X-ray source technology

The X-ray source technology we are developing for AXSIS can be categorized in different projects that are explained in more detail below.

Conceptual overview

X-ray sources with greater than keV photon energy and sufficient brilliance to perform X-ray crystallography are based on relativisitic electron bunches from synchrotrons or linear accelerators. The relativistic motion confines the angle of emission to a narrow cone dramatically increasing source brightness. The electron beams are produced by thermionic or photo cathodes, and are sent into radio-frequency (RF) driven accelerators made up of metallic RF structures operating often at S-band frequencies, i.e. around 1-3 GHz. Frontier facilities like LCLS in the US and the future European XFEL project are linear accelerator (LINAC) based and accelerate electrons to highly relativistic (10 GeV) energies. A LINAC relies on room temperature or super-conducting RF technology. The accelerating gradients in either case are limited to several tens of MeV per meter limited by field emission from cavity walls. The LINAC length, therefore, must be in the km regime and facility costs are in the billion Euro category.

The production of femtosecond electron bunches has seen steep advances over the last decades. Bunches can be created by photoemission from the cathode in the presence of a strongly accelerating field and can be many picoseconds long. These pulses are then injected into a bunch compressor and low charge bunches, 1–10 pC, may be compressed down to 3–10 μm in bunch length which correspond to pulse lengths of 10–30 fs. Compression factors exceeding 100 are routinely achieved at LCLS. At a given accelerating field strength and RF frequency, compression is limited by space charge to a few femtoseconds. These short electron bunches from conventional LINACS are sent into periodic magnetic fields, undulators, with a typical period of 3 cm. The undulators force the electrons on sinusoidal paths to emit photons. Under certain conditions, a coherent, exponential self-amplification of the spontaneously emitted radiation is achieved, by the so-called self-amplified spontaneous emission (SASE) principle. For this process to occur, typically more than 1,000 undulator periods are necessary, putting the length of these radiation structures in the range of 500 m and longer. One consequence of this large length is a very small fractional bandwidth of emission, which is inversely proportional to the number of undulator periods. Therefore, a typical XFEL bandwidth is less than 0.1% of the center wavelength. In addition, it requires an extraordinary energy stability of 0.01% in the electron beam. LINAC-based FELs have enabled significant advances in X-ray science and pioneered the field of femtosecond X-ray crystallography. The FEL process, as sketched above, is constantly being improved for increased scientific reach. So-called seeding techniques have been implemented to improve the coherence properties of the FEL output. Seeding at UV wavelength and high gain harmonic generation (HGHG) and direct seeding with EUV radiation generated via high-order harmonic generation has led to fully coherent output in the EUV at FERMI [Allaria 2012]. Efforts are underway at LCLS to use emittance spoiling of the electron bunch to achieve sub-fs-duration X-ray pulses. However, most of these advances go along with strongly reduced overall photon yield, while the cost of the facility and its complexity stay the same. Furthermore, self-seeding is only successful in a small fraction of the fs pulses, and only this small fraction of the images can be used for data analysis [Barends 2015].

In the AXSIS project, we combine the optimum X-ray pulse specifications for attosecond serial crystallography and spectroscopy, which are co-evolving as the new methodology, and develop a new accelerator and radiator technology that leads to a highly optimized and compact source for this application. Key aspects of the design principle of the new X-ray source is the implementation of 200 times higher accelerator frequencies, i.e., THz frequencies and correspondingly shorter pulse durations, which increases the threshold for field emission, and thus we expect to utilize 10–100 times higher field strength in the range of 1 GV/m, both in the gun and accelerator. The higher operating frequencies and field strength will enable low bunch emittance and compression to attosecond duration with high charge. In addition, we use optical undulators, i.e., inverse Compton scattering, with a period on the order of 1 μm, 30,000 times shorter compared to magnetic undulators. This in turn allows reduction of the required electron beam energy from 10 GeV to tenth of MeV to reach sub-Ångström radiation. This comes at a cost, which is that the SASE FEL regime is more difficult to access under those conditions. However, new developments in ultrafast nanostructured photocathode arrays combined with precision optical field control eventually enables the production of an electronic Coulomb crystal (Wigner crystal). An ideal electron crystal can be used to produce a coherent emission in an undulator leading to a fully temporally and spatially coherent pulse with attosecond duration from a compact source, while maintaining or even surpassing the peak brilliance of the current hard XFELs. Fig. 1 shows a schematic layout of the proposed AXSIS source and experimental setup currently under construction in the SINBAD-facility at DESY.

Fig. 1. Schematic layout of the AXSIS setup currently under construction at the SINBAD-facility at DESY in Hamburg.

Over the past few years, key demonstrations have been made showing the feasibility of this concept. The electronic Coulomb crystal can be generated from a coherently controlled nanostructured field emitter array (FEA) of various types [Hommelhoff 2006, Hobbs 2014, Keathley 2013, Ropers 2007]. The FEA might be driven by a rectangularly shaped multi-cycle IR or mid-IR laser pulse, that is polarized along the in-plane field emitter elements. The incoming field strength is tuned such that the resulting tip field is typically enhanced by a factor of 10–100 beyond the incoming field, sufficient for field emission of ideally one electron per emitter and cycle. This process results in a three dimensional electron crystal emitted from the FEA, which is further accelerated by the strong (~1 GV/m) THz pulse. Within a few mm of propagation distance the accelerated electron bunch reaches close to relativistic velocities and leaves the gun. The multi-cycle IR-field is shaped to conform with the THz field, such that the exact time of the electron emission is coherently controlled and leads to an equal arrangement of electrons with an approximate bunch length of 100 fs. After leaving the gun, the bunches are already as short as the shortest bunches achievable in a typical S-band accelerator. As an example, we aim for a typical bunch charge of about 3 pC (20 million electrons) that are sliced into 20 discs each containing about a million electrons. This micro-structured bunch is then further accelerated and undergoes velocity bunching by injection into a THz-powered waveguide. The waveguide ideally provides phase and velocity matching between THz radiation and the electron bunch to allow for a highly relativistic electron bunch.

THz-based acceleration of 3 pC electron bunches to about 20 MeV, i.e., a bunch energy of 60 μJ, requires on the order of 20 mJ accelerating THz pulse energy [Wong 2013] , limiting the acceleration to 0.3% extraction efficiency without excessive beam loading effects. Up to date, such high THz energies have only been available using electron beam sources as the driver, which is not compatible with the idea of a compact source. However, very recently theoretical analysis and experiments have shown more than 1% conversion efficiency from sub-ps optical pulses to single-cycle THz pulses and 0.1 % conversion to multi-cycle pulses both in the 0.5-THz wavelength range [Fülöp 2011, Fülöp 2012, Huang 2013, Carbajo 2015]. Further increases in conversion efficiencies towards the multi-percent range seem possible. Thus 1-J-level optical ps-pulses as are also necessary for an efficient ICS-interaction should be sufficient for the THz generation.

A proof-of-principle THz-acceleration experiment based on laser-generated single-cycle THz pulses at 0.45 THz has been performed recently [Nanni 2015]. Fig. 2 a) shows the setup. The single-cycle THz pulse with a center frequency of about 450 GHz (see Figs. 2(b) and (c)) was generated with a sub-ps Yb:KYW regenerative amplifier by optical rectification in cryogenically cooled LiNbO3 using the tilted-pulse-front technique. Of the generated 10 μJ THz energy, about 1 μJ was ultimately coupled into a dielectrically loaded metal waveguide optimized for accelerating sub-relativistic electron bunches provided by a 60-keV DC electron gun. Figs. 2 (d) and (e) shows the electron beam on a multi-channel plate (MCP) after a deflecting dipole magnet without and with accelerating THz pulse. The electron bunches are generated by a UV-photocathode in the electron gun. The generated electron pulses have spread beyond one ps when reaching the THz-powered waveguide and therefore both acceleration and deceleration of electrons occur, which leads to a split beam on the MCP when the THz acceleration is on. Figs. 2(f) and (g) shows the measured and simulated electron energy spectra for both cases. From the measured electron energy spectra, we infer that the 1-μJ single-cycle THz pulse readily leads to a 7 keV electron beam energy modulation.

Fig. 2. (a) THz LINAC and source with the THz acceleration chamber and accompanying power supplies, chillers and pumps on a portable optical cart. (b) THz acceleration pulse generated by optical rectification in cryogenically cooled LiNbO3 . (c) THz beam profile. (d) Electron beam profile without THz pulse. (e) Electron beam profile with THz pulse on. (f) and (g) comparison between measured (black) and modelled (red) electron spectra without and with THz acceleration pulse [Nanni 2015].

The current goal is to achieve >5% percent of optical-to-THz generation efficiency. Close to a 1-Joule laser pulse will be necessary for generating the required 20 mJ THz pulse energy in the waveguide including transport losses. Such energy is also necessary for providing an optical undulator with an equivalent undulator parameter close to 1 as desired for efficient X-ray generation. Thus we aim for a few-ps 1-J laser capable of operating ultimately at up to 1 kHz pulse repetition rate. Cryogenically cooled Yb-based amplifiers are the laser technology of choice for this application. Cryogenic Yb:YAG amplifiers with 1 J of pulse energy operating at 100 Hz repetition rate have already been demonstrated [Rocca 2012]. We have recently demonstrated a compact composite thin-disk cryogenic Yb:YAG amplifier with up to 160 mJ of pulse energy at 100 Hz repetition rate [Zapata 2015] and scaling of this technology to the 1-J level at 1 kHz repetition rate is currently in progress.

The development of the proposed AXSIS source will of course face challenges and detailed start-to-end simulations are necessary to support the desired design principles and specifications outlined in this paper as well as to develop the machine. Such simulations are well on its way but surpass greatly the page limit set for this paper and therefore we defer them to a forthcoming article. However, back off the envelope simulations as well as first simulations indicate that the AXSIS source produces already as a low risk inverse Compton scattering source, i.e. no bunching of electrons is needed, 106 X-ray photons per shot at 12 keV within an opening angle of 1 mrad and at 1 kHz repetition rate. When operating in the regime where micro bunching is achieved either due to the use of an electron crystal generated in the gun or in combination with partial bunching due to FEL action, 108 to 109 photons per shot at 12 keV within an opening angle of less than 100 μrad and at 1 kHz repetition rate seem possible.