Experimental Optimization of Electron Beams for Generating THz CTR and CDR with PITZ

Transcription

1 Experimental Optimization of Electron Beams for Generating THz CTR and CDR with PITZ Introduction Outline Optimization of Electron Beams Calculations of CTR/CDR Pulse Energy Summary & Outlook Prach Boonpornprasert DPG-Frühjahrstagung 2017 TU Dresden, Dresden

2 PITZ Facility Photocathode RF Gun Booster (Linac) Deflecting Cavity ~7 MeV/c ~22 MeV/c Quadrupole magnet 0 m Dipole magnet Screen HEDA2 ~21.7 m PITZ Beamline Layout The Photo-Injector Test facility at DESY Zeuthen site (PITZ). Develop, study and optimize high brightness electron sources for linacbased FELs. Working closely with FLASH and the European XFEL. 2 UV photocathode laser systems Cylindrical pulse shape (Gaussian, flat-top, comb-like) Beamline length Cathode laser pulse duration Electron bunch charge Maximum electron beam momentum Important Parameters ~22 m few ps to ~22 ps (FWHM) sub pc to > 5 nc ~24 MeV/c 3D-ellipsoidal pulse shape Adjustable by using laser pulse shaper Prach Boonpornprasert DPG-Frühjahrstagung Page 2

3 Motivations for Studies of IR/THz Production at PITZ X-ray Pump & Probe experiment IR/THz IR/THz sources Conventional laser Accelerator based Requirements of the IR/THz Pulses Same time structure and repetition rate as those of the X-ray pulses (XFEL pulses / sec) Possible to have precise synchronization with each of the X-ray pulses High stability (intensity and phase) High (various) pulse energy (µj - mj) Wide wavelength tunability: from 15 µm (20THz) to 3 mm (0.1 THz) Variety of temporal and spectral patterns Variety of polarizations PITZ is an ideal facility for the development of a prototype of such accelerator based IR/THz source Strengths Over a decade of experience in high brightness photo injector research and development. PITZ has same type of electron source as EXFEL same time structure of radiation pulse merit for precise synchronization. The site of a PITZ-like setup is small enough to fit in the experimental hall for the EXFEL users. short IR/THz transport Born to be a test facility, the beam time and the accelerator are adaptable to new research ideas and proposals. 3 methods of radiation production have been studied. Single-pass FEL SASE FEL for λ rad 100 µm (f 3 THz) Coherent Transition Radiation(CTR) for λ rad 100 µm (f 3 THz) Coherent Diffraction Radiation(CDR) for λ rad 100 µm (f 3 THz) Progresses on CTR and CDR studies are presented in this presentation. Prach Boonpornprasert DPG-Frühjahrstagung Page 3

4 CTR and CDR Coherent Transition Radiation (CTR) E-beam CTR radiator Coherent Diffraction Radiation (CDR) E-beam CDR radiator Backward transition radiation θ Backward diffraction radiation θ Backward TR energy (U TR ) emitted in the frequency range dω into the solid angle dω can be calculated by Ginzburg-Frank Formula. CTR energy from an electron bunch can be expressed as d 2 U CTR N 2 F dωdω long ω d2 U TR bunch dωdω 1 electron where N is number of electrons in the bunch and + F long ω = ρ long t e iωt dt is longitudinal form factor of the electron bunch. 2 The DR energy (U DR ) from a circular aperture radius r can be calculated by d 2 U DR d 2 U TR = D ω, r dωdω 1 electron dωdω 1 electron where D ω, r is the correction for DR. CDR energy from an electron bunch can be expressed as d 2 U CDR dωdω bunch N 2 F long ω d2 U DR dωdω 1 electron Prach Boonpornprasert DPG-Frühjahrstagung Page 4

5 Optimization of Electron Beams Machine Parameters Laser diameter on the cathode 2.0, 2.5 mm Peak E-field in gun 60 MV/m Peak E-field in booster 17.2 MV/m Gun phase* 0 degree Booster phase* 0 to -60 degree *w.r.t. Maximum Mean Momentum Gain phase Corresponding <P z > ~ 22 to 15 MeV/c Short Gaussian Beam Optimization Illuminated the cathode with a Gaussian laser pulse with temporal length of ~2.4 ps FWHM. Velocity bunching Went off-crest booster phase for lower momentum at the head and higher momentum at the tail of the electron bunch. Measured the longitudinal profile by using the deflecting cavity, Experiments were done with bunch charges of 20 pc, 100 pc and 250 pc. Comb-like Beam Optimization Illuminated the cathode with a comb-like laser pulse. In this experiments, the laser pulse shaper was adjusted for a comb-like laser pulse with 6 teeth. Also went off-crest booster phase for velocity bunching. Measured the longitudinal profile by using the deflecting cavity, Experiments were done with bunch charges of 100 pc, 250 pc and 500 pc. Prach Boonpornprasert DPG-Frühjahrstagung Page 5

6 Examples of Optimized Bunch Profiles Short Gaussian Beams Longitudinal Bunch Profiles Comb-like Beams Longitudinal Bunch Profiles <P z > ~ 14MeV/c <P z > ~ 21MeV/c Corresponding Form Factors Corresponding Form Factors Prach Boonpornprasert DPG-Frühjahrstagung Page 6

7 Calculations of CTR/CDR Pulse Energy Radiation Pulse Energy vs Ω vs f Total Radiation Pulse Energy Short Gaussian beam, 250 pc CTR CDR Short Gaussian beam Comb-like beam CTR CDR Comb-like beam, 250 pc CTR CDR Prach Boonpornprasert DPG-Frühjahrstagung Page 7

8 Design of the CTR/CDR station The station is placed ~13 m downstream from the cathode. Acceptance angle from the radiator to the viewport is 0.4 rad. The viewport is made of z-cut crystal quartz. THz radiation diagnostics system is placed in the tunnel, normal room environment. The system will be used to measure: Radiation energy/power Radiation spatial profile Radiation polarization Radiation spectrum (Michaelson interferometer) The detectors is pyroelectric detector. P = off-axis parabolic mirror F = flat mirror Acknowledgement: G.Koss, S.Philipp Prach Boonpornprasert DPG-Frühjahrstagung Page 8

9 Summary and Outlook Summary Optimizations of electron beams (Short Gaussian and Comb-like) for CTR/CDR were done. The calculated radiation has pulse energy in the range of sub µj within the frequency range of THz. The bunch lengths are still too long to cover THz frequency. Design, machining and installation of CTR/CDR station is ongoing. Outlook First experimental generation of (sub)thz CTR/CDR at PITZ is planned to take place in May-Jun 2017 Ways to achieve shorter bunch length Sub-ps cathode laser pulse from the 3D-ellipsoidal laser system Bunch compressor Prach Boonpornprasert DPG-Frühjahrstagung Page 9

10 Backup Slides Prach Boonpornprasert DPG-Frühjahrstagung Page 10

11 Experiments of Velocity Bunching Prach Boonpornprasert DPG-Frühjahrstagung Page 11

12 Form factor Form factor Form factor Comb Beams: Profiles and Form Factors FFT FFT FFT Prach Boonpornprasert DPG-Frühjahrstagung Page 12

13 Photo Injector: RF-Gun Bucking solenoid RFgun: L-band (1.3 GHz) nc (copper) standing wave 1½-cell cavity Main solenoid, Bz_peak~0.2T Photo cathode (Cs 2 Te) QE~0.5-5% UHV Vacuum mirror Cathode laser 262nm 20ps (FWHM) F gun L-band 1.6-cell copper cavity Cs 2 Te photocathode (QE~5-10%) Coaxial RF coupler Dry ice cleaning low dark current (<100uA@6MW) LLRF control for amplitude and phase stability Solenoid for emittance compensation Electron bunch <1 pc 5 nc, ~5-7MeV Prach Boonpornprasert DPG-Frühjahrstagung Page 13

14 Variation of the pulse shape by using a single different Lyot filter (UV, measured with OSS) no Lyot filter Lyot: 6 mm YLF Lyot: 16 mm YLF Lyot: 4 mm YVO 4 FWHM = 2.5 ps FWHM = 4 ps FWHM = 7 ps FWHM ~ 16 ps without shaper 2 5 ps 2 5 ps 2 5 ps 2 5 ps edges ~ 2ps edges ~ 3ps edges ~ 6 ps ~ 14 ps with shaper (13 crystals) 2 4 ps ~ 2 3 ps ~ 19 ps 2 5 ps 2 5 ps 2 5 ps 2 5 ps Edges of the flat-top pulses are slightly shorter than FWHM of the Gaussian pulse (measured without shaper) Smoothening of the Modulations in the flat-top region of the pulse Laser temporal profile for high TR PWA experiment V I. Will, G. Klemz Increasing the flexibility in pulse shape of a Yb:YAG photocathode laser Prach Boonpornprasert DPG-Frühjahrstagung Page 14

15 Characterizations of 4 nc Beams for SASE FEL Options ( ) The measurements were done with beam momenta of about 15 and 22 MeV/c for λ FEL of 100 and 20 µm, respectively. Longitudinal profiles were measured by using transverse-deflecting cavities. The measured parameters were used as input of the GENESIS1.3 code for FEL calculations. <P z >~15MeV/c <P z >~22MeV/c Measured slice emittance Helical undulator with period length of 40 mm Measured longitudinal phase space (LPS) Measured slice momentum spread Calculated SASE FEL radiations using the measured beam parameters Prach Boonpornprasert DPG-Frühjahrstagung Page 15