LCLS Injector Laser System Paul R. Bolton, SLAC April 24, 2002

1. LCLS Injector Laser SystemPaul R. Bolton, SLACApril 24, 2002 • UV Pulse Requirements • Laser System Overview • Manipulation of Temporal Envelopes (ir) • Some Key Diagnostics • UV Conversion • UV Pulse Delivery to Photocathode • Meeting the Specifications Paul R. Bolton, SLAC

2. UV Pulse requirements • Energy • 260 nm < l < 266 nm • > 0.5 mJ to cathode (for nC electron production with 10-5 QE) • energy stability < 2 % rms (for ir < 0.7 % rms) • Timing • 120 Hz rep rate • 10 psec FWHM pulse duration with 0.5 psec rise/fall time • nominally ‘square’ temporal envelope but adjustable • laser-RF timing jitter < 500 fsec rms • Spatial • profile homogeneity at cathode < 10 % ptp • spot diameter = 2 mm (hard edge) with jitter < 1 % ptp • pointing jitter < 1% of radius rms (< 10 microns for 1 mm radius) Paul R. Bolton, SLAC

3. LaserSystem Overview Ti:Sapphire Choice -high bandwidth available -high average power output -stable, industrial standard for broadband TiS mode-locked oscillator -lock to 79.33 MHz ext source ~ 15 nJ pulse energy 5 nJ Pulse Shaper Pulse Stretcher 2nd Stage Bowtie Amplifier -100 mJ pump level @ 532 nm average single pass gain = 6.3 1st Stage Bowtie Amplifier -100 mJ pump level @ 532 nm average single pass gain = 7 Vacuum spatial filtering & profile flattening 18 mJ 12 mJ • UV Pulse Delivery • to Photocathode • relay imaged • - grazing incidence Pulse Compression 7 mJ UV Conversion (two crystals) 1 mJ (uv) Paul R. Bolton, SLAC

4. UV Pulse Shape Fourier Transform Sinc2(aDn) a = 10 ps (fwhm) < 0.5 ps aDn Manipulation of Temporal Envelopes • Frequency Domain Pulse Shaping • performed at low pulse energy • minimizes optical damage and uses ir wavelengths • efficient use of amplifier stored energy • shaper efficiency ~50% • alters spectral shape and effective bandwidth • computer-programming of transform-limited pulse shapes Paul R. Bolton, SLAC

5. Pulse Stretching suppresses nonlinear behaviour (including damage) in amplifier imposes positive linear chirp on ir waveform: w ( t) = w0 + 2 pt pulse duration stretched by a factor up to 104 efficiency 50 – 65 % Pulse Compression imposes a negative chirp to undo the positive dispersion for ‘matched’ condition recover transform limited pulsewidths incomplete compression is an additional shaping option efficiency 50 – 65 % Manipulation of Temporal Envelopes Paul R. Bolton, SLAC

6. Some Key Diagnostics • Amplifier Probe Pulse • ultrashort pulse verifies amplifier total gains • cross-correlation with shaped oscillator (ir) pulses (using a low energy diagnostic compressor) • cross-correlation with shaped uv pulses • Frequency-Resolved Optical Gating (FROG) • ultrafast single pulse diagnostic (ir and uv) • provides ‘local’ uv pulse measurement at photocathode location • Electron Beam Generated Phase Error Signal • ‘bottom line’ type phase error wrt injector RF • generated using a beamline resonator Paul R. Bolton, SLAC

7. UV Conversion • BaB2O4 (BBO) crystals of thickness near 0.5 mm • ir to uv in 2 steps: (SHG in 1st crystal) + (SFG in 2nd crystal) • for 6 nm bandwidth (120 fsec pulsewidth) conversion efficiency is 11 - 21 % • modified (polarization) Type II phase matching technique will be tested (has demonstrated conversion efficiencies in excess of 30 % above 0.5 GW/cm2 with 140 psec pulses using KDP) • saturate conversion: improves uv pulse energy stability (given ir stability) • damage threshold limits: measurements (ongoing) at J/cm2 level mean spotsizes > 1 mm • document effects of ir pulse shaping and profile flattening Paul R. Bolton, SLAC

8. UV Pulse Delivery to Photocathode • grazing incidence, P polarization • image hard edged aperture (trimmer) onto cathode • specular reflection diagnostics • spatial filtering • launch grating corrects anamorphic magnification and time slew : Launch Grating b ct 2 mm Photocathode E-Beam Specular Reflection To Joulemeter/Camera Paul R. Bolton, SLAC

9. Meeting the Specifications • Pulse Energy • oscillator • Diode pumping in TEMoo mode • environmental controls – temperature,humidity,mechanical isolation,beam covering • amplifier • Fourier relay imaging and saturation of ir gain (final passes) • Feed forward stabilized Nd:YAG pumping of TiS • uv conversion (rms energy jitter < 2%) • saturate conversion & measure detailed pulse characteristics at photocathode • Spatial Profile • ir spatial filtering & profile flattening (optional uv spatial filtering) • overfill hard edge aperture (relay imaged onto photocathode) • Timing • oscillator bandwidth enables ‘square’ pulse generation • oscillator –RF timing jitter (integrated to 40 MHz) < 480 fsec (measured) • additional (external) phase-locked loop uses sampled ir pulse energy (oscillator) and e- beam generated phase error signal Paul R. Bolton, SLAC