1. 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

2. Page 2 FACET Beamline THz source: Coherent transition radiation from two 1-µm-thick Ti foils  10–14 m before main focus at experiments on the IP (Interaction Point) Table  Allows parasitic operation and use of THz for beam diagnostics

3. Page 3 Coherent Transition Radiation σ e-bunch

4. 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

5. Page 5 THz Table with Dry-Air Enclosure  Continuous purge of dry air to remove attenuation of THz by water vapor

6. Page 6 Optics for Upstream Foil OAP Foil Ref. Pyro Joulemeter To Michelson THz Polarizers 800nm Line

7. Page 7 Michelson Interferometer THz beamsplitter Scanning mirror Fixed mirror Pyroelectric detectors

8. Page 8 Interferogram, Spectrum, and Bunch Profile  New “double waist” electron optics from Yr. 2013 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

9. 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

10. 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”

11. 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/Å * M.K. Liu et. al., Nature (487), 345, 2012

12. 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/Å (At LCLS, ~ 9um bunch length, 50um spot size and 350 pC gave 4.4 GV/m) FACET FACET-II THz

13. 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 σ z = 30um, pulse energy flat out below 100um spot size Spot size in the ~ 30um range to reach 1V/Å at 1mJ and σ z = 3 um

14. 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 …

15. Page 15 Narrow-band Dielectric Tubes 1.2 THz (TM 02 ) 400 GHz (TM 01 ) e-beam Cylindrical SiO 2 Tube ID= 450µm,OD= 640µm, L= 10cm Cladding Cu layer = 25µm Tubes  = 35 MeV  Ez > 1 GV/m  ~150 mJ energy (0.4 THz)

16. Page 16 Silica dual grating structure (ε r = 4.0) 30 um periodicity, 55 periods 15 um tooth width, 15 um gap width eBeam: 3 nC charge, 30 um σ 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