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Strategies for achieving femtosecond synchronization in Ultrafast Electron Diffraction John Byrd R. B. Wilcox, G. Huang, L. R. Doolittle Lawrence Berkeley.

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Presentation on theme: "Strategies for achieving femtosecond synchronization in Ultrafast Electron Diffraction John Byrd R. B. Wilcox, G. Huang, L. R. Doolittle Lawrence Berkeley."— 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

2 We have been focused on synchronization issues at FELs where one of the main issues is stable timing distribution and synchronization of remote lasers. Ill try to concentrate on issues relevant to lab- scale experiments for UED. 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. 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 3 valid data range pump probe detect timing, bin data by time wasted shots jitter statistics

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

5 Jitter from electron bunch compression Path-Length Energy- Dependent Beamline Path-Length Energy- Dependent Beamline z i i z i z i V = V 0 sin(kz) z space charge chirpspace charge chirp early late z 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. t rf-laser t sample

6 RF field stability: low-level 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. Sample Beam diags RF Control HV Modulator Gun Buncher Forward, Reverse and Cavity power probes Master Clock

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. 7

8 Example: LCLS Linac (F.J. Decker) 8 8 –0.35 deg to 0.03 deg LCLS Jitter Status in 2012 Sample images HV=300kVBC1: E =250 MeV Un-SLEDed, HV=340kV ?

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 9

10 How good does the clock have to be? Determined by delay difference t D = t A – t B High frequency: differential noise, frequency >1/(2t D ) Low frequency: phase delay change t = t D x ( f/f) Example: 200m fiber –t D is 1 S –High frequency noise above 500kHz < 1fs –Long term frequency drift < clock experiment 10

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

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

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

14 Synching mode-locked lasers with RF ML Laser T rep BP H slave n*f rep Master Clock Basic Phase-locked loop 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 Shelton (14GHz) Bartels (456THz) present work (5THz) repetition rate n*f rep carrier/envelope offset m*f rep +f ceo frequency 0 Shelton et al, O.L. 27, 312 (2002) Bartels et al, O.L. 28, 663 (2003) ML Laser ML Laser T rep BP T rep BP H master slave n*f rep Detection and bandpass filter

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) 16 Transfer function: amplitude phase 39kHz resonance laser DAC step response step response ADC

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

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 18 Schultze et al, Opt. Exp. 18, (2010) 3 J 6fs 100kHz 88as 240as

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

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. 20

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 kHz 100Hz

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 V/V~10 -4 and ~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 ParameterValue Energy750keV Charge1-3x10 5 fC laser spot (rms) μm repetition rate Hz emittance μm min. bunch length (rms) 100fs

23 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. Master PC laser RF control Timing diagnostics Timing diagnostics Seed lasers User lasers Laser heater Stabilized link

24 NGLS Approach: RF and BB Feedback 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) 3.9 CM1 CM2,3 CM4 CM9 CM10 CM27 BC1 210 MeV BC2 685 MeV GUN 0.8 MeV Heater 100 MeV L0 L1 Lh L2 L3 SPREADER 2.4 GeV ΔE Δσ τ SP ΔE Δσ τ SP ΔE τ SP ΔE

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 V/V~10 -4 and ~0.01 deg –Potential for significant improvement in laser lock Further improvement using beam-based feedback to stabilize source. –High rep-rate will help. 25

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