FPGA-based High Performance Diagnostics For Fusion

High performance diagnostics are an important aspect of fusion research. Increasing shot-lengths paired with the requirement for higher accuracy and speed make it mandatory to employ new technology to cope with the increasing demands on digitization and data handling. Field programmable gate arrays (FPGAs) are well known in high performance applications. Their ability to handle multiple fast data streams whilst remaining programmable make them an ideal tool for diagnostic development. Both the improvement of old and the design of new diagnostics can benefit from FPGA-technology and increase the amount of accessible physics significantly. In this work the developments on two FPGA-based diagnostics are presented. In the first part a new open-hardware low-cost FPGA-based digitizer is presented for the MAST-Upgrade (MAST-U) integral electron density interferometer. The system is shown to have an optically limited phase accuracy and a detection bandwidth of over 3.5 MHz. Data is acquired continuously at 20 MS/s and streamed to an acquisition PC via optical fiber. By employing a dual-FPGA approach real-time processing of the density signal can be achieved despite severly limited resources, thus providing a control signal for the MAST-U plasma control system system with less than 8 μs latency. Due to MAST-U being still inoperable, in-situ testing has been conducted on the ASDEX Upgrade, where fast wave physics up to 3.5 MHz could first be observed. The second part presents developments to the Synthetic Aperture Microwave Imaging (SAMI) diagnostic. In addition to improving the utilization of long shot-lengths and enabling dual-polarized acquisition the system has been enhanced to continuously acquire active probing profiles for 2D Doppler back-scattering (DBS), a technique recently developed using SAMI. The aim is to measure pitch angle profiles to derive the edge current density. SAMI has been transferred to the NSTX-Upgrade and integrated into the experiment’s infrastructure, where it has been acquiring data since May 2016. As part of this move an investigation into near-field effects on SAMI’s image reconstruction algorithms was conducted

High-definition velocity-space tomography of fast-ion dynamics

Velocity-space tomography of the fast-ion distribution function in a fusion plasma is usually a photon-starved tomography method due to limited optical access and signal-to-noise ratio of fast-ion Dα (FIDA) spectroscopy as well as the strive for high-resolution images. In high-definition tomography, prior information makes up for this lack of data. We restrict the target velocity space through the measured absence of FIDA light, impose phase-space densities to be non-negative, and encode the known geometry of neutral beam injection (NBI) sources. We further use a numerical simulation as prior information to reconstruct where in velocity space the measurements and the simulation disagree. This alternative approach is demonstrated for four-view as well as for two-view FIDA measurements. The high-definition tomography tools allow us to study fast ions in sawtoothing plasmas and the formation of NBI peaks at full, half and one-third energy by time-resolved tomographic movies

Development of FPGA controlled diagnostics on the MAST fusion reactor

Field Programmable Gate Array technology (FPGA) is very useful for implementing high performance digital signal processing algorithms, data acquisition and real-time control on nuclear fusion devices. This thesis presents the work done using FPGAs to develop powerful diagnostics. This has been achieved by developing embedded Linux and running it on the FPGA to enhance diagnostic capabilities such as remote management, PLC communications over the ModBus protocol and UDP based ethernet streaming. A closed loop real-time feedback prototype has been developed for combining laser beams onto a single beam path, for improving overall repetition rates of Thomson Scattering systems used for plasma electron temperature and density radial profile measurements. A controllable frequency sweep generator is used to drive the Toroidal Alfven Eigenmode (TAE) antenna system and results are presented indicating successful TAE resonance detection. A fast data acquisition system has been developed for the Electron Bernstein Wave (EBW) Synthetic Aperture Microwave Imaging system and an active probing microwave source where the FPGA clock rate has been pushed to the maximum. Propagation delays on the order of 2 nanoseconds in the FPGA have been finely tuned with careful placement of FPGA logic using a custom logic placement tool. Intensity interferometry results are presented on the EBW system with a suggestion for phase insensitive pitch angle measurement

Experimental fast-ion transport studies on the Mega-Amp Spherical Tokamak

Nuclear fusion holds the promise of a sustainable means of electrical power generation. The technical challenge posed by controlled nuclear fusion however is formidable. One key aspect of research into magnetically-confined fusion plasmas is the study of the behaviour of fast ions. Produced by auxiliary heating systems including neutral beam injection (NBI) and ion cyclotron resonance heating, as well as by the fusion reactions themselves, the energies of these particles range from tens of keV to several MeV. Four fast-ion diagnostics on a medium-sized experimental fusion research device, namely the Mega-Amp Spherical Tokamak (MAST), have been used to investigate the transport of NBI-generated fast ions under the influence of various magnetohydrodynamic (MHD) instabilities. These include frequency-chirping fast-ion-driven modes known as toroidal Alfv\'{e}n eigenmodes (TAE) and fishbones, as well as saturated internal kink modes and sawtooth reconnection events. The frequencies of these modes in the plasma frame ranged from 0 - 150 kHz in MAST. The effects of these modes on the fast ions have been investigated with the use of a fission chamber, a collimated neutron detector, a fast-ion deuterium alpha spectrometer and a charged fusion product detector. Data from each diagnostic are examined and compared for consistency in the presence of each type of instability. Fast-ion transport modelling is coupled with synthetic diagnostics to model the observed signals based on calculated fast-ion distributions. The data reveal a significant enhancement of fast-ion transport due to each of the MHD modes. It is found that the transport of fast ions in the presence of TAE and fishbones, averaged over the chirping mode cycle, is well-represented by assuming anomalous radial diffusion of these ions. A simple model for fast-ion mixing during sawtooth reconnection is found to reproduce partially the observations accompanying such events. The effects of the saturated internal kink mode cannot be modelled in such a simple way, and partial consistency with the measurements requires the use of tailored transport coefficients. Shortcomings and possible enhancements of the diagnostic capability are discussed in the light of these results