Cascaded Multicycle Terahertz-Driven Ultrafast Electron Acceleration and Manipulation

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Cascaded Multicycle Terahertz-Driven Ultrafast Electron Acceleration and Manipulation

Dongfang Zhang, Moein Fakhari, Huseyin Cankaya, Anne-Laure Calendron, Nicholas H. Matlis, and Franz X. Kärtner

Phys. Rev. X 10, 011067 – Published 19 March 2020

Abstract

Terahertz (THz)-based electron acceleration and manipulation has recently been shown to be feasible and to hold tremendous promise as a technology for the development of next-generation, compact electron sources. Previous work has concentrated on structures powered transversely by short, single-cycle THz pulses, with millimeter-scale, segmented interaction regions that are ideal for acceleration of electrons in the sub- to few-MeV range, where electron velocities vary significantly. However, in order to extend this technology to the multi-MeV range, an investigation of approaches supporting longer interaction lengths is needed. Here, we demonstrate first steps in electron acceleration and manipulation using dielectrically lined waveguides powered by temporally long, narrow-band, multicycle THz pulses that copropagate with the electrons. This geometry offers centimeter-scale single-stage interaction lengths and offers the opportunity to further increase interaction lengths by cascading acceleration stages that recycle the THz energy and rephase the interaction. We prove the feasibility of THz-energy recycling for the first time by demonstrating acceleration, compression, and focusing in two sequential ${Al}_{2} O_{3}$ -based dielectric capillary stages powered by the same multicycle THz pulse. Since the multicycle THz energy achievable using laser-based sources is currently a limiting factor for the maximum electron acceleration, recycling the THz pulses provides a key factor for reaching relativistic energies with existing sources and paves the way for applications in future ultrafast electron diffraction and free-electron lasers.

Received 5 July 2019
Revised 25 October 2019
Accepted 6 February 2020

DOI:https://doi.org/10.1103/PhysRevX.10.011067

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Electron beams & optics Light-matter interaction Photonics Ultrafast optics Ultrafast phenomena

Electron sources

Terahertz techniques

Accelerators & BeamsAtomic, Molecular & Optical

Authors & Affiliations

Dongfang Zhang ^1,*, Moein Fakhari^1,2, Huseyin Cankaya^1,2, Anne-Laure Calendron¹, Nicholas H. Matlis¹, and Franz X. Kärtner^1,2

¹Center for Free-Electron Laser Science, Deutsches Elektronen Synchrotron, Notkestrasse 85, 22607 Hamburg, Germany
²Department of Physics and The Hamburg Centre for Ultrafast Imaging, University of Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany

^*To whom correspondence should be addressed. dongfang.zhang@cfel.de

Popular Summary

Terahertz-based electron acceleration has recently emerged as a promising candidate for next-generation compact electron sources. In these devices, pulses of terahertz radiation accelerate electrons across a relatively short distance. Although initial work in this field has demonstrated impressive capabilities for acceleration and beam manipulation, the electron energy gain is still limited by the short interaction distance and low terahertz energy for powering the device. Here, we demonstrate the first steps toward creating a scheme to overcome these hurdles.

Our setup uses dielectric-lined waveguides powered by narrow-band, long-duration terahertz pulses (known as multicycle pulses) that copropagate with the electrons. The terahertz phase velocity is matched with the electron speed, which enables effective beam manipulation across interaction lengths roughly a centimeter long in the nonrelativistic regime—about an order of magnitude greater than other methods. Moreover, we demonstrate a novel scheme of cascaded acceleration and manipulation by terahertz-energy recycling and rephasing the interaction, which offers the opportunity to further increase the interaction length.

Increased interaction length is important because the efficiency of current laser-based technologies for terahertz generation is still rather low, which requires high-power laser systems for electron acceleration to mega-electron-volt energies. Our cascading scheme will greatly lower the demand on the required laser system for electron acceleration in the nonrelativistic regime, opening new possibilities for the design of terahertz-based accelerators.

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Vol. 10, Iss. 1 — January - March 2020

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Figure 1
Experimental setup. A small fraction of the 1030-nm infrared optical beam is converted to 257-nm based on two-stage second harmonic generation. The 257-nm UV pulse is directed onto a gold photocathode generating electron pulses, which are accelerated to 53 keV by the dc electric field with around 1 fC charge. The same infrared laser also drives a multicycle THz generation stage and two single-cycle THz stages for the DLW manipulator and the STEAM streaker, respectively.
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Figure 2
${TM}_{01}$ —mode parameters of DLW. Group (a) and phase (b) velocity of the ${TM}_{01}$ mode at 260 GHz in a circular copper waveguide with dielectric loading. The red dots represent the design point for electron beam manipulation with $250 μ m$ vacuum radius and $140 μ m$ dielectric thickness. Transverse (c) and longitudinal (d) cross section of the electric field distribution at ${TM}_{01}$ mode with 43% phase velocity at the design parameter for the experiment simulated with cst microwave studio [36]. The dielectric wall thickness is $140 μ m$ , and the vacuum channel diameter is $510 μ m$ .
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Figure 3
DLW-based electron manipulation. (a) Measured electron energy spectra for the input beam (black curve) and accelerated beam (blue curve) in the acceleration mode. (b) Measured (hollow square) and simulated (red line) electron bunch length as a function of the applied THz field in compression mode. The simulation is performed with astra with around 1 fC charge injected into the DLW. (c) Time-dependent deflection diagrams measured by varying the delay between the arrival time of the electron bunch and the deflecting THz pulse for initially uncompressed and compressed electron bunches. (d) Electron beam diameter as a function of the THz field strength in focusing condition. (e) Electron peak intensity change as a function of the THz-electron delay.
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Figure 4
Cascaded manipulation. (a) Schematic of the cascaded acceleration. Because of the low group velocity of the accelerating THz field inside the DLW, after THz-electron interaction, the electrons overtake the THz wave at the exit of DLW I. In a vacuum, the THz pulse catch up and overtake the electron bunch again before both enter DLW II for a second interaction, i.e., acceleration stage. (b) Measured electron energy spectra for the input beam (black curve), single-stage accelerated beam (blue curve), and cascaded electron acceleration (red curve). (c) Measured pulse duration via the single-stage compression (blue curve) and two-stage compression (red curve). (d) Top: Simulation of the experimental condition with two-stage cascaded acceleration that couples 20 nJ THz pulses with $0.11 MV / m$ longitudinal electric fields into the DLW I. The THz coupling efficiency from the first to second stage is approximately 10%. Bottom: Simulation of two-stage cascaded acceleration with approximately $100 MV / m$ field gradients inside the DLW I and approximately 90% THz coupling from DLW I to DLW II. (e) Simulation of the electron pulse duration (FWHM) during propagation with a single DLW stage and two DLW stages. The coupling of THz into the second stage is assumed to be approximately 10%. The simulation is performed with astra with around 1 fC charge injected into the DLW corresponding to the experimental conditions.
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