High Gradient Dielectric Wakefield Experiments Brendan O’Shea, Oliver Williams, Gerard Andonian, Jere Harrison, Kristin Fitzmorris, James Rosenzweig, Mark Hogan, Vitaly Yakimenko September 14 th 2015 Two Bunch Measurements

2 Structures Used at FACET ParameterValue Beam Energy20.35 GeV Charge3 nC Beam IP30x30 μm β-function0.15x1.5 m Bunch Length20-100s μm

3 Experiment Measurement of Cerenkov radiation spectral content Measurement of beam energy

4 Coherent Cherenkov Radiation (CCR) We are ringing the right modes Are we ringing other undesirable HEM modes? TM GHz TM THz HEM GHz HEM GHz a)1 cm tube Autocorrelation Trace b)Spectrum of a) c)10 cm 450/640 tube spectrum d)10 cm 400 um steel tube spectrum

5 CCR Damping Kramers-Kronig minimum phase retrieval Unusually sharp fall off in field strength – the pulse is too short Expect: 18 mm Measured: ~5 mm Possible Sources: Halo electron induced conductivity High field induced conductivity Beam CCR Transition Region

6 High Gradient Measurements 5 GV/m breakdown limit previously measured 1) 300 um (2a) ID, 400 um (2b) OD, 15 cm long Energy change of 200 MeV Measured average decelerating gradient of 1.35 GV/m Peak gradient expected to be 2.8 GV/m 1) Thompson et al, PRL (2008) a)Beam Energy b)Simulation of Fields

7 Drive-Witness Measurement 400 um (2a) ID, 530 um OD (2b), 10 cm long Witness bunch gradient of 320 MV/m Drive bunch gradient of 250 MV/m Separation: 250 um 640 MV/m peak unloaded gradient ~80% efficiency a)Drive Beam Energy b)Witness Beam Energy c)Simulation of Fields Driver: um Witness: um

8 Dipole Modes Thus far difficult to measure via CCR. Do show up in BPMs downstream of DWA. Effects in energy measurement can be corrected for using BPMS and spectrometer. Systematic studies not yet performed.

9 Summary Very high gradients have been measured in DWAs, 1.3 GV/m Witness bunches have been accelerated at 320 MV/m Future Research: Dipole mode studies Exotic structures (slabs, Braggs, woodpiles), i.e. structures with favorable mode composition Examine source of CCR losses Positrons

10 Dielectric Wakefield Accelerators (DWA) Goal: Move away from km scale systems Potential source of THZ radiation Takes advantage of nanofabrication techniques Capable of GV/m fields, show 1.3 GV/m deceleration and 300 MV/m acceleration here Long lifetimes (many 10000s of shots) 1) Gai et al, Phys. Rev. Lett. 61, 2756 (1988) 2) Andonian et al, Phy.s Rev. Lett. 108, (2012) 1) 2)

11 Beam Quality Drive Beam Energy Spread: 1.1% -> 1.08% Witness Beam Energy Spread 0.44% -> 0.73%

12 Bunch Length and Separation

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