1 / 25

Strategies for achieving femtosecond synchronization in Ultrafast Electron Diffraction

Strategies for achieving femtosecond synchronization in Ultrafast Electron Diffraction. John Byrd R. B. Wilcox, G. Huang, L. R. Doolittle Lawrence Berkeley National Laboratory Workshop On Ultrafast Electron Sources For Diffraction And Microscopy Applications UCLA, December 14-16 2012.

leon
Download Presentation

Strategies for achieving femtosecond synchronization in Ultrafast Electron Diffraction

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Strategies for achieving femtosecond synchronization in Ultrafast Electron Diffraction John Byrd R. B. Wilcox, G. Huang, L. R. Doolittle Lawrence Berkeley National Laboratory Workshop On Ultrafast Electron Sources For Diffraction And Microscopy Applications UCLA, December 14-16 2012

  2. When in the Course of human events, it becomes necessary for one people to dissolve the political bands which have connected them with another, and to assume among the powers of the earth, the separate and equal station to which the Laws of Nature and of Nature's God entitle them, a decent respect to the opinions of mankind requires that they should declare the causes which impel them to the separation. We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness. That to secure these rights, Governments are instituted among Men, deriving their just powers from the consent of the governed, That whenever any Form of Government becomes destructive of these ends, it is the Right of the People to alter or to abolish it, and to institute new Government, laying its foundation on such principles and organizing its powers in such form, as to them shall seem most likely to effect their Safety and Happiness. Prudence, indeed, will dictate that Governments long established should not be changed for light and transient causes; and accordingly all experience hath shewn, that mankind are more disposed to suffer, while evils are sufferable, than to right themselves by abolishing the forms to which they are accustomed. But when a long train of abuses and usurpations, pursuing invariably the same Object evinces a design to reduce them under absolute Despotism, it is their right, it is their duty, to throw off such Government, and to provide new Guards for their future security. • We have been focused on synchronization issues at FELs where one of the main issues is stable timing distribution and synchronization of remote lasers. • I’ll try to concentrate on issues relevant to lab-scale experiments for UED. Check here if you agree

  3. <10fs pump/probe experiments drive timing system design • ≤10fs X-ray pulses already on LCLS, FLASH • Want timing uncertainty ≤ pulse width • Otherwise pulse is statistically widened • Or, timing range is statistically sampled (then “binned” if measured) • And/or shots are wasted, reducing effective reprate detect timing, “bin” data by time probe pump jitter statistics wasted shots valid data range

  4. Sources of jitter in a UED system Laser control RF Control • Assume RF gun-based to achieve <50 fsec bunches for UED Master Clock Laser HV Modulator Buncher Sample Beam diags Dispersive drift Gun Timing distribution: Master clock jitter Link jitter Electron beam: Gun voltage Amp+phase BuncherAmp+phase PC laser arrival time Laser: Oscillator phase noise Amplifier

  5. d d Jitter from electron bunch compression d DE/E ‘space charge chirp’ szi Dtrf-laser z z late z early Dtrf-laser Dtrf-laser Dtrf-laser sdi • Relative phase jitter of the electron bunch and RF is converted to energy jitter. • The time jitter is compressed by the compression factor • Early and late bunches have different compression • Overfocused beams begin to increase time jitter. V = V0sin(kz) Dtsample Path-Length Energy- Dependent Beamline

  6. RF field stability: low-level RF control RF Control • Use modern digital RF controller to measure and stabilize the cavity field. • Feedback within RF pulse can only occur for long RF pulses >20 microseconds • Feedback cannot control shot-to-shot variable noise from the RF source • Modern RF controllers can achieve <10-4 amplitude and 0.01 deg phase stability. Master Clock Forward, Reverse and Cavity power probes HV Modulator Buncher Sample Beam diags Gun

  7. RF source stability • For pulsed RF sources: • Variable charging of the PFN delivers variation of the high voltage to the klystron • Variable firing of the thyratron switch • Klystron is often run near saturation so HV variation usually results in a phase shift. • “Breakdown” in any part of the RF path (klystron, SLED, waveguide, cavity, load) can cause plasma induced reflections, phase shifts. These “breakdowns” can be well below the limit for an RF trip and may be already a part of “normal” operations.

  8. Example: LCLS Linac (F.J. Decker) Un-SLEDed, HV=340kV ? Sample images BC1: E =250 MeV HV=300kV 8 • 0.35 deg to 0.03 deg LCLS Jitter Status in 2012

  9. RF source stability • For CW or quasi-CW RF sources: • Klystron must be operated with some overhead to provide feedback control • AM/PM conversion from variable cavity tuning • HV PS harmonics • RF clock phase noise

  10. How good does the clock have to be? signal path A • Determined by delay difference tD = tA – tB • High frequency: differential noise, frequency >1/(2tD) • Low frequency: phase delay change Dt = tD x (Df/f) • Example: 200m fiber • tD is 1mS • High frequency noise above 500kHz < 1fs • Long term frequency drift < 10-9 experiment clock signal path B

  11. Optical clocks are good enough ~10-15 freq. stability <0.1fs jitter above 500KHZ • RF and optical frequencies, at exact integer multiples • Commercially available Kubina et al, Opt. Expr. 13, 904 (2005) Song, et al, Opt. Expr. 19, 14518 (2011) 2e6, 2e6+1... reprate 2 3 4 5... amplitude RF optical 100MHZ 200THz frequency Menlo Systems

  12. Pulsed lasers are naturally quiet Er:fiber laser: • <1fs above 100kHz • Electro-optic modulators have ~1MHz BW J. A. Cox et al, Opt. Lett. 35, 3522 (2010)

  13. Stabilized optical link timing distribution VCO or laser receiver transmitter RF phase detect, correct wRF CW laser • RF clock controls remote oscillator • ~10fs is about the limit • 0.01 degree phase error • 10fs at 3GHz • Currently used in LCLS and Fermi@Elettra AM wRF FS Rb ref optical delay sensing wRF Out-of-loop resuts: Controlling VCXO, 200m fiber delay error, fs 8.4fs, 20 hours to 2kHz (loop BW) time, hours

  14. Synching mode-locked lasers with RF Trep n*frep slave BP ML Laser Basic Phase-locked loop Df Master Clock H • ML Oscillator is a sub-harmonic of the clock frequency. • Best performance if the photo-detected harmonic of oscillator frequency is the clock frequency. Otherwise, additional frequency multiplication is needed, reducing resolution. • Possible AM/PM conversion at the PD • ML oscillator is a dynamic device. Feedback response H should be designed to dynamic response of oscillator (piezo, piezo driver, etc.)

  15. Laser-laser synchronization Trep Trep slave master n*frep n*frep BP BP ML Laser m*frep+fceo n*frep Df H Detection and bandpass filter carrier/envelope offset repetition rate 0 frequency Shelton (14GHz) Bartels (456THz) Shelton et al, O.L. 27, 312 (2002) Bartels et al, O.L. 28, 663 (2003) present work (5THz) ML Laser

  16. Optimizing RF lock for ti:sapphire laser • Use modern control techniques • Determine open loop transfer function • Add filter to prevent oscillation with high gain (30kHz LPF) Transfer function: laser 39kHz resonance amplitude DAC ADC step response phase

  17. RF locking results with tisaf • In-loop measurement compared with difference between two externally referenced measuements FFT of noise In-loop: 21fs RMS 1Hz to 170kHz Jitter spectral density of laser and reference Integrated RMS jitter Out- of- loop: control bandwidth 26fs RMS 30Hz to 170kHz

  18. Effect of amplifiers on CEP • CEP thru example optical parametric amp, 240as long term • Dispersion changes CEP • Carrier and envelope velocity are different • Dispersion controlled to minimize pulse width, thus stable 3mJ 6fs 100kHz Schultze et al, Opt. Exp. 18, 27291 (2010) 88as 240as

  19. Out-of-loop lock diagnostics • Compare ML phase with measured buncher phase Laser control RF Control Master Clock Laser HV Modulator Buncher Beam diags Dispersive drift Gun

  20. Post-sample diagnostics • Measure electron charge, position and angle following sample • Use deflecting cavity to measure beam-RF jitter. • Use magnetic spectrometer to measure energy jitter. Should be correlated to energy jitter induced by timing jitter at buncher.

  21. Noise measurement and control depends on repetition (sample) rate • High reprate enables high bandwidth feedback • Control BW ≈ sample rate/10 • Integrated jitter above sample rate is “shot to shot” 100Hz 100kHz

  22. A high rep-rate RF gun for UED(Daniele Filippetto) • APEX Phase I RF gun has been built as R&D for a high rep-rate FEL • CW 187 MHz gun, 750 keV, 1 MHz laser rep-rate (could be higher), low emittance • Because of low frequency RF gun, beam dynamics quasi-DC. 1.3 GHz buncher. • Expected RF stability DV/V~10-4 and Df~0.01 deg • Deflecting cavity and spectrometer diagnostics. • High rep-rate allows for broadband RF and beam-based feedback. • If laser pump/electron probe jitter can be reduced to <10 fsec, diffraction images can be integrated. • Expected operation in 2013.

  23. NGLS@Berkeley • The eventual goal is to provide remote synchronization between all FEL driver systems: x-rays, lasers, and RF accelerators. Our current focus is to synch user laser systems with timing diagnostics. Timing diagnostics PC laser Laser heater RF control Seed lasers Stabilized link Stabilized link Stabilized link Stabilized link Stabilized link User lasers Master Stabilized link

  24. NGLS Approach: RF and BB Feedback GUN 0.8 MeV Heater 100 MeV SPREADER 2.4 GeV BC1 210 MeV BC2 685 MeV L0 L2 Lh L3 L1 CW SCRF provides potential for highly stable beams… • Measure e- energy (4 locations), bunch length (2 locations), arrival time (end of machine) • Feedback to RF phase & amplitude, external lasers • Stabilize beam energy (~10-5 ?), peak current (few %?), arrival time (<20 fs) CM1 CM2,3 CM9 CM27 CM4 CM10 3.9 ΔE ΔE ΔE Δστ Δστ ΔEτ SP SP SP SP

  25. Conclusions • UED is the ideal setup for pump-probe • Pump and probe generated by same laser • Laser-RF stability requires careful control of RF and laser with out-of-loop comparisons. • Greatest potential for improvement. • CW RF can be stabilized to DV/V~10-4 and Df~0.01 deg • Potential for significant improvement in laser lock • Further improvement using beam-based feedback to stabilize source. • High rep-rate will help.

More Related