The FEL Facility Optical Diagnostics and FEL Characterization M. Shinn, C. Behre, S. Benson, J. Coleman, R. Evans, A. Grippo, J. Gubeli, D. Hardy, K. Jordan, G. Neil, S. Zhang G. Williams Jefferson Lab Work supported by U.S. Dept. of Energy under contract DE-AC05-84-ER40150, the Office of Naval Research, the Commonwealth of Virginia, and the Laser Processing Consortium
OUTLINE A look backwards: optical diagnostics for the IR Demo FEL User Facility Performance of the IR Demo FEL Diagnostics fielded Looking forward, diagnostics and characterization for the FEL Upgrades Expanded wavelength range Higher power (as well as pulse energy) Pulseshape determinations Conclusions
IR DEMO PERFORMANCE Specification Achieved (not simultaneously) Average Power 600 1000 W 2100 W Wavelength range 6.5 3 µm 6.5-2.9 µm + harmonic lasing Micropulse energy ~25 µj 28 µj Pulse length ~2 ps FWHM nominal 0.2-1.7 ps PRF 37.425, 18.7 MHz 74.85, 37.425, 18.7 MHz Bandwidth ~ 0.2 0.5% 0.2-3.3% Timing jitter < 0.2 ps not yet measured Amplitude jitter < 20% p-p <5% p-p Wavelength jitter 0.02% RMS not yet measured Polarization linear, > 100:1 >6000:1 Transverse mode quality < 2x diffraction limit <2x Beam diameter at lab 2-4 cm 1.5-3.5 cm
FEATURES OF THE IR DEMO FEL IR Demo FEL Characteristics Superconducting RF, energy-recovering linac Reliable operation, ~ 80% availability. high average power ~2 kw in fundamental range (2 8.0 µm) Operation in 3 spectral regions: 3, 5, & 6 µm extension in 2.1 to 1.0 µm with 3rd harmonic operation 340 W @ 1.05 µm demonstrated in October 2000 extensions through the UV with nonlinear conversion crystals 14 W @ 350 nm & 262.5 nm DC photocathode gun enabled unique pulse flexibility sub-ps micropulse at high (18.7, 37.4, 75 MHz) repetition rate Macropulses as short as 0.8 µs, or as long as seconds PRF from 0.5-60 Hz, locked to AC, or 20 khz, externally triggered Macropulses had relatively low amplitude jitter (< 5% p-p)
OPTICAL DIAGNOSTICS CAPABILITIES Performs continuous diagnostics on CW or pulsed laser output Lasing spectrum (λ peak, FWHM) Output power Pulsewidth (via autocorrelation) Beam Profile, Beam Quality, Pointing Stability Provides optical beam dump if beam is not required by users Diagnostics Power: Molectron PM3, PM5K (for CW output) Energy: Molectron J-25 pyroelectric detector Pulsewidth of micropulse: Two-photon absorption or Type I autocorrelation Beam Profile: Spiricon Pyrocam Type I & III, Coherent Modemaster Spectrum: Acton SP300I (0.3 m fl) EG&G Judson InSb and MCT Sensors Unlimited InGaAs array Hamamatsu PMT
EXAMPLES OF DIAGNOSTIC OUTPUTS
OPTICS CONTROL ROOM
END OF LINE DIAGNOSTICS SUITE Brewster window Pick-off window CCD camera Optical Beam Position Monitor (hidden in this photo) IR camera Coherent Modemaster Water-cooled power meter
10 KW IR AND 1 KW UV
IR UPGRADE PERFORMANCE 35 60 30 50 25 40 Power(kW) 20 15 10 Power(kW) Gain(%) 30 20 Gain(%) 5 10 0 2 4 6 8 10 12 14 Wavelength(µm) 0
UV UPGRADE PERFORMANCE 5 500 4 400 Power(kW) 3 2 Power(kW) Gain(%) 300 200 Gain(%) 1 100 0 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Wavelength(µm) 0
UPGRADE FEL OUTPUT CHARACTERISTICS As shown in the graphs, the new FELs will have far higher output than the IR Demo Plots are for 74.85 MHz operation (CW) Operation at lower PRFs is possible (4.68 MHz vs 18.71 MHz) Spot size will be roughly the same size as before: 25-40 mm 1/e 2 diameter for IR Upgrade
UPGRADE DIAGNOSTICS Upgrades required by the increased power, spectral range of IR Upgrade and UV FEL, and our experience operating the IR Demo. Status Designed vacuum-compatible power meter for higher power rating (IR). Add IR array for FEL wavelength stabilization. Working on integration. Purchased quad detectors and IR cameras for more information on beam quality and position in the user labs. Working on integration. Purchased grating for far-ir operation. Purchased ultrafast laser system for electron and FEL beam diagnostics Added hardware for FROG (Frequency Resolved Optical Gating) analyses of pulseshape. Pending Optical transport system delayed due to funding. Turning mirror system designed, must complete design of collimator.
WHY ADD PULSESHAPE ANALYSIS? Unlike other laser systems, a FEL generally has a more complicated pulseshape. The pulseshape depends on the degree of cavity detuning. S.V. Benson et al, Phys Rev Lett 48, 235 (1982) C.W. Rella et al Opt. Comm 157, 335 (1998) A.M. MacLeod, et al Phys Rev E 62, 4216 (2000) Because of this, determination of the pulsewidth via autocorrelation is problematic. Employing FROG is the most direct technique (although nontrivial to implement) to determine the pulseshape. MacLeod et al, Phys Rev E (2000)
FROG FOR FEL UPGRADE A single-shot FROG has been setup and tested with Ti:sapphire Laser 1.0 0.8 E Field and Phase 14 12 E field (A.U.) 0.6 0.4 0.2 10 8 6 4 2 Phase (rad) 0.0 0-100 -50 0 50 100 Time delay(fs) PH Analyzer Camera SP Intensity(A.U.) 1.0 0.8 0.6 0.4 Spectra and Phase 25 20 15 10 Phase(Rad) 0.2 5 CR 0.0 790 800 810 Wavelength(nm) 820 0 830 1.0 0.8 Auto-correlation (Calibration FROG ) PH CR Intensity (A.U.) 0.6 0.4 Xtal CR 0.2 0.0-200 -100 0 100 200 Time Delay(fs)
INSERTABLE MIRROR ASSEMBLY Routes 50 kw to diagnostic beam dump or pulsed beam to diagnostics. In first light (pulsed) configuration, second insertable mirror not shown Air-actuated mirror routes pulsed beam downward to diagnostics Output through Brewster window (1st light config.) Diagnostic beam dump location (not installed) First light diagnostics will be mounted here
FIRST LIGHT DIAGNOSTICS SCHEMATIC This setup lets us measure energy per macropulse, average power, wavelength, and profile Pyrocam Spectrometer Flip in target Pyro Power Meter
CONCLUSIONS We have a set of optical diagnostics for the IR Upgrade FEL Macropulse energy (and amplitude stability) Average power (once we have cw capability) Spectrum Beam profile We have upgraded our ability to measure the FEL pulseshape Conventional FROG Polarization FROG (using a synchronized S-P Tsunami) Poised to do thorough check of beam quality Will implement after we install the optical transport system.