Thomas Jefferson National Accelerator Facility (Jefferson Lab) Newport News, Virginia Simulation, Measurement and Analysis of Photoemission Simulation, Measurement and Analysis of Photoemission from Dispenser Cathodes, Metals and Coated Materials from Dispenser Cathodes, Metals and Coated Materials K. L. Jensen Code 6841, ESTD JLAB CASA Seminar Naval Research Laboratory Washington, DC 20375-5347 10:30 - 11:30 AM EM: kevin.jensen@nrl.navy.mil October 1, 2004 D. W. Feldman, P. G. O’Shea, Host: N. Moody, D. Demske Carlos Hernandez Garcia Jefferson Lab Inst. for Res. in El. & Appl. Phys. 12000 Jefferson Ave MS6A U. of Maryland, Newport News, VA 23606 College Park, MD 20742 http://www.ipr.umd.edu/ We gratefully acknowledge funding by the Joint Technology Office and the Office of Naval Research JLAB 1
HISTORY The ability to predict QE of pure metals / materials is hard. • Fowler DuBridge Theory: ⇒ R. H. Fowler, Phys. Rev. 38, 45 (1931); ⇒ L. A. DuBridge, Phys. Rev. 43, 727 (1933). If coatings are involved, it is far harder. Consider: • A. H. Sommer, “The Element of Luck in Research - Photocathodes 1930 to 1980” (Gaede-Langmuir Award), J. Vac. Sci. Technol. A1, 119 (1983). “Six photocathode materials were developed during the period from 1930 to 1963 to provide the spectral response and other characteristics needed for such applications as photometry, television, scintillation counters, and night vision devices. The history and the essential properties of these materials are reviewed and it is shown that all the cathodes resulted from lucky accidents and not from the application of scientific insight. The period of empirical innovation came to an end in the late 1960’s when negative electron affinity (NEA) materials became the first photocathodes that were developed on a strictly scientifc basis.” What is involved? Is a predictive model possible? JLAB 2
INTRODUCTION Photocathodes: • Sources for Electron Beams, From Free Electron Lasers (FELs) to Accelerator Applications Due to the High Quality Electron Beams • Ideal Photocathode Has High QE at Longest Possible Wavelength, Capable of in Situ Repair or Rehabilitation, Demonstrates Good Lifetime To meet particular needs of a megawatt (MW) class FEL, a photocathode… • …should produce 1 nC of charge in a 10-50 ps pulse every ns (100 A peak and 1 A average current) in 10-50 MV/m and 0.01 mTorr for several seconds. Even if such a photocathode were available… • Making predictions of performance is complex: Useful models must account for cathode surface conditions and material properties, as well as drive laser parameters. ⇒ surface conditions (coating, field enhancement, reflectivity), ⇒ laser parameters (duration, intensity, wavelength), and ⇒ material characteristics (reflectivity, laser penetration depth, scattering) • Focus: dispenser photocathodes, but also discuss other photocathodes PRESENT PROGRAM: Develop and validate with experiment a predictive and quantitative theory of photoemission & quantum efficiency. JLAB 3
PHOTOCATHODES DRIVE LASER • Reliability <=> System Reliability: UV Unsuitable for Hi-duty Non-linear Crystals Decrease λ by 2-4; Efficiency Very Low for UV • • Conversion by 2 From IR to Green ok: Seek High QE Photocathode in Visible PHOTOCATHODE • Bulk & Surface of Complex Materials Produced by Empirical Techniques; Short Lifetime, Complex Replacement Process. • Cathode Selection Influences Drive Laser Chosen (e.g., λ , spot bandwith, laser energy, QE) METALLIC: • Hi ave power, drive laser w/ 5 - 500 µJ/pulse req. • Rugged but require UV, have lower QE ( ≤ 0.01%). • For low duty factor, low rep rate UV pulses • Fast response time (fs-structure On Laser Appears on Beam) DIRECT BAND-GAP P-TYPE SEMICONDUCTORS: • Highest QE photocathodes ⇒ alkali antimonides (Cs 3 Sb, K 2 CsSb); visible, PEA, RF gun ⇒ alkali tellurides (Cs 2 Te, KCsTe) UV, PEA, RF gun ⇒ Bulk III-V wCs + oxidant (O or F); IR - visible, NEA, DC guns • Emission time is long (10-20 ps) for NEA sources: insufficiently responsive for pulse shaping. • ALL chemically reactive: Easily poisoned by H 2 0 & C0 2 (Protection at expense of QE); “Harmless" H 2 & CH 4 damage by ion back bombardment (greater issue for DC guns) JLAB 4
EMISSION NON-UNIFORMITY Environmental Conditions Can 31 Oct 01 – before 1 st cleaning QE • Erode low work function coatings • Deposit material that degrades performance • Damage the surface (ion bombardment) Re-cleaning / Reconditioning does not necessarily restore original performance • QE scans of LEUTL Photoinjector Mg Cathode Courtesy of John W. Lewellen, Argonne National Lab • Details: images from APS photoinjector. Blue = 2xYellow; pixels =10 micron^2; image = (300 pixels)^2 Operation: 6 Hz for 30 days (1.55E7 pulses total); macropulse = 1.5 µ s 5 Nov 2001 - after 1 st cleaning 4 Dec 2001 - after 1 st cleaning 10 Dec 2001 - after 2 nd cleaning JLAB 5
PHOTOCATHODE RESPONSE TIME 1.2 Pulse Shaping τ = 0.2 ps Emitted Current [a.u.] 1 • Optimal Shape for emittance: τ = 0.8 ps beer-can (disk-like) profile • Laser Fluctuations τ = 3.2 ps 0.8 occur (esp. for higher τ = 12.8 ps harmonics of drive laser) 0.6 • Fast response: laser hash reproduced 0.4 • Slow response: beer-can profile degraded • Optimal: 1 ps response time 0.2 0 Mathematical Model ( ω n = 2 π n/T ) 0 5 10 15 20 25 30 time [ps] ∑ ( ) ( ) = I o θ t ( ) θ T − t ( ) N c n cos ω n t I λ t n = 0 ( ) e T / τ − 1 ( ) + ω n τ sin ω n t ( ) ⎧ ⎡ ⎤ ⎦ e − t / τ cos ω n t t < T ( ) exp − t − s ⎡ ⎤ ⎪ ⎛ ⎞ ⎣ N ( ) = QE c n ∑ ∫ t ∝ ⎨ ⎜ ⎟ I e t I λ s ⎢ ⎥ ( ) e − t / τ ( ) e T / τ − 1 ⎝ ⎠ τ τ 2 1 + ω n τ −∞ ⎣ ⎦ t ≥ T ⎪ n = 0 ⎩ JLAB 6
FN AND RLD DOMAINS DOMAINS 10 0 FN ( Φ =4.4 eV) • RLD: Corrupted When Tunneling Field Near Barrier Maximum Is Non- 10 -1 Field [eV/Å] Field [eV/�] negligible FN ( Φ =2 eV) 4/3 ⎛ ⎞ 2 m Q 1/3 F < ⎜ ⎟ 10 � k B T RLD ( Φ =2 eV) 10 -2 ⎝ ⎠ Photocathode 10 -3 • FN: Corrupted When Barrier Maximum Is Too Close to Fermi Thermionic 10 -4 Level or Slope of ln( T(E) ) > ln( f(E) ) 400 800 1200 1600 ⇒ Maximum Field: βφ > 6 Temperature [K] F < 1 ( ) 2 4 Q Φ − 6 k B T For high intensity lasers incident on photocathodes, emission is ⇒ Minimum Field: c fn < 2 β NOT field OR thermal OR photo, F > 4 but it is ALL of these processes ( ) 2 m Φ k B T acting in concert. � JLAB 7
QUANTUM EFFICIENCY (3-D) 1 ∞ ∞ ( ) , T ρ , t ( ) ∫ ∫ J F ρ 2 πρ d ρ dt ⎡ ⎤ ⎣ ⎦ Quantum Efficiency is ratio of −∞ 0 q QE = total # of emitted electrons with 1 ∞ ∞ ( ) ∫ ∫ total # of incident photons I ρ , t 2 πρ d ρ dt h ω −∞ 0 Emitted Current J(F,T) Field significantly exaggerated to show detail • Photoemission component 10 0 λ ο = 1064 nm T(E); f(E) [10 16 #/cm 2 ] ∞ ( ) f E ( ) T(E); f(E) [10 16 #/cm 2 ] ∫ D E + h ω ; ρ dE 10 -1 q ( ) f λ I ρ ( ) h ω 1 − R 0 ∞ ( ) ∫ f E dE 10 -2 T(E) T(E+4h ν ) 0 • Thermal-field component (limit: RLD) T(E+3h ν ) 10 -3 q ∞ ( ) f E ( ) ∫ T(E+2h ν ) D E dE 2 π h 0 10 -4 ⇒ Richardson Eq. (High T, Low F) T(E+h ν ) f(E) ⇒ Fowler Nordheim (High F, Low T) 10 -5 12 14 16 18 20 22 To estimate local time-dependent current density as a Energy [eV] function of local temperature and field, we use: ( ) ( ) dy ) I λ ( t ) U β h ω − φ ( ) = ⎡ ⎤ “Fowler factor” ∫ x ⎣ ⎦ ln 1 + e y q ( ) = f λ ( U x J T , F , Φ h ω 1 − R −∞ [ ] U βµ ( ) ( ) x 1 − be ⎧ x ≤ 0 ax e ⎪ + A RLD T 2 exp − βφ [ ] = ⎨ 2 + π 2 1 ( ) ( ) − x 1 − be 6 − e − ax x > 0 ⎪ 2 x ⎩ ⎡ ⎤ βφ = Φ − 4 QF ⎦ / k B T e ⎣ JLAB 8
DISPENSER CATHODE AS PHOTOCATHODE Surface Interpore ≈ 6 µm; Grain Size ≈ W Image 4.5 µm; Pore Diam. ≈ 3 µm Top View [0.1 mm] 2 Side Ba View O Nd:Yag PROFILIMETRY DATA 1064 nm 0.6 Field Enhancement At Local Emission Sites (e.g., Hemisphere: β = 3) 0.4 Vertical [ µ m] 0.2 Scandate & Ba Dispenser Cathode 0.0 Work Function: 1.8 - 2.1 eV -0.2 DISPENSER CATHODES -0.4 • Used in radar & communications • Porous tungsten matrix w/ impregnates which diffuse to surface 0 20 40 60 80 • Radial [ µ m] Emitting region constantly renewed (self - r ejuvenating in situ ) • Robust and long - lived, can operate at elevated temperatures JLAB 9
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