Electro optic sampling as a timing diagnostic at Pegasus lab

Transcription

1 Electro optic sampling as a timing diagnostic at Pegasus lab Cheyne M. Scoby Particle Beam Physics Lab, UCLA 13 January 2009 High Power High Brightness Workshop Los Angeles, CA

2 Outline Motivation for EOS Pump probe diffraction Quick EO effect theory ZnTe response Single shot geometry Pegasus EOS setup Timing and jitter Outlooks Conclusion EOS Greek goddess of the dawn

3 Timing at Pegasus Non correlation expected There are three independent sources of jitter Drive laser s Synchrolock, rms jitter > 250 fs Klystron for gun rf, rms jitter > a few hundred fs Modulator, rms jitter ~1 ps Can eliminate 2 jitter sources for time resolved electron diffraction EOS diff. sample arrivals are always fixed separation in time

4 Electro optic theory: a 1 slide version An electro optic material experiences an additional birefringence linear in applied electric fields Linearly polarized laser light sent through the birefringent material will experience phase modulation Γ E *Source: Van Tilborg Ph.D. thesis The laser phase modulation can be converted to an intensity modulation with a polarizer I = I 0 sin 2 (Γ)

5 EOS Frequency response Sensor BW determined by response function ZnTe/GaP capable terahertz sensors Thickness/Bandwidth tradeoff Source: Q. Wu and X.C. Zhang, Appl. Phys. Lett. 70 (1997) Current sensors can detect E field pulses as short as 30 fs, corresponding to 40 THz bandwidth EO sensing is non destructive, ideal for high current (ultrashort) beams

6 Single shot EOS layout T. Srinivasan Rao, et al. Phys. Rev. STAB 5, (2002) Spatial encoding of the EOS signal Signal proportional to bunch transverse electric field Signal measured with filtered CCD

7 Pegasus accelerator complex Source:

8 EOS experimental chamber setup x Lab frame coordinate system y CCD broadband absorptive filters analyzer ( polarization in z ) e- bunch 25 pc, 100-fs elliptical [110] direction diffraction sample imaging screen polarizer ( polarization in x ) IR delay line (110)-oriented ZnTe 10 x 10 x 0.5 mm EOS laser pulse ( polarization in x ) diffraction sample pump pulse

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10 Future at Pegasus: Finish developing EOS system Reduce BG, automate For PAC 09 Wakefield simulations > oopic shows promise to explain wave like signals observed Calculate entire 2D structure of observed signals

11 Team Pegasus: Acknowledgements Prof. Pietro Musumeci Another grad student: Josh Moody Undergrads: Tan Tran and Michael Gutierrez Funding provided by Office of Naval Research

12 Backup slides Timing at Pegasus Calibration scan Resolution estimates (2 slides) Jitter estimates

13 micrometer linear positioner analyzer signal CCD alignment cam

14 Zinc telluride (ZnTe) Optically isotropic under nominal conditions (residual birefringence) Zincblende (diamond) crystal structure _ Member of cubic 43m crystal symmetry class Refr. index = 800 nm High 1 st order nonlinear tensor electric susceptibility r 43 = 3.9 x m/v Induced birefringence Fast THz response Our crystal: 10 mm x 10 mm x 0.5 mm (110) cut

15 Calibration scan Different peaks moved on/off screen during scan Scanned delay over ~25 ps range Many peaks per scan explained by wakefield in crystal shield Direct beam field not detected Wakefields cite: M.J. Fitch, et al. PAC 1999 Proceedings, Volume: 3, pp vol.3

16 Compared the center of the leftmost peak with the centers of the other two in single shot Assuming shot to shot uniformity of the wakefield, this uncertainty defines the resolution for time of arrival measurements

17 Time resolution estimate from peak center deviations Subtract data points from linear fit (residuals) 20 shots yield 40 residual points Should be able to measure time resolution to ~200 fs Not bad, but want to improve to <100 fs

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19 Who s using EOS for time of arrival? Synchronization for single shot pump probe xray diffraction exp ts SPPS at SLAC: A.L. Cavalieri, et al. Phys Rev Lett 94, (2005) * LCLS at SLAC: K.J. Gaffney and H.N. Chapman, Science 316, (2007) FLASH at DESY: G. Berden, et al. PRL 99, (2007) Time of arrival EOS for (a) 20 consec. Shots (b) 1000 shots Relative jitter when compared to an X ray based pump probe exp t *Images taken from Cavalieri, et al.