1. 1 E206 Terahertz Radiation from the FACET Beam SAREC Review SLAC 2013 July 26 Alan Fisher and Ziran Wu SLAC National Accelerator Laboratory

2. 2 Topics Fisher: E206 THz  Changes to the layout of the THz table  Effect of smaller size at foil  Collaborative measurements with Smith-Purcell (E203)  Collaborative measurements with plasma wakefield (E200)  Effect of notch collimator  Comparison to transverse cavity  Terahertz transport using Sommerfeld’s mode  Plans for next run

3. 3 Changes to the THz Table Layout Fisher: E206 THz  Downstream THz foil given to TCAV for OTR imaging  Reference pyroelectric detector moved to upstream foil  Used both for THz and as bunch-length monitor  Must be robust and not sensitive to THz alignment  Beamsplitter after upstream THz foil  Used to provide light to OTR camera  Now shared with reference pyroelectric detector and camera  Spherical mirror added to focus light onto pyro, after observing orbit sensitivity  Knife-edge beam-size scanner replaced with test of THz transport along a copper wire  No change to interferometer

4. 4 OAP Pyro e-e- Rotating Mirror THz BS Reference Signal Interference Signal 1-µm Ti foil CCD for TCAV Insertable Mirror THz CCD Pyro Spherical Mirror Insertable silicon plate Side View 4.5-mm-Diameter Copper Tubing THz Table Layout during the 2013 Run From ChicaneTo IP Table OAP Michelson Interferometer Fisher: E206 THz Pyro Si

5. 5 Smaller Beam on the Transition-Radiation Foil Fisher: E206 THz Effect of beam size at THz foil from different electron optics  In 2012 run, simulation gave sizes for:  “Normal optics”: 1200 µm  6 µm  “Double-waist”: 320 µm  36 µm  In reasonable agreement with sizes seen using OTR from upstream THz foil  Test in 2013 to learn if smaller size would give more high-frequency content  On downstream THz OTR foil:  260 µm  130 µm for usual 2013 optics  113 µm  65 µm with special configuration  Quite similar THz radiation observed  Both gave 37 µJ per pulse  Almost the same transverse size at focus  Similar THz spectra and reconstructed waveforms  Transverse size was already small and was not the limiting factor

6. 6 Misalignment of Foil Fisher: E206 THz After the 2012 run, pneumatic actuators with single foils were replaced with motorized “ladders” with multiple foils.  Evidence that the upstream THz foil was misaligned when installed:  THz pulse energy was significantly lower than last year  37 µJ this year with 10 10 electrons versus 400 to 600 µJ last year with 2  10 10  Expect 100 µJ (scaling for charge), or more due to smaller beam size  Repeatedly maximized THz energy when collimating off-axis parabolic mirror (OAP) was 8 mm upstream of the middle of the THz window  Broken radial symmetry: Affects coupling to Sommerfeld mode (discussed later)  HeNe laser, at 90° to beamline, reflected from back of upstream THz foil; light hits beampipe before reaching downstream THz foil (<1 m away)  Camera at upstream THz window could not see OTR beam image  Faint image on a YAG was seen, but no OTR: More directional?  OTR beam image was easily seen at downstream THz foil  No opportunity for vacuum break after confirming problem

7. 77 Comparing THz and Smith-Purcell Fisher: E206 THz THz and Smith-PurcellTHz Reconstruction of a compressed bunch

8. 8 Notch Collimator: THz Measurements With notch collimator: incomplete split With notch collimator: full split Without notch collimator: wide beam Fisher: E206 THz

9. 9 THz Notch Collimator: Comparing THz and TCAV Fisher: E206 THz  TCAV data was taken immediately before starting THz interferometer scan  Some evidence for residual vertical dispersion, which would affect TCAV calibration  May account for discrepancy in peak separation THz and TCAVTCAV Δt = 518 fs (Δz = 155 µm) σ left = 72 fs σ right = 106 fs σ left = 82 fs σ right = 70 fs

10. 10 Sommerfeld Mode: THz Transport along a Wire Fisher: E206 THz  THz diffracts quickly in free space  Waveguides are far too lossy  Two options:  Free-space propagation with large mirrors and frequent refocusing  Confined mode  Testing Sommerfeld’s mode (1899)  Transports a radially polarized wave outside a cylindrical conductor  Low loss and low dispersion  Mirror can reflect fields at corners  Collaborating with Daniel Mittleman (Rice University), who first applied this to THz

11. 11 Testing Sommerfeld’s Mode  Began test during 2013 run  4.5-mm-diameter copper tubing  0.8-m straight path on the THz table  Suspended by thin nylon fishing line  Transmission observed but not yet fully characterized or optimized  No time for several reconfigurations  Asymmetry from misalignment of CTR foil reduces coupling to wire mode  Plans for next run  Optimize coupling and transmission  Add a 90° bend  Recollimate and measure transmitted spectrum with interferometer  Look for enhanced field at tapered tip  Goubau (1950) modified wire surface  May increase transport distance while reducing radial spread Sommerfeld Calculations for a 4.8-mm Copper Wire Fisher: E206 THz

12. 12 THz Timing Diagnostic Fisher: E206 THz Investigating the “switched mirror” concept  THz incident on silicon at Brewster’s angle: full transmission  Fast laser pulse creates electron-hole pairs  Rapid transition to full reflection  Time of transition slewed across surface by different incident angles  Pyroelectric camera collects both transmitted and incident THz pulses  Measures temporal profile and laser-electron jitter, shot by shot  Goal: ~20 fs resolution  Depends on laser absorption depth and carrier dynamics on a fs timescale  Bench tests this summer  Begin beam tests in next run

13. 13 Summary Fisher: E206 THz During the spring 2013 run:  Tested smaller beam size at foil  Compared measurements with Smith-Purcell (E203)  Longitudinal profile measured with notch collimator  Started testing guided THz-transport mode Plans for next run in October:  More transport tests  Testing shot-by-shot profiles and time jitter using a switched mirror Longer range:  Possible start of a transport line to laser room in the Gallery