Abstract-Performance analysis of the prototype THz-driven electron gun for the AXSIS project

G. Vashchenko (1), R. Assmann (1), U. Dorda (1), M. Fakhari (1 and 2), A. Fallahi (1), K. Galaydych (1), F. Kaertner (1 and 2 and 3), B. Marchetti (1), N. Matlis (1), T. Vinatier (1), W. Qiao(1), C. Zhou (1 and 2), ((1) Deutsches Elektronen-Synchrotron, (2) University of Hamburg, (3) The Hamburg Centre for Ultrafast Imaging)

https://arxiv.org/abs/1804.01279

The AXSIS project (Attosecond X-ray Science: Imaging and Spectroscopy) aims to develop a THz-driven compact X-ray source for applications e.g. in chemistry and biology by using ultrafast coherent diffraction imaging and spectroscopy. The key components of AXSIS are the THz-driven electron gun and THz-driven dielectric loaded linear accelerator as well as an inverse Compton scattering scheme for the X-rays production. This paper is focused on the prototype of the THz-driven electron gun which is capable of accelerating electrons up to tens of keV. Such a gun was manufactured and tested at the test-stand at DESY. Due to variations in gun fabrication and generation of THz-fields the gun is not exactly operated at design parameters. Extended simulations have been performed to understand the experimentally observed performance of the gun. A detailed comparison between simulations and experimental measurements is presented in this paper.

Abstract-Beam dynamics and tolerance studies of the THz-driven electron linac for the AXSIS experiment

K. Galaydych, R. Assmann, U. Dorda, B. Marchetti, G. Vashchenko, I. Zagorodnov, 

https://www.sciencedirect.com/science/article/pii/S016890021830439X

A dielectric-loaded linac powered by THz-pulses is one of the key parts of the “Attosecond X-ray Science: Imaging and Spectroscopy” (AXSIS) project at DESY, Hamburg. As in conventional accelerators, the AXSIS linac is designed to have phase velocity equal to the speed of light which, in this case, is realized by tuning the thickness of the dielectric layer and the radius of the vacuum channel. Therefore, structure fabrication errors will lead to a change in the beam dynamics and beam quality. Additionally, errors in the bunch injection will also affect the acceleration process and can cause beam loss on the linac wall. This paper numerically investigates the process of electron beam acceleration in the AXSIS linac, taking into account the aforementioned errors. Particle tracking simulations were done using the code ECHO, which uses a low-dispersive algorithm for the field calculation and was specially adapted for the dielectric-loaded accelerating structures.

AXSIS: FRONTIERS IN ATTOSECOND X-RAY SCIENCE: IMAGING AND SPECTROSCOPY

https://axsis.desy.de/

X-ray crystallography yields atomic-resolution 3D images of the whole spectrum of molecules ranging from small inorganic clusters to large protein complexes consisting of hundred-thousands of atoms that constitute the macromolecular machinery of life. Life is not static, and many of the most important reactions in chemistry and biology are light induced and occur on ultrafast timescales. These have been studied with high time resolution primarily by optical spectroscopy, enabled by ultrafast laser technology, but they reduce the vast complexity of the process to a few reaction coordinates.

In the AXSIS project at CFEL-Hamburg, funded by the European Research Council, we develop the new method of attosecond serial X-ray crystallography and spectroscopy, to give a full description of ultrafast processes atomically resolved in real space and on the electronic energy landscape, from co-measurement of X-ray and optical spectra, and X-ray diffraction. This technique will revolutionize our understanding of structure and function at the atomic and molecular level and thereby unravel fundamental processes in chemistry and biology like energy conversion processes.

For that purpose, we develop a compact fully coherent, THz driven, attosecond X-ray source based on coherent inverse Compton scattering off a free-electron crystal, to outrun radiation damage effects due to the necessary high X-ray irradiance required to acquire diffraction signals. This highly synergistic project starts from a completely clean slate rather than conforming to the specifications of a large free-electron laser (FEL) user facility, to optimize the entire instrumentation towards fundamental measurements of the mechanism of light absorption and excitation energy transfer.

A multidisciplinary team formed by laser-, accelerator,- X-ray scientists and bio-chemists optimizes X-ray pulse parameters, in tandem with sample delivery, crystal size, and advanced X-ray detectors. Ultimately, the new capability, attosecond serial X-ray crystallography and spectroscopy, will be applied to one of the most important problems in structural biology, which is to elucidate the dynamics of light reactions, electron transfer and protein structure in photosynthesis.

Abstract-Beam dynamics in THz dielectric-loaded waveguides for the AXSIS project

T Vinatier1, R W Assmann1, U Dorda1, F Lemery2 and B Marchetti1

http://iopscience.iop.org/article/10.1088/1742-6596/874/1/012042/meta;jsessionid=C02E64AFEB4527A91F3E6D34A998989A.c2.iopscience.cld.iop.org

In this paper, we investigate with ASTRA simulations the beam dynamics in dielectric-loaded waveguides driven by THz pulses, used as linac structure for the AXSIS project. We show that the bunch properties at the linac exit are very sensitive to the phase velocity of the THz pulse and are limited by the strong phase slippage of the bunch respective to it. We also show that the bunch properties are optimized when low frequencies (< 300 GHz) are used inside the linac, and that the longitudinal focal point can be put several tens of cm away from the linac exit thanks to ballistic bunching. However, a strong asymmetry in the bunch transverse sizes remains for which a solution is still to be found.