Dielectric Wakefield Accelerators at FACET (II) Brendan O’Shea October 15 th, 2015

2 Dielectric based THz sources Fabrication is cheap and can be fast Relies on stock at fiber companies or wafer manufacturers Fabrication time on the order of 1 week. Total cost: ~$15

Experimental Layout 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

How is data taken?

7 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

8 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

9 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. Divergence Growth: 100% in Drive, 61% in Witness Energy Spread Change: Drive: 1.1% -> 1.08 % Witness: 0.44% - > 0.73%

FACET-II Beam Break-Up Longer Beta Functions: >1m ideal for examining beam break- up and ideas for mitigation. Multi-Bunch Investigate Wakefield Damping and loading and implications for colliders. First need to describe + overcome damping! Use DWA to modulate energy before FEL – Two Colors Two bunches generated with energy difference on the order of the energy spread. DWA used to transfer energy from drive to witness. Ex: 10 GeV -> GeV -> keV Photons High acceptance chicane sets bunch spacing