Dielectric Wakefield Acceleration at CLARAVELA Toby Overton Dielectric

Dielectric Wakefield Acceleration at CLARA/VELA Toby Overton

Dielectric Wakefield Acceleration (DWA) • Electrons travelling at β≃1 emit Cherenkov radiation perpendicular • Can use a dielectric material to reflect radiation back into the structure • Accelerate a witness bunch with gradients ∼GV/m • The strength of the accelerating gradient is dependent on: • Separation between dielectric plates (either planar or radius) • Charge of the driving bunch • Bunch length (negative relationship) S. S. Baturin, PRL 113, 214801 (2014)

Experimental Outlook Experiments have mainly been focused in the US SLAC FACET, Argonne National Lab, Jefferson Lab etc. Set of experiments last year in Daresbury were the UK’s first DWA experiment Highest accelerating gradient of 1. 3 GV/m measured Measured at SLAC 15 cm long cylindrical device Need to achieve high gradients over longer distances Transverse effects become more and more important Testing modelling methods suggested by theorists Potential to speed up simulation time

Transverse Field Effects – Driving Beam • With the bunch in the centre of the structure, only see quadrupole field • Defocusing in y, focusing in x • When off-centre, dipole terms become more prominent • Shows the potential use of DWA as kickers • Effects approx. order of magnitude lower than accelerating field • Considerable deflection • Need to control transverse kicks to stop BBU • How to reduce? • Elliptical beam • Equivalent FODO cell of DWAs Driving particle at structure centre Driving particle off centre

CLARA/VELA Facility for DWA First experiments conducted in 2017 Poor beam quality and low bunch charge affected the quality of results Still the first demonstration of DWA in the UK Now able to solve these problems ~35 Me. V electron beam with bunch charge Bunch length 200 fs is still limiting The tail of the bunch will experiencing the wake of the head increases transverse effects Longer driving beam decreases the accelerating gradient Large beam area for diagnostics and the DWA structures Ability to provide variable bunch length, longitudinal profiles and chirps BA 1

Dechirper Setup Planar structure of variable dielectric gap Want to control the deceleration gradient longitudinally Not ruin the beam quality either Transverse effects important Interested in measuring energy spread and distribution More general use to conventional accelerators Both from Pacey, Saveliev, Xia NIMIP 2018

DWA Structures One of the main focuses will be transverse effects in DWA Not looked at in detail before by other experiments Large coffin in BA 1 allows for more measurements Focus on measuring the beam emittance dilution – overall and slice Comparing planar and cylindrical (+ wildcard square) of similar proportions Should achieve higher accelerating gradient in cylindrical structure Comparing structures with same half-gap and radius is a direct comparison between the two structure shapes Elliptical (high aspect) beam as a method of reducing BBU Hopefully measure accelerating gradients ~100 MV/m No witness beam but from the deceleration of the driving bunch

Next Steps Hopefully we see controllable transverse effects Ability to develop novel ways to control DWA Want a controlled driving beam deceleration Having a beam of almost constant energy at the end is desirable Testing methods for calculating the transverse force Going from fast methods being point-like charge to a real bunch Investigating other structure types with these methods

Extra Slides

Single Particle Transverse Fields

Transverse Force within a bunch

Conformal Mapping Method • Parameters from CLARA slide • Focusing force for an electron 100μm from structure centre • Defocusing force is equal and symmetric