Electron and Ion Sources - CERN · Electron and Ion Sources Electrons –– Photo CathodesPhoto Cathodes Quantum Efficiencyyp[ = Electrons/photon [ Q e(λ)) ] GaAs:Cs=17% , CsTe=12.4%

Electron and Ion SourcesElectron and Ion SourcesLayoutLayoutLayoutLayout

Electron SourcesElectron SourcesThermionicThermionicPhoto-CathodesChild-Langmuir Current Limitation

Ion SourcesIon SourcesPenning Ion SourceECR Ion SourceECR Ion SourceNegative Ions

Electron and Ion SourcesElectron and Ion SourcesElectronsElectrons Thermionic EmissionThermionic EmissionElectrons Electrons –– Thermionic EmissionThermionic Emission

When a material is When a material is h t d th l th t d th l theated, the electrons heated, the electrons energy distribution shifts energy distribution shifts from the zero from the zero t t F it t F i

T=0KT=1000K

1.5

2.0

2.5

temperature Fermi temperature Fermi distribution.distribution.

T=1000K T=2000K

nits

) 0.0

0.5

1.0

⎤⎡

0.01

0.1

1

10

Ele

ctro

ns (a

rb u

n

0 2 4 6 8

dE

kTEE

EhmdEEn

Fermi

e

⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢

⎣

⎡

⎟⎠⎞

⎜⎝⎛ −+

⎥⎦

⎤⎢⎣

⎡=

exp1

)2(4)( 3

2/3π

1E-7

1E-6

1E-5

1E-4

1E-3

eφwork

Free

E

Uwork

E

EFermi

0 2 4 6 81E-8

1E 7

Electron Energy (eV)METAL VACUUM


Therefore at high temperatures there is an ELECTRON CLOUD Therefore at high temperatures there is an ELECTRON CLOUD around the material. The current density can then be found by around the material. The current density can then be found by i t ti th il bl l t d th ii t ti th il bl l t d th iintegrating the available electrons and their energy.integrating the available electrons and their energy.

⎟⎠⎞

⎜⎝⎛ −⋅=

kTeUTAJ workexp2 This electron current is available

to be pulled off the surface…⎠⎝ kT

22-63

2

Am102.14 −×≈= Kh

kemA eπ

Richardson-Dushmann equationRev. Mod. Phys. 2, p382 (1930)This factor A is not achievedI ti3h In practice.

The current density is further increased by the The current density is further increased by the SchottkySchottky effect effect ––the electric field on the surface, used to extract the electrons, the electric field on the surface, used to extract the electrons, allows electron tunnelingallows electron tunnelingallows electron tunnelingallows electron tunneling

⎟⎠⎞

⎜⎝⎛×= − T

EJJ SDR

139exp

Where Es is in kV/cm => 15%for 1kV/cm @1000K

Uwork

E

EFermi⎠⎝ T

METAL VACUUM


A Uwork 100Cs

Cs/O/WMixed Oxide

Thoriated WLaB6

Acm-2K-2work

eV

W 60 4.54

W 3 2.63

10Ta

W

2 )W Thoriated

3 2.63

MixedOxide

0.01 10.1

1

W Thoriated W Mixed OxideCaesiumm

issi

on (A

cm-2

Cesium 162 1.81

Ta 60 4.12

C /O/W 0 003* 0 72* 500 1000 1500 2000 25001E-3

0.01

Caesium Ta Cs/O/W LaB6

E

Melting pointsCs: 301.6 K

Cs/O/W 0.003* 0.72*

LaB6 29 2.66

500 1000 1500 2000 2500

Temperature (K)

Ta: 3290 KW: 3695 KLaB6: ~2800 K (decomp)

*- A and work function depend on the Cs/O layer

Thickness and purity

Electron and Ion SourcesElectron and Ion SourcesElectronsElectrons A GunA GunElectrons Electrons –– A GunA Gun

CATHODE GRID BUCKING COILPUMPING PORT

CATHODE

ANODE

CATHODE GRID

ANODE

INSULATOR

Electron and Ion SourcesElectron and Ion SourcesElectronsElectrons Photo EmissionPhoto EmissionElectrons Electrons –– Photo EmissionPhoto Emission

The energy of an electron in a material can be increased above The energy of an electron in a material can be increased above the vacuum energy by absorbing photons the vacuum energy by absorbing photons -- photoelectric effect.photoelectric effect.PhotocathodePhotocathode

UE

UEEaUwork

METAL VACUUM

EFermi

Uwork

SEMI-COND VACUUM

EFermiEGAP

workworkc UeU

hc 8.1239==λ

METAL VACUUM SEMI COND VACUUM

aGAPaGAPc EEEE

hc+

=+

= 8.1239λ

Uwork (eV) λc (nm)W 4.5 275M 3 67 340

Eg+Ea (eV) λc (nm)GaAs 5.5 225Cs Te 3 5 350Mg 3.67 340

Cu 4.65 267Cs2Te ~3.5 350K2CsSb 2.1 590

Electron and Ion SourcesElectron and Ion SourcesElectronsElectrons Photo InjectorPhoto InjectorElectrons Electrons –– Photo InjectorPhoto Injector

Photo cathodes can produce bunch structure of the same Photo cathodes can produce bunch structure of the same length as the light pulselength as the light pulselength as the light pulse.length as the light pulse.Photo Injector Test Facility Photo Injector Test Facility -- ZeuthenZeuthen

RF Injection – 1.5GHzCs2Te Photo-Cathode

or Mo

j

262nm Laser

Pico-second pulses @ 1.5GHz

freq quadrupled

Δ=0.67ns

Electron and Ion SourcesElectron and Ion SourcesElectronsElectrons Photo CathodesPhoto CathodesElectrons Electrons –– Photo CathodesPhoto Cathodes

Quantum Efficiency = Electrons/photon [ QQuantum Efficiency = Electrons/photon [ Qee((λ) ]λ) ]y p [y p [ ee(( ))GaAs:Cs=17% , CsTe=12.4% , K2CsSb=29%, Cu~0.01%,

METALSMETALSIf desired can be almost “blind” to optical or infra redIf 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.At high optical powers, a plasma is formed.

SEMICONDUCTORSSEMICONDUCTORSCan find materials optical wavelengths with high quantum efficiency (cf Photo Cathode Tubes)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).Common material=Cs2Te (Cesium Telluride)– High Quantum 2 ( ) gefficiency & stable.

Electron and Ion SourcesElectron and Ion SourcesElectronsElectrons ChildChild Langmuir LawLangmuir LawElectrons Electrons –– ChildChild--Langmuir LawLangmuir Law

ChildChild--Langmuir law (3/2 power law) gives the limit of current that Langmuir law (3/2 power law) gives the limit of current that can be removed from a surface.can be removed from a surface.Need electric field to remove electrons from surfaceNeed electric field to remove electrons from surfaceNeed electric field to remove electrons from surface.Need electric field to remove electrons from surface.Electrons set up their own space charge field.Electrons set up their own space charge field.

These electrons create an electric field 1.4

V/Vo - No Space Charge V/Vo - With Space Charge V/Vo - Space Charge LimitedE No Space Charge;

CATHODE ANODE

ese e ect o s c eate a e ect c e dThat repels these electrons

0.8

1.0

1.2

E - No Space Charge E - With Space Charge E - Space Charge Limited

d/Vo

)

;

0 2

0.4

0.6

0.8

(V/V

o) o

r (Ed;

v0.0 0.2 0.4 0.6 0.8 1.0

0.0

0.2

x/d

2

2 ρ−=UdvJ ρ= 21 mvqU =

E0

2 εdxvJ ρ=

2mvqU =

0)0(;)(;0)0( ======dxxdUVdxUxU

Electron and Ion SourcesElectron and Ion SourcesElectronsElectrons ChildChild Langmuir LawLangmuir LawElectrons Electrons –– ChildChild--Langmuir LawLangmuir Law

Hence there is a MAXIMUM current density that can be extracted Hence there is a MAXIMUM current density that can be extracted f i lt df i lt dfor a given voltage and gap.for a given voltage and gap.

2

2/32/1

02

94

dVqJ LC ⎟

⎠⎞

⎜⎝⎛=− ε

d : Cathode to Anode distanceV : Cathode to Anode voltageq : particle charge

If th th dIf th th d d lt id lt i

29 dm ⎠⎝ q : particle chargem : particle massThis is not relativistic

2 5If the cathodeIf the cathode--anode voltage isanode voltage isvaried, so is the electrode current.varied, so is the electrode current.

If the cathodeIf the cathode--anode voltage isanode voltage is1.5

2.0

2.5

sity

(Acm

-2)

If the cathodeIf the cathode--anode voltage isanode voltage isZERO, no current is extractedZERO, no current is extracted--> Cathode Grid.> Cathode Grid.

0.0

0.5

1.0C

urre

nt D

en

0 2 4 6 8 10

Voltage over 1cm (kV)

Electron and Ion SourcesElectron and Ion SourcesIon SourcesIon Sources BasicsBasicsIon Sources Ion Sources -- BasicsBasics

An Ion Source requires an “ion production” region An Ion Source requires an “ion production” region q p gq p gand an “ion extraction” system.and an “ion extraction” system.In most (but not all) cases, ion production occurs In most (but not all) cases, ion production occurs in a plasmain a plasmain a plasma.in a plasma.

Electron and Ion SourcesElectron and Ion SourcesIon SourcesIon Sources BasicsBasicsIon Sources Ion Sources -- BasicsBasics

Plasma ProcessesPlasma Processes

Electron heatingElectron heating

Plasma confinement (electric and magnetic)Plasma confinement (electric and magnetic)

Collisions (eCollisions (e--e, ee, e--i, ii, i--e, ie, i--i + residual gas)i + residual gas)

Atomic processes (ionisation, excitation, disassociation, Atomic processes (ionisation, excitation, disassociation, recombination)recombination)eco b at o )eco b at o )

Surface physics (coatings + desorbtion, eSurface physics (coatings + desorbtion, e--emission)emission)

Mechanical processes (chamber heating+cooling, erosion)Mechanical processes (chamber heating+cooling, erosion)

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

Electron and Ion SourcesElectron and Ion SourcesPlasma Particle MotionPlasma Particle MotionPlasma Particle MotionPlasma Particle Motion

B

E

B

meB

eBmE

LL == ⊥ ωρ ,2

2BBEvdrift

×=

Electron and Ion SourcesElectron and Ion SourcesPlasma Particle MotionPlasma Particle MotionPlasma Particle MotionPlasma Particle Motion

2

2/1

2/12/1

2/3

2

2 ~12~~

Tm

mm

TeBEm

D p

p

epcL ⎟

⎟⎠

⎞⎜⎜⎝

⎛⎟⎟

⎠

⎞

⎜⎜

⎝

⎛⊥υρ

B cf: opposite to classicalenergy – velocity equation !

2/12⎟⎠⎞

⎜⎝⎛=

mEv⎠⎝ m

Electron and Ion SourcesElectron and Ion SourcesECR SourceECR Source Magnetic MirrorMagnetic MirrorECR Source ECR Source –– Magnetic MirrorMagnetic Mirror

A force acts in the opposite direction to the

Increasing B fieldy

B1F2

Increasing B field

x

B2F1

v

Vdrift

Energy is transferred

from Vdrift to Vecr

( )BKm

vdrift2 2/1

⎭⎬⎫

⎩⎨⎧ −= μ

Bmv2

2⊥=μ μ = magnetic moment

K = total kinetic energy

Electron and Ion SourcesElectron and Ion SourcesIon SourceIon Source Penning / PIGPenning / PIG 2mKvIon Source Ion Source –– Penning / PIGPenning / PIG

Penning or Philips Ionisation Gauge Penning or Philips Ionisation Gauge (PIG) source (PIG) source Gas Pressure 10-3 > 1 mbar

eV1@μm30

2

≈

== ⊥⊥

L

cL eB

mKv

ρω

ρ

CATHODEGas Pressure 10-3 -> 1 mbarArc Voltage ~1kVArc Current 0.1 -> 50 AMagnetic Field >0.1T ANODE

Cathode can be Hot or ColdCathode can be Hot or ColdElectrons are accelerated by the arc Electrons are accelerated by the arc voltage across the cathode sheath voltage across the cathode sheath layerlayer GAS

B

layer.layer.Magnetic field stops cathode Magnetic field stops cathode electrons reaching the anode (>0.1T electrons reaching the anode (>0.1T required).required).Some electrons strike the antiSome electrons strike the anti

GAS

Some electrons strike the antiSome electrons strike the anti--cathode.cathode.Otherwise they may oscillate in the Otherwise they may oscillate in the Penning Trap and ionise the gas.Penning Trap and ionise the gas.

High Current Supply High Voltage Supply

Electrons go to the anode by Electrons go to the anode by diffusion processes, plasma diffusion processes, plasma oscillations and the plasmaoscillations and the plasma--anode anode potential.potential.

Electron and Ion SourcesElectron and Ion SourcesIon SourceIon Source Penning / PIGPenning / PIGIon Source Ion Source –– Penning / PIGPenning / PIG

The Rutherford ISIS Penning The Rutherford ISIS Penning source source –– John ThomasonJohn Thomason

Electron and Ion SourcesElectron and Ion SourcesIon SourceIon Source ECRECRIon Source Ion Source –– ECRECR

Electron Cyclotron Resonance Ion Electron Cyclotron Resonance Ion S (ECR)S (ECR)Source (ECR)Source (ECR)

For a given magnetic field, nonFor a given magnetic field, non--relativistic electrons have a fixed relativistic electrons have a fixed revolution frequency.revolution frequency. ]kG[82]GH[ Bf

meB

ecr

×

=ωq yq y

The plasma electrons will absorb energy The plasma electrons will absorb energy at this frequency.at this frequency.If confined in a magnetic bottle, the If confined in a magnetic bottle, the electrons can be heated to the keV andelectrons can be heated to the keV and

]kG[8.2]GHz[ Bfce ×=

electrons can be heated to the keV and electrons can be heated to the keV and even MeV range.even MeV range.Ions also trapped by the charge of the Ions also trapped by the charge of the electrons, but for millielectrons, but for milli--seconds allowing seconds allowing mutliple ionisation.mutliple ionisation.mutliple ionisation.mutliple ionisation.The solenoid magnetic field still allows The solenoid magnetic field still allows losses on axis losses on axis –– these ions make the these ions make the beam.beam.


CERN ECR4 – Built by GANIL


Scan of Bending magnet Current -11/04/03 -JCh

No filament is needed, greatly No filament is needed, greatly increasing the source lifetime.increasing the source lifetime.

60

80

100

120

140

n Fa

rard

ay c

up 1

(µA

)

Pb27+ Pb25+

Pb26+ & O2+

Singly, multiply and highly Singly, multiply and highly charged ions can be produced by charged ions can be produced by these sources (although the these sources (although the

0

20

40

65 70 75 80 85 90 95 100Bending Magnet Current (A)

Cur

rent

in

Pb34+O3+source construction will influence source construction will influence

this).this).A A A+ A+ A2+ A2+ A3+ A3+ Stepwise ionisation.Stepwise ionisation.

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

14.5GHz Forward Power

Ion Current (In21+)

for uranium).for uranium).

In the afterglow mode, the ion In the afterglow mode, the ion intensity increases AFTER intensity increases AFTER switching off the microswitching off the micro waveswaves

0 1 2 3 4 5

Time (ms)switching off the microswitching off the micro--waves.waves.

Electron and Ion SourcesElectron and Ion SourcesIon SourcesIon Sources Negative IonsNegative IonsIon Sources Ion Sources –– Negative IonsNegative Ions

Negative ion sources allow charge exchange injection into Negative ion sources allow charge exchange injection into synchrotrons.synchrotrons.

Electron AffinityElectron Affinity(eV)(eV)

The bonding energy for an electron onto The bonding energy for an electron onto an atom is the Electron Affinitan atom is the Electron Affinit(eV)(eV)

HH 0.75420.7542HeHe <0<0

an atom is the Electron Affinity.an atom is the Electron Affinity.Ea < 0 for Noble GassesEa < 0 for Noble GassesLarge Ea for HalogensLarge Ea for HalogensT t i f ti iT t i f ti iLiLi 0.61820.6182

BeBe <0<0BB 0.2770.277

Two categories of negative ion sourcesTwo categories of negative ion sourcesSurface – an atom on a surface can be desorbed with an extra electron (whose wave-function overlapped the atom).V l Th h lli i t dCC 1.26291.2629

NN <0<0OO 1.4621.462

Volume – Through collisions, e-capture and molecular dissociation, negative ions can be formed.

AB + e → A- + B A + B → A- + B+FF 3.3993.399

AB e A B A B A B

AB* + e → A- + B A+ + B → A- + B2+

Electron and Ion SourcesElectron and Ion SourcesHH Surface Ion ProductionSurface Ion ProductionHH-- Surface Ion ProductionSurface Ion Production

CATHODE

CsCs

H+, H2+H-

e-e-e-

Protons from the plasma are accelerated to the cathode, Protons from the plasma are accelerated to the cathode, which has a coating of caesiumwhich has a coating of caesiumwhich has a coating of caesium.which has a coating of caesium.The protons desorbed from the low work function surface, The protons desorbed from the low work function surface, with an additional electron.with an additional electron.The plasma must not be too hot to avoid ionising the HThe plasma must not be too hot to avoid ionising the HThe plasma must not be too hot, to avoid ionising the HThe plasma must not be too hot, to avoid ionising the H--..Penning, Magnetron, etc, sources produce H this way.Penning, Magnetron, etc, sources produce H this way.

Electron and Ion SourcesElectron and Ion SourcesIon SourcesIon Sources Negative IonsNegative IonsIon Sources Ion Sources –– Negative IonsNegative Ions

CATHODE

ANODE

GAS +CAESIUMB

-

Electrons are extracted along with Electrons are extracted along with negative ions! Electron current negative ions! Electron current can be reduced with a dipole B can be reduced with a dipole B

Vextraction

Vbeam+

B

field in extraction.field in extraction. +

Electron and Ion SourcesElectron and Ion SourcesSummarySummarySummarySummary

Electron Source SummaryElectron Source SummaryThermionic Source. Some thermal electrons are above the Work-Function.Use low work-function or high melting point materials to obtain the most electronsPhoto-cathodes – Use photons above the work-function or Eg+Ea.M t l St bl b t h l t ffi iMetals – Stable but have a low quantum efficiencySemiconductors – high Q, but can be unstable and degrade in use.Require an field to extract electrons J ~ V3/2 / d2 .

Ion Source SummaryIon Source SummaryIon Source SummaryIon Source SummaryA vast array of ion source type. Using surfaces, sputtering, plasmas and different heating configurations.PIG/Penning – Cathode-Anode discharge in a magnetic field, where electrons oscillate in a plasma ionizing the rest gaselectrons oscillate in a plasma, ionizing the rest gas.ECR – Heating of electrons on the ECR resonance, producing a plasma. Electrons and ions are confined in a magnetic bottle. Confinement leads to multiple collisions and highly charged-ions.N ti i f l t ith hi h l t ffi it b d dNegative ions of elements with a high electron affinity can be produced. H- requires a warm plasma to excite H2. In a cooler plasma region, electron attachment and disassociation occurs.

Electron and Ion SourcesElectron and Ion SourcesFurther ReadingFurther ReadingFurther ReadingFurther Reading

HandbookHandbook ofof IonIon Source,Source, BB.. Wolf,Wolf, BocaBoca Raton,Raton, FLFL::CRCCRC Press,Press, 19951995IonIon Sources,Sources, ZhangZhang HuaHua Shun,Shun, BerlinBerlin:: Springer,Springer,1999199919991999..The Physics and Technology of Ion Source, I. G. The Physics and Technology of Ion Source, I. G. Brown, New York, NY: Wiley, 1989 Brown, New York, NY: Wiley, 1989 LargeLarge IonIon BeamsBeams:: FundamentalsFundamentals ofof GenerationGeneration andandPropagation,Propagation, TT.. AA ..Forrester,Forrester, NewNew York,York, NYNY:: Wiley,Wiley,19881988CASCAS –– 55thth GeneralGeneral SchoolSchool (CERN(CERN 9494--0101 )) andandCyclotrons,Cyclotrons, LinacsLinacs…… (CERN(CERN--9696--0202 ))

Electron and Ion Sources - CERN · Electron and Ion Sources Electrons –– Photo CathodesPhoto Cathodes Quantum Efficiencyyp[ = Electrons/photon [ Q e(λ)) ] GaAs:Cs=17% , CsTe=12.4%

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