Dielectric Wakefield Accelerators at FACET II Brendan OShea

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

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 2

Experimental Layout Measurement of Cerenkov radiation spectral content Measurement of beam energy

Coherent Cherenkov Radiation (CCR) • We are ringing the right modes • Are we ringing other undesirable HEM modes? • • a) b) c) d) TM 01 400 GHz TM 02 1. 2 THz HEM 11 350 GHz HEM 12 585 GHz 1 cm tube Autocorrelation Trace Spectrum of a) 10 cm 450/640 tube spectrum 10 cm 400 um steel tube spectrum 4

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 5

How is data taken?

1) Thompson et al, PRL 100 214801 (2008) High Gradient Measurements • 5 GV/m breakdown limit previously measured 1) • 300 um (2 a) ID, 400 um (2 b) OD, 15 cm long • Energy change of 200 Me. V • Measured average decelerating gradient of 1. 35 GV/m a) Beam Energy b) Simulation of Fields • Peak gradient expected to be 2. 8 GV/m 7

Drive-Witness Measurement • 400 um (2 a) ID, 530 um OD (2 b), 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 Driver: 1. 53 n. C @ 55 um Witness: 0. 96 n. C @ 30 um a) Drive Beam Energy b) Witness Beam Energy c) Simulation of Fields 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 Divergence Growth: 100% in Drive, 61% in Witness not yet performed. Energy Spread Change: Drive: 1. 1% -> 1. 08 % Witness: 0. 44% - > 0. 73% 9

FACET-II Beam Break-Up • Longer Beta Functions: >1 m ideal for examining beam breakup 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 Ge. V -> 11. 2 + 8. 8 Ge. V -> 5 + 3. 2 ke. V Photons • High acceptance chicane sets bunch spacing