Presentation on theme: "John Byrd R. B. Wilcox, G. Huang, L. R. Doolittle"— Presentation transcript:
1 Strategies for achieving femtosecond synchronization in Ultrafast Electron Diffraction John ByrdR. B. Wilcox, G. Huang, L. R. DoolittleLawrence Berkeley National LaboratoryWorkshop On Ultrafast Electron Sources For Diffraction And Microscopy ApplicationsUCLA, December
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 agreeWe 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.
3 <10fs pump/probe experiments drive timing system design ≤10fs X-ray pulses already on LCLS, FLASHWant timing uncertainty ≤ pulse widthOtherwise pulse is statistically widenedOr, timing range is statistically sampled (then “binned” if measured)And/or shots are wasted, reducing effective repratedetect timing,“bin” data by timeprobepumpjitterstatisticswastedshotsvalid data range
4 Sources of jitter in a UED system Assume RF gun-based to achieve <50 fsec bunches for UEDRF ControlMaster ClockLaser controlLaserHV ModulatorBuncherSampleBeam diagsGunDispersive driftElectron beam:Gun voltage Amp+phaseBuncher Amp+phasePC laser arrival timeTiming distribution:Master clock jitterLink jitterLaser:Oscillator phase noiseAmplifier
5 Jitter from electron bunch compression d DE/Ezsdiszidz‘space charge chirp’V = V0sin(kz)Dtrf-laserlateearlyDtrf-laserDtrf-laserDtrf-laserdzRelative phase jitter of the electron bunch and RF is converted to energy jitter.The time jitter is compressed by the compression factorEarly and late bunches have different compressionOverfocused beams begin to increase time jitter.DtsamplePath-Length Energy-Dependent Beamline
6 RF field stability: low-level RF control Master ClockForward, Reverse and Cavity power probesHV ModulatorBuncherSampleBeam diagsGunUse modern digital RF controller to measure and stabilize the cavity field.Feedback within RF pulse can only occur for long RF pulses >20 microsecondsFeedback cannot control shot-to-shot variable noise from the RF sourceModern RF controllers can achieve <10-4 amplitude and 0.01 deg phase stability.
7 RF source stability For pulsed RF sources: Variable charging of the PFN delivers variation of the high voltage to the klystronVariable firing of the thyratron switchKlystron 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) 0.35 deg to 0.03 degUn-SLEDed, HV=340kV?Sample imagesBC1: E =250 MeVHV=300kV8LCLS 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 controlAM/PM conversion from variable cavity tuningHV PS harmonicsRF clock phase noise
10 How good does the clock have to be? signal path Aclockexperimentsignal path BDetermined by delay difference tD = tA – tBHigh frequency: differential noise, frequency >1/(2tD)Low frequency: phase delay change Dt = tD x (Df/f)Example: 200m fibertD is 1mSHigh frequency noise above 500kHz < 1fsLong term frequency drift < 10-9
11 Optical clocks are good enough Song, et al, Opt. Expr. 19, (2011)<0.1fs jitter above 500KHZ~10-15 freq. stabilityKubina et al, Opt. Expr. 13, 904 (2005)RF and optical frequencies, at exact integer multiplesCommercially available2e6, 2e6+1...reprateamplitudeRFoptical100MHZ200THzfrequencyMenlo Systems
12 Pulsed lasers are naturally quiet Er:fiber laser:J. A. Cox et al,Opt. Lett. 35, 3522 (2010)<1fs above 100kHzElectro-optic modulators have ~1MHz BW
13 Stabilized optical link timing distribution VCO or lasertransmitterreceiverRF phasedetect,correctwRFCWlaserAMFSwRFRbrefopticaldelaysensingwRFRF clock controls remote oscillator~10fs is about the limit0.01 degree phase error10fs at 3GHzCurrently used in LCLS andOut-of-loop resuts:Controlling VCXO, 200m fiberdelay error, fs8.4fs, 20 hours to 2kHz (loop BW)time, hours
14 Synching mode-locked lasers with RF n*frepTrepslaveMaster ClockBPML LaserBasic Phase-locked loopDfHML 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 PDML oscillator is a dynamic device. Feedback response H should be designed to dynamic response of oscillator (piezo, piezo driver, etc.)
15 Laser-laser synchronization DfML LaserTrepBPHmasterslaven*frepDetection and bandpass filterML LaserShelton (14GHz)Bartels (456THz)presentwork (5THz)repetition raten*frepcarrier/envelopeoffsetm*frep+fceofrequencyShelton et al, O.L. 27, 312 (2002)Bartels et al, O.L. 28, 663 (2003)
16 Optimizing RF lock for ti:sapphire laser Use modern control techniquesDetermine open loop transfer functionAdd filter to prevent oscillation with high gain (30kHz LPF)Transfer function:laseramplitude39kHzresonanceDACADCstepresponsephase
17 RF locking results with tisaf In-loop measurement compared with difference between two externally referenced measuementsFFT of noiseIn-loop:21fs RMS1Hz to 170kHzJitter spectral densityof laser and referenceIntegratedRMS jitterOut-of-loop:controlbandwidth26fs RMS30Hz to 170kHz
18 Effect of amplifiers on CEP 3mJ6fs100kHzSchultze et al,Opt. Exp. 18, (2010)88as240asCEP thru example optical parametric amp, 240as long termDispersion changes CEPCarrier and envelope velocity are differentDispersion controlled to minimize pulse width, thus stable
19 Out-of-loop lock diagnostics Compare ML phase with measured buncher phaseRF ControlMaster ClockLaser controlLaserHV ModulatorBuncherBeam diagsGunDispersive drift
20 Post-sample diagnostics Measure electron charge, position and angle following sampleUse 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 feedbackControl BW ≈ sample rate/10Integrated jitter above sample rate is “shot to shot”100Hz100kHz
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 FELCW 187 MHz gun, 750 keV, 1 MHz laser rep-rate (could be higher), low emittanceBecause of low frequency RF gun, beam dynamics quasi-DC. 1.3 GHz buncher.Expected RF stability DV/V~10-4 and Df~0.01 degDeflecting 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.ParameterValueEnergy750keVCharge1-3x105fClaser spot (rms)μmrepetition rate1-106Hzemittancemin. 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.TimingdiagnosticsPC laserLaser heaterRF controlStabilized linkSeed lasersStabilized linkUser lasersMaster
24 NGLS Approach: RF and BB Feedback GUN0.8 MeVHeater100 MeVBC1210 MeVBC2685 MeVSPREADER2.4 GeVL0L1LhL2L3CM1CM2,33.9CM4CM9CM10CM27ΔEΔEΔEΔστΔστΔEτSPSPSPSPCW 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 lasersStabilize beam energy (~10-5 ?), peak current (few %?), arrival time (<20 fs)
25 Conclusions UED is the ideal setup for pump-probe Pump and probe generated by same laserLaser-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 degPotential for significant improvement in laser lockFurther improvement using beam-based feedback to stabilize source.High rep-rate will help.