Terahertz Generation at FACET, and Potential Enhancement at FACET-II Ziran Wu, Alan Fisher, Matthias Hoffmann, Mike Litos Steve Edstrom, Christine Clarke, Stefano Bonetti, Gerard Andonian, Hermann Durr, Mark Hogan SLAC National Accelerator Laboratory Page 2 FACET Beamline THz source: Coherent transition radiation from two 1-m-thick Ti foils 1014 m before main focus at experiments on the IP (Interaction Point) Table Allows parasitic operation and use of THz for beam diagnostics Page 3 Coherent Transition Radiation e-bunch Page 4 IR Laser on the THz Table 800nm, ~150fs, 9Hz, 1mJ CCD P. Diode BS ND Filter /2 Polarizer Pyro EO Crystal VO 2 Sample PEM Det. Pyrocam Translation Stage /4 PD W. Polarizer Page 5 THz Table with Dry-Air Enclosure Continuous purge of dry air to remove attenuation of THz by water vapor Page 6 Optics for Upstream Foil OAP Foil Ref. Pyro Joulemeter To Michelson THz Polarizers 800nm Line Page 7 Michelson Interferometer THz beamsplitter Scanning mirror Fixed mirror Pyroelectric detectors Page 8 Interferogram, Spectrum, and Bunch Profile New double waist electron optics from Yr to reduce spot size at THz foils (113um x 65um) Three different pyrometers as the autocorrelator detectors THz content up to about 4 THz, with features from water vapor and etalon effect of the detector Reconstruction in time domain using K-K Relation Low-frequency compensation due to finite optical apertures Bunch length estimation compares well with TCAV Page 9 Pulse Energy, Field Strength, and Spot Profile Improved SLC machine tuning and bunch compression Measured 1.7 mJ pulse energy (1.75 mJ from calculation) 1.24mm by 1.40mm focal spot size Estimated 0.6 GV/m peak E-field strength (can improve by tighter focus) Pyrocam Image After linear polarizer Page 10 Time Domain Spectroscopy Coarse timing done by a PEM detector for both 800nm and THz (~100 picosecond uncertainty) Fine timing by EO effect of nonlinear crystal GaP and ZnTe (1 to 2 picosecond) Provide 1-ps level electron and photon synchronization A direct THz waveform measurement Scan needed, e- and photon pointing and timing jitter add noise time zero Page 11 THz Induced Material Switching VO 2 film shows IOM transition on ps timescale Direct THz field induction allows transition dynamics study Sample transmission change of ~5% under current THz field and room T. Need one order of magnitude higher peak field strength 1V/ * M.K. Liu et. al., Nature (487), 345, 2012 Page 12 Potential Enhancement at FACET-II Shorter bunch to push the peak E-field (Compression in 3 dimensions) ~ 30um bunch length Peak at 1 THz, 0.6 GV/m e-field To boost to 10 THz, we need ~ 3um bunch length, may reach tens of V/ (At LCLS, ~ 9um bunch length, 50um spot size and 350 pC gave 4.4 GV/m) FACET FACET-II THz Page 13 Potential Enhancement at FACET-II Smaller e- spot size on the foil Typical spot size 250 x 150 um 2 at FACET THz foil Measured 33% pulse energy increase from ~ 1000 x 100 um 2 spot (old optics) Drilled holes and eventually tore up the foil CDR may be a solution z = 30um, pulse energy flat out below 100um spot size Spot size in the ~ 30um range to reach 1V/ at 1mJ and z = 3 um Page 14 Narrow Band THz Radiation Courtesy of SPIE.org 5~10 THz, 10 mW or more output power is highly desired Excitation of molecular vibrational and rotational modes, Spectroscopy of lossy samples, enhanced SNR, etc. Long e- beam microbunched at about 50um Undulator period would be too large for a 10 GeV beam Structures for narrow-band radiation generation Corrugated metal tubes Dielectric tubes Dielectric grating structures Page 15 Narrow-band Dielectric Tubes 1.2 THz (TM 02 ) 400 GHz (TM 01 ) e-beam Cylindrical SiO 2 Tube ID= 450m,OD= 640m, L= 10cm Cladding Cu layer = 25m Tubes = 35 MeV Ez > 1 GV/m ~150 mJ energy (0.4 THz) Page 16 Silica dual grating structure ( r = 4.0) 30 um periodicity, 55 periods 15 um tooth width, 15 um gap width eBeam: 3 nC charge, 30 um z e- k E0E0 Field Monitor TR at the grating entrance Multi-cycle radiation ~ 0.6 GV/m Broadband from TR Multi-cycle from grating 4.4 THz 3.41 mJ pulse energy in the 4.4 THz band FWHM: 162 GHz Dual-layer Grating Radiator