An RF photogun for external injection of electrons in a Laser Wakefield Accelerator
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Transcript of An RF photogun for external injection of electrons in a Laser Wakefield Accelerator
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An RF photogun for external injection of electrons in a Laser Wakefield Accelerator
Seth Brussaard
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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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29-06-2011Daresbury 5
Injection
0 -
50plasma
laserplasma
min 10
max 1000
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29-06-2011Daresbury 6
How many electrons can we get in?
What will come out?
External Injection
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29-06-2011Daresbury 7
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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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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29-06-2011Daresbury 9
RF Photogun
Coaxial S-band input coupler: scaled down version L-band design DESY
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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.
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RF Photoguns
cathode platefirst (half) cell
second cell
Clamped construction:cavity parts
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RF Photoguns
Clamped construction:cavity parts single-diamond turning
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RF Photoguns
Clamped construction:cavity inside stainless steel vacuum can
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RF Photogun
Cavity mounted inside main magnet:
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RF Photoguns
RF characterization: resonances
f0=2.9980 GHz
-mode
0-mode
f0=2.9918 GHz
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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
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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!
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1811-02-2011ZFEL Workshop
RF Photoguns
• Water cooling for 1 kHz PRF
• Presently operating @ 100 Hz (limited by Modulator/Klystron)
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Emittance
Quadrupole scan:
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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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29-06-2011Daresbury 34
focus 20 mm inside plasmafocus at entrance of plasma
Simulations
Einj = 3.71 MeVPlaser = 25 TW
Eout = 900 MeV
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29-06-2011Daresbury 35
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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29-06-2011Daresbury 36
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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29-06-2011Daresbury 40
Timing
Coherent Transition Radiation (CTR)
RF phase
100 fs jitter