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