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

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 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.

<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

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

Jitter from electron bunch compression d  DE/E z sdi szi d z ‘space charge chirp’ V = V0sin(kz) Dtrf-laser late early Dtrf-laser Dtrf-laser Dtrf-laser d 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. Dtsample Path-Length Energy- Dependent Beamline

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

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.

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

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

How good does the clock have to be? signal path A clock experiment signal path B 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

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

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

Stabilized optical link timing distribution VCO or laser transmitter receiver RF phase detect, correct wRF CW laser AM FS wRF Rb ref optical delay sensing wRF 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 Out-of-loop resuts: Controlling VCXO, 200m fiber delay error, fs 8.4fs, 20 hours to 2kHz (loop BW) time, hours

Synching mode-locked lasers with RF n*frep Trep slave Master Clock BP ML Laser Basic Phase-locked loop Df 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.)

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

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 amplitude 39kHz resonance DAC ADC step response phase

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

Effect of amplifiers on CEP 3mJ 6fs 100kHz Schultze et al, Opt. Exp. 18, 27291 (2010) 88as 240as 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

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

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.

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

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. Parameter Value Energy 750 keV Charge 1-3x105 fC laser spot (rms) 50-1000 μm repetition rate 1-106 Hz emittance 0.03-0.6 min. bunch length (rms) 100 fs

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 Stabilized link Seed lasers Stabilized link User lasers Master

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

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.