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Terahertz-driven accelerators for high-brightness electron- and X-ray based instruments
Nicholas H. Matlis1, Dongfang Zhang1, Tobias Kroh1,2,4, Christian Rentschler1,2,3, Umit Demirbas1,5, Moein Fakhari1, Mostafa Vahdani1,2, Reza Bazrafshan1, Junhao Zhang1, Koustuban Ravi1, Timm Rohwer1, Zhelin Zhang1,5, Marvin Edelmann1, S. M. Mohamadi1, Simon Reuter1, Mikhail Pergament1, and Franz X. Kärtner1,2,4
1Center for Free-Electron Laser Science CFEL, Deutsches Elektronen-Synchrotron DESY, Notkestr. 85, 22607 Hamburg, Germany
2Department of Physics, University of Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany
3Max Planck School of Photonics, Hans-Knöll-Straße 1, 07745 Jena, Germany
4The Hamburg Centre for Ultrafast Imaging, University of Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany
5Tsung-Dao Lee Institute, Shanghai Jiao Tong University, Shanghai 200240, China
5Antalya Bilim University, 07190 Dosemealti, Antalya, Turkey
In the last decades, X-ray free electron lasers (XFELs) and ultrafast electron diffraction (UED) instruments have emerged as preeminent tools for decoding the mysteries of molecular and material processes and paving the way for development of new technologies to address the exponentially-increasing technological demands of society. Guiding this revolution is the idea of tracking the atomic and electronic structure of matter on atomic spatial and temporal scales during fundamental processes like chemical reactions in molecules or evolution of collective modes and phase changes in solid-state materials. To observe these quantum phenomena with sufficient spatial and temporal resolution requires coherent probes of extreme characteristics including Ångstrom-scale wavelengths and femtosecond-scale durations. Until recently, such probes, in the form of either electrons or X-rays have been produced primarily using large-scale accelerators available at national laboratories. These accelerators, which have undergone a century of development and painstaking refinement, can now produce ultrabright electron
beams of exquisite performance that have opened the door to the quantum world and are fueling world-wide development of new XFELs and UED instruments. Nevertheless, hints that the limits of this technology, which is powered by radio-frequency (RF) electromagnetic waves, are on the horizon is motivating development of alternative “advanced” laser-based accelerator technologies with novel properties that have potential to extend the performance in one or more aspects. A key accelerator performance metric is the brightness of the electron bunches. This parameter is strongly affected by the strength of the initial electric fields experienced during the creation of the beam at the photocathode (i.e., the extraction fields) as well as of the fields used to accelerate the electrons to their final energy. This dependence is a result of mutual-repulsion of fermionic electrons via the Coulomb interaction which can be increasingly mitigated via relativistic effects by maximizing the field strength and hence minimizing the time the electron bunches remain at low energies. The field strength that can be applied
to an accelerator structure, however, is limited by field-induced discharge which dramatically impacts performance and can destroy the device. The maximum field strength achievable is dependent on the material properties and the surface preparation as well as the frequency and duration of the applied field which govern the probability of electron emission. The drive to increase field strengths has therefore sparked development in multiple directions, including extension of conventional accelerators from RF (1 – 3 GHz) to higher frequencies (10 GHz and above) where the breakdown threshold is higher as well as exploration of alternate materials to form the accelerating structures like laser-driven plasmas which can sustain enormous fields and are impervious to destruction.
A new approach that appeared within the last decade and is rapidly gaining visibility is extension of traditional accelerator concepts to terahertz (THz) frequencies [1]. This approach, pioneered in our group, is expected to enable increases in field strength by one to two orders of magnitude, with corresponding improvements in the brightness of the electrons and ultimately the spatiotemporal resolution of the associated instrument. THz-driven accelerators, especially those powered by laser-based THz sources, present a number of additional advantages. First, like other laser-based accelerator concepts, THz-driven accelerators benefit from intrinsic synchronization between the laser-derived fields accelerating the electrons and the laser-derived pump beams used to induce material dynamics. Temporal jitter between the electron (or derived X-ray) probes and the laser pumps, which remains a limiting factor in the temporal resolution of RF-based devices, is thus to a large degree mitigated. Second, the shorter wavelengths of THz relative to RF fields combined with the increased field-strengths provide several-orders-of-magnitude higher field gradients which can be used to perform extreme manipulations of the electron-bunch phase-space, bringing novel capabilities not only to THz accelerators, but also to THz-enhanced RF accelerators. Finally, the smaller wavelength of the radiation leads to a corresponding reduction in the dimensions and hence volume of the accelerator structures. As a result, far less power is required to generate intense fields leading to dramatically reduced heating and the potential for scaling repetition rates (and hence average flux) to unprecedented levels.
These benefits, however, do not come without a cost. First, the smaller scale of the electromagnetic waves significantly increases the difficulty of controlling and maintaining the phase of wave experienced by the electron bunch which is necessary for reaching high energies. Second, the smaller waves and accelerator dimensions limit the number of electrons that can be packed into the bunch before Coulomb repulsion deteriorates its quality. Third, the smaller scale of the accelerator presents significant challenges for manufacturing the required structural features and achieving the required tolerances in terms of alignment and surface quality. Finally, the difficulty in generating sufficient THz energy strongly limits the electron energies currently achievable which is currently a primary factor limiting the general acceptance of the technology.
Nevertheless, significant progress has been made with increasingly frequent demonstration of key milestones. Following proof-of-principle demonstrations of THz-driven electron acceleration in photoguns [2] and LINACs [3], development of practical, multi-functional THz-driven electron accelerator modules operating at the 100 keV level was achieved [4–6]. Extension of these concepts to relativistic electron beams from conventional accelerators came shortly after, and very recently, use of THz-driven photogun technology for UED and electron microscopy at low energies (15 – 50 keV) has been demonstrated [7,8]. In parallel, enormous progress has been made in development of laser-based THz sources using nonlinear down conversion, both for single-cycle and multicycle THz generation. To continue this rapid pace of progress and bring THz-driven accelerator technology to a state suitable for mainstream use in ultrafast instrumentation, further development of multiple technologies is required, including high-quality THz-accelerator structures (Fig. 1a) and high-energy, high repetition-rate THz sources (Fig. 1b) as well as the lasers that power them. In this talk, I review the work in our group on these three frontiers and discuss specific technical and physical challenges to be overcome.
Figure 1: THz accelerator technology. a) Partially assembled prototype of a THz photogun mounted in an assembly station. The red spot is from a HeNe laser used for alignment of the multi-layered structure. Dielectric inserts used to tune the time of arrival of the THz pulse in each layer are visible on either side of the red spot. b) Photo of a cryostat used for high energy single-cycle THz generation by nonlinear down conversion of laser light. The prominent green color is unwanted second harmonic light parasitically generated in the interaction. In the center of the cryostat, a large aperture (35 mm tall) lithium niobate prism, which is the nonlinear conversion medium, is visible.
References
[1] F. X. Kärtner, et al., “AXSIS: Exploring the frontiers in attosecond X-ray science, imaging and spectroscopy,” Nucl. Instruments Methods Phys. Res. Sect. A Accel. Spectrometers, Detect. Assoc. Equip. 829, 24–29 (2016)
[2] W. Ronny Huang, A. Fallahi, X. Wu, H. Cankaya, A.-L. Calendron, K. Ravi, D. Zhang, E. A. Nanni, K.-H. Hong, and F. X. Kärtner, “Terahertz-driven, all-optical electron gun,” Optica 3, 1209–1212 (2016)
[3] E. A. Nanni, W. R. Huang, K. H. Hong, K. Ravi, A. Fallahi, G. Moriena, R. J. Dwayne Miller, and F. X. Kärtner, “Terahertz-driven linear electron acceleration,” Nat. Commun. 6, 1–8 (2015)
[4] D. Zhang, A. Fallahi, M. Hemmer, X. Wu, M. Fakhari, Y. Hua, H. Cankaya, A. L. Calendron, L. E. Zapata, N. H. Matlis, and F. X. Kärtner, “Segmented terahertz electron accelerator and manipulator (STEAM),” Nat. Photonics 12, 336–342 (2018)
[5] D. Zhang, A. Fallahi, M. Hemmer, H. Ye, M. Fakhari, Y. Hua, H. Cankaya, A. Calendron, L. E. Zapata, N. H. Matlis, and F. X. Kärtner, “Femtosecond phase control in high-field terahertz-driven ultrafast electron sources,” Optica 6, (2019)
[6] D. Zhang, M. Fakhari, H. Cankaya, A. L. Calendron, N. H. Matlis, and F. X. Kärtner, “Cascaded Multicycle Terahertz-Driven Ultrafast Electron Acceleration and Manipulation,” Phys. Rev. X 10, 011067 (2020)
[7] D. Zhang, T. Kroh, F. Ritzkowsky, T. Rohwer, M. Fakhari, H. Cankaya, A.-L. Calendron, N. H. Matlis, and F. X. Kärtner, “THz-Enhanced DC Ultrafast Electron Diffractometer,” Ultrafast Sci. 2021, 9848526 (2021)
[8] J. Ying, X. He, D. Su, L. Zheng, T. Kroh, T. Rowher, M. Fakhari, G. Kassier, J. Ma, P. Yuan, N. H. Matlis, F. X. Kärtner, and D. Zhang, “High-gradient Terahertz-driven ultrafast photogun,” Nat. Photonics (2024)