The development of ultra bright electron sources: An overview

B.L. Militsyn

Accelerator Science and Technology Centre

Science & Technology Facility Council, UK

B.L. Militsyn, Ultra bright electron sources workshop, 29 June – 1July 2011, Daresbury, UK

Production of ultra bright electron beams. Outline of problems.

• Emission of fs electron bunches• Time structure definition and synchronisation• Acceleration with the preservation of bunch length

and emittance• Bunch compression• Transportation and manipulation• Characterisation

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B.L. Militsyn, Ultra bright electron sources workshop, 29 June – 1July 2011, Daresbury, UK

Emission of fs-scale electron beams

• Thermal emission – Medium brightness – High repetition rate – Poor controllability

• Photoemission from metal photocathodes– Medium brightness – Medium repetition rate– Good controllability– Generation of high power fs-scale laser pulses is required

• Field emission– High brightness, – Low repetition rate– Poor controllability

• Non-traditional electron emitters– Plasma emitters– ...

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Emission. Photoemitters. Metal photocathodes

B.L. Militsyn, Ultra bright electron sources workshop, 29 June – 1July 2011, Daresbury, UK

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F. Le Pimpec et al. , Workshop on Photocathodes for RF Guns, 1-2 March 2011, Lecce, Italy

• Very fast – few fs response time• Relatively high QE – 10-6 – 10-4

• Very robust at a vacuum of 10-10 mbar

Emission of high brightness beam. Demands to the cathode field.

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B.L. Militsyn, Ultra bright electron sources workshop, 29 June – 1July 2011, Daresbury, UK

Field strength

Technology 0.01 nC 0.1 nC 1.0 nC

10 MV/m DC gun 0.11 0.34 1.08

20 MV/m VHF gun 0.08 0.24 0.77

50 MV/m L-band gun 0.05 0.15 0.48

100 MV/m S-band gun 0.03 0.11 0.34

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B.L. Militsyn, Ultra bright electron sources workshop, 29 June – 1July 2011, Daresbury, UK

Acceleration technologies

• DC gun – Low field– Highly stability– Acceptable environment for photocathodes– Unlimited repetition rate (limited by the laser repetition rate and

photocathode thermal load)• NC RF guns

– High field– Medium stability– Relative low repetition rate

• SRF guns– Medium field– High repetition rate (limited by the RF frequency)

• Non-traditional accelerator schemes– Plasma wake-field acceleration

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B.L. Militsyn, Ultra bright electron sources workshop, 29 June – 1July 2011, Daresbury, UK

Acceleration. PITZ/FLASH L-band NCRF photocathode gun

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Distribution of accelerating RF and focusing magnetic field in the gun

B.L. Militsyn, Ultra bright electron sources workshop, 29 June – 1July 2011, Daresbury, UK

Transportation and manipulation

• Emittance preservation– Emittance compensation– Transportation– Precise beam optical system

• Velocity bunching– Require relative low energy– Linearity– High demands to phase stability of the bunching cavity

• Magnetic compression– High energy– CSR at high bunch charge– Physical length of the chicane

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Transportation and manipulation. MAX-IV high brightness injector.

B.L. Militsyn, Ultra bright electron sources workshop, 29 June – 1July 2011, Daresbury, UK

Periodic sectionState 156 cells

Output coupling cell

Input coupling cell

1.5-cell FERMI type S-band gun with Cu photocathode

5 m long S-band 2π/3 travelling wave linac

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MAX-IV high brightness injector. Emittance compensation scheme optimisation results

B.L. Militsyn, Ultra bright electron sources workshop, 29 June – 1July 2011, Daresbury, UK

Beam size (rms) 0.317 mmProjected emittance 0.396 mm mradAverage slice emittance 0.350 mm mradPeak current 19 ABunch length (rms) 1.79 psBunch length (full) 7.87 psEnergy spread (full) 1.59 MeVEnergy 104.2 MeV

Bunch charge 100 pCLaser spot diameter 1 mmLaser pulse width 5 psLaser rise/fall times 1 psInitial thermal energy 0.75 eVGun peak field 100 MV/mGun phase - 5 °Solenoid peak field 0.192 TLinac entrance position 1.85 mLinac peak field 26.25 MV/mLinac phase + 5 °

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B.L. Militsyn, Ultra bright electron sources workshop, 29 June – 1July 2011, Daresbury, UK

Beam characterisation

• Emittance– Projection and slice emittance <0.1 mm·mrad at a bunch charge of 1-10

pC• Bunch length

– Bunch length 10-100 fs• Energy spread

– Less than 10-4

• Arrival time– Precision of 10’s fs

• Slice parameters

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B.L. Militsyn, Ultra bright electron sources workshop, 29 June – 1July 2011, Daresbury, UK

Beam characterisation. PITZ diagnostic beam line

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J. W. Bähr et al., Recent upgrade of the PITZ facility,

2010 Beam Instrumentation Workshop

B.L. Militsyn, Ultra bright electron sources workshop, 29 June – 1July 2011, Daresbury, UK

Time structure definition and synchronisation

• Laser system– Laser pulse duration– Laser pulse repetition rate– Laser pulse stability– Temporal pulses structure– Spatial laser pulse structure– Laser pulse temporal stability relative to the master oscillator

• RF system– Amplitude stability– Phase stability relative to the master oscillator

• Synchronisation system– Temporal stability– Thermal stability– Etc.

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B.L. Militsyn, Ultra bright electron sources workshop, 29 June – 1July 2011, Daresbury, UK

Time structure definition and synchronisation. The LCLS laser system

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B.L. Militsyn, Ultra bright electron sources workshop, 29 June – 1July 2011, Daresbury, UK

Where we are?• We can deliver electron pulses from RF guns with

– A pulse duration of 300 fs and less– A bunch charge of 100 pC and more– Relative emittance of up to 0.4 mmmrad/mm

• We can generate UV laser pulses with – An energy of several mJ– Pulse duration of 10’s fs scale– Temporal stability of 200-300 fs

• We can compress the high energy bunches to a 10’s fs scale • We can accelerate and transport beams with emittance of 1

mmmrad scale• We can measure

– Emittance in 0.1 mmmrad range– Bunch length with a precision of 100’s fs

• We begin to develop non-traditional electron sources• We still have order(s) of magnitude to go!

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B.L. Militsyn, Ultra bright electron sources workshop, 29 June – 1July 2011, Daresbury, UK

Welcome to discussion!

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