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Work Function of Caesiated Surfaces in H2/D2 Low Temperature Plasmas Correlated with Negative Ion Formation

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Work Function of Caesiated Surfaces in H2/D2 Low Temperature Plasmas Correlated with Negative Ion Formation Sofia Cristofaro Work function of caesiated surfaces in H2/D2 low temperature plasmas correlated with negative ion formation IPP 2019-05 März 2019 Work function of caesiated surfaces in H2/D2 low temperature plasmas correlated with negative ion formation Dissertation zur Erlangung des akademischen Grades Dr. rer. nat eingereicht an der Mathematisch-Naturwissenschaftlich-Technischen Fakultät der Universität Augsburg von Sofia Cristofaro Augsburg, September 2018 Tag der mündlichen Prüfung: 15.01.2019 Erster Gutachter: apl. Prof. Dr.-Ing. U. Fantz Zweiter Gutachter: Prof. Dr. H. Höppe 3 Abstract An important component of future fusion machines like ITER and DEMO is the neutral beam injection system based on negative hydrogen or deuterium ions (NNBI). The NNBI system for ITER will deliver 16.5 MW of power by means of a 1 MeV accelerated and homogeneous beam with pulses of up to one hour. A key element of the NNBI is the ion source, which will deliver an accelerated D− current density of 200 A/m2 with a co-extracted electron to extracted negative ion current density ratio below unity. The ion source relies on the surface production of negative hydrogen ions via conversion of atomic hydrogen and positive hydrogen ions impinging on a caesiated low work function surface. At the ion sources a main issue is the temporal instability of the co-extracted electron current, which constantly increases during long pulses. Such instability is expected to be connected to a degradation or vanishing of the Cs layer at the converter surface, causing a variation of the work function. This issue is more evident in deuterium operation, where stronger dynamics of the electron current with respect to hydrogen operation is observed. Fundamental investigations of the negative ion formation and of the work function of caesiated surfaces are thus performed in the flexible laboratory experiment ACCesS, operating at ion source relevant vacuum (background pressure: 10−6 mbar) and plasma conditions. The setup consists of an inductively coupled plasma, of a Cs oven for the Cs evaporation, and of several diagnostic systems, in order to investigate the work function of a surface, the Cs dynamics, the H− density in front of the surface, and the plasma parameters. It is known that during vacuum phases the moderate vacuum leads to the formation of non-metallic Cs compounds at the surface, leading to work functions of 2.7 eV in minimum. The current investigations have shown the impact of plasma exposure on the work function of a caesiated surface due to the impinging plasma particles and the UV/VUV radiation. On the one hand, the plasma cleans the Cs layer within 5–10 seconds leading to a decrease of the work function from 2.7 to 2.1 eV (corresponding to pure Cs). On the other hand, the plasma surface interaction also removes the Cs layer if not enough Cs flux towards the surface is sustained. Thus, the removal of the Cs layer must be counteracted by a Cs flux of at least 1.5×1016 m−2s−1. Furthermore, a similar temporal variation of the work function in hydrogen and deuterium plasmas is observed. Moreover, controlled and reproducible investigations allowed to correlate for the first time the negative ion density in front of the surface to the work function, showing a steep increase of the negative ion density for work functions below 2.7 eV both in H2 and in D2 operation. Finally, due to the high Cs reactivity and complex redistribution dynamics, alternative materials were considered in view of the demonstration fusion reactor DEMO: LaB6, lanthanated materials (W and Mo), a tungsten dispenser cathode, and europium were tested. However, none of the considered materials is a valid alternative to Cs evaporation due to either too high work function under ion source conditions or even higher dynamics compared to Cs. 5 Contents 1 Introduction9 2 Ion sources for negative hydrogen ions 11 3 Low temperature low pressure hydrogen plasmas 25 3.1 Plasma properties . 25 3.2 Low temperature low pressure plasmas . 26 3.2.1 Electron energy distribution . 27 3.2.2 Plasma processes . 28 3.3 Plasma sheath . 32 3.4 Hydrogen plasmas . 34 3.4.1 Atomic hydrogen . 34 3.4.2 Hydrogen molecule . 35 3.4.3 Hydrogen plasma processes . 37 4 Negative hydrogen ions and the converter surface 41 4.1 Volume formation . 41 4.2 Surface formation . 42 4.3 Destruction mechanisms . 46 4.4 Converter surfaces for negative ion sources . 49 5 Caesium properties 53 5.1 Atomic properties . 53 5.2 Work function of caesiated surfaces . 54 5.3 Cs desorption and adsorption . 55 5.4 Cs compounds and ion source relevant conditions . 58 6 Experimental setup and applied diagnostics 61 6.1 Experimental setup . 61 6.2 Sample holder . 64 6.3 Caesium source . 66 6 Contents 6.4 Monitoring systems . 68 6.4.1 Cs diagnostics . 68 6.4.2 Optical emission spectroscopy (OES) for hydrogen species 74 6.4.3 Langmuir probe . 78 6.4.4 Residual gas analyzer (RGA) . 82 7 Determination of WF and nH− & experimental procedure 83 7.1 Work function diagnostic . 83 7.1.1 Fowler method . 83 7.1.2 Setup for work function measurements at ACCesS . 86 7.1.3 Benchmark and typical procedure for the work function measurements . 89 7.2 H− density diagnostic . 92 7.2.1 Cavity ring-down spectroscopy (CRDS) . 92 7.2.2 CRDS setup at ACCesS . 94 7.3 Typical plasma operation . 97 7.3.1 H− volume production at ACCesS . 99 7.3.2 H− surface production at ACCesS . 101 8 Low work function materials 103 8.1 Lanthanum hexaboride . 103 8.2 Lanthanated materials . 106 8.3 Tungsten dispenser cathode . 107 8.4 Europium . 112 8.5 Summary . 116 9 Work function of caesiated surfaces 119 9.1 Cs evaporation at ACCesS . 119 9.1.1 Evaporation under vacuum conditions . 121 9.1.2 Evaporation during plasma operation . 124 9.2 Surface caesiation in vacuum . 126 9.3 Plasma exposure on caesiated surfaces . 128 9.4 Caesiation during plasma . 134 10 Correlation between H−/D− density and work function 137 10.1 Caesiation in H2 plasma at 2 Pa/450 W . 138 10.2 Correlation between H− density and work function . 142 10.3 Comp. between H2 and D2 plasmas regarding surf. prod. of neg. ions145 Contents 7 11 Application to negative ion sources for fusion 151 12 Conclusions 159 Bibliography 163 Acknowledgements 177 9 1 Introduction Thermonuclear fusion is one of the promising candidates for a sustainable energy production, and the first reactor-scale nuclear fusion experiment ITER1 is currently under construction in France (Cadarache). ITER represents an intermediate step towards a commercial fusion power plant, whose feasibility will be proved by the demonstration power plant DEMO. An important component of such fusion machines is the neutral beam injection system based on negative hydrogen2 ions (NNBI). The NNBI consists of a negative hydrogen ion source, where negative hydrogen ions are created in a low pressure low temperature hydrogen plasma by plasma surface interaction. The negative hydrogen ions are then extracted and accelerated, producing a high energetic ion beam. This beam is afterwards neutralized and injected into the fusion plasma. ITER will include two high energetic neutral beam injection systems delivering 16.5 MW each. A 1 MeV accelerated and homogeneous D− beam of 40 A is required with an accelerated negative ion current density of 200 A/m2 and for pulses of up to 1 hour. In particular, since electrons are inevitably co-extracted together with the negative ions, it is required that the ratio between the co-extracted electron current and the extracted negative ion current must not exceed unity. At present, no NNBI system fulfills simultaneously all the ITER requirements, hence research and extensive investigations are carried on to further develop the ion sources and the other NNBI components. The NNBI source for ITER relies on the surface production mechanism of negative hydrogen ions via conversion of atomic hydrogen and positive hydrogen ions impinging on a low work function converter surface. To achieve the low work function, the converter surface is coated with caesium (i.e. the element with the lowest work function) by evaporation within the source. Cs easily reacts with the residual gases during vacuum phases, affecting the work function of the Cs layer, and the interactions of the plasma particles, together with the UV/VUV plasma 1ITER: latin word meaning ’the way’. 2Summarizing the hydrogen isotopes 1H and 2D. 10 Chapter 1 Introduction emission, lead to a cleaning and a removal of the Cs layer. Hence, a continuous evaporation of fresh Cs during plasma operation is mandatory for a stable bulk Cs layer. Nevertheless, particular attention has to be paid to minimize the Cs consumption in view of DEMO. The prototype source for the ITER NNBI is the RF-driven ion source devel- oped at the Max-Planck-Institut für Plasmaphysik (IPP) in Garching, Germany. Experiments carried out at the IPP test facilities demonstrated that the ITER requirements can be achieved for short plasma pulses, while the performance decreases during long plasma operation. A big issue is the co-extracted electron current, which constantly increases during long pulses. This problem is particu- larly relevant during deuterium operation, since the co-extracted electron current is much higher than during hydrogen operation. In order to gain insight into the behavior of the negative ion current and the co-extracted electron current in correlation with the work function of a caesiated surface (diagnostically not easily accessible at the ion sources), fundamental investigations are performed at the flexible laboratory experiment ACCesS3, which can reproduce ion source relevant vacuum and plasma parameters at controlled conditions.
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