Gigavolt-per-meter Gradients in a Dielectric Wakefield Accelerator Brendan O’Shea August 5 th, 2016 Advanced Accelerator Conference 2016 Experimental Results

2 Dielectric Wakefield Acceleration 1) 1) Cook et al, Phys. Rev. Lett (2009) 2) O’Shea et al, Nat. Commun. to be published ~2 GV/m gradients have been shown 2) Potential source of high energy terahertz Straight forward to fabricate Wakefield Damping Deflecting Modes 15 cm

Final Focus Spectrometer Terahertz Mirror DWFA Beam Direction Beam Position Monitors 3 Experimental Description ParameterValue Beam Energy20.35 GeV Charge3 nC Beam IP30x30 μm β-function0.15x2.5 m Bunch Length20-100s μm

4 High-Field Induced Wakefield Damping e-ze-z Damping associated with increase in fields Fits into on-going research in electron-field resonances in insulating materials. Two bunch will not notice but may be problematic for bunch trains.

5 Spectra of CCR

6 …but not impact ionization is the attenuation parameter

7 High Field Phenomenon Virtual Photoconductivity (Franz-Keldysh like) Not studied at terahertz! Typically: FACET: 1) 1) Yablonovitch et al, Phys. Rev. Lett., Vol. 63, N. 9 (1989) Dielectric “Metalization” FACET operates here 2) 2) Schiffrin et al, Nature 493, (2013) At FACET effect is strongly adiabatic due to low photon energy

8 Breakdown? Breakdown: Sudden termination in CCR resulting in drop in collected energy Observations: Not sure outliers are breakdown Even so, no apparent increase in number of outliers as field increases Outliers appear with more energy as well as less!

9 Excitation of Higher Order Modes Higher Order Modes interferometer resolves ~ 5.9GHz noise is important Feedback on beam- structure alignment HEM 11 TM 01 HEM 21 TM 02 Cylindrical Structure: Dipole and Quadrupole are seen!

10 Slab Symmetry Slab structures mitigate deflecting forces 1) No CCR signal loss with change in aspect ratio Damping wasn’t obvious ZTA, Diamond, SiO2 Band gap:10.6, 5.5, 9 eV Flat beam Round beam dielectric metal 1) A. Tremaine, et al, Phys. Rev. E 56, 7204 (1997)

11 Transverse Modes in SLABs TMs Deflecting Modes Simulation Data Not Dielectric

12 Positron Experiments DWFA are indifferent to charge. Measurements so far agree. Comparison of hard vs soft walled structures: DWFA vs Hollow Channel PWFA. Deflection of Positrons vs Offset in DWFA

13 Future Work and Conclusions FACET DWA Research: High Gradient ( 2 GV/m) 300 MV/m accelerating gradient Damping Positrons Future Work: Higher efficiency Beam Break-Up Origin of damping Staging Studies Resonant THz excitation FACET II provides these opportunities

14 Thank You UCLA: Oliver Williams, Gerard Andonian, Phuc Hoang, Sam Barber, Kristin Fitzmorris, Sahel Hakimi, Jere Harrison, Brian Naranjo, JB Rosenzweig SLAC: Mark Hogan, Vitaly Yakimenko

15 CCR Collection Geometry e-e- Dielectric Structure Terahertz CCR TM 01 HEM Mirror Interferometer

16 Directivity of SLABs + Angle Cuts Acceptance of optics in vacuum chamber

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