Outline of a dielectric laser acceleration experiment at SwissFEL

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Abstract

The Accelerator on a Chip International Program (ACHIP), funded by the Gordon and Betty Moore Foundation for a 5 year period, pursues basic research and development for a super-compact accelerator on a chip, where the accelerating structure is a dielectric microstructure excited by femtosecond laser pulses. The Paul Scherrer Institute (PSI) will contribute to this by providing the international collaboration access to the high-brightness electron beams in SwissFEL, where we plan to do a proof-of-principle demonstration of the acceleration of a highly relativistic beam. In this contribution we present the conceptual layout of the experiment, in which we will focus the beam down to sub-micrometer beam sizes. Start-to-end simulation results of the tracked electron beam, and first calculations of the accelerating field of the microstructure will be shown.

Introduction

Dielectric laser acceleration (DLA), a modern technique based on a laser powering a dielectric microstructure, can achieve gradients in the GV/m range, implying a potential reduction of conventional accelerators’ length by one order of magnitude or more [1]. The Accelerator on a Chip International Program (ACHIP) has the goal of carrying out research and development activities for DLA. The ACHIP program includes performing a proof-of-principle experiment of DLA at relativistic energies using the high-brightness beam delivered by SwissFEL [2] at the Paul Scherrer Institute (PSI). The goal is to demonstrate a gradient of 1 GV/m for a dielectric length of 1 mm, resulting in an acceleration of 1 MeV. In this document we present the current design of the DLA experiment that will be performed at SwissFEL from 2017, including the experiment location, the laser, the chamber and diagnostics. The main parameters of the experiment are shown in Table 1. We also present results of electron beam start-to-end simulations and acceleration calculations at the dielectric structure.

DLA has already been demonstrated in several experiments—see for instance Refs. [1], [3], [4]. In all previous work, done at significantly lower energies than the ones that will be available at SwissFEL, the transverse dimensions of the beam were too large to travel through the structure without significant losses. Now at SwissFEL the goal is to accelerate all electrons that reach the structure. To achieve that, for the double-pillar structures of our experiment, the total transverse beam size should be smaller than 1.2μm in the horizontal (x) and smaller than 7.0μm in the vertical plane (y).

Section snippets

Experiment location

The DLA experiment will be carried out at SwissFEL, a free-electron laser (FEL) facility presently under construction at PSI that will serve two beamlines: Aramis, a hard X-ray beamline that will produce FEL radiation in 2017, and Athos, a soft X-ray beamline expected to lase by 2021. Fig. 1 shows a schematic overview of the facility. The electrons are generated in an RF photoinjector. After that the beam is accelerated with S-band technology (f3 GHz) to an energy of 330 MeV and compressed in

Electron beam

For the studies presented here, corresponding to the first experiments, the electron bunch length is longer than the laser wavelength. Consequently, the beam will not experience a net energy gain but an energy spread increase—an energy gain of 1 MeV corresponds to an rms energy spread increase of 0.71 MeV. To have a good resolution, the initial beam energy spread should be much smaller than the expected energy spread increase in the structure. In the near future we will investigate how to

Laser and timing

A commercial Ti:Sapphire oscillator with a regenerative amplifier (Coherent, Astrella) will likely be used as the laser source. The output from the amplifier system will be employed to pump an optical parametric amplifier (Light Conversion, TOPAS Prime) with a final output wavelength of 2μm, a pulse energy of 500μJ, a FWHM pulse length of 100 fs, and a repetition rate of 1 kHz. To match the longitudinal length of the dual-pillar structure, the spot size of the laser will be 1 mm in the direction

Structure

Fig. 4 (left) shows the generic layout of the double-pillar structure that will be used for the experiment. It consists of a polycrystalline sapphire (Al2O3) substrate, on which there is a double row of pillars of the same material. The electron beam traverses the structure between these rows. The pillars, spaced at the free-space wavelength of 2μm, longitudinally modulate the electric field, as necessary for net acceleration. Furthermore, we have a TE monopole type resonance inside the pillar,

Experimental chamber

The experimental chamber is displayed in Fig. 5. It consists of the dielectric microstructure, two triplets of permanent quadrupoles, an alignment stage for the structure, the laser coupling and relevant instrumentation in a two-meter vacuum chamber that can be placed on a girder. As shown in the figure, the vacuum chamber is laid out symmetrically around the interaction point. The chamber will be constructed during the summer and fall of 2016, and should be ready for installation in 2017.

The

Diagnostics

The diagnostics installed in the experimental chamber should allow verification of the electron beam optics as well as serve for the spatial and temporal alignment of the electron beam, the structure and the laser at the focus. Given the expected beam size, the spatial resolution should ideally reach 100 nm and cover an area of about 1 mm for both horizontal and vertical directions. The beam should be observable at different locations in the vicinity of the focal plane. For this purpose, we plan

Conclusion and outlook

We have presented the initial design of the DLA experiment to be performed at SwissFEL from 2017 within the ACHIP collaboration. All key components such as the laser, the dielectric structure, and the chamber with its diagnostics have been specified and will be available for the experiments. Our simulation results show that the electron beam will traverse the structure without losses, and first calculations indicate that we should be able to demonstrate an acceleration gradient in excess of 1 

Acknowledgments

Many thanks go to Thomas Schietinger for a careful proof-reading of the manuscript. This work is supported by the Gordon and Betty Moore Foundation.

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