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Electron and Ion Sources Layout Electron and Ion Sources Layout • Electron Sources o Thermionic o Photo-Cathodes • Ion Sources o Particle motion in plasmas o Protons o ECR Ion Source o Negative Ions Richard Scrivens, BE Dept, CERN. CAS@CERN, February 2017 1 Electron and Ion Sources Every accelerator chain needs a source! 2 Electron and Ion Sources Every accelerator chain needs a source! Protons Ions Principles of the electron guns, with thermionic and photo Principles of ion sources, and the cathodes types used at CERN. 3 Electron and Ion Sources • Electron Sources o Thermionic o Photo-Cathodes • Ion Sources o Particle motion in plasmas o Protons o ECR Ion Source o Negative Ions o Radioactive Ions 4 Electron and Ion Sources Electron Sources - Basics Insulator Chamber E-field Beam Cathode (Electron source) HT Power Supply The classic Cathode Ray Experiment 5 Electron and Ion Sources • Electron Sources o Thermionic o Photo-Cathodes • Ion Sources o Particle motion in plasmas o Protons o ECR Ion Source o Negative Ions o Radioactive Ions 6 Electron and Ion Sources Electrons – Thermionic Emission Electrons within a material are heated to energies above that needed to escape the material. Cathode emission is dominated by the Richardson Dushmann equation. Energy difference Energy between highest energy electron and vacuum Electrons Work Function fs Material 7 Electron and Ion Sources Electrons – Thermionic Emission (the maths) Conducting materials contain free electrons, who follow the Fermi-Dirac These electrons can energy distribution inside the material. escape the material When a material is heated, the electrons 8 energy distribution shifts from the zero 8 T=2000K T=1000K temperature Fermi distribution. T=0K 6 6 Fermi work E f e 4 4 (eV) Energy Electron 3/ 2 4 (2me ) E 2 2 n(E)dE 3 dE h E EFermi 1 exp kT 0 0 1 10 0.1 0.0 0.5 1.0 1.5 2.0 2.5 0.01 1E-8 1E-7 1E-6 1E-5 1E-4 1E-3 Numberunits) of (arb Free ElectronsElectrons Free 8 Electron and Ion Sources Electrons – Thermionic Emission (the maths) 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 nev This electron current is available eU to be pulled off the surface… J AT 2 exp work Richardson-Dushmann equation kT Rev. Mod. Phys. 2, p382 (1930) 4em k 2 This factor A is not achieved in A e 1.2106 Am-2K 2 h3 practice (some electrons are reflected from the inner surface) Electron and Ion Sources Electrons – Thermionic Emission Cs/O/W LaB6 Thoriated W Cs Mixed Oxide 100 A Uwork Ta Acm-2K-2 eV 10 W ) W 60 4.54 -2 1 W 3 2.63 W 0.1 Thoriated W Thoriated Mixed Oxide Caesium Emission (AcmEmission Ta 0.01 Mixed 0.01 1 Cs/O/W LaB6 Oxide 1E-3 Cesium 162 1.81 500 1000 1500 2000 2500 4000 Temperature (K) 3500 Element melting point v work Ta 60 4.12 function for selected metals : 3000 Nature does not provide an Cs/O/W 0.003* 0.72* 2500 ideal solution 2000 LaB6 29 2.66 1500 1000 Element Melting Point (C) 500 0 *- A and work function depend on the Cs/O layer 1 2 3 4 5 6 Thickness and purity Work Function (eV) 10 Electron and Ion Sources • Electron Sources o Thermionic o Photo-Cathodes • Ion Sources o Particle motion in plasmas o Protons o ECR Ion Source o Negative Ions o Radioactive Ions 11 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 Ea Uwork Uwork EFermi EGAP EFermi METAL VACUUM SEMI-COND VACUUM hc Photon Energy hc 1239.8 hc 1239.8 c c eU work U work EGAP Ea EGAP Ea Uwork (eV) c (nm) Eg+Ea (eV) c (nm) W 4.5 275 GaAs 5.5 225 Mg 3.67 340 Cs2Te ~3.5 350 Cu 4.65 267 K2CsSb 2.1 590 12 Electron and Ion Sources Electrons – Photo Cathodes Quantum Efficiency = Electrons/photon [ Qe() ] GaAs:Cs=17% , CsTe=12.4% , K2CsSb=29%, Cu~0.01%, Strongly wavelength dependent. c =590 nm 13 Electron and Ion Sources Electrons – Photo Cathodes • METALS o Lower quantum efficiency requires high power lasers. o But at high optical powers, a plasma is formed. o Very robust and simple to use cathode material. • SEMICONDUCTORS o Can find materials optical wavelengths with high quantum efficiency (cf Photo Cathode Tubes). o Difficult to use in a high radiation area of an electron-gun (x-rays and ions cause decomposition and surface damage). o Cs2Te (Cesium Telluride) – High Quantum efficiency but needs UV lasers. 14 Electron and Ion Sources CTF3 – Electron Guns CTF3 has three electron guns. 1. A thermionic Gun for the drive beam generation 2. A test photo-emission and RF gun as a test facility for the drive beam. 3. A photo-emission and RF gun for the probe beam. 15 Electron and Ion Sources CTF3 Thermionic Gun Sintered tungsten dispenser cathode. Electron Current 5-10 A Ohmic heating. Pumping Port 16mm diameter. Electron Energy 140 keV Cathode Grid Emittance 15-20 mm.mrad Bucking Coil Pulse 1.4us @ 5 Hz Cathode -140 kV A cathode + grid Insulator Anode Disclaimer: These are not actual CTF3 systems 16 Electron and Ion Sources CTF3 Thermionic Gun – bunching the beam RF bunching o The thermionic gun produces a 1.5us pulse of electrons. o RF cavities are then used to produce bunches, which can lead to transverse emittance growth. 17 Electron and Ion Sources CTF3 – CALIFES – probe beam photo gun Nd:YLF – 4x frequency -> UV o The RF gun accelerates to 5MeV in ~15cm, which combats space charge forces. Photo cathode o The short laser pulses (~6ps) generate short electron bunches from the CsTe photo cathode. o The laser can pulse at a different harmonic of the RF system. 1.5GHz laser-electron bunches are created, using RF 3GHz acceleration. 18 Electron and Ion Sources CTF3 – CALIFES – RF Photo injector Electron Current 0.9 A Electron Energy 5-6 MeV Emittance 20 mm.mrad Pulse 150ns @ 5 Hz 19 Electron and Ion Sources CTF3 – Photo Emission … and you need a laser… 20 Electron and Ion Sources • Electron Sources o Thermionic o Photo-Cathodes • Ion Sources o Particle motion in plasmas o Protons o ECR Ion Source o Negative Ions o Radioactive Ions 21 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. A plasma or discharge chamber A hole to let the ions out! Material input Power to create a plasma / discharge An extraction system 22 Electron and Ion Sources Ion Sources - Basics 23 Electron and Ion Sources Ion Sources - Basics o Hydrogen plasma (for protons or H-) from an RF source. o Hydrogen plasma emits a pink light from an atomic transition. 24 Electron and Ion Sources Ion Sources - Basics • Plasma Processes o Electron heating o Plasma confinement (electric and magnetic) o Collisions (e-e, e-i, i-e, i-i + residual gas) o Atomic processes (ionisation, excitation, disassociation, recombination) o Surface physics (coatings + desorbtion, e-emission) o 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. 25 Electron and Ion Sources • Electron Sources o Thermionic o Photo-Cathodes • Ion Sources o Plasmas and their particle’s motion o Protons o ECR Ion Source o Negative Ions o Radioactive Ions 26 Electron and Ion Sources The recipe for ions • In many ion sources we use electron impact ionization. • We need to create electrons, accelerate them to a few times the ionization potential of the material, and get them to interact with atoms. Ionization Cross Section by Electron Impact -15 10 H He Ne Ar Some ion sources will use -16 Kr 10 photo-ionization, or surface Xe interactions. ) 2 -17 10 CrossSection (cm R. Rejoub, B. G. Lindsay, and R. F. -18 10 Stebbings, Phys. Rev. A 65, 042713 (2002) Except H: Y.-K. Kim and M.E. Rudd, Phys. Rev. A 50, 3954 (1994) -19 10 1 2 3 10 10 10 Electron Energy (eV) 27 Electron and Ion Sources Controlling the ions • In order to control the electrons and ions, we make use of magnetic and electric fields to alter their paths. 28 Electron and Ion Sources Plasma Particle Motion E B B 2mE eB E B , vdrift c eB c m B2 29 Electron and Ion Sources Plasma Particle Motion 2 1/ 2 1/ 2 2mp E 1 m m D ~ 2 ~ e ~ p c c eB T 3/ 2 m T 1/ 2 B p cf: opposite to classical energy – velocity equation ! 1/ 2 2E v m 30 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 Energy is transferred vdrift K B) m from Vdrift to Vecr 2 mv = magnetic moment 2B K = total kinetic energy 31 Electron and Ion Sources Ion Source – Gas Discharge • Many sources work on the principle of a cathode – anode gas discharge • The gas can be a compound form (e.g. Carbon from CO) or from a vapour (e.g. lead vapour from an oven).
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