plasma signal. To address this challenge, this thesis introduces new methods for CTS background estimation and subtraction, applicable to both filterbank and fast-digitizer data. These methods significantly improve the fidelity of CTS signal isolation and are particularly successful in exploiting power modulations of the CTS probe gyrotron. In principle, this processing enables continuous CTS measurements without affecting the quality of the background subtraction.

To address hardware limitations, this thesis introduces an ultrafast digitizer with novel continuous measurement capabilities. Indeed, existing data acquisition systems are often limited to lower sampling rates or to sparse acquisitions due to an insufficient data throughput. By incorporating a field-programmable gate array (FPGA) into the design, this limitation was entirely eliminated, resulting in a modular digitizer capable of delivering state-of-the-art performance continuously throughout an AUG discharge. This advancement significantly enhances the flexibility of the CTS radiometers, both for CTS applications and for dedicated PDI studies, as demonstrated in this work. Moreover, the FPGA’s onboard computing power creates opportunities for the development of real-time CTS signal processing.

Preliminary experimental results highlight the performance of CTS in detecting fast-ion dynamics with high temporal resolution in AUG, including the detection and quantification of Alfvén eigenmode-induced fast-ion transport with CTS for the first time. The thesis also reports further on the first fast-ion measurements using CTS at reactor-relevant densities in tokamaks. Additionally, we present the first radial profile measurements of fast ions with CTS, achieved by sweeping a single CTS measurement volume across the plasma radius, confirming the feasibility of fast-ion density profiles with CTS.

These results demonstrate the applicability of the methods developed in this thesis and the resulting extension of the operational regime of the CTS diagnostic for fast-ion measurements at AUG. Beyond addressing immediate challenges, this work establishes a foundation for advancing CTS capabilities in ITER and future reactor-grade devices. Through technological and methodological advancements, this work contributes to the development of CTS as a robust and reliable tool for investigating fast-ion physics in fusion plasmas.