Layout

Layout

Electrons Thermionic Emission (the maths)Conducting materials contain free electrons, who follow the Fermi-Dirac energy distribution inside the material.When a material is heated, the electrons energy distribution shifts from the zero temperature Fermi distribution.

7

EFermi

But we want to know what will be the current of electrons.Electron and Ion Sources2 non-integer spin particles cannot occupy same energy level (Pauli exclusion)Hence Fermi-Dirac equation gives the occupation of conduction electrons at different energy levels.Integrate the equation above the work-functionElectrons 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.

This electron current is availableto be pulled off the surfaceRichardson-Dushmann equationRev. Mod. Phys. 2, p382 (1930)This factor A is not achieved in practice (some electrons are reflected from the inner surface)

Electron and Ion SourcesAvailable electrons are those above the work function.A is a pseudo constant but NOT temperature dependent.3Electron SourcesThermionicPhoto-Cathodes

Ion SourcesParticle motion in plasmasProtonsECR Ion SourceNegative IonsRadioactive Ions

Electron and Ion SourcesElectron Sources - Basics4HTPower SupplyInsulatorCathode(Electron source)BeamE-fieldChamber

The classic Cathode Ray ExperimentElectron and Ion Sources5Electron SourcesThermionicPhoto-Cathodes

Ion SourcesParticle motion in plasmasProtonsECR Ion SourceNegative IonsRadioactive Ions

Electron and Ion SourcesElectrons Thermionic Emission6Electrons within a material are heated to energies above that needed to escape the material.Cathode emission is dominated by the Richardson Dushmann equation.

MaterialElectronsEnergyEnergy difference betweenhighest energy electron and vacuumWork Function fsElectron and Ion SourcesElectrons Thermionic EmissionAAcm-2K-2UworkeVW604.54W Thoriated32.63MixedOxide0.011Cesium1621.81Ta604.12Cs/O/W0.003*0.72*LaB6292.66*- A and work function depend on the Cs/O layerThickness and purity

Element melting point v work function for selected metals : Nature does not provide an ideal solution9Electron and Ion Sourceswww.peakpeak.com/~jdavis/boron/LaB.htm for LaB6 melting point - Richardson-Dushmann A for LaB6 from Charged Particle Beams book, chp7Metal melting point v work function from (http://www.lns.cornell.edu/~hoff/LECTURES/08S_688/08S_688_080225.pdf)Very hot cathodes can lead to a thermal limited emittance.

10Electron SourcesThermionicPhoto-Cathodes

Ion SourcesParticle motion in plasmasProtonsECR Ion SourceNegative IonsRadioactive Ions

Electron and Ion SourcesElectrons Photo EmissionThe energy of an electron in a material can be increased above the vacuum energy by absorbing photons - photoelectric effect.

Uwork (eV)lc (nm)W4.5275Mg3.67340Cu4.65267UworkMETALVACUUMEEFermiUworkSEMI-CONDVACUUMEEFermiEGAPEa

Eg+Ea (eV)lc (nm)GaAs5.5225Cs2Te~3.5350K2CsSb2.159011Electron and Ion SourcesK2CsSb is bialkali, used as the photo cathode in photo multiplier tubes Sb=Antimony. K=Potassium.Cs2Te requires UV laser (highest powers are not yet available), Cs:GaAs and K2CsSb can work with green lasers.Electrons Photo CathodesQuantum Efficiency = Electrons/photon [ Qe(l) ]GaAs:Cs=17% , CsTe=12.4% , K2CsSb=29%, Cu~0.01%, Strongly wavelength dependent.12

lc =590 nmElectron and Ion SourcesK2CsSb is bialkali (spelling) Looks excellent, but low lifetime and difficult to use. Sb=Antimony.

13Electrons Photo CathodesMETALSLower quantum efficiency requires high power lasers.But at high optical powers, a plasma is formed.Very robust and simple to use cathode material.SEMICONDUCTORSCan 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).Cs2Te (Cesium Telluride) High Quantum efficiency but needs UV lasers.Electron and Ion SourcesTechnology of photocathode production, Braem, Joram, Piuz, Schyns, Seguinot.http://www-nds.iaea.org/publications/indc/indc-nds-0322.pdf

CTF3 Electron Guns14

CTF3 has three electron guns.1. A thermionic Gun for the drive beam generation2. 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.Electron and Ion Sourceshttp://www-pnp.physics.ox.ac.uk/~jaiweb/slides/2012_Doebert.pdfhttp://project-clic-cdr.web.cern.ch/project-CLIC-CDR/CDR_Volume1.pdfCTF3 Thermionic Gun15

CathodeAnodeInsulatorCathode GridBucking CoilPumping Port-140 kV

A cathode + grid

Disclaimer: These are not actual CTF3 systemsElectron Current5-10 AElectron Energy 140 keVEmittance15-20 mm.mradPulse1.4us @ 5 HzSintered tungsten dispenser cathode.Ohmic heating.16mm diameter.

Electron and Ion SourcesCTF3 Thermionic Gun bunching the beam16

The thermionic gun produces a 1.5us pulse of electrons.RF cavities are then used to produce bunches, which can lead to transverse emittance growth.

RF bunchingElectron and Ion Sourceshttp://www-pnp.physics.ox.ac.uk/~jaiweb/slides/2012_Doebert.pdf

CTF3 CALIFES probe beam photo gun17

Photo cathode

The RF gun accelerates to 5MeV in ~15cm, which combats space charge forces.The short laser pulses (~6ps) generate short electron bunches from the CsTe photo cathode.The laser can pulse at a different harmonic of the RF system. 1.5GHz laser-electron bunches are created, using RF 3GHz acceleration.

Nd:YLF 4x frequency -> UV Electron and Ion SourcesCONSTRUCTION OF THE PROBE BEAM PHOTO-INJECTOR OF CTF3, Proceedings of EPAC 2006, Edinburgh, Scotland, MOPLS114See also https://indico.cern.ch/getFile.py/access?contribId=28&resId=22&materialId=poster&confId=177701

CTF3 CALIFES RF Photo injector18

Electron Current0.9 AElectron Energy 5-6 MeVEmittance20 mm.mradPulse150ns @ 5 HzElectron and Ion SourcesStatus and progress of the CTF3 probe beam, http://indico.cern.ch/getFile.py/access?contribId=26&sessionId=1&resId=2&materialId=slides&confId=45580

CTF3 Photo Emission and you need a laser19

Electron and Ion Sourceshttps://indico.cern.ch/getFile.py/access?contribId=28&resId=22&materialId=poster&confId=177701

20Electron SourcesThermionicPhoto-Cathodes

Ion SourcesParticle motion in plasmasProtonsECR Ion SourceNegative IonsRadioactive Ions

Electron and Ion SourcesIon Sources - BasicsAn Ion Source requires an ion production region and an ion extraction system.In most (but not all) cases, ion production occurs in a plasma.21

A plasma or discharge chamberMaterial inputPower to create a plasma / dischargeA hole to let the ions out!An extraction systemElectron and Ion SourcesIon Sources - Basics

22Electron and Ion SourcesIon Sources - Basics

23Hydrogen plasma (for protons or H-) from an RF source.Hydrogen plasma emits a pink light from an atomic transition. Electron and Ion SourcesIon Sources - BasicsPlasma Processes

Ion Source Goal -> Optimise these processes to produce the required ion type and pulse parameters.AND maximize reliability, minimize emittance, power and material consumption.

Electron heatingPlasma 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)24Electron and Ion Sources25Electron SourcesThermionicPhoto-Cathodes

Ion SourcesParticle motion in plasmasProtonsECR Ion SourceNegative IonsRadioactive Ions

Electron and Ion SourcesPlasma Particle Motion

BEB

26Electron and Ion SourcesPlasma Particle MotionB

cf: opposite to classical energy velocity equation !27Electron and Ion SourcesECR Source Magnetic MirrorB1B2F1F2vA force acts in the opposite direction to theIncreasing B field VdriftEnergy is transferredfrom Vdrift to Vecr

= magnetic momentK = total kinetic energyxy28Electron and Ion Sources29

R. Rejoub, B. G. Lindsay, and R. F. Stebbings, Phys. Rev. A 65, 042713 (2002)Except H: Y.-K. Kim and M.E. Rudd, Phys. Rev. A 50, 3954 (1994)Ionization Cross Section by Electron ImpactIn 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.

Some ion sources will use photo-ionization, or surface interactions.

Electron and Ion SourcesIon Sources - BasicsIon Sources at CERN.

Linac2 Protons - DupolasmatronLinac3 Ions (Pb, O, Ar) ECRISOLDE Radioactive ions Surface, laser, Electron Bombardment.Linac4 Negative Hydrogen RF30Electron and Ion SourcesIon Source Gas Discharge31Many sources work on the principle of a cathode anode gas dischargeThe gas can be a compound form (e.g. Carbon from CO) or from a vapour (e.g. lead vapour from an oven).Electrons from a hot cathode are accelerated into the gas by a cathode to anode voltage, and ionize the gas atoms/molecules with electron impact ionization.At low gas pressures, most electrons do not cause ionization and the ion density remains low.At higher pressures, the electrons cause ionization, which also leads to new electrons to be accelerated and cause ionization.

Electron and Ion SourcesMagnetic = 2 ROLES. Stop electrons to walls. Increase path length

32

By applying an magnetic field, electrons can have longer path lengths inside the source, and the chance of ionization is increased.

Electron and Ion Sources33Electron SourcesThermionicPhoto-Cathodes

Ion SourcesParticle motion in plasmasProtonsECR Ion SourceNegative IonsRadioactive Ions

Electron and Ion SourcesIon Source Duoplasmatron Linac234

SolenoidCathodeAnodeGas feedExpansion CupProton Current200 mAProton Energy 90 keVEmittance~0.4 mm.mradPulse for LHC20us @ 1 Hz# protons / pulse2.5x1013# LHC bunches~24 ** Creation of LHC bunches is a complicated process, this is an example for 50ns LHC bunchesElectron and Ion SourcesMagnetic = 2 ROLES. Stop electrons to walls. Increase path length.

35Electron SourcesThermionicPhoto-Cathodes

Ion SourcesParticle motion in plasmasProtonsECR Ion SourceNegative IonsRadioactive Ions

Electron and Ion SourcesIon Source ECR Linac3Electron Cyclotron Resonance Ion Source (ECR)For a given magnetic field, non-relativistic electrons have a fixed revolution frequency.The plasma electrons will absorb energy at this frequency (just as particles in a cyclotron).If confined in a magnetic bottle, the electrons can be heated to the keV and even MeV range.Ions also trapped by the charge of the electrons, but for milli-seconds allowing mutliple ionisation.The solenoid magnetic field still allows losses on axis these ions make the beam.

36FrfFrfElectron orbit RF period laterElectron and Ion SourcesAfter Fecr go to drawing

Ion Source ECR

CERN ECR4 Built by GANIL

37Electron and Ion SourcesIon Source ECR High charge states

No filament is needed, greatly increasing the source lifetime.

Singly, multiply and highly charged ions can be produced by these sources (although the source construction will influence this).A A+ A2+ A3+ Stepwise ionisation.

Gaseous ions are easily made. Metallic ions come from an OVEN or from a compound gas (e.g UF6 for uranium).

In the afterglow mode, the ion intensity increases AFTER switching off the micro-waves.

38Electron and Ion Sources39

Ion Source ECR High charge states + industry solutionsPlasma density increases with frequency and associated magnetic field. Example: VENUS source and Berkeley, Ca, uses superconducting solenoid and sextapole magnets.

Industry can now provide turnkey solutions for ECR ions sources, usually using permanent magnets.

Electron and Ion Sources40ECR What happens when the oven is refilled

Electron and Ion Sources41Electron SourcesThermionicPhoto-Cathodes

Ion SourcesParticle motion in plasmasProtonsECR Ion SourceNegative IonsRadioactive Ions

Electron and Ion SourcesIon Sources Negative IonsNegative ion sources allow: Charge exchange injection into synchrotrons. Charge exchange extraction from cyclotrons. Tandem accelerators.Electron Affinity(eV)H0.7542He