Development of Smart, Compact Fusion Diagnostics using Field-Programmable Gate Arrays Jack Lovell A Thesis presented for the degree of Doctor of Philosophy Centre for Advanced Instrumentation Department of Physics Durham University United Kingdom November 2017 Development of Smart, Compact Fusion Diagnostics using Field-Programmable Gate Arrays Jack Lovell Submitted for the degree of Doctor of Philosophy November 2017 Abstract: Fusion research requires high quality diagnostics to understand the complex physical processes involved. Traditional analogue systems are complex, large and expensive, and expansion of diagnostic capabilities is often impossible without building a completely new system at considerable expense. Field-programmable gate array (FPGA) technology can provide a solution to this problem. By implementing complex functionality and digital signal processing on an FPGA chip, diagnostic hardware can be greatly simplified and compacted. In this thesis we describe the enhancements of two diagnostics for the MAST-Upgrade tokamak using FPGA technology. Firstly, the design of the back end electronics for the new divertor bolometer is described. Results of tests of the new electronics at a number of sites, including lab-based testing and tokamak installations, are also presented. We demonstrate the correct functionality of the electronics and illustrate a number of important effects which must be taken into account when interpreting bolometer data on MAST-U. Secondly, we describe the new control and acquisition electronics developed for the MAST-U divertor Langmuir probe diagnostic. Much of the analogue control circuitry of the previous system has been upgraded to a digital implementation on an FPGA, which results in a significantly more compact and cost effective design. Given that MAST-Upgrade will feature around 850 Langmuir probes, these improvements are extremely important to keep the diagnostic manageable. Again, results are presented from the testing of the system at several sites, which both demonstrate the correct functionality of the new system and provide information on the diagnostic behaviour which needs to be accounted for when interpreting the probe data during MAST-U experiments. Declaration The work in this thesis is based on research carried out at the Centre for Advanced Instrumentation in the Department of Physics at Durham University. No part of this thesis has been submitted elsewhere for any degree or qualification. The original design and specification of the BOLO8 hardware, and a conceptual outline of the new calibration procedure, were already in place before I started this work. Similarly, the design and specification of the Langmuir probe electronics described herein had already began before I started this work. The refinement and implementation of these specifications is however my own work. Collaborations with other researchers and institutions have been indicated at appro- priate points in the text. Copyright © 2017 Jack Lovell. “The copyright of this thesis rests with the author. No quotation from it should be published without the author’s prior written consent and information derived from it should be acknowledged.” Acknowledgements The production of this thesis has relied on a large number of people. Thanks are of course due to my supervisors Ray Sharples of Durham University and Graham Naylor of Culham Centre for Fusion Energy (CCFE) for their help and advice throughout the project, including the many hours of proof reading and suggestions for improvement of the thesis text itself. Anthony Field, Peter Drewelow and Mike Stamp provided assistance with the bolometer development at CCFE and JET, whilst Matt Reinke has proved to be an invaluable contact first at York University and then Princeton Plasma Physics Laboratory on behalf of Oak Ridge National Laboratory to enable collaborations with these institutions. Robert Stephen, Stuart Bray, Sarah Elmore and James Harrison have provided assistance with the Langmuir Probe system at CCFE, with Hannah Willett and Kieron Gibson of York University and Matej Peterka, Aleš Hávranek, Megi Dimitrova and Jirka Adámek of IPP CAS providing assistance with tests of the system at the York Linear Plasma Device and the COMPASS tokamak respectively. Much of the data processing and plotting in the results chapters uses Python [1], together with the Scipy [2], Matplotlib [3] and xarray [4] libraries. Many diagrams are also drawn using the PGF/TikZ LATEX package [5] and the Inkscape graphics editor [6]. Thanks are due to M Imran and J Lawson who prepared the LATEX template which this thesis uses. This work was supported by the Engineering and Physical Sciences Research Council, grants EP/L01663X/1 and EP/P012450/1. Parts of this work have been carried out within the framework of the EUROfusion Consortium and have received funding from the Euratom research and training programme 2014–2018 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. Parts of this work are supported by Czech Ministry of Education, Youth and Sports project LM2015045. Parts of this work are supported by US Department of Energy grants DE-AC05-00OR22725 and DE-AC02-09CH11466. Dedicated to The strong force, without which all this would be futile. Contents Abstract iii List of Figures xv 1 Introduction 1 1.1 Nuclear Fusion . .1 1.1.1 Physical process . .2 1.1.2 An ideal energy source? . .5 1.2 Plasma confinement . .7 1.3 The tokamak . .8 1.3.1 Tokamak principle . .8 1.3.2 Tokamak Diagnostics . 12 1.3.3 The divertor . 14 1.4 MAST-Upgrade . 15 1.4.1 Spherical tokamaks . 17 1.4.2 The Super-X divertor . 18 1.5 Field-programmable gate arrays . 20 1.5.1 The principle of FPGAs . 20 1.5.2 Xillybus . 24 1.6 FPGAs and fusion diagnostics . 24 xii Contents 2 Design of the Bolometer diagnostic 27 2.1 The bolometer principle . 27 2.1.1 Resistive bolometers . 28 2.1.2 AC synchronous detection . 30 2.1.3 Calculation of absorbed power . 31 2.1.4 Calibration . 32 2.2 Hardware . 34 2.2.1 New calibration procedure . 39 2.3 FPGA design: Xillybus version . 41 2.3.1 Digital Signal Processing . 47 2.3.2 Jetblack software interface . 51 2.4 FPGA redesign: BOLODSP module . 53 2.5 Improvements over previous systems . 56 3 Results from the Bolometer system 59 3.1 Lab-based measurements at the York Plasma Institute . 59 3.1.1 Comparison of FPGA and software signal processing . 60 3.1.2 Calibration dependence on bias voltage . 63 3.1.3 Calibration dependence on pressure . 65 3.1.4 Summary of York tests . 67 3.2 Installation on JET . 68 3.2.1 Direct voltage comparison . 70 3.2.2 Optimisation of drive frequency . 72 3.2.3 Power calculation . 74 3.2.4 Tomography comparison . 80 3.2.5 Total power calculation . 83 3.2.6 Summary of JET tests . 86 3.3 Installation on TCV . 86 3.4 Lab-based measurements at PPPL . 88 3.4.1 Laser profile . 88 Contents xiii 3.4.2 Sensitivity calculation: manual calibration method . 91 3.4.3 Sensitivity calculation: BOLO8 calibration method . 92 3.4.4 Absolute power accuracy . 93 3.4.5 Laser power scan . 95 3.4.6 Laser frequency scan . 97 3.4.7 Cable length scan . 104 3.4.8 Drive frequency scan . 106 3.4.9 Summary of PPPL tests . 113 3.5 Implications for installation on MAST-Upgrade . 114 4 The Langmuir Probe diagnostic 115 4.1 Principles . 115 4.1.1 The IV characteristic . 116 4.1.2 MAST-U’s divertor Langmuir probes . 118 4.2 Hardware . 119 4.2.1 The multiplexer . 121 4.3 FPGA design . 123 4.3.1 Web server with CherryPy . 126 4.4 Improvements over previous system . 127 5 Results from the Langmuir Probe system 129 5.1 Installation on the York Linear Plasma Device . 129 5.1.1 Floating potential measurements . 131 5.1.2 Sweep measurements . 133 5.1.3 Plasma parameters from sweeps . 137 5.1.4 Summary of YLPD tests . 139 5.2 Installation on the COMPASS tokamak . 139 5.2.1 Experimental setup . 140 5.2.2 Floating potential measurements . 141 5.2.3 Sweep waveform . 146 5.2.4 Sweep measurements . 149 5.2.5 Summary of the COMPASS tests . 151 5.3 Implications for installation on MAST-U . 152 xiv Contents 6 Conclusions 155 6.1 Summary . 155 6.2 Future work . 158 6.3 Final word: the potential of integrated FPGA diagnostics in fusion . 160 Bibliography 163 List of Figures 1.1 Average binding energy per nucleon for a range of nuclei . .3 1.2 The cross sections for the 3 most relevant terrestrial nuclear fusion reactions . .4 1.3 The principle of the magnetic mirror . .9 1.4 The magnetic field configuration in a tokamak . 11 1.5 The magnetic flux surfaces in a divertor configuration for the JET tokamak. 16 1.6 Comparison of conventional and Super-X divertor configurations in MAST-U. 19 1.7 FPGA architecture . 21 1.8 Schematic of the Zynq architecture . 23 2.1 Schematic of the bolometer circuit . 29 2.2 Schematic of circuit typically used to calibrate bolometers . 32 2.3 The BOLO8 and ACQ2006 hardware . 36 2.4 Diagram showing the interaction between components of the bolo- meter system . 37 2.5 Schematic of the BOLO8 module . 38 2.6 Comparison of the spectra of raw ADC voltage measurements, with and without the Schottky diodes . 38 2.7 Electrical setup of the BOLO8 calibration procedure . 40 2.8 Schematic showing the modular design of the FPGA firmware . 44 2.9 Flow diagram showing how the AC synchronous detection is performed on the FPGA . 47 2.10 The user-programmable DSP interface . 54 xvi List of Figures 3.1 Comparison of floating-point and fixed-point signal processing for a calibration shot .