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Field Emission Measurements From Cesiated Titanium and Stainless Steel Electrodes K.Surles-law, P.Adderley, J.Brittian, D.Charles, J.Clark, J.Grames, J.Hansknecht, M.Poelker, M.Stutzman Field Emission from 25deg Electrodes POISSON model of 25deg electrode E-fields shown are for V = -100kV Field Emission Electrons are confined in a metal by a potential well Energy of electron insufficient to escape from metal Electron must be given extra energy to escape (thermal, photoemission) QM demonstrates the electron wavefunction attenuates rapidly outside potential barrier Field Emission Application of external fields lowers and thins the potential barrier Fowler- Nordheim used quantum mechanics to demonstrate that some electrons can tunnel through the potential barrier and escape into the vacuum Field Emission: Fowler-Nordheim Equation 6 I (E) 1.54 x10 A ( E ) 2 ( Exp 3 3/ 2 6.83 x10 E ) This is the Fowler-Nordheim expression for field emission E is the electric field (MV/m) is the work function of the material (eV) is the field enhancement factor A is the effective emitting area Field Emission: Fowler-Nordheim Equation F-N with Cesium Work Function = 2.12eV 2 Field Emission Current Density (Amps/m ) 1600 1400 1200 1000 800 600 400 200 0 -200 0 1 2 3 4 5 6 E (MV/m) Plot of Fowler-Nordheim equation with b = 200. The electric field gradients were generated from POISSON model of the 25 degree electrode. The potential varied from 10 to 125kV Field Emission: Fowler-Nordheim Equation One can use exponential form of FN to compare field emission from Ti and SS Field Emission: Fowler-Nordheim Plot Ti and SS 7th Application of Cs 8000 7000 FE Current (pA) 6000 5000 SS (7th Cs) 4000 Ti (7th) 3000 2000 1000 0 0 20 40 60 80 100 120 HV (kV) 210min of Cs for Ti and SS 140 Field Emission: Fowler-Nordheim Plot More interesting to plot ln(I/V^2) vs. 1/V The slope of the fitted data is proportional to the work function to the 3/2 power If work function changes, the slope of FN plot changes Field Emission: Fowler-Nordheim Plot 2 d ln( J / E ) 3/ 2 Slope a 1 d( ) E Field Emission: Fowler-Nordheim Plot Now let’s observe FN plots for Ti and SS with no Cs and 210min of Cs Field Emission: Fowler-Nordheim Plot SS No Cs Linear Fit of Data10_B -7.5 SS 210min Cs Linear Fit of Data10_I -3.5 -4.0 -8.0 -4.5 -8.5 ln ( I / V ) -9.0 2 2 ln ( I / V ) -5.0 -9.5 -6.0 slope = -975 slope = - 934 -10.0 -5.5 -6.5 -7.0 -10.5 -7.5 0.0080 0.0085 0.0090 0.0095 1/V 0.0100 0.0105 0.0110 0.0115 0.0080 0.0085 0.0090 0.0095 1/V 0.0100 0.0105 0.0110 0.0115 Field Emission: Fowler-Nordheim Plot Ti no Cs Linear Fit of Data14_B Ti with 210min Cs Linear Fit of Data12_I -0.5 -7.9 -8.0 -1.0 -8.1 ln ( I / V ) -8.2 2 2 ln ( I / V ) -1.5 -8.3 -8.4 -2.0 -2.5 slope = - 816 slope = - 352 -8.5 -3.0 -8.6 -3.5 -8.7 0.0080 0.0085 0.0090 1/V 0.0095 0.0100 0.0080 0.0085 0.0090 0.0095 1/V 0.0100 0.0105 0.0110 0.0115 Field Emission: Fowler-Nordheim Plot SS no Cs SS 210min Cs Ti no Cs Ti 210min Cs 0 -2 2 ln ( I / V ) -4 -6 -8 -10 0.0080 0.0085 0.0090 0.0095 1/V 0.0100 0.0105 0.0110 0.0115 Field Emission: Fowler-Nordheim Equation Next, decrease cathode-anode spacing Take FE data for Ti and SS with no Cs Compare FE vs. spacing: Is Ti or SS better, or same?