Investigation of Molecular Assisted Recombination Detachment Physics in Fusion Relevant Hydrogen Plasmas
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Investigation of Molecular Assisted Recombination Detachment Physics in Fusion Relevant Hydrogen Plasmas Joseph Branson Doctor of Philosophy University of York Physics September 2019 ii Abstract The Molecular Assisted Recombination (MAR) regime is investigated in hydrogen plas- mas in both a linear divertor simulator device, the York Linear Plasma Device, and a Gaseous Electronics Conference standardised reference cell. The aim of this work has been to charac- terise both the negative ion population within the YLPD and the significance of the surface material choice in divertors by using a well known plasma source in the form of the GEC. Hollow radial profiles with varying profile shapes and absolute maxima have been found for negative ion densities on the YLPD with good spatial resolution. The fraction of H- has been found to typically lay within 10% of the electron density for most pressures which is in agreement with previous work. Rotational gas temperatures have been found to be around 400-700 K on the YLPD in an MAR state while vibrational temperatures were estimated at 2000-3500 K, also in agreement with literature. The results on the GEC cell for surface effect on negative ion production for stainless steel, tungsten and molybdenum have shown significant differences, the causes of which are not yet fully understood. There was enhance- ment of negative ion production for both steel and tungsten when compared to molybdenum at 25 Pascals pressure. The trends of negative ion density with applied radio frequency power 9 mm above the samples were similar between materials however the curves appear to be shifted along the power axis based on the specific material as well as scaled in their respective maxima. It is hoped that these measurements will assist in the improvement of MAR understanding and modelling for future research with applications in tokamak diver- tor design, linear device study and simulation as well as potential broader applications in negative ion devices. The overarching aim is to improve the understanding of MAR negative ion production for divertor optimization. Contents Abstract ii Contents iii List of Tables vii List of Figures viii Acknowledgements xxii Declaration xxiii 1 Introduction1 1.1 Nuclear fusion....................................1 1.1.1 Introduction................................1 1.1.2 Fusion Definition..............................2 1.1.3 Fusion Process...............................2 1.1.4 Plasma basics................................4 1.1.5 Tokamaks..................................5 1.2 Thesis outline.................................... 12 2 Background and theory 14 2.1 Detachment overview................................ 14 2.2 Electron Ion Recombination (EIR) detachment................. 15 2.3 MAR detachment.................................. 17 2.3.1 Initial suggestion of MAR......................... 17 2.3.2 MAR processes............................... 17 iii CONTENTS iv 2.4 Conditions and rate constant........................... 19 2.5 Existing work on detachment in tokamaks and tokamak relevant experiments 21 2.5.1 Detachment research in tokamaks..................... 21 2.5.2 Divertor simulators............................. 22 2.5.3 Relation to hydrogen plasmas in the GEC................ 32 2.6 Existing work on RF plasmas with emphasis on the GEC reference cell.... 33 2.6.1 Negative ion experiments overview.................... 33 2.6.2 Negative ion production for particle accelerators............ 33 2.6.3 Negative ion production in the microelectronics industry........ 34 2.6.4 Surface production in fusion devices................... 34 2.6.5 Effect of material type on surface negative ion production - general research................................... 35 2.6.6 Effect of material type on surface negative ion production - GEC cell studies.................................... 37 2.6.7 Summary of work on negative ion production in the wider plasma research community............................ 39 2.7 Theory of plasma diagnostics and techniques used............... 39 2.7.1 Electrical probes - Langmuir Probes................... 39 2.7.2 Laser photodetachment theory...................... 52 2.7.3 Optical emission spectroscopy diagnostic theory............. 56 2.8 Summary of chapter................................ 63 3 Experimental methodologies 64 3.1 Experimental technique for measuring negative ions on the GEC....... 64 3.1.1 Overview of GEC Reference cell..................... 64 3.1.2 Laser photodetachment experiments on the GEC reference cell.... 69 3.2 Experimental techniques performed on the YLPD................ 80 3.2.1 Overview of the YLPD linear divertor simulator............ 80 3.2.2 Measurements of Ion flux to the end/target plate............ 87 3.2.3 Fulcher band optical emission spectroscopy............... 89 3.2.4 Fulcher band rotational temperature analysis.............. 92 3.2.5 Core electron density and temperature measurements......... 94 3.2.6 Linear device laser photo-detachment experiments........... 94 CONTENTS v 4 GEC negative ion study results 111 4.1 Overview of chapter................................ 111 4.2 Negative ion results for Tungsten electrode grounding surface......... 112 4.2.1 Electron density and temperature variation with pressure and power (W)..................................... 113 4.2.2 Negative ion density variation with pressure and power (W)...... 117 4.2.3 Negative ion fraction variation with pressure and power (W)..... 119 4.3 Negative ion results for molybdenum electrode grounding surface....... 122 4.3.1 Electron density and temperature variation with pressure and power (Mo)..................................... 122 4.3.2 Negative ion density variation with pressure and power (Mo)..... 124 4.3.3 Negative ion fraction variation with pressure and power (Mo)..... 127 4.4 Negative ion results for varying surface material at 25 Pa........... 130 4.4.1 Electron density and temperature variation with material and power. 130 4.4.2 Negative ion density variation with material and power........ 133 4.4.3 Negative ion fraction variation with material and power........ 136 4.5 Discussion of GEC results overall......................... 138 4.5.1 Similarity to other experiments and results............... 138 4.5.2 Potential sources of increased negative ions in the GEC........ 138 4.5.3 Discussion of experiments performed on the GEC in the context of aforementioned processes......................... 144 4.6 Summary...................................... 152 5 Results of experiments performed on the YLPD 156 5.1 Chapter introduction................................ 156 5.2 Ion flux measurements............................... 156 5.2.1 Central plasma column electron density and temperature profiles... 158 5.2.2 Fulcher band results from the YLPD................... 162 5.3 Radial profiles of charged species beyond the central core........... 167 5.3.1 Negative ion profiles vs pressure..................... 172 5.3.2 Variation at single position vs intermediate electrode voltage..... 175 5.3.3 Variation with upstream electron temperature - V(ie)......... 178 5.3.4 Variation of negative ions with applied field............... 183 CONTENTS vi 5.3.5 Discussion of results obtained on the Linear Device........... 189 6 ITER core TS filter design 191 6.1 Outline....................................... 191 6.2 Thomson scattering on ITER........................... 192 6.3 Approach to estimating ITER background light................. 192 6.3.1 Overview of techniques and posed questions............... 192 6.3.2 Analysis considerations for JET-ILW and CHERAB data....... 193 6.4 Results........................................ 201 6.4.1 Results concerning reflectivity in the CHERAB ray tracing simulation. 201 6.4.2 Analysis of fit between the JET-ILW background and CHERAB... 203 6.4.3 Identification of strong lines for filtering................. 203 6.5 Conclusions and additional work......................... 209 7 Conclusions and future work 211 7.1 Summary and conclusions............................. 211 7.2 Future work - GEC device............................. 216 7.3 Future work - Linear device............................ 216 7.3.1 Potential future experiments....................... 216 7.3.2 Modelling.................................. 218 List of Tables 3.1 Fulcher band transition peak wavelengths (nm) (vf=vi). The exact wave- lengths reported may differ slightly from source to source within the literature. 92 vii List of Figures 1.1 Fusion reaction cross section obtained from [1]. This shows the relative ease of access of the various candidate fusion reactions and thus the reasoning behind the use of deuterium and tritium as the fuel mixture...............3 1.2 This diagram illustrates the basic differences in the geometry associated with a toroidal device and a linear device. It can be seen that the magnetic field runs along the Z axis in the linear case whereas in the toroidal geometry, the field components are in the φ and θ directions. The Z axis in the toroidal case represents the height of a point above or below a given plane, sometimes the mid-plane of the device.............................6 1.3 Basic Tokamak layout, magnetic confinement systems. (Structure not shown) [2]..........................................7 1.4 Basic schematic of nested flux surfaces based on image from [3].........9 1.5 (a): Basic schematic of limiter configuration and (b): basic layout of divertor configuration - [3,4].................................9 2.1 Calculations of: (A) the rate coefficient and (B) the