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  1. 1. Pontifications, Punditry and a few Pejoratives on Photoinjector Drive Lasers G. Travish UCLA Department of Physics and Astronomy Special thanks to Marcus Babzien, Nick Barov, Paul Bolton, Mark Hogan, Dinh Nguyen, and James Rosenzweig <ul><li>Outline : </li></ul><ul><li>Beam parameters </li></ul><ul><li>Beam propagation </li></ul>
  2. 2. Laser System Cartoon
  3. 3. Effects of Laser Fluctuations Agreement on what laser should deliver is a good idea
  4. 4. Laser Characterization: Energy <ul><li>Energy detector generally superior to photodiode </li></ul><ul><li>Shot-shot measurement needed </li></ul><ul><li>Correlation with beam charge needed </li></ul><ul><li>Best parameter for shot rejection </li></ul>
  5. 5. Laser Characterization: Spatial <ul><li>CCDs are ubiquitous </li></ul><ul><li>Modal decomposition can be a useful rejection trigger </li></ul><ul><li>Spot size and pointing can be tracked </li></ul><ul><li>Can measure M 2 </li></ul>
  6. 6. Laser Characterization: Temporal <ul><li>Single Shot Autocorrelator </li></ul><ul><ul><li>Useful and “easy” in IR </li></ul></ul><ul><ul><li>Changes in pulse length easy to detect and reject </li></ul></ul><ul><ul><li>Excellent relative measure of differing operating modes </li></ul></ul><ul><li>Spectrum </li></ul><ul><ul><li>Can often reveal same info especially in CPA systems </li></ul></ul><ul><ul><li>Critical for shaped beams </li></ul></ul><ul><li>Streak Camera </li></ul><ul><ul><li>Expensive </li></ul></ul><ul><ul><li>Hard to use </li></ul></ul><ul><ul><li>Limited dynamic range </li></ul></ul><ul><ul><li>Great for 2-axis information </li></ul></ul>
  7. 7. Some Solutions to Laser Fluctuations All of them require x2 more money and staff
  8. 8. 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… <ul><li>Transport lines: </li></ul><ul><ul><li>As short as possible </li></ul></ul><ul><ul><li>Fewest surfaces (“less glass is better”) </li></ul></ul><ul><ul><li>Enclosed (Vacuum or Helium) </li></ul></ul><ul><ul><li>Relay Imaged (crystal to cathode) </li></ul></ul>
  9. 9. Propagation of Gaussian Photon Beams Diffraction Limited Beams: Angular spread of beam can be described in limit…
  10. 10. Spatial Mode Decomposition <ul><li>Hermite-Gaussian </li></ul><ul><ul><li>Functions which are their own Fourier transform </li></ul></ul><ul><ul><li>Used to describe TEM modes </li></ul></ul><ul><li>Laguere (flat top) </li></ul><ul><ul><li>Suited to shaped beams </li></ul></ul><ul><li>Bessel (e-beams) </li></ul><ul><ul><li>More useful for satisfying boundary conditions </li></ul></ul>
  11. 11. Propagation of Real Photon Beams M 2 : A measure of the deviation from pure TEM 00 Define “real” beam via a multiple M of diffraction-limit: Then, M 2 is a useful measure of how far from ideal a beam is. The goal is to have Note: The M 2 for a flat-top beam is big (bad Gaussian)!
  12. 12. Far-Field Limit <ul><li>Fraunhofer vs Fresnel: </li></ul><ul><ul><li>At long range, phase fronts become “ordered” and propagation properties can be measured </li></ul></ul><ul><ul><li>Energy, mode structure and profile can be measured in the near-field (Fresnel) </li></ul></ul><ul><ul><li>Crude approximation of far field (Fraunhofer): </li></ul></ul>Example: UV laser with 1mm effective source
  13. 13. Measuring M 2 Sample beam along propagation path Move Sample Point Move Focal Point Sample Multiple Point “ Quad Scans” “ Three Screen Emittance”
  14. 14. Two Formalisms In general, electron and photon beams are described by different formalisms, but these two can be unified. (e.g. Rosenzweig 2002). What about M 2 ? But, we don’t work with quantum electron beams…
  15. 15. Laser Characterization: Wavefronts <ul><li>Pure M 2 diagnostics are similar to quad scans, and Wavefront detectors are similar to emittance pepper-pots. </li></ul><ul><ul><li>Hartman sensor uses a 2D array of lenslettes and a CCD array </li></ul></ul><ul><ul><li>Wavefront phase shifts are measured through displacement of spots </li></ul></ul>
  16. 16. Reliability & Conclusions <ul><li>Two classes of drive lasers: </li></ul><ul><ul><li>Research & user facility </li></ul></ul><ul><ul><li>Time to move to user facility style </li></ul></ul><ul><li>Spend the money </li></ul><ul><ul><li>Automation, Diagnostics </li></ul></ul><ul><ul><li>Swappable parts </li></ul></ul><ul><ul><li>Online historical data </li></ul></ul><ul><li>Staff the machine </li></ul><ul><li>Over-spec the laser </li></ul><ul><ul><li>Parameter changes can be made in the UV </li></ul></ul>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