Electron Beam Based Laser Diagnostics

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Electron Beam Based Laser Diagnostics

• X.J. Wang, M. Babzien and Z. Wu
• National Synchrotron Light Source,
• Brookhaven National Laboratory
• Upton, NY 11973, USA
• Presented at SLAC Workshop on Laser Issues for
  Electron RF Photoinjectors
• October 25, 20002

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What We would like Our Laser To be

• No timing jitter
• No energy fluctuation
• Perfect point stability
• 7/24 available
• Remote controllable
• NO laser physicist.

Programmable in both transverse and longitudinal
distribution
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Outline

• Introduction - Photocathode RF Gun injection
  system
• Beam based laser diagnostics
• Timing jitter requirements and measurements.
• Longitudinal and transverse laser distribution.
• Summary

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SASE FEL Noise Amplifier
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Introduction Photocathode RF Gun Injection System

• Photocathode RF gun injection system
• 1. RF gun.
• 2. Solenoid Magnet.
• 3. RF gun associate beam diagnostics.
• 4. Laser system and optics.
• 5. Cathode technology
• 6. Operating principle

Stability and Reliability
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The HGHG Experiment
Seed Laser l10.6mm Ppk0.7 MW
HGHG FEL l5.3 mm Ppk35 MW
Dispersion Section L0.3 m
Radiator Section Bw0.47T lw3.3cm L2 m
Modulator Section Bw0.16T lw8cm L0.76 m
Electron Beam Input Parameters E 40 MeV
en 4p mm-mrad dg/g0.043 I 110A te 4 ps
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VISA SASE Saturation
Microbunching _at_ 845nm
Microbunching _at_ 422nm
?n ? 2.0 mm-mrad
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Stability and Reliability Leads To Better
Performance
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Laser and Laser optics
YbS-FAP ?? 1047 nm ??LD?? 900 nm ????
1.26 ms ???? 3 J/cm2 ????? gt5
Oblique incident is preferred for better laser
diagnostics and less effect on the beam.
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Photocathode
1. QEgt10-4 2. Life time gt weeks 3.Spatial
Uniformity p-p lt 10
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Thermal Emittance
Electrons are emitted with a kinetic energy Ek
laser spot assumed uniform
with radius r
Example of measurement for Cu-cathode
(Courtesy of W. Graves)
Nonlinear fit gives brf3.1/-0.5,
Fcu4.73/-0.04 eV, and Ek0.40 eV
Linear fit gives Ek0.43 eV
ICFA/BD Sardinia July 2002
Ph. Piot, DESY
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Thermal Emittance of Mg Cathode
Laser wave length match to work funtion is not
good idea.
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Longitudinal Emittance Compensation
Longitudinal emittance compensation demands lower
RF gun phase, which lead to tighter timing jitter
requirements.

• Phys. Rev. E. 54, R3121 (1996)
• PAC 97

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The Advanced FEL Photoinjector Operates at 20
MV/m Gradient and 200 mA Average Current

• 1300 MHz
• Eb 15-20 MeV
• Imacro 100-400 mA
• Q 1-4 nC
• erms 1.6 mm-mrad
• Dg/g 0.2
• Injection f 30o
• Solenoid 300A
• Bucking Sol. 310A

(D. Nguyens talk at BNL PERL workshop, Jan 2001)
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Emittance Optimization at the BNL ATF
Smaller Phase ? tight timing jitter
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Photo-injector Beam Diagnostics

• Energy
• Charge
• RF Gun Phase

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Quantum Efficiency Measurements
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Photo-injector Diagnostics
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Beam Profile and Point Stability
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Non-cylindrical symmetry non-uniform
lasers

• Non-uniform beam fine grained and randomly
  Five special laser masks are used to produce
  different laser uniformities.

90 11.2
80 15
100 7.3
70 28
60 33
50 40
F. Zhou et al, Phys. Rev. ST Accel. Beams 5,
094203 (2002).
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Emittance measurements
F. Zhou et al, Phys. Rev. ST Accel. Beams 5,
094203 (2002).
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Laser and Photoinjector Characterization
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Summary

• The stability and reliability of the
  photocathode RF gun is dominated by the laser
  system. Our experience shows good and on-line
  diagnostics is critical for the stable and
  reliable operation. Electron beam can be used for
  all aspects of the laser diagnostics, i.e.,
  transverse and longitudinal distribution, energy
  stability, point stability and timing jitter.

Thank You!