Electron and Ion Sources Layout Electron Sources Thermionic Photo-Cathodes Child-Langmuir Current Limitation Ion Sources Particle motion in plasmas Penning Ion Source ECR Ion Source Negative Ions Richard Scrivens, BE Dept, CERN. CAS, Varna, September 2010 1
Electron and Ion Sources Electrons – Thermionic Emission Conducting materials contain free electrons, who follow the Fermi-Dirac E Fermi 2.5 energy distribution inside the material. 2.0 When a material is heated, the 1.5 electrons energy distribution shifts T=0K T=1000K from the zero temperature Fermi 1.0 T=2000K distribution. 0.5 Free Electrons (arb units) 0.0 π 3 / 2 4 ( 2 m ) E = e n ( E ) dE dE 0 2 4 6 8 − 3 E E h 10 + Fermi 1 exp 1 kT 0.1 Electrons 0.01 above the 1E-3 E work 1E-4 U work 1E-5 e φ function E Fermi 1E-6 work energy, can 1E-7 be removed 1E-8 0 2 4 6 8 from the METAL VACUUM Electron Energy (eV) material. 2
Electron and Ion Sources Electrons – Thermionic Emission Therefore at high temperatures there is an ELECTRON CLOUD around the material. The current density can then be found by integrating the available electrons and their energy. J = nve This electron current is available − eU to be pulled off the surface… = ⋅ 2 work J A T exp Richardson-Dushmann equation kT Rev. Mod. Phys. 2, p382 (1930) π 2 4 em k − = ≈ × 6 - 2 2 This factor A is not achieved e A 1 . 2 10 Am K 3 h In practice. The current density is further increased by the Schottky effect – the electric field on the surface, used to extract the electrons, allows electron tunneling Where E s is in kV/cm => 15% for 1kV/cm @1000K E 139 E = × S J J exp U work − R D E Fermi T METAL VACUUM 3
Electron and Ion Sources Electrons – Thermionic Emission LaB 6 Cs/O/W Thoriated W Mixed Oxide Cs 100 Ta 10 A U work W Acm -2 K -2 eV -2 ) Emission (Acm 1 W 60 4.54 W Thoriated W 0.1 Mixed Oxide W 3 2.63 Caesium Thoriated Ta 0.01 Cs/O/W LaB6 Mixed 0.01 1 1E-3 Oxide 500 1000 1500 2000 2500 Temperature (K) Cesium 162 1.81 Element melting point v work 4000 function for selected metals : 3500 Element Melting Point (C) Ta 60 4.12 Nature does not provide an ideal solution 3000 2500 Cs/O/W 0.003* 0.72* 2000 LaB 6 29 2.66 1500 1000 500 0 *- A and work function depend on the Cs/O layer 1 2 3 4 5 6 Thickness and purity Work Function (eV) 4
Electron and Ion Sources Electrons – A Gun PUMPING PORT BUCKING COIL CATHODE GRID CATHODE ANODE INSULATOR 5
Electron and Ion Sources Electrons – Photo Emission The energy of an electron in a material can be increased above the vacuum energy by absorbing photons - photoelectric effect. E E E a U work U work E Fermi E GAP E Fermi SEMI-COND METAL VACUUM VACUUM hc 1239 . 8 hc 1239 . 8 λ = = λ = = c + + c eU U E E E E work work GAP a GAP a λ c (nm) λ c (nm) E g +E a (eV) U work (eV) GaAs 5.5 225 W 4.5 275 Cs:GaAs * * Mg 3.67 340 Cs 2 Te ~3.5 350 Cu 4.65 267 K 2 CsSb 2.1 590 Cs:GaAs – Surface Caesiated GaAs can be used with 532nm radiation. Requires 6 Recaesiation after a few hundred C extraction.
Electron and Ion Sources Electrons – Photo Cathodes Quantum Efficiency = Electrons/photon [ Q e ( λ) ] GaAs:Cs=17% , CsTe=12.4% , K2CsSb=29%, Cu~0.01%, METALS If desired, can be almost-“blind” to optical or infra-red. Using the thermal electrons above the Fermi Energy, can make a very low current source using optical wavelengths. At high optical powers, a plasma is formed. SEMICONDUCTORS Can find materials optical wavelengths with high quantum efficiency (cf Photo Cathode Tubes). Difficult to use in a high radiation area of an electron-gun (x- rays and ions cause decomposition and surface damage). GaAs:Cs has high QE at 532nm – High power lasers available. Cs surface not suited to RF guns. Cs 2 Te (Cesium Telluride)– High Quantum efficiency but needs UV lasers. 7
Electron and Ion Sources Electrons – Photo Injector Photo cathodes can produce bunch structure of the same length as the light pulse. Photo Injector Test Facility - Zeuthen RF Injection – 1.3GHz Cs 2 Te Photo-Cathode or Mo ∆ =0.77ns 262nm Laser Pico-second pulses @ 1.3GHz freq quadrupled with LBO and BBO crystals 8
Electron and Ion Sources Cornell DC Photoemission gun. laser = 520nm, 1.3GHz Cathode Cs:GaAs 9
Electron and Ion Sources Limitations in Emittance Thermal Emittance: Electron and ion source have a minimum emittance that can be produced, due to the excess thermal energy of the particles before they are brought into vacuum. ε = βγσ σ n x ' x γ β σ σ ( m c ) ε = 0 x ' x n m c 0 β m σ Is the transverse momentum. Can be assess for c 0 x ' particle sources. Normalised emittance for photoelectrons 2 E kin ε = σ E kin : Electron excess kinetic energy th laser 2 3 m c σ laser : Laser beam spot 0 Typically values for the thermal emittance are 0.1 – 1 mm.mrad Can use λ laser to change E kin . But E kin and high Q e are not compatible. 10
Electron and Ion Sources Electrons – Child-Langmuir Law Child-Langmuir law (3/2 power law) gives the limit of current that can be removed from a surface. Need electric field to remove electrons from surface. Electrons set up their own space charge field. V/Vo - No Space Charge V/Vo - With Space Charge V/Vo - Space Charge Limited 1.4 These electrons create an electric field ; E - No Space Charge E - With Space Charge That repels these electrons 1.2 E - Space Charge Limited CATHODE ANODE 1.0 (V/Vo) or (Ed/Vo) 0.8 ; 0.6 0.4 0.2 v 0.0 0.0 0.2 0.4 0.6 0.8 1.0 ρ x/d 2 d U 1 mv = − = ρ qU = 2 J v ε 2 dx 2 0 = dU ( x 0 ) = = = = = E U ( x 0 ) 0 ; U ( x d ) V ; 0 11 dx
Electron and Ion Sources Electrons – Child-Langmuir Law Hence there is a MAXIMUM current density that can be extracted for a given voltage and gap. d : Cathode to Anode distance 1 / 2 3 / 2 4 2 q V V : Cathode to Anode voltage = ε J − C L 0 2 q : particle charge 9 m d m : particle mass This is not relativistic 2.5 If the cathode-anode voltage is varied, so is the electrode current. 2.0 -2 ) Current Density (Acm 1.5 If the cathode-anode voltage is 1.0 ZERO, no current is extracted -> Cathode Grid. 0.5 0.0 0 2 4 6 8 10 Voltage over 1cm (kV) 12
Electron and Ion Sources Ion Sources - Basics An Ion Source requires an “ion production” region and an “ion extraction” system. In most (but not all) cases, ion production occurs in a plasma. 13
Electron and Ion Sources Ion Sources - Basics Plasma Processes Electron heating Plasma confinement (electric and magnetic) Collisions (e-e, e-i, i-e, i-i + residual gas) Atomic processes (ionisation, excitation, disassociation, recombination) Surface physics (coatings + desorbtion, e-emission) Mechanical processes (chamber heating+cooling, erosion) Ion Source Goal -> Optimise these processes to produce the required ion type and pulse parameters. AND maximize reliability, minimize emittance, power and material consumption. 14
Electron and Ion Sources Plasma Particle Motion E B B × ⊥ ω 2 mE eB E B ρ = = = v drift , L L 2 B eB m 15
Electron and Ion Sources Plasma Particle Motion 2 1 / 2 1 / 2 2 m E m 1 m ⊥ ρ υ p 2 p e D ~ ~ ~ L c 3 / 2 1 / 2 eB T m T p cf: opposite to classical B energy – velocity equation ! 1 / 2 2 E = v m 16
Electron and Ion Sources ECR Source – Magnetic Mirror A force acts in the opposite direction to the y Increasing B field x F2 B1 F1 B2 v Vdrift 1 / 2 2 ( ) = − µ v drift K B Energy is transferred m from Vdrift to Vecr 2 µ = magnetic moment mv µ = ⊥ 2 B K = total kinetic energy 17
Electron and Ion Sources 2 mK Ion Source – Penning / PIG v ρ = = ⊥ ⊥ ω L eB Penning or Philips Ionisation Gauge c ρ ≈ μm 30 @ 1 eV (PIG) source L CATHODE Gas Pressure 10 -3 -> 1 mbar Arc Voltage ~1kV Arc Current 0.1 -> 50 A ANODE Magnetic Field >0.1T B Cathode can be Hot or Cold Electrons are accelerated by the arc voltage across the cathode sheath layer. GAS Magnetic field stops cathode electrons reaching the anode (>0.1T required). Some electrons strike the anti- cathode. Otherwise they may oscillate in the High Current Supply High Voltage Supply Penning Trap and ionise the gas. Electrons go to the anode by diffusion processes, plasma oscillations and the plasma-anode 18 potential.
Electron and Ion Sources Ion Source – Penning / PIG The Rutherford ISIS Penning source – John Thomason 19
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