Study of cold electron emission sources for a cold cathode electron gun

1. Study of cold electron emission sources for a cold cathode electron gun Bruno GalanteTopical Workshop – Low energy facility design and optimization through diagnostics6-7 February 2019 “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

2. Table of contents • ELENA and Electron cooling • Electron Cooler and Electron gun • Electron mission processes • Novel Emission Source • The Cold Cathode Test Bench • Results • Improvements & Next steps bruno.galante@cern.ch

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

4. Electron Cooling • However, emittance blow-up is caused by: • Intra-beam scattering • Scattering with residual gases • Deceleration process. • Electron cooling reduces: • Longitudinal and Transverse energy spread (Momentum spread) • Size of the beam • Divergence of the ions Reaching thermal equilibrium between electrons and ions (e.g. antiprotons) in a time in the order of seconds. Result:A very intense and brilliant beam can be delivered to the experiments. [1][2][21] bruno.galante@cern.ch

5. E-Cooling Theory: a Brief Overview • Let’s consider an ion beam (in our case an antiproton beam) and an electron beam as 2 plasmas of different T. • High T Plasma:Ion beam - Low T Plasma:Electron beam. • If there is a big density of electrons and they are continuously generated, after a certain time Ion Beam T = Electron Beam T. • Electron-Ion interactions:Series of small-angle Rutherford scattering via Coulomb interaction where little momentum and energy is transferred. • Single ion-electron interaction: •  • And the energy transferred from the ion to the electron would be: [1][2][21] bruno.galante@cern.ch

6. E-Cooling Theory: a Brief Overview Multiple collisions case.The effect of many scatters can be considered statistically using the statistical expectation value of the product of the momentum transfer components: Energy lost by the ion in the longitudinal direction - Cooling Force (frictional): Diffusion Coefficient: Rearranging: [1][2][21] bruno.galante@cern.ch

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

8. 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 transverse Tof the emitted beam.  Adiabatic expansion. Requires additional solenoid to generate a large magnetic field at the gun. Photocathodes suffer of an usually quite low lifetime, stability issues and are quite complicated to operate. Alternative solution: Field Emission due to have a Cold Cathode. E-Gun and Adiabatic expansion Effect of the expansion solenoid on the longitudinal field inside the drift solenoid [3] bruno.galante@cern.ch

9. 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 transverse Tof the emitted beam.  Adiabatic expansion. Requires additional solenoid to generate a large magnetic field at the gun. Photocathodes suffer of an usually quite low lifetime, stability issues and are quite complicated to operate. Alternative solution: Field Emission due to have a Cold Cathode. Effect of the expansion solenoid on the longitudinal field inside the drift solenoid [3] bruno.galante@cern.ch

10. Field Emission Thermionic Emission, Photoemission: Generation of electronsonce the electrons have energy enough to overtake the potential barrier. Field Emission: Tunneling of electrons through the barrier applying a very large electric field (~ 107 V/cm). Fowler-Nordheim Theory. Number of electrons incident on a surface of unit area per unit time: And hence the current I is: And in field emission conditions, hence at low T: W: kinetic energy of the electron moving in direction normal to the surfaceF: Electric Field, k2=8π2m/h2 = 0, x <0. [4][5][6] bruno.galante@cern.ch

11. Carbon Nanotubes • For flat surfaces the required electric field is too strong. • Possible solution: Field Enhancement with tips Multi-Walled carbon NanoTube (MWNT) • 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 Single-Walled NanoTube (SWNT) [4][5][6][7][8][9] bruno.galante@cern.ch

12. Vertically Aligned CNTs Best field emission performances have been achieved with perfectly aligned CNTs. However, several parameters have to be taken into account: • Screening Effect: Uniform penetration of the electric field is complicated for high density forests. Minimization and dense enough forests  S= between 2h and 3h • Length distribution and burn-out  Conditioning • Degradation  MWNTs show better stability Effect of nanotubes density on the field emission performances Uniformity of the electric field on the carbon nanotubes forest and edge effect Screening effect in a COMSOL simulation. [10][11] bruno.galante@cern.ch

13. CNT Arrays Probably best solution to enhance FE performance. Also, more studies are present regarding these structures Growth Methods: • PECVD (Plasma-Enhanced Chemical Vapor Deposition) and TCVD (Thermal CVD) are the methods mostly used • Patterning via use of catalyst (e.g. Fe, Ni) with different shapes on the substrate. Parameter to optimize: • Spacing • 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. h SEM images. 1 - Array of squared island of nanotubes. 2 - A single island. a, b - Array with hexagonal pattern. c – cross-section image. d – TEM image. e – CNT forest. f – Field simulation. [12][13] bruno.galante@cern.ch

14. CNT Arrays Probably best solution to enhance FE performance. Also, more studies are present regarding these structures Growth Methods: • PECVD (Plasma-Enhanced Chemical Vapor Deposition) and TCVD (Thermal CVD) are the methods mostly used • Patterning via use of catalyst (e.g. Fe, Ni) with different shapes on the substrate. Parameter to optimize: • Spacing • 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. Outgassing and pressure control have to be carefully taken into account. a, b - Array with hexagonal pattern. c – cross-section image. d – TEM image. e – CNT forest. f – Field simulation. [12][13] bruno.galante@cern.ch

15. Exotic 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. Illustration of doped CNTs. Effect of decoration with different elements on the FE performances. Composite structure: Few-Layer Graphene on the top of Carbon nanotubes [14]-[20] bruno.galante@cern.ch

16. Cold Cathode Test Bench Flange with 3 different CNT arrays Vacuum tank with the flange mounted in front bruno.galante@cern.ch

17. Cold Cathode Test Bench • The first test bench has been designed to characterize: • Necessary Conditioning process • Emitted current in function of the applied electric field • Stability of the emitted current • Lifetime Molybdenum Anode Carbon Nanotubes (CNT) Ceramic spacer bruno.galante@cern.ch

18. 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

19. Fowler-Nordheim Plot Determination of material parameters and enhancement factor: “a” and “b” are necessary parameters for simulation of field emission. bruno.galante@cern.ch

20. Further Improvements & Next steps • Improvements: • Annealing of samples at 450 degrees in Ammonia atmosphere • Bake-out of the test bench • 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

21. Cold Cathode Test Bench 2 • Allows measurements of: • Longitudinal Energy • Transverse Energy of the electron beam depending on the inter-electrode distance. • Longitudinal Energy: • Deceleration of the electron applying an inverse electric field. • Transverse energy: • Measure of the emission spot on a phosphor screen. Linear Motion Feedthrough Support for CNT Extracting Grid Anode (not present in the reality) bruno.galante@cern.ch

22. 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

23. 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[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[21] – Electron cooling: Theory, Experiment, Application – Helmuth Poth bruno.galante@cern.ch

24. bruno.galante@cern.ch

25. bruno.galante@cern.ch

26. bruno.galante@cern.ch

27. Adiabatic Expansion • Single electron case. If:- axially symmetric field geometry, B = B‖(z)- electron moves slowly enough • We can consider the following adiabatic invariance: • When the field is reduced from the value Bi to a value B: • Also, the magnetic flux contained in the cross section of the assumed cylindrical electron beam is an adiabatic invariant, therefore it will increase when the B decreases: • Multiple electrons. For multiple, not independent electrons, the Coulomb interaction breaks the invariance. However, the invariance stands for the average transverse energy: • Therefore: Electron cooling and recombination experiments with an adiabatically expanded electron beam – S.Pastuszka, U.Schramm, M.Grieser, C.Broude, R.Grimm, D.Habs, J.Kenntner, H.-J.Miesner, T.Schussler, D.Schwalm, A.Wolf bruno.galante@cern.ch

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

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

30. Longitudinal and Transverse Energy ~ -2kV ~ -2kV Longitudinal Energy Transverse Energy r Starting from: 2) TE ~ +1kV 0 to ~ -2kV 1 - The Commissioning of TESS: An experimental facility for measuring the electron energy distribution from photocathodes – L.B. Jones, R.J. Cash, B.D. Fell, T.C.Q. Noakes, B.L. Militsyn2 - Electron transverse energy distribution in GaAs negative electron affinity cathodes: calculations compared to experiments – G.Vergara, A. Herrera-Gomez, W.E. Spicer bruno.galante@cern.ch

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

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