Evaluation of TiN-Coated Aluminum Electrodes for DC High Voltage Electron Guns M.A. Mamun1,2, E. Forman3, R. Taus4, M. Poelker3, and A.A. Elmustafa1,2 1Department of Mechanical and Aerospace Engineering, Old Dominion University, Norfolk, Virginia 23529, USA 2Applied Research Center, Thomas Jefferson National Accelerator Facility, Newport News, VA 23606, USA 3Thomas Jefferson National Accelerator Facility, Newport News, VA 23606, USA 4Frank R. Seaver College of Science and Engineering, Loyola Marymount University, Los Angeles, CA 90045, USA 41st ICMCTF, San Diego, CA April 28 - May 2, 2014

Spin polarized electron beams via photoemission from a photocathode biased at high voltage Thomas Jefferson National Accelerator Facility (Jefferson Lab) is one of 17 national laboratories funded by the U.S. Department of Energy. The lab also receives support from the City of Newport News and the Commonwealth of Virginia. The primary mission of the Lab is to utilize its unique Continuous Electron Beam Accelerator Facility (CEBAF) to study the atomic nucleus to gain a deeper understanding of the structure of matter. Jefferson Lab also carries out applied research with industry and university partners using a free-electron laser that was developed by the lab based on its superconducting technology. Jefferson Lab also applies its unique technical capabilities to developing radiation detectors and medical imaging devices. Proton Electron Neutron

Electron Beams via Photoemission Photocathode preparation chamber High voltage chamber Pressure ~ 10-12 Torr Even nA of field emission makes pressure increase noticeably Cathode electrode e- hν Beam quality, including transmission, improves at higher gun voltage Higher voltage = better beam quality But higher voltage often leads to field emission Field emission degrades vacuum Bad vacuum degrades photocathode yield 41st ICMCTF, April 28 - May 2, 2014, San Diego, CA, USA

New initiatives require higher voltage Higher operating voltage leads to a less complicated, shorter injector beam line Further reduce space charge induced emittance growth Easier to maintain a small transverse beam profile and short bunch length Biggest Obstacle: Field Emission and HV Breakdown… which lead to bad vacuum and photocathode death For DC guns Interested in higher bias voltage (500kV) & Gradient >10MV/m Stiff beam, smaller dimension, immune to space charge forces Need it for high bunch charge applications 130 kV photogun Goal : 350 kV or higher 41st ICMCTF, April 28 - May 2, 2014, San Diego, CA, USA

Field Emission (FE) Field Emission Fowler-Nordheim E.L. Murphy, and R.H. Good, Physical Review 102, 1464 (1956) An unregulated release of electrons from the cathode electrode when biased at high voltage. A quantum mechanical tunneling effect explained by Fowler-Nordheim, 1928. The theory assumes a single field emitter Not very realistic to implement for large electrodes as it requires knowledge about emitter areas and shapes which is unrealistic to quantify beforehand Hence, not useful to predict the onset of FE Mainly used for post-emission data analysis Field emission(FE): The unregulated release of electrons from the cathode electrode when biased at high voltage. Electron emission based mechanism: Micro-protrusions Micro-Particle based Mechanisms: Embedded impurity particles Dust particles Filamentary structures torn from the surface Gas Desorption Regenerative Ionization: Local desorption of gas pockets in high field 41st ICMCTF, April 28 - May 2, 2014, San Diego, CA, USA

Sources of Field Emission Electron emission based mechanism: Micro-protrusions Micro-Particle based Mechanisms: Embedded impurity particles of the polishing medium(Alumina, Diamond,…) Filamentary structures that have been torn from the surface. Dust particles attached to surface by Van der Waals forces from handling or assembling Regenerative Ionization: Local desorption of gas pockets after the bake out Ion Exchange Mechanism Liberating electrons from contaminated areas 41st ICMCTF, April 28 - May 2, 2014, San Diego, CA, USA

Material Aspects of Electrode Impurity and material defects Grain structure and grain boundary Surface roughness smooth surface does not guarantee cathode performance free of field emission, microscopic protrusions definitely hurts Work function of the material Thermal conductivity Electrical conductivity, etc. 41st ICMCTF, April 28 - May 2, 2014, San Diego, CA, USA

Choice of Electrode Materials Traditionally, we choose hard metals: Stainless steel, titanium-alloy, or molybdenum And Smooth surface Diamond paste polishing typically employed Time consuming In this work our goal is to fabricate a good electrode using inexpensive materials and techniques, and in a short amount of time. 41st ICMCTF, April 28 - May 2, 2014, San Diego, CA, USA

High Voltage Test Stand Materials studied: Ti-alloy (Ti-6AL-4V) Al 6061 TiN coated Al 6061 Max.voltage 225 kV Max. field strength Gap, mm F, MV/m 50 12.8 40 13.7 30 15.4 20 18.7 10 28.9 Introduce test stand. List materials that have been studied. Buffered chemical polished of niobium. “Inverted” insulator: no SF6 and no HV breakdown outside the chamber 41st ICMCTF, April 28 - May 2, 2014, San Diego, CA, USA

Field Emission Results 1200 grit SiC polished High FE at field strength < 15 MV/m Not a good electrode to meet the requirement 800 and 1200 grit SiC polished 1 µm IBED TiN coating Low FE at field strength < 15 MV/m Easy to fabricate and coat Diamond-paste polished Low FE at field strength < 15 MV/m Inconsistently good electrode But expensive material and labor intensive polishing 41st ICMCTF, April 28 - May 2, 2014, San Diego, CA, USA

SEM Results: Fabrication Al: 400 grit Al: 600 grit Al: 800 grit Al: 1200 grit TiN coated: 1200 grit 41st ICMCTF, April 28 - May 2, 2014, San Diego, CA, USA

Surface Appearance before HV testing 1200 grit polished Uncoated Al TiN Coated Al 41st ICMCTF, April 28 - May 2, 2014, San Diego, CA, USA

Al 6061 after HV conditioning and gas processing Intense localized heating from sputtering & ion bombardment during gas processing 11/9/2018

Surface images of bare aluminum SEM images of HV conditioned Al electrode (a) prior to He processing and (b) post-He processing. Lots of craters developed during gas conditioning at the field emitted sites 41st ICMCTF, April 28 - May 2, 2014, San Diego, CA, USA

Surface images of bare aluminum SEM and EDS of post-He processed and HV conditioned bare Al electrode depicting (a) a field emitted area and (b) the chemical analysis of the emitted area. No silica contamination Lots of oxide formation in the FE sites 41st ICMCTF, April 28 - May 2, 2014, San Diego, CA, USA

Surface images of TiN coated Al6061 electrode As fabricated and before HV application (a) prior to TiN coating and (b) post-TiN coating. TiN coating effectively serves to hide the underlying material defects 800 grit 1200 grit 1200 grit uncoated uncoated TiN coated RMS roughness 20 nm 16 nm 13 nm (form 2 mm line scans using profilometer on representative coupons) 41st ICMCTF, April 28 - May 2, 2014, San Diego, CA, USA

Surface images of TiN coated Al6061 After HV test TiN coated 1200 grit finish Al Electrode after (a) HV conditioning and He processing (max. gradient = 18.7 MV/m) (b) HV conditioning (max. gradient= 22.4 MV/m) Breakdown occurred when subjected to higher field strength 41st ICMCTF, April 28 - May 2, 2014, San Diego, CA, USA

Coupon assessment RMS roughness and average grain height were measured as 7 and 20 nm respectively for the scan area of 1µm×1µm. Topography of 3µm IBED TiN coating on 1200 grit polished 6061Al revealed by AFM GPa Al6061 TiN Hardness 2 18 Modulus 70 270 Hardness and Modulus as a function of contact depth for uncoated and TiN coated 6061Al. The error bars represent 3 standard error. 41st ICMCTF, April 28 - May 2, 2014, San Diego, CA, USA

Coupon assessment No signs of delamination or peeling off 3 µm indent 500 nm indent Load versus displacement into surface for 2 µm indents on an uncoated and TiN coated 6061 Al coupons with 1200 grit surface finish. SEM images of the residual impressions of (left) 500 nm and (right) 3 µm berkovich indents on the TiN coated 1200 grit polished 6061Al coupon. The occurrence of cracks for the larger indent was observed as pop-in events during indentation. No signs of delamination or peeling off 41st ICMCTF, April 28 - May 2, 2014, San Diego, CA, USA

Properties of electrode materials Commonly accepted values from literature: Al 6061 TiN Ti-6Al-4V Electrical Resistivity, ohm-cm 4E-6 30~70E-6 180E-6 Thermal Conductivity,  W/m-K 167 28.8 6.7 Modulus, Gpa 70 270 114 Hardness, GPa 2 18 6~7 Work function (ɸ), eV 3.5 5.0 4.5

Summary Maximum voltage and field strength for <100 pA: Maximum field strength at different cathode-anode gap: Field Al (He) TiN/Al (He)  Strength (MV/m) 1200 grit 800 grit 50mm 8.99 12.57 >12.79 40mm 8.51 13.33 >13.73 30mm 9.02 >15.35 20mm 9.4 18.20 17.97 Maximum voltage and field strength for <100 pA: Ti-alloy Bare Al TiN/Al 225 kV 160 kV 225 kV at 50 mm gap >18.7 MV/m 9.4 MV/m 18.2 MV/m at 20 mm gap Future works: Coat a dummy electrode ball for evaluation at 350kV Planning to test TiN/Cu too, it’s easy to polish and even better thermal conductor Study even rougher electrode surfaces coated with TiN 41st ICMCTF, April 28 - May 2, 2014, San Diego, CA, USA

Conclusion A reasonably polished inexpensive aluminum electrode can meet the goal of a good electrode by applying TiN coating in a short amount of time. 41st ICMCTF, April 28 - May 2, 2014, San Diego, CA, USA

Thank You Welcome for Q&A 11/9/2018 41st ICMCTF, April 28 - May 2, 2014, San Diego, CA, USA