Towards a Practical Multi-Meter Long Dielectric Wakefield Accelerator: Problems and Solutions Evgenya I. Simakov, Dmitry Yu. Shchegolkov Los Alamos National Laboratory Alexander A. Zholents Argonne National Laboratory AAC2016 August 4th, 2016 Operated by Los Alamos National Security, LLC for NNSA
Outline Motivation for the multi-meter long DWA Production of shaped bunches for a DWA Multi-meter propagation of the shaped bunches in a DWA Conclusions Operated by Los Alamos National Security, LLC for NNSA
Motivation for the multi-meter long dielectric wakefield accelerator Operated by Los Alamos National Security, LLC for NNSA
DWAs and high transformer ratios A schematic of the dielectric High transformer ratio wakes excited by wakefield accelerator double-triangular beams in DWAs TR=16, f=850 GHz By shaping the drive electron beam in a DWA into a double- triangular shape one may achieve high transformer ratios, way higher than TR=2. Operated by Los Alamos National Security, LLC for NNSA
A concept of a multi-user FEL facility experimental end stations Undulators Spreader SRF: 2.5 GeV Compact E-gun ~1 MHz ~300 m ~200 m ~50 m ~50 m Inexpensive Flexible Operated by Los Alamos National Security, LLC for NNSA 5
Production of shaped bunches for the DWA Operated by Los Alamos National Security, LLC for NNSA
DWA with a double-triangular drive bunch and a trapezoidal witness bunch High transformer ratio wake: Approximate dimensions: Tolerances. For ∆ G/G < 10 -4 we must have Operated by Los Alamos National Security, LLC for NNSA
Production of the shaped bunches Transverse particle distribution in Elegant: The shape of the mask that cuts out the correct bunch shapes out of the Gaussian distribution. Operated by Los Alamos National Security, LLC for NNSA
Emittance exchanger (EEX) Operated by Los Alamos National Security, LLC for NNSA
Issues • Nonlinearities. • Space charge. • Beam loss at the mask, X-rays, etc. Beam aberrations due to nonlinearities of Large momentum spread due to space the beamline: charge: Operated by Los Alamos National Security, LLC for NNSA
Alternative ideas: diamond field emitter arrays? We measured ~20 μ A currents • Exquisitely sharp diamond emitted by single diamond pyramids. pyramids. • Current > 1 A/mm 2 . • Emittance < 1 mm*mrad. • Naturally suited for production of shaped electron bunches. 10 µm Operated by Los Alamos National Security, LLC for NNSA
DWA afterburner for a multi-user FEL facility Operated by Los Alamos National Security, LLC for NNSA
Limitation: beam break up of the drive bunch Cumulative collective instability develops due to exposure of tail electrons to transverse wake field. F z ~ Q/a 2 F ~ Q/a 3 Operated by Los Alamos National Security, LLC for NNSA
BBU suppression with quadrupoles BBU can be controlled by a quadrupole wiggler. Since the main bunch is getting decelerated, the strength of the wiggler must be tapered to match the beam. Tapered quadrupole gradient E 0 Main E 0 Drive dump Wakefield accelerator *) C Li et al., to be published Operated by Los Alamos National Security, LLC for NNSA
BNS damping of BBU • Energy chirp in the bunch results in particles of different energies having different oscillation periods in FODO lattice. • No resonant excitation of the dipole mode. After 8 m of DWA Transverse oscillation of particles of a chirped beam (no wake) main Initial energy chirp ~15 % Operated by Los Alamos National Security, LLC for NNSA
Illustration: 0 m main 3.5 ps Note an off-center shift • Space charge effects are not included. • Witness bunch is not optimized. Operated by Los Alamos National Security, LLC for NNSA
Illustration: 4 m main • The main bunch accelerates, the drive bunch develerates. Operated by Los Alamos National Security, LLC for NNSA
Illustration: 17 m main slow particles • New effect: some particles in the tail of the drive bunch start lagging behind. Operated by Los Alamos National Security, LLC for NNSA
Illustration: 34 m main slow particles • The tail of the drive bunch decelerates, mixes with the main bunch and now sees the accelerating field. Operated by Los Alamos National Security, LLC for NNSA
Possible solutions for de-phasing • Move main bunch to second maximum ( can be difficult if done using the main mask ). s • Make adaptive frequency channel ( easy ). • Use drive bunch with higher energy ( affects facility cost and energy 1 2 3 efficiency ). 1 2 3 Operated by Los Alamos National Security, LLC for NNSA
Summary of options Propagation Energy Energy of Particle distance with loss of the witness loss at 20 0% particle the drive bunch at m loss (PD0) bunch at PD0 PD0 No FODO 0.72 m 2.8% 464 MeV 100% FODO with no 4.5 m 18% 797 MeV severe chirp FODO with 15% 20 m 80% 2.03 GeV 0% chirp FODO with reduced chirp 19.68 m 77.5% 1.97 GeV 0.5% (7%) + parabolic current content Operated by Los Alamos National Security, LLC for NNSA
Facilities, equipment and Conclusions Operated by Los Alamos National Security, LLC for NNSA
Conclusions Dielectric wakefield accelerators with shaped electron bunches may become effective afterburners (for example, for the future X-ray FEL facilities). We developed (although not ideal) means for production of arbitrary shaped electron bunches. Beam breakup may severely limit energy extraction from the drive bunch. Effective means for suppressing beam breakup: tapered FODO lattice and energy chirp. More experiments required. Operated by Los Alamos National Security, LLC for NNSA
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