High Current Density Advanced Cold Cathode Facility * ECE Department, University of Wisconsin, Madison a ; University of Michigan ， Ann Arbor b * Research Supported through AFOSR by a USDOD MURI05 grant on the Nano-physics of High Current Density Cathode and Breakdown Abstract We report measurements and analyses of field emission from both copper and aluminum cathodes. The copper cathodes have been micromachined to have raised ridges or knife edges to enhance the surface electric field. Raised ridges have also been formed into the aluminum cathodes, using Ablation Line Focus (ALF) laser-micromachining, developed at the University of Michigan. The measurements are conducted in the Madison Cathode eXperiment (MACX) facility after baking and processing to a UHV base pressure of Torr. The anode-cathode gap is variable from 0-10 mm with 50 μm accuracy and the cathode voltage is variable from 0 – 20 kV (negative). Pulse lengths are variable between 1 μs – 500 ms. The MACX facility also provides the ability to heat the cathode from ~ 300 – 650 o K while measuring electron emission currents. Research Objectives  UHV ( Torr) facility for high current density cathode research  Develope TMM model to predict the entire range of emission from thermionic to field emission and mixed emission (field + thermionic)  Analyze the experimental data with TMM model codes  Design high-current density cathode with high local field enhancement factor (β=E loc /(V/d)) and low work function Vacuum System Introduction  Vacuum System: A scroll pump, turbo pump and a VacIon pump are used to bring the whole system to Torr with the system baked to 450 ºC. Cathode and Anode: The cathodes are made of Al, Cu, C or Mo and may be coated with CsI. The anode is made of stainless steel. It is a tube about 1- cm in diameter and 8-cm long. A tiny second anode is applied to measure the emission current density distribution. Cathode Electrical Test Facility System TMM Method Model References (1) R.H.Fowler and L.W.Nordheim, Proc. Roy.Soc, (London), A119, 173 (1928) (2) E.L.Murphy and R.H.Dood, Phys Rev, 102, 1464, (1956) (3) Kevin L. Jensen, Patrick G. O’Shea, and Donald W. Feldman, Appl. Phys. Lett. 81, 3867, (2002) (4) Kevin L. Jensen, Marc Cahay, Appl. Phys. Lett, 88, (2006) (5) Kevin L. Jensen, J. Vac. Sci. Technol. B 21, (2003) X. He a, J. Scharer a, J. Booske a, S. Sengele a, V. Vlahos a ; N. Jordan b, R. Gilgenbach b Electrical System  ~ -20 kV, 1 μs -500 ms long duration with <60 ns rise time are applied between the cathode and anode.  2. To reduce the transient current spike, a high-voltage current limiting resistor and two forward /reverse Schottky power diodes in parallel are added.  3. At low current level,(<1 μA), a three stage operational amplifier is utilized to provide 1000 times of amplification in amplitude. A three stage low pass filter is used to improve the SNR (Signal-Noise Ratio) when necessary. TMM Model  1. Assumes a Fermi-Dirac distribution with a non-zero temperature in electron supply  2.. Electron tunneling is calculated exactly (within a 1D assumption) using a transfer matrix method to solve the steady-state Schrödinger’s equation (without WKB approximation)  3. The entire range of emission from thermionic to field emission and mixed emission (field + thermionic) is obtained  4 Experiments on a copper knife-edge cathode show close agreement with the TMM model. Cathode Anode Structure 1.25 cm diameter ALF cathode with β ~4 which works well on UM HPM (Several Hundred MW) magnetron. University of Michigan Collaboration Cathode Structure Copper Knife Edge (CKE) Cathode Fabrication Structure The experimental data is investigated with an accurate TMM (Transfer Matrix Method) model. With this model we can predict the entire range of emission from thermionic to field emission by varying the surface electric field. A new definition of TE (thermionic emission), FE (field emission) and ME (mixed emission/transition range) is provided. Experimental result from CKE cathode fitted with TMM model  Temperature=330 ºC  Work function Φ=1.73 eV  Effective Local Field Enhancement Factor β eff = E surface / E gap =7  Emission Area Ratio S emission /S knifeedge =1 Bench mark with Fowler-Nordheim and Richardson’s law TE, FE and ME Regime S =E 0,T=T 0 ; J s =E 0,T=0 K; J S =0,T=T 0 High electric field required for transition at high temperature TMM Method Model Experimental result from CKE cathode fitted with FN model  Work function Φ=4.5 eV  Effective Local Field Enhancement Factor β eff = E surface / E gap =602  Emission Area Ratio S emission /S knifeedge =3.56* Summary  Experimental result can be explained by the TMM model well  Unique and physically reasonable cathode parameters like effective beta, β eff, effective work function Φ, and emission area ratio R are obtained with TMM model  TMM code shows good agreements with Fowler-Nordhem law and Richardson law at very high or very low electric fields applied 9.06 mm