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1 E206 Terahertz Radiation from the FACET Beam SAREC Review SLAC 2013 July 26 Alan Fisher and Ziran Wu SLAC National Accelerator Laboratory
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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
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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
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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
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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
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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
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77 Comparing THz and Smith-Purcell Fisher: E206 THz THz and Smith-PurcellTHz Reconstruction of a compressed bunch
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8 Notch Collimator: THz Measurements With notch collimator: incomplete split With notch collimator: full split Without notch collimator: wide beam Fisher: E206 THz
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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
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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
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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
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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
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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
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