Cold Cathode Development for Electron Coolers

1. Cold Cathode Development for Electron Coolers Bruno Galante BE-BI-EABI Day 04-12-2018 bruno.galante@cern.ch

2. ELENA [1][2] bruno.galante@cern.ch

3. ELENA e-Cooler Electron Gun Collector [3] bruno.galante@cern.ch

4. Electron Gun It must produce a • Cold (T⊥ < 0.1eV, T// < 1meV) • Intense electron beam (ne ≈ 1.5x1012 cm-3) Thermionic cathodes limit the performance of electron cooling due to high Tof the emitted beam while photocathodes suffer of an usually quite low lifetime. Alternative solution: Field Emission due to have a Cold Cathode. bruno.galante@cern.ch

5. Field Emission • Thermionic Emission: Heating of the cathodethat leads the generation of an electron beamonce the electrons have energy enough to overcome the potential barrier. • Photoemission: Generation of electron by irradiation with a light source with proper energy. • Field Emission: Tunneling of electrons through the barrier applying avery large electric field (~ 107 V/cm) [4][5][6] bruno.galante@cern.ch

6. Carbon Nanotubes • For flat surfaces the required electric field is too strong. • Possible solution: Field Enhancement with tips • PRO: • High aspect ratio -> High enhancement • Emit at low field, in order of some V/μm • Scalable production techniques • Chemical inertness and stable structure • CONS: • Small emitted current per tip • Screening effects • Impurities and defects [4][5][6][7][8][9] bruno.galante@cern.ch

7. Vertically Aligned CNTs • Best performances achieved with perfectly aligned CNTs • Screening minimization and dense enough forests  S=2h • Length distribution and burn-out  Conditioning • Degradation  MWNTs better [10][11] bruno.galante@cern.ch

8. CNT Arrays • Probably best solution -> More studies • Use of catalyst (e.g. Fe, Ni) with different shapes on the substrate • Parameter to optimize: spacing and size of the forests • In Fig. above -> 30µm x 30µm Fe catalyst pattern at pitch distance 125 µm. Best performance achieved: 80 mA/cm2 at about 3 V/µm • In Fig. below -> 1 mA/cm2 at 1,5 V/µm and current densities up to 1,5 A/cm2. FE evaluated using diode configuration at a pressure of 1x10-8 mbar. Emission area: 4x4mm2. Inter-electrode distance: 0.25mm (spacers). [12][13] bruno.galante@cern.ch

9. CNT Arrays • Probably best solution -> More studies • Use of catalyst (e.g. Fe, Ni) with different shapes on the substrate • Parameter to optimize: spacing and size of the forests • In Fig. above -> 30µm x 30µm Fe catalyst pattern at pitch distance 125 µm. Best performance achieved: 80 mA/cm2 at about 3 V/µm • In Fig. below -> 1 mA/cm2 at 1,5 V/µm and current densities up to 1,5 A/cm2. FE evaluated using diode configuration at a pressure of 1x10-8 mbar. Emission area: 4x4mm2. Inter-electrode distance: 0.25mm (spacers). [12][13] bruno.galante@cern.ch

10. Doping, Decoration and Composite Structures • Doping: Modification of the crystalline structure introducing different elements, e.g. N, O • Decoration: Metal coating using different metals with lower work function • Composite Structures: Add of different structure on the top of the nanotubes to modify work function and/or increase enhancement factor [14]-[20] bruno.galante@cern.ch

11. Cold Cathode Test Bench Copper Anode Alumina Spacer Copper foil Substrate + CNT Array Vespel Insulator bruno.galante@cern.ch

12. Cold Cathode Test Bench bruno.galante@cern.ch

13. Conditioning Test 0.91 V/µm 1.94 V/µm 2.51 V/µm 1.48 V/µm 2.4 V/µm 2.63 V/µm bruno.galante@cern.ch

14. Fowler-Nordheim Plot bruno.galante@cern.ch

15. Further Improvements & Next steps • Improvements: • Annealing of samples at 450 degrees in Ammonia atmosphere • Bake-out of the test bench • Better vacuum • Resistor in series with the power supply  Ballast Resistor • Next: • Measurement of Longitudinal and Transverse Energy depending on the inter-electrode distance bruno.galante@cern.ch

16. Thank you “AVA has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 721559.” bruno.galante@cern.ch

17. References • [1] - https://espace.cern.ch/elena-project/SitePages/Home.aspx[2] - ELENA: the extra low energy anti-proton facility at CERN – S.Maury, W.Oelert, W.Bartmann, P.Belochitskii, H.Breuker, F.Butin, C.Carli, T.Eriksson, S.Pasinelli, G.Tranquille[3] - The ELENA electron cooler: parameter choice and expected performance – G.Tranquille, A.Frassier, L.Joergensen[4] - Electron emission in intense electric fields – R.H.Fowler, Dr.L.Nordheim[5] - Carbon Nanotube Electron Source: from electron beams to energy conversion and optophotonics – AlirezaNojeh[6] - Electron field emission from carbon nanotubes – Y.Cheng, O.Zhou[7] - Vacuum nanoelectronics devices: Novel electron sources and applications – A. Evtukh, H. Hartnagel, O. Yilmazoglu, H. Mimura, D. Pavlidis[8] - Carbon nanotubes for cold electron sources – P.Groning, P.Ruffiex, L.Schlapbach, O.Groning[9] - Carbon Nanotube and related field emitters: Fundamentals and applications – Yahachi Saito [10] - Array geometry, size and spacing effects on field emission characteristics of aligned carbon nanotubes – Y.M.Wong, W.P.Kang, J.L.Davidson, B.K.Choi, [11] - Maximizing the electron field emission performance of carbon nanotube arrays – R.C.Smith, S.R.P.Silva[12] - Patterned selective growth of carbon nanotubes and large field emission from vertically well-aligned carbon nanotube field emitter arrays – J.Sohn, S.Lee, Y.-H.Song, S.-Y.Choi, K.-S.Nam[13] - High emission current density, vertically aligned carbon nanotube mesh, field emitter array – C.Li, Y.Zhang, M.Mann, D.Hasko, W.Lei, B.Wang, D.Chu, D.Pribat, G.Amaratunga, W.I.Milne[14] - The doping of carbon nanotubes with nitrogen and their potential applications – P.Ayala, R.Arenal, M.Rummeli, A.Rubio, T.Pichler[15] - Oxygen and nitrogen doping in single wal carbon nanotubes: An efficient stable field emitter – A.Kumar, S.Parveen, S.Husain, M.Zulfequar, Harsh, M.Husain[16] - Improved field emission properties of carbon nanotubes decorated with Ta layer – Z.Wang, Y.Zuo, Y.Li, X.Han, X.Guo, J.Wang, B.Cao, L.Xi, D.Xue bruno.galante@cern.ch

18. References • [17] - Highly improved field emission from vertical graphene-carbon nanotube composites – J.-H. Deng, R.-N. Liu, Y.Zang, W.-X. Zhu, A-L.Han, G.-A. Cheng[18] - Highly improved field emission from vertical graphene-carbon nanotube composites – J.-H. Deng, R.-N. Liu, Y.Zang, W.-X. Zhu, A-L.Han, G.-A. Cheng [19] - Enhanced field emission properties of a reduced graphene oxide/carbon nanotube hybrid film – D.D.Nguyen, Y.-T.Lai, N.-H.Tai[20] - Excellent field emission characteristics from few-layer graphene-carbon nanotube hybrids synthesized using radio frequency hydrogen plasma sputtering deposition – J.-H.Deng, R.-t. Zheng, Y.-M.Yang, Y.Zhao, G.-A.Cheng bruno.galante@cern.ch

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22. Fowler-Nordheim Plot bruno.galante@cern.ch

23. SEM of CNT Array bruno.galante@cern.ch

24. Longitudinal and Transverse Energy ~ -2kV ~ -2kV r 2) TE 1) LE ~ +1kV ~ -2kV bruno.galante@cern.ch

25. Grid vs Hole Grid Hole bruno.galante@cern.ch

26. Grid Effect Grid No Grid bruno.galante@cern.ch