Towards a Practical Multi-Meter Long Dielectric Wakefield - - PowerPoint PPT Presentation

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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

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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

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Motivation for the multi-meter long dielectric wakefield accelerator

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A schematic of the dielectric wakefield accelerator

DWAs and high transformer ratios

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.

High transformer ratio wakes excited by double-triangular beams in DWAs

TR=16, f=850 GHz

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5

~50 m ~50 m ~200 m

SRF: 2.5 GeV ~1 MHz

E-gun Undulators ~300 m Spreader

experimental end stations

Compact Inexpensive Flexible

A concept of a multi-user FEL facility

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Production of shaped bunches for the DWA

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Approximate dimensions: High transformer ratio wake:

• Tolerances. For ∆G/G < 10-4 we must

have

DWA with a double-triangular drive bunch and a trapezoidal witness bunch

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The shape of the mask that cuts out the correct bunch shapes out of the Gaussian distribution. Transverse particle distribution in Elegant:

Production of the shaped bunches

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Emittance exchanger (EEX)

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Issues

• Nonlinearities.
• Space charge.
• Beam loss at the mask, X-rays, etc.

Beam aberrations due to nonlinearities of the beamline: Large momentum spread due to space charge:

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Alternative ideas: diamond field emitter arrays?

• Exquisitely sharp diamond

pyramids.

• Current > 1 A/mm2.
• Emittance < 1 mm*mrad.
• Naturally suited for production
• f shaped electron bunches.

We measured ~20 μA currents emitted by single diamond pyramids.

10 µm

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DWA afterburner for a multi-user FEL facility

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Cumulative collective instability develops due to exposure

• f tail electrons to transverse wake field.

Fz ~ Q/a2 F ~ Q/a3

Limitation: beam break up of the drive bunch

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*) C Li et al., to be published Tapered quadrupole gradient Main dump Wakefield accelerator Drive

E0 E0

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.

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• 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.

Transverse oscillation of particles of a chirped beam (no wake)

Initial energy chirp ~15 %

main After 8 m of DWA

BNS damping of BBU

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main Note an off-center shift 3.5 ps

• Space charge effects are not

included.

• Witness bunch is not
• ptimized.

Illustration: 0 m

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main

• The main bunch

accelerates, the drive bunch develerates.

Illustration: 4 m

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Illustration: 17 m

main

• New effect: some particles in

the tail of the drive bunch start lagging behind. slow particles

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Illustration: 34 m

main slow particles

• The tail of the drive bunch

decelerates, mixes with the main bunch and now sees the accelerating field.

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1 2 3 1 2 3

s

• Move main bunch to

second maximum (can be difficult if done using the mask).

• Make adaptive frequency

channel (easy).

• Use drive bunch with

higher energy (affects facility cost and energy efficiency).

main

Possible solutions for de-phasing

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Summary of options

Propagation distance with 0% particle loss (PD0) Energy loss of the drive bunch at PD0 Energy of the witness bunch at PD0 Particle loss at 20 m No FODO 0.72 m 2.8% 464 MeV 100% FODO with no chirp 4.5 m 18% 797 MeV severe FODO with 15% chirp 20 m 80% 2.03 GeV 0% FODO with reduced chirp (7%) + parabolic current content 19.68 m 77.5% 1.97 GeV 0.5%

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• Facilities, equipment and

Conclusions

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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.