Experimental Studies on FEL Gain Controlled by Laser-induced - PowerPoint PPT Presentation

Introduction Lasing Suppression July 2014 Studies . Experimental Studies on FEL Gain Controlled by Laser-induced Longitudinal Space Charge Amplification Christoph Lechner University of Hamburg 6th Microbunching Instability Workshop Trieste; October 7, 2014 Supported by BMBF under contract No.s 05K13GU4 and 05K13PE3 and by DFG GRK 1355 1 Christoph Lechner, University of Hamburg

Introduction Lasing Suppression July 2014 Studies . FLASH at DESY (Hamburg) Soft X-ray RF Stations Accelerating Structures Photon sFLASH Undulators Diagnostics THz FLASH1 RF Gun Bunch Compressors Lasers 5 MeV 150 MeV 450 MeV 1250 MeV Beam Dump FEL Experiments 315 m 2 Christoph Lechner, University of Hamburg

Introduction Lasing Suppression July 2014 Studies . Outline • Lasing suppression • Longitudinal space charge amplifier (LSCA) • Recent results 3 Christoph Lechner, University of Hamburg

Introduction Lasing Suppression July 2014 Studies . Establishing Temporal Overlap ORS technique: Use Modulator-radiator arrangement to find precise temporal overlap. Coherent undulator radiation emitted in radiator signals laser-electron overlap. 4 Christoph Lechner, University of Hamburg

Introduction Lasing Suppression July 2014 Studies . Establishing Temporal Overlap II Observation of coherent undulator radiation to find precise temporal overlap overlap1signal1[arbh1units] 51h0 0 1h0 scan1time1[ps] FEL1pulse1energy1[µJ] 120 110 ORS1overlap1causes 100 drop1of1observed 90 first1scan 80 FLASH1SASE1energy 70 01:03:21 01:06:14 01:09:07 01:12:00 01:14:52 01:17:45 time1[hh:mm:ss] 5 Christoph Lechner, University of Hamburg

− − − Introduction Lasing Suppression July 2014 Studies . Applications of this Lasing Suppression Effect Laser manipulation of electron bunches offers many possibilities: Tailor electron bunches without touching machine • short photon pulse generation in the few-fs range • better synchronization for pump-probe experiments 6 Christoph Lechner, University of Hamburg

− − − Introduction Lasing Suppression July 2014 Studies . Applications of this Lasing Suppression Effect Laser manipulation of electron bunches offers many possibilities: Tailor electron bunches without touching machine • short photon pulse generation in the few-fs range • better synchronization for pump-probe experiments 6 Christoph Lechner, University of Hamburg

Introduction Lasing Suppression July 2014 Studies . Applications of this Lasing Suppression Effect Laser manipulation of electron bunches offers many possibilities: Tailor electron bunches without touching machine • short photon pulse generation in the few-fs range • better synchronization for pump-probe experiments 10 energy5deviation5(10 − 3 ) Study longitudinal profile of the electron 5 bunches 0 • sample bunches longitudinally • which part is qualified to lase? − 5 − 500 0 500 longitudinal5coordinate5(fs) Longitudinal5phase5space5distribution ... measured5with transverse5deflecting5structure5(TDS) 6 Christoph Lechner, University of Hamburg

Introduction Lasing Suppression July 2014 Studies . Impact of Energy Spread on FEL Performance 7 Christoph Lechner, University of Hamburg

Introduction Lasing Suppression July 2014 Studies . LSC Amplification • Laser pulse energy too small for direct suppression of lasing • Amplification process required to explain results • LSC (Longitudinal Space Charge) effects in a combination of focusing channel and chicane can amplify initial density fluctuations 8 Christoph Lechner, University of Hamburg

Introduction Lasing Suppression July 2014 Studies . LSC Amplification focusing channel dedicated chicane shot noise • In focusing channel, charge inhomogeneities generate varying longitudinal electric field → plasma oscillation → growth of energy modulation driven by field • Chicane converts energy modulation into bunching → bunching after chicane can be stronger than initial bunching 9 Christoph Lechner, University of Hamburg

Introduction Lasing Suppression July 2014 Studies . LSC Amplification • In focusing channel, charge inhomogeneities generate varying longitudinal electric field → plasma oscillation → growth of energy modulation driven by field • Chicane converts energy modulation into bunching → bunching after chicane can be stronger than initial bunching • Here, LSC laser-initiated at λ = 800 nm using modulator-chicane combination 9 Christoph Lechner, University of Hamburg

Introduction Lasing Suppression July 2014 Studies . Some LSCA-Related Experiments Marinelli, et al. [5th MBI WS; Phys Rev Lett 110, 264802 (2013)] • Measured at NLCTA at SLAC • Experiment with three-stage LSCA, starting from shot-noise • Emission of undulator radiation • High overall gain ( ∼ 10 4 ) Seletskiy, et al. [X. Yang, this workshop; Phys Rev Lett 111, 034803 (2013)] : • SDL at Brookhaven [Fig 3 from Seletskiy, et • Initial density modulation generated at al., PRL 111, 034803 photo-injector (2013) removed] • Strong bunching at wavelengths suitable for THz generation observed 10 Christoph Lechner, University of Hamburg

Introduction Lasing Suppression July 2014 Studies . LSCA Simulations 5 6 xP10 xP10 0.8 8 4 4 0.6 6 energyPdeviationP[MeV] energyPdeviationP[MeV] EnergyPDeviationP[eV] 0.4 4 EnergyPDeviationP[eV] 2 2 0.2 2 0 0 0 0 -0.2 − 2 − 2 -2 -0.4 − 4 − 4 -4 -0.6 − 6 -0.8 − 8 − 6 -6 − 1 -10 − 0.5 0 0 0.5 10 1 − 1 -10 − 0.5 0 0 0.5 10 1 LongitudinalPPos.PzP[m] longitudinalPpos.P[µm] LongitudinalPPos.PzP[m] longitudinalPpos.P[µm] − 5 − 5 x 10 x 10 Simulation: M. Dohlus 11 Christoph Lechner, University of Hamburg

Introduction Lasing Suppression July 2014 Studies . Studies of the Amplification Process • Variation of R 56 of first chicane (second chicane was set to R 56 = 40 µ m , third chicane off) • Inject density-modulated electron bunch into beamline ( L = 24 m from chicane to TDS) • Impact of initial modulation on electron bunches studied on LOLA transverse deflecting structure (TDS) TexText font size 1.5 Parameter Value electron energy E 0 700 MeV peak current I pk 0 . 3 kA rms bunch duration σ t 0 . 3 ps laser pulse duration 200 fs FWHM 12 Christoph Lechner, University of Hamburg

Introduction Lasing Suppression July 2014 Studies . Measured Effect of Seeded LSC 200 fs long laser pulses 1.35 ps tail laser off head R56=50 um E R56=147 um 1 MeV R56=294 um t 13 Christoph Lechner, University of Hamburg

Introduction Lasing Suppression July 2014 Studies . Scan of R 56 In the limit of large gain, we expect σ E , tot ∼ | R 56 | [figures removed] � σ 2 E , LSCA + σ 2 E , 0 + σ 2 σ E , tot = E , PW 60 fs FWHM laser pulses 14 Christoph Lechner, University of Hamburg

Introduction Lasing Suppression July 2014 Studies . Lasing Suppression • After studying LSC at 0 . 3 kA , we compressed electron bunch for SASE conditions ( I pk ≃ 0 . 9 kA ) • Laser pulse duration 200 fs : stable temporal overlap FEL pulse energy [ µ J] 100 50 0 23:15 23:16 23:17 23:18 23:19 23:20 time [hh:mm] 15 Christoph Lechner, University of Hamburg

Introduction Lasing Suppression July 2014 Studies . Lasing Suppression: Spectra • Laser modulation enabled/disabled every few seconds → impact of any drifts removed • FWHM of average ( n = 500) spectrum: ∆ ω/ω 0 = 0 . 0113 (off), ∆ ω/ω 0 = 0 . 0076 (on), energy reduction by factor of 3. [figure removed] 16 Christoph Lechner, University of Hamburg

Introduction Lasing Suppression July 2014 Studies . Lasing Suppression: Study of Two-Stage LSCA • In beamline after first chicane, growth of energy modulation amplitude at expense of bunching • Chicane C 3 in front of TDS generates strong bunching from amplified initial modulation amplitude • Especially for small initial energy modulation amplitudes, bunching after first chicane limited by slice energy spread 17 Christoph Lechner, University of Hamburg

Introduction Lasing Suppression July 2014 Studies . Lasing Suppression: Study of Two-Stage LSCA [figure removed] Reduced impact of laser in single-stage configuration 18 Christoph Lechner, University of Hamburg

Introduction Lasing Suppression July 2014 Studies . Where do we hit the Bunch? 10 10 8 8 energy deviation (10 −3 ) energy deviation (10 −3 ) 6 6 4 4 2 2 0 0 −2 −2 −4 −4 −6 −6 −500 0 500 −500 0 500 longitudinal coordinate (fs) longitudinal coordinate (fs) For the relative timing with maximum SASE suppression effect, determine position of laser modulation using TDS 19 Christoph Lechner, University of Hamburg

Introduction Lasing Suppression July 2014 Studies . Summary • Hardware of FLASH1 seeding experiment is well-suited for studies of longitudinal space charge amplification • 3 chicanes, 2 modulators • controlled initiation of LSCA • TDS for slice-resolved analysis • Experimental investigation of single LSCA stage using TDS to characterize final longitudinal phase space distribution • We used this laser-initiated instability to suppress FEL lasing process • Stronger lasing suppression in two-stage LSCA configuration • Implications of LSC forces on seeded operation: next presentation 20 Christoph Lechner, University of Hamburg