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Pontifications, Punditry and a few Pejoratives on Photoinjector Drive
Lasers
G. TravishUCLA Department of Physics and Astronomy
Special thanks to Marcus Babzien, Nick Barov,Paul Bolton, Mark Hogan, Dinh Nguyen, and James Rosenzweig
Outline:1) Beam parameters2) Beam propagation
Laser System Cartoon
Oscillator Amplifier(s) Conversion
Transport
Diagnostics Diagnostics
Diagnostics Diagnostics Gun/Cathode
Stretcher/Compressor
Effects of Laser FluctuationsMeasurable Laser Fluctuations Effect on E-Beam
Energy Charge, Space charge
Transverse size (breathing) Size, Emittance
Transverse profile (shape changes) Profile, Emittance
Transverse position (pointing) Beam trajectory and profile, Emittance
Temporal length Emittance, Bunch length
Temporal profile Complex interaction
Timing (synchronization) Energy spread, Bunch length, Charge
Agreement on what laser should deliver is a good idea
Laser Characterization: Energy Energy detector generally superior to photodiode
Shot-shot measurement needed
Correlation with beam charge needed
Best parameter for shot rejection
QuickTime™ and aGIF decompressor
are needed to see this picture.
Laser Characterization: Spatial
CCDs are ubiquitous
Modal decomposition can be a useful rejection trigger
Spot size and pointing can be tracked
Can measure M2
Laser Characterization: TemporalSingle Shot Autocorrelator
Useful and “easy” in IR
Changes in pulse length easy to detect and reject
Excellent relative measure of differing operating modes
Spectrum Can often reveal same info especially in CPA systems
Critical for shaped beams
Streak Camera Expensive
Hard to use
Limited dynamic range
Great for 2-axis information
Some Solutions toLaser Fluctuations
Fluctuating Parameter Typical Solutions
EnergyTemperature stabilization, feedback,new laser
Transverse size (breathing) Block air currents, spatial filter
Transverse profile (shape changes) Fix mode purity, spatial filter
Transverse position (pointing) Relay imaging, move laser closer
Temporal lengthCompressor issues, stabilize rest ofparameters
Temporal profile ? Spectral Filtering
Timing (synchronization)Thermal stabilization and noisereduction, better feedback, better RF
All of them require x2 more money and staff
Beam Propagation
Facilities that have done it right have either limited the scope or poured tremendous effort into the projects.
You’ve made such a good beam; now you better keep it…
Transport lines:
As short as possible
Fewest surfaces (“less glass is better”)
Enclosed (Vacuum or Helium)
Relay Imaged (crystal to cathode)
Propagation of Gaussian Photon Beams
Diffraction Limited Beams:
w(z) w0 1zw02
2
1/ 2
w0
2w0
ZR
w(z)
(z)
Angular spread of beam can be described in limit…
(z)w(z)z
w0
for z zR w0
2
Spatial Mode Decomposition Hermite-Gaussian
Functions which are their own Fourier transform
Used to describe TEM modes
Uij (x,y)Hi2x
H j
2x
e (x
2y 2) /
Laguere (flat top)Suited to shaped beams
Bessel (e-beams)More useful for satisfying boundary conditions
TEM00 TEM10 TEM11
Propagation of RealPhoton Beams
M2: A measure of the deviation from pure TEM00
Define “real” beam via a multiple M of diffraction-limit:
M W0 Mw0
Then,
W0 M2 2
M 2 w0 min
M2 is a useful measure of how far from ideal a beam is. The goal is to have
M 2 1
Note: The M2 for a flat-top beam is big (bad Gaussian)!
Far-Field Limit
Fraunhofer vs Fresnel: At long range, phase fronts become “ordered” and propagation
properties can be measured
Energy, mode structure and profile can be measured in the near-field (Fresnel)
Crude approximation of far field (Fraunhofer):
ZF ZR w0
2
ZF (110 3)2
26310 910m
Example: UV laser with 1mm effective source
Measuring M2
Sample beam along propagation pathMove Sample Point Move Focal Point
Sample Multiple Point
“Quad Scans”
“Three Screen Emittance”
Two FormalismsIn general, electron and photon beams are described by different formalisms, but these two can be unified. (e.g. Rosenzweig 2002).
Electron Beam Photon Beam
*
ZR
n
4
It's the laser It's the cathode / gun
What about M2?
Meff2 min
* 2*
But, we don’t work with quantum electron beams…
Laser Characterization: Wavefronts
Pure M2 diagnostics are similar to quad scans, and Wavefront detectors are similar to emittance pepper-pots.
•Hartman sensor uses a 2D array of lenslettes and a CCD array
•Wavefront phase shifts are measured through displacement of spots
Focus
CollimatorLens Array
Relay Lens CCD
Hartman Spot
Reliability & Conclusions
Two classes of drive lasers: Research & user facility Time to move to user facility style
Spend the money Automation, Diagnostics Swappable parts Online historical data
Staff the machine
Over-spec the laser Parameter changes can be made in the UV
Treat diagnostics and transport to cathode as integral to laser system. Simulate your e-beam so you know the sensitivities.
Don’t let this be your drive laser
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