Electron Emission from Nano and Micro Structured Materials for Plasma Applications

Transcription

1 Electron Emission from Nano and Micro Structured Materials for Plasma Applications Marlene Patino Department of Mechanical and Aerospace Engineering University of California, Los Angeles Yevgeny Raitses Princeton Plasma Physics Laboratory Richard Wirz Department of Mechanical and Aerospace Engineering University of California, Los Angeles 58 th Annual Meeting of the APS Division of Plasma Physics November 3, 2016

2 Outline Secondary Electron Emission (SEE) Background Motivation for SEE from Structured Materials Experimental Setup Results of Total SEE Yield Conclusions and Implications 2

3 Secondary electron emission (SEE) from plasma-facing walls adversely affects plasma performance [1] Wall Plasma Wall Plasma sheath ~3T e i plasma e - sheath ~ Te i plasma e - SEE e - current balance ion continuity e- continuity Plasma sheath at wall without SEE i const i plasma e SEE e plasma e SEE e const Reduces plasma sheath potential Increases plasma e - power loss Increases plasma cooling SEE becomes important as σ 1 Plasma sheath at wall with SEE 1 e sheath kte ln 2 me / mi 2kTe Q niovo E e sheath 1 I I SE PE [1] G.D. Hobbs and J.A. Wesson, Plasma Physics 9 85 (1967) 3

4 Types of secondary electrons and dependence for smooth materials Primary electron bombardment of materials produces [1-3] Primary electron - Elastically backscattered secondary Primary electron - Inelastically backscattered secondary Primary electron - True secondaries Dependence on Incident Energy [1-9] σ σ max σ = 1 more generated generated deeper Dependence on Incident Angle [1-3,6,9-10] 0 : ( ) escape depth Primary electron generation zone escape depth θ Primary electron generation zone E PE I E PE max cos( ) E PE II E PE [1] H. Bruining, Physics and Applications of Secondary Electron Emission (1954) [2] H. Seiler, J. Appl. Phys (1983) [3] C.E. Hueta and R.E. Wirz, 52 nd AIAA Joint Propulsion Conf. AIAA [4] R. Cimino et al, Phys. Rev. Lett (2004) [5] A.G.Lye and A.J. Dekker, Phys. Rev (1957) [6] J.R.M. Vaughan, IEEE Trans. Ele. Dev (1989) [7] E.W. Thomas, Int. Nucl. Data Comm. (1995) [8] J.J. Scholtz, D. Dijkkamp, and R.W.A. Schmitz, Philips J. Res (1996) [9] C.A. Ordonez and R.E. Peterkin, J. Appl. Phys (1996) [10] M.A. Furman and M.T.F. Pivi, Phys. Rev. AB (2002) 4

5 C velvet affects plasma discharge in a Hall-effect thruster Total SEE Yield Higher T e and reduced plume divergence [1] Reduced SEE from microstructured materials [2-7] Boron Nitride C velvet Boron Nitride Graphite C velvet Primary Electron Energy, ev [1] Y. Raitses et al, J. Appl. Phys (2006) [2] Y. Raitses et al, IEPC [3] A. Duneavsky et al, Phys. Plasmas (2003) [4] J.P. Bugeat and C. Koppel, IEPC [5] M. Pivi et al, J. Appl. Phys (2008) [6] M. Ye et al, J. Appl. Phys (2013) [7] A.N. Curren, IEEE Trans. Elec. Devices (1986) 5

6 W walls for fusion devices Leading candidate for the divertor in ITER [1] Source: ww.tue.nl Forms nanostructures when bombarded by He ions [2-6] [1] R.A. Pitts et al, J. Nucl. Mater. 438, S48 (2013) [2] S. Kajita et al, Nucl. Fusion (2009) [3] M.J. Baldwin et al, J. Nucl. Mater (2009) [4] G. De Tammerman et al, J. Vac. Sci. Technol. A (2012) [5] G.M. Wright et al, Nucl. Fusion (2012) [6] F.W. Meyer et al, Phys. Scr. T (2016) 6

7 Objective Quantitative measurements of SEE from plasma-facing structured materials at plasma relevant energies Plasma-Generated W fuzz [1] d fiber = 25-50nm L fiber = nm 5-10d fiber apart Engineered C velvet [2] d fiber ~ 5μm L fiber = 1-2mm ~4d fiber apart 1 sample x Smooth W Graphite [1] M. Patino et al, Appl. Phys. Lett. (accepted) [2] Y. Raitses et al, IEPC

8 fuzz analyzer smooth Hemispherical energy analyzer Experimental facilities sample W Princeton Univ. (10-9 Torr) secondary electrons secondary electrons sample +V sample 0V primary electrons primary electrons σ C PPPL (10-7 Torr) capability to measure SEE from conductive and non-conductive materials A. Dunaveasky et al, Phys. Plasmas (2003) X-ray Ion Gun Sputter clean X-ray Photoelectron Spectroscopy Composition sample Scanning Electron Microscope Topography X-ray Photoelectron Spectroscopy Composition M. Patino et al, Appl. Phys. Lett. (accepted) 8

9 Intensity, a.u. Intensity, a.u. W fuzz has more C and O impurities and oxidation Na1s O KLL 4s O1s 4p 1/2 4p 3/2 C1s 4d 3/2 4d 5/2 4f W 4f 5/2 W 4f 7/2 5s WO x Binding Energy, ev % W % C % O WO x /W Smooth W ~ Binding Energy, ev W fuzz

10 Total SEE Yield SEE from smooth W at 0 matches literature Smooth W, 0 1 Ref 1 Ref [1] A.J Ahearn, Phys. Rev (1931) [2] K.G. McKay, Phys. Rev (1942) Primary Electron Energy, ev 10

11 Total SEE Yield SEE is reduced by >40% for W fuzz at Smooth W, 0 1 W fuzz, [1] M. Patino et al, Appl. Phys. Lett. (accepted) Primary Electron Energy, ev 11

12 Total SEE Yield SEE from smooth W follows a typical angular dependence 2.5 Smooth W, 45 2 calculated Smooth W, W fuzz, [1] H. Bruining, Physics and Applications of Secondary Electron Emission (1954) [2] H. Seiler, J. Appl. Phys (1983) Primary Electron Energy, ev 12

13 Total SEE Yield SEE from W fuzz independent of incident angle 2.5 Smooth W, Smooth W, 0 1 W fuzz, 0 & θ global [1] M. Patino et al, Appl. Phys. Lett. (submitted) Primary Electron Energy, ev 13

14 Conclusions and Implications Direct measurements of secondary electron emission from self-generated W fuzz Reduced by >40% over smooth W Nearly independent on incident angle Implications for plasma-surface interactions (PSI) and plasma applications (e.g., aerospace applications such as plasma thrusters, multipactor discharges, arcing of solar panels etc., particle accelerators, material processing devices) 1. SEE adverse effects on plasmas may be reduced with W fuzz 2. Lack of angular dependence important for plasmas were electrons likely incident at glancing angles 14

15 Acknowledgements Princeton/PPPL Surface Science and Technology Lab Bruce Koel Xiaofang Yang Yao-Wen Yeh Luxherta Buzi Yuxin Yang PPPL Hall Thruster Lab Alex Merzhevskiy The College of New Jersey David Caron (student intern) MIT Plasma Science and Fusion Center Dennis Whyte Graham Wright Support: 1. DOE Office of Science Graduate Student Research Award 2. Air Force Office of Scientific Research 3. DOE/PPPL Off-Site University Research Support Program 4. UCLA Mechanical and Aerospace Engineering 15