1

E206Terahertz Radiation

from the FACET Beam

SAREC ReviewSLAC

2013 July 26

Alan Fisher and Ziran WuSLAC National Accelerator Laboratory

2Fisher: E206 THz

Topics

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

3Fisher: E206 THz

Changes to the THz Table Layout

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

4Fisher: E206 THz

OAP

Py

ro

e-

Rotating Mirror

THz BS

ReferenceSignal

InterferenceSignal

1-µm Ti foil 1-µm Ti foil

CCD for TCAV

Insertable Mirror

THz CCD

Pyro

Spherical MirrorInsertable

silicon plate

Side View

4.5-mm-DiameterCopper Tubing

THz Table Layout during the 2013 Run

From Chicane To IP Table

OAP

OAP

MichelsonInterferometer

Pyro

Si

5Fisher: E206 THz

Smaller Beam on the Transition-Radiation Foil

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-2 0 2 4 6 8 10 12

-0.2

-0.1

0

0.1

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0.5

Time (ps)

E-f

ield

(M

V/c

m)

New OpticsNorm. Optics

0 0.5 1 1.5 2 2.5 30

0.1

0.2

0.3

0.4

0.5

0.6

0.7

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0.9

1

Frequency (THz)

Nor

mal

ized

Pow

er D

ensi

ty

New OpticsNorm. Optics

6Fisher: E206 THz

Misalignment of Foil

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 1010 electrons versus 400 to 600 µJ last year with 21010

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

7Fisher: E206 THz 7

Comparing THz and Smith-Purcell

THz and Smith-PurcellTHz

Reconstruction of a compressed bunch

8Fisher: E206 THz

Notch Collimator: THz Measurements

With notchcollimator:

incomplete split

With notchcollimator:

full split

Without notchcollimator:wide beam

9Fisher: E206 THz

THz

Notch Collimator: Comparing THz and TCAV

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 TCAV TCAV

Δt = 518 fs(Δz = 155 µm)

σleft = 72 fsσright = 106 fs

σleft = 82 fsσright = 70 fs

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

11Fisher: E206 THz

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

12Fisher: E206 THz

THz Timing Diagnostic

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 absorptiondepth and carrier dynamicson a fs timescale

Bench tests this summer Begin beam tests in next run

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