An RF photogun for external injection of electrons in a Laser Wakefield Accelerator

Seth Brussaard

29-06-2011Daresbury 2 2

People

Xavier Stragier Marnix van der Wiel (AccTec)

Willem op ‘t Root Jom LuitenWalter van Dijk Seth BrussaardWalter Knulst (TUDelft)Fred Kiewiet

Eddy Rietman Bas van der Geer (Pulsar Physics)

Ad Kemper Marieke de Loos (TU/e & Pulsar)Harry van DoornIman KooleJolanda van de Ven

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Outline

• Laser Wakefield Acceleration

• External Injection

• RF Photogun Design

• RF Photogun Performance

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Laser Wakefield Acceleration

Accelerating Fields:100-1000 GV/m

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Injection

0 -

50plasma

laserplasma

min 10

max 1000

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How many electrons can we get in?

What will come out?

External Injection

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Setup

RF- photogun

Parabolic mirror

Solenoid (focusing electron bunch)

Plasma channel

Incoming laser pulse:300 mJ, 200 ps , 800 nm

Compressed laser pulse:150 mJ, 50 fs, 800 nm

UV-pulse for photogun: 266-400 nm

1.2 meter

8

RF Photoguns

Our approach:Emittance growth due to non-linear acceleration fields:

• full cylindrical symmetry• no tuning plungers• on-axis RF coupling

single-diamond turning

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RF Photogun

Coaxial S-band input coupler: scaled down version L-band design DESY

10

RF Photoguns

Approach:Emittance growth due to non-linear acceleration fields:

• full cylindrical symmetry• no tuning plungers• on-axis RF coupling

single-diamond turning

2nd generation:• Elliptical irises

– Highest field strength on cathode;• Cavity parts are clamped, not braized

– Easily replaced;• Copper cavity inside stainless vacuum can.

11

RF Photoguns

cathode platefirst (half) cell

second cell

Clamped construction:cavity parts

12

RF Photoguns

Clamped construction:cavity parts single-diamond turning

13

RF Photoguns

Clamped construction:cavity inside stainless steel vacuum can

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RF Photogun

Cavity mounted inside main magnet:

15

RF Photoguns

RF characterization: resonances

f0=2.9980 GHz

-mode

0-mode

f0=2.9918 GHz

16

RF Photoguns

RF characterization: on axis field profile

0

0,2

0,4

0,6

0,8

1

0 20 40 60 80 100

z (mm)

E/Em

ax

1711-02-2011ZFEL Workshop

RF Photoguns

High power RF commissioning:

• 80 MV/m at cathode (after one month of training)• Still occasional breakdown• 3 MeV electrons• QE ≈ 3·10-5 → bunch charge Qmax ≈ 300 pC

Conclusion: clamping is OK!

1811-02-2011ZFEL Workshop

RF Photoguns

• Water cooling for 1 kHz PRF

• Presently operating @ 100 Hz (limited by Modulator/Klystron)

19

Emittance

Quadrupole scan:

20

Emittance

Quadrupole scan: Q = 5 pC

εn = 0.40(5) mm·mrad

σx,cathode= 0.43 mm

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The RF photogun: 2.5 Cell

Three coupled pillboxesResonant frequency of 2998 MHz

RF power source:10 MW peak power klystron

Electron source:Photo-emission from cavity wall

266nm, 50fs

RF power

E-bunch

Injector for Laser Wakefield Acceleration

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Setup

RF- photogun

Parabolic mirror

Solenoid (focusing electron bunch)

Plasma channel

Incoming laser pulse:300 mJ, 200 ps , 800 nm

Compressed laser pulse:150 mJ, 50 fs, 800 nm

UV-pulse for photogun: 266-400 nm

1.2 meter

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Beamline

266nm50fs

correction coilsRF

spectrometer

phosphor screen

pulsed solenoid Faraday

cup

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Bunch Energy

spectrometer

0

1

0.5

Inte

nsity

(a.u

.)

3.61 3.67 3.73 3.79Energy (MeV)

E = 3.71 ± 0.03 MeVσEmax

= 2 keV

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Spot Size

0.0 0.1 0.2 0.3 0.40

100

200

300

400

500

600

700

RM

S R

adiu

s [μ

m]

focal length [m]

εn ~ 1-3 mm·mrad.

pulsed solenoid

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Bunch Size at the Focus

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Spot Size & Stability

1 mm

0.75 mm

pulsed solenoid

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Focus Stability

300 μm

100 μm

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Spot Size & Stability

1 mm

0.75 mm

-12 -6 0 6 120

5

10

15

20

Cou

nts

ΔY centre focus [μm] ΔX centre focus [μm]

-12 -6 0 6 12

pulsed solenoid

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focus 20 mm inside plasmafocus at entrance of plasma

Simulations

Einj = 3.71 MeVPlaser = 25 TW

Eout = 900 MeV

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focus 20 mm inside plasmafocus at entrance of plasma

Simulations

Einj = 3.71 MeVPlaser = 25 TW

Eout = 900 MeV

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focus 20 mm inside plasmafocus at entrance of plasma

Simulations

Einj = 3.71 MeVPlaser = 25 TW

Eout = 900 MeV

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focus 20 mm inside plasmafocus at entrance of plasma

Simulations

Einj = 3.71 MeVPlaser = 25 TW

Eout = 900 MeV

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focus 20 mm inside plasmafocus at entrance of plasma

Simulations

Einj = 3.71 MeVPlaser = 25 TW

Eout = 900 MeV

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How many electrons can we get in?

What will come out?

External Injection

1 pC @ 3.7 MeV

@ 25 TW:8 fs bunch900 40 MeV

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Conclusions & Outlook

RF Photogun as external injector feasible~ 1 pC accelerated bunches realistic

Next:Condition to 6.5 MeVInject behind the laser pulse

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Timing

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Timing

CTR: radially polarized

Coherent Transition Radiation (CTR)

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Bunch Length

Q = 70 pCτbunch < 2 ps

THz power & energy in focus

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Timing

Coherent Transition Radiation (CTR)

RF phase

100 fs jitter