1. Group and phase velocity matching in THz IFEL interaction
Emma Curry
University of California, Los Angeles
EAAC
Sept. 28, 2017

2. Acknowledgments Funding
Prof. Pietro Musumeci1 Asst. Prof. Jared Maxson1,2 Prof. Avraham Gover3 Siara Fabbri1
1Department of Physics and Astronomy, UCLA, Los Angeles, California 90095, USA 2Cornell Laboratory for Accelerator-Based Sciences and Education, Cornell University, Ithaca, New York 14853, USA 3Department of Physical Electronics, Tel Aviv University, Tel Aviv 69978, Israel
Funding

3. Outline Motivations Zero-slippage IFEL interaction
Tunable IFEL resonance
Longitudinal phase space measurements
Bunch length compression
Conclusions

4. THz-driven beam manipulation
Motivations
Sustain high gradient fields, orders of magnitude larger than conventional RF based accelerators
Large phase acceptance and guiding structures compared to optical frequencies
Single cycle THz pulse from optical rectification
Synchronized to electron beam
Extend duration of interaction?
Total e-bunch capture
High acceleration gradient
Low Frequency High Frequency
THz field
Fabiańska, J., et al. Scientific Reports 4 (2014) Fallahi, A., et al. PRSTAB 19, (2016) Nanni, E., et al. Nature Comm. 6 (2015).

5. Zero-slippage IFEL interaction
Phase velocity matching for IFEL resonance
Single frequency
Group velocity matching for zero-slippage interaction
Tangential intersection indicates broadband interaction
Extend to meter-scale
THz
Electron
beam
Undulator Magnets
CPPWG
Two degrees of freedom:
Tune electron beam energy to achieve IFEL resonance
Adjust waveguide aperture to control THz group velocity matching

6. Undulator and Waveguide
3 cm period Halbach array
0.45 T peak field on-axis
10 periods with additional entrance and exit section
Curved Parallel Plate Waveguide (CPPWG)
Composed of two distinct aluminum plates fabricated at UCLA using conventional machining techniques
Plate spacing 1.5 – 4 mm range, limited by undulator magnet gap
Plate curvature acts as focusing element to minimize diffraction losses through gap
Vertically polarized THz focused into waveguide entrance using off-axis parabolic mirror
Mounting plates
Magnet arrays

7. Undulator and Waveguide
CPPWG mount
Undulator
3 cm period Halbach array
0.45 T peak field on-axis
10 periods with additional entrance and exit section
Curved Parallel Plate Waveguide (CPPWG)
Composed of two distinct aluminum plates fabricated at UCLA using conventional machining techniques
Plate spacing 1.5 – 4 mm range, limited by undulator magnet gap
Plate curvature acts as focusing element to minimize diffraction losses through gap
Vertically polarized THz focused into waveguide entrance using off-axis parabolic mirror
CPPWG

8. PEGASUS beamline THz source Beam control Diagnostics Beam Parameters
Solenoid focusing and steering magnets align beam through hole in OAP and CPPWG
Diagnostics
Vertical deflection in X-band cavity maps longitudinal position on the y-axis
Horizontal dispersion in dipole magnet maps beam energy on the x-axis
Intensified CCD camera
Beam Parameters
Bunch energy
4-9 MeV
Undulator period
3 cm
Undulator parameter, K
1.27
# of undulator periods 10
CPPWG spacing mm
Plate curvature radius
2 mm
Peak frequency
0.8 THz
Pulse energy
1 μJ
THz source
Pulse-front-tilted optical rectification of 800 nm laser in sLN

9. Tunable IFEL resonance
Resonant beam energy shifts with CPPWG spacing
Larger spacing → smaller resonant energy
THz phase velocity decreases
Consecutive beam images show energy spectrum for THz off and on
Small vertical slice used to determine the FWFM of the distribution
Timing between THz and beam synchronized via generation by same laser pulse
CPPWG spacing
2.4 mm
1.8 mm
Measured up to 150 keV energy modulation
Lines indicate the FWFM energy modulation from simulation

10. Longitudinal Phase Space Measurements
Beam energy on the x-axis and temporal position on the y-axis
Energy modulation reflects the waveform of the evolving THz pulse
See dispersed THz profile predicted by EOS measurements
Peak-to-peak energy modulation vs. THz pulse energy shows expected linear relationship
Time-of-arrival jitter evident in the position of the energy modulation with respect to the electron beam
400 fs rms temporal jitter for PEGASUS

11. Bunch length compression
Short electron beam injected at the zero-crossing of waveform for energy chirp
Plot RMS bunch length vs. energy spread
TOA jitter results in wide range of THz injection phase
Color indicates the energy-position correlation in the LPS image
Decompressed beam has both large energy spread and bunch length
Compression by factor of two: decreases to a minimum of 200 fs
still shows positive energy correlation
Undulator to streak camera distance (1 m) too short for maximum compression
Timing synchronization

12. Longitudinal Profile Diagnostic
Future Work
Longitudinal Profile Diagnostic
Proof-of-principle THz-streaking experiment planned at PEGASUS for later this year
CPPWG and undulator assembly built for IFEL experiment will be re-used
TE02 mode excited in CPPWG for interaction between transversely oscillating beam trajectory and longitudinal B-field
THz amplification
Pick off the THz pulse at exit of the CPPWG
Real-time interferometer for single shot THz pulse measurement
Thangaraj, J., et al. "Demonstration of a real-time interferometer as a bunch-length monitor in a high-current electron beam accelerator." Rev. Sci. Instrum. 83.4 (2012):
Increase bunch charge
Simulate tapered undulator configuration
For more information see Curry et al, New J. Phys. 18, (2016)
Curry et al, Nucl. Instr. Meth. Phys. Res. A (2017).

13. Conclusions
Group and phase velocity matching achieves a broadband interaction with an initially near-single cycle THz pulse over 30 cm
TOA synchronized via generation from same laser pulse
Resonant beam energy shifts with CPPWG spacing for tunable IFEL interaction
Measured up to 150 keV energy modulation
Longitudinal phase space measurements show well-defined sinusoidal energy modulation
Bunch compression by factor of two
Scales linearly with THz field
Time-stamping with energy modulation
Future Directions
THz-driven longitudinal profile diagnostic
THz amplification via FEL interaction
Undulator Magnets
CPPWG